Photographs of Flooding: Courtesy of Ken Mason

Passumpsic River Flood Mitigation Study- Volume 1 of 2

Prepared for:Town of Lyndon, VT

55 North Stark HighwayWeare, NH 03281 603-529-4400

Prepared by:

Main Report

November 2006

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Executive Summary Background The Town of Lyndon and village of Lyndonville, VT (hereafter referred to as the “town”) are located in Caledonian County in the northern Vermont. The town is located in close proximity to the Passumpsic River as shown in Figures 1.0-1 and 1.0-2 (see Volume 2-Figures). The East and West Branches combine to form the beginning of the Passumpsic River. From Lyndon the Passumpsic River meanders through a wide flat valley floor. For purposes of this report, the term “project reach” represents the Passumpsic River from its origin to the Vail Dam. The river is flanked on the east by the village of Lyndonville which also sits in the flat valley floor. After passing through town, the Passumpsic River passes over the Vail Dam and then the Great Falls Dam before traveling several miles to its confluence with the Connecticut River in Barnet, VT.

Over the last few years, most notably June 2002 and January 2000, the Passumpsic River has overflowed its banks and caused considerable flooding within the town. These recent floods and others have resulted in considerable property damage, power interruptions, traffic interruptions, and disruption to the residents and businesses located within the town. The repeated flooding has raised the town’s awareness of its connection to the Passumpsic River. The town and public are seeking answers and potential solutions to reduce flooding. After

the June 2002 flood event, the town convened a meeting in which members of the public voiced their opinions including potential solutions to flooding. The purpose of this study is to assist the municipality, residents and businesses in sorting out the varying advice given by prior flood study efforts. In addition to a review of past findings, an independent analysis was conducted to help understand flooding issues. Finally, a recommendations section is provided that outlines potential structural and non-structural alternatives that Lyndon may wish to reduce flooding in Lyndon. Causes of Flooding There are many factors that can contribute to flooding in Lyndon as summarized below.

• Precipitation and the corresponding volume of runoff from the watershed area that drains into Passumpsic River within Lyndon directly impacts flooding.

• Land use development above Lyndon- specifically the conversion of forest land and wetlands to

urban development can influence flooding downstream in Lyndon. This report contains an analysis of long-term flow trends on the East Branch of the Passumpsic River. It appears that over time East Branch flows are slowly increasing, which may be a function of encroachment on the East Branch floodplain, inability of the East Branch to access its floodplain, timber harvesting, natural variability, or a host of other possibilities.

• Land use development within the rivers floodplain can contribute to flooding. Historic topographic

maps in this report show progressive development within the Passumpsic River floodplain in Lyndon.

Looking down on manufactured homes- North Lyndon

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Development in the floodplain reduces the area available to convey flow through the floodplain resulting in increased inundation.

• Changes in river channel morphology (plan, profile and dimension) and sediment deposition can

impact flooding in Lyndon. It is suspected that because the Passumpsic River is extremely flat and moves slowly through Lyndon, sediment from incoming tributaries (East Branch, West Branch, Miller Run) deposits in the river. Sediment influx to the Passumpsic River is also a function of the instability of upstream tributaries such as the East Branch. Over time, increased sediment deposition can result in “filling” the channel in Lyndon resulting in less cross-sectional area to convey flood flows and thus increased floodplain inundation.

• Upstream development and human disturbances

in the floodplain of major tributaries (East Branch, West Branch, Miller Run) entering Lyndon can contribute to flooding. On the East Branch there have been human activities (road and building development in the floodplain, gravel mining from the river bed, straightening of the river, etc) that have impacted the ability of the East Branch to access its floodplain. Access to a river’s floodplain is critical to storing and attenuating flood flows. In some locations the East Branch is incising- meaning the bottom of the river is lowering through erosional processes over time. Erosion of the channel bed results in the transport of sediment downstream in the Passumpsic River as noted in the above bullet. Because the East Branch can not access its floodplain, there is less cross-sectional area in which to store/attenuate flood flows resulting in more water being conveyed to Lyndon. In addition, the incising of the East Branch river channel causes sediment deposition in the Passumpsic River in Lyndon where the water velocities slow. It is unknown if similar conditions are occurring on the West Branch or Miller Run whereby the river can not access its floodplain.

• Hydraulic controls such as bridges and the Vail Dam can also contribute to flooding. From the

beginning of the Passumpsic River to the Vail Dam there are seven bridges that span the Passumpsic River. Debris and ice jams on these structures can exacerbate flooding. In addition to man-made hydraulic controls, natural hydraulic controls may exist in the river.

National Flood Insurance Program-Flood Insurance Study Lyndon is enrolled in the National Flood Insurance Program, which offers flood insurance to Lyndon residents and businesses. As part of the program, the Federal Emergency Management Agency (FEMA) conducted a flood insurance study (FIS) which was initially completed in 1977, and then updated in 1988. As part of this study FEMA estimated the extent of the 100-year floodplain, which highlights those sections of town having a 1% chance of being flooded (inundated) in any given year. As part of the FIS, FEMA developed a “hydraulic model” of the Passumpsic River. The hydraulic model is used to predict the water surface elevation at various points along the Passumpsic River in Lyndon for various flood flows such as the 100-year flood. The estimated water surface elevation data is then used to estimate the land area susceptible to inundation from various flood flows. Past Flood Studies In addition to FEMA’s FIS, over the past six years there have been several studies that evaluated the causes of flooding in Lyndon. Most of these studies relied on the original hydraulic model developed by

~ 20+ feet

East Branch Passumpsic River

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FEMA, but each user applied different assumptions. Studies were conducted by the Federal Energy Regulatory Commission (FERC), which oversees hydroelectric projects such as Vail Dam. In addition, the US Army Corps of Engineers (Corps), the Vermont Department of Transportation (VTrans), and the US Geological Survey (USGS) completed flood investigations. These studies were conducted independently and in some cases yielded varying results based on different assumptions. Most of the past flood studies (FERC, USGS, Corps) focused on the contribution of flooding caused by the Vail Dam and bridges. The FERC and Corps used the original hydraulic model to determine if removal of the Vail Dam would reduce flooding in Lyndon. Although FERC and the Corps made different assumptions in their hydraulic models, their analysis showed that removing the Vail Dam resulted in a reduction of water levels upstream of the Vail Dam under the 100-year flood. However, both agencies concluded that the reduction of water levels upstream did not significantly reduce the area of inundation. In addition, both agencies noted that the reduction in the area of inundation did not reduce damage enough to justify the removal of the Vail Dam. During the FERC and Corps studies, the Vermont Agency of Natural Resources (VANR) and others noted that there may be bedrock ledge or a series of large boulders located within the Vail Dam impoundment. The purpose for raising this issue is if the bedrock or large boulders (which would serve as a natural hydraulic control) were only a few feet lower than the Vail Dam spillway crest elevation and spanned the river there may be limited flood benefits of removing the Vail Dam. It should be noted that neither the FERC nor the Corps studies described above incorporated any natural hydraulic control above the Vail Dam, as this information was not available at the time of their studies. Because of this concern, the Corps contracted with the USGS to conduct ground penetrating radar (GPR) in the area above Vail Dam. One of the purposes of this field effort was to determine the presence of bedrock or boulders above Vail Dam that could serve as the new hydraulic control if the Vail Dam were removed. Using GPR, the USGS collected a series of river profiles and cross-sections throughout the Passumpsic River from the Vail Dam to the beginning of the Passumpsic River. More detailed testing was conducted in the vicinity of the Vail Dam. Using the GPR, the USGS located bedrock/boulders approximately 410 feet upstream of the Vail Dam that nearly spanned the river channel as shown in USGS cross-section 6 below. In their report, the USGS concluded that “cross-section 6 may represent the

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most shallow depth to bedrock or boulder stream bed which could serve as a natural control of the water-surface if the dam is removed”. Although cross-section 6 does not show boulders or bedrock along the right side of the river, it was assumed that some natural control would exist, but not necessarily spanning the entire river channel. As discussed later, for modeling purposes, it was assumed that if the Vail Dam were removed, cross-section 6 would act as a ridged boundary. In addition to the Corps’ hydraulic analysis (completed in 2002), the Corps also conducted an evaluation of structural measures (levees, plastic sheeting) to control flooding in Lyndon (completed in 2006). The village of Lyndonville requested the assistance of the Corps under Section 205 authority to study flooding problems in the northern and southern parts of town. The Corps assessed the cost/benefit of a potential flood control project by estimating the cost and the associated flood protection benefit it provides. If the cost/benefit ratio is less than one, then the Corps concludes that the flood protection project is not warranted. In the case of Lyndon flood study, the cost/benefit ratio was close to 0.3, well below the ratio of one. Given this, the Corps did not move forward with any structural measures. It should also be noted that in the Corps’ 2006 report, they concluded that “either removal of the Vail dam or a bypass channel around the dam was ruled out as being ineffective and expensive, since ground penetrating radar (GPR) analysis test determined there is ledge in the river bed approximately 100 feet upstream of the Vail Dam, which would act as a natural dam even if Vail Dam is removed”. Independent Hydraulic Analysis An independent hydraulic analysis was conducted using FEMA’s original hydraulic model. The major change to the model was the inclusion of the Corps’ cross-section 6. It should be noted that when the USGS conducted the GPR, cross-section 6 (and other cross-sections) were not surveyed into a common vertical datum. Instead, the depth to bedrock/boulder at cross-section 6 was relative to the water surface elevation at the time of the survey. When conducting hydraulic models and floodplain mapping all cross-sections in the hydraulic model must be on the same vertical datum. Thus, several assumptions were made (as described more fully in Section 7.1) relative using cross-section 6 in a revised hydraulic model. As noted above, one assumption is that bedrock and boulders along a portion of cross-section 6 likely serve as some type of hydraulic control in the river, however, the hydraulic control does not necessarily span the entire river channel. The following alternatives were evaluated in the hydraulic model: Alternative 1- Existing Conditions- cross-section 6 was not added to the model, such that the original

FIS results (developed in a different version of a hydraulic model) could be compared to the most current hydraulic model.

Alternative 2- Existing Conditions- same as Alternative 1, but cross-section 6 was added to the hydraulic model.

Alternative 3- same as Alternative 2, but the Vail Dam, including the powerhouse and entire cross-section was removed.

Alternative 4- same as Alternative 2, but all bridges1 and encroachments along the bridges such as roadway fill leading to the bridges were removed and the Vail Dam remained.

Alternative 5- Natural Conditions- cross-section 6 was added to the model, Vail Dam and all bridges were removed. This alternative represents the lowest possible flood elevations.

Alternative 6- Bypass channel. This alternative included simulating a bypass channel around the Vail Dam- approximately 50% of the 100-year flood was bypassed around Vail Dam.

1 It should be noted that initially individual bridges were removed on an incremental basis in the hydraulic model to determine the flood impacts of individual bridges. However, because the net effect of removing all bridges was minimal, only the model results for the removal of all bridges is provided.

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Alternative 7- Lowering the Passumpsic River river channel in the project reach. This alternative was added to illustrate how channel cross-section area can impact flooding. It is recognized that lowering the entire channel within the project reach is not practicable. For purposes of this analysis, the channel cross-sections from the beginning of the Passumpsic River to just upstream of cross-section 6 were lowered an arbitrary 4 feet.

Summary of HEC-RAS modeling Alternatives (Source: Gomez and Sullivan, 2006)

Options Alt. 1 Alt. 2 Alt. 3 Alt. 4 Alt. 5 Alt. 6 Alt. 7 Cross-Section 6 added - X X X X X X Vail Dam removed - - X - X - - All Bridges removed - - - X X - - Bypass 50% of 100-year flood

- - - - - X -

Lowering river channel in project reach

- - - - - - X

For each alternative, the hydraulic model predicted the water surface profile for the 100-year flood within the project reach. In addition, for each alternative the area of inundation for the 100-year flood was displayed in plan view. The water surface profile along the Passumpsic River was plotted for each alternative and compared to Alternative 2, which represents the most up-to-date conditions. On each plot the water surface profile for existing conditions (Alt 2) is shown along with one alternative for comparison purposes to illustrate the difference in water levels. The following figures were developed and are available in Volume 2: Figure 7.3-1: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) Figure 7.3-2: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and All Bridges Removed (Alt 4) Figure 7.3-3: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Vail Dam and All Bridges Removed (Alt 5) Figure 7.3-4: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Bypass Channel (Alt 6) Figure 7.3-5: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and lowering the river channel by four feet within the project reach (Alt 7) The above figures provide information only on the vertical change in water surface elevation- it does not show the corresponding horizontal change in the area of inundation. The question is whether a reduction in the area of inundation reduces flooding of infrastructure (roads, houses, businesses, etc). The area of inundation for the 100-year flood was plotted for each alternative. On each plot the area of inundation for existing conditions (Alt 2) is shown along with one alternative for comparison purposes to illustrate the difference in the inundation area. The following figures were developed and are available in Volume 2: Figure 7.3-6: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) Figure 7.3-7: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and All Bridges Removed (Alt 4) Figure 7.3-8: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam and All Bridges Removed (Alt 5)

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Figure 7.3-9: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Bypass Channel (Alt 6) Figure 7.3-10: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and and lowering the river channel by four feet within the project reach (Alt 7) Figure 7.3-11: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) in the area around Vail Dam Discussion of Results Alternatives 1 and 2 As noted above, Alternative 1 reflected the same conditions as the original FIS. Because the predicted water surface elevations between the original FIS2 and HEC-RAS models were nearly identical, it was assumed that the HEC-RAS model was acceptable. There were negligible differences in the water surface elevations between Alternative 1 and 2. Because Alternative 2 included cross-section 6, reflecting more up-to-date conditions, it was used as a point of comparison for the other Alternatives. Alternative 3 Alternative 3 includes cross-section 6 and the full removal of the Vail Dam cross-section (dam, powerhouse, and upland topography) from the hydraulic model. It is important to understand that removing the entire Vail Dam cross-section also assumes that there is no bedrock beneath the dam. Based on Figure 7.3-1, removal of the Vail Dam reduces the water surface elevation by approximately 7.6 feet at a point 31 feet upstream of the dam. The water surface elevation drops by approximately 3.3 feet at cross-section 6, located 410 feet above the dam. The water surface elevation drops by approximately 1.4 feet at cross-section D, which is located between Vail Dam and Chapel Street Bridge. Above Chapel Street Bridge, the water surface elevation at the other cross-sections dropped on the order of 0.3 to 0.8 feet (less as you move upstream). As shown in Figure 7.3-6 removal of the Vail Dam does reduce the area of inundation primarily in the area just upstream of the dam, and just upstream of the Chapel Street Bridge. Shown in Figure 7.3-11 is a closer plan view of the inundation area with and without the Vail Dam. As these figures show, there are a few buildings (houses) on Town Highway 66 and Red Village Road as well as on Back Center Road (above Chapel Street Bridge) that would benefit from removal of the Vail Dam. Alternative 4 Alternative 4 includes the removal of all bridges (including encroachments due to roadway fill leading up to the bridges), but leaving the Vail Dam in place. As shown in Figure 7.3-2 the water surface profile for Alternative 2 closely matched existing conditions until the area of the Main Street Bridge. With the Main Street Bridge removed, the water levels decreased around 0.6 feet upstream. Although the hydraulic model showed that the Main Street Bridge contributes somewhat to flooding; the model does not account for debris jamming. Experience has shown that in fact debris jamming at the bridges has occurred and exacerbates flooding, thus the modeling effort does not reflect worst case conditions. As shown in Figure 7.3-7 there is some differences in the inundation area between Alternative 4 and existing conditions near the Main Street Bridge and along Miller Run. Alternative 5 Alternative 5 includes the removal of all bridges and the Vail Dam, which basically mimics “natural” conditions. As shown in Figure 7.3-3 the water surface profile for Alternative 5 shows a reduction in water levels immediately above the Vail Dam until the Chapel Street Bridge. From the Chapel Street Bridge to the Main Street Bridge, the reduction in water levels is minimal relative to existing conditions. 2 The original hydraulic model was called HEC-2- this was replaced with a new version called HEC-RAS.

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Upstream of the Main Street Bridge the water levels decreased by approximately 0.6 feet. As shown in Figure 7.3-8, there is some reduction in the 100-year flood inundation area near the Vail Dam and at a few locations upstream. Alternative 6 Alternative 6 includes the addition of a bypass channel that would divert flow from upstream of the Vail dam to a location below the dam. In the analysis, it was assumed that 50% of the 100-year flood flow would be conveyed into the bypass channel. As shown in Figure 7.3-4 the water surface profile for the effect of the bypass channel on flooding shows a reduction in water levels above the Vail Dam to the Chapel Street Bridge and diminishes thereafter. As shown in Figure 7.3-9, there is a reduction in the 100-year flood inundation area particularly in the area of Vail Dam and Chapel Street Bridge. Alternative 7 Alternative 7 includes the arbitrary lowering of the channel bed by four feet throughout the entire project reach up to cross-section 6. As shown in Figure 7.3-5 the water surface profile for Alternative 7 shows a reduction in water levels on the order of 1.8 feet near the Chapel Street Bridge and slowly reducing further upstream where the water levels drop as much as 4 feet in the upper reach (FEMA cross-section K). As shown in Figure 7.3-10, there is a reduction in the 100-year flood inundation area throughout the entire project reach, with greater reductions occurring further upstream. This alternative illustrates that the channel cross-sectional area has a direct relationship to the level of flooding. For example, consider the effect of adding four feet of sediment to each cross-section as opposed to removing it as in this alternative. Four feet of sediment added to the cross-sections would raise the water surface elevations much higher. Note that the cross-sections used in this model were obtained in 1977, which reflects only a snapshot in time. These cross-sections are likely different today as there has likely been sediment deposition within the project reach as summarized below. This is discussed further below. Geomorphic Impacts on Flooding in Lyndon The discussion above focused primarily on measures within Lyndon that could potentially reduce flooding. However, it does not include factors that occur within the watershed above Lyndon that may also influence flooding in Lyndon. There are three main tributaries that empty into the Passumpsic River within Lyndon- East Branch, West Branch and Miller Run. On the East Branch a geomorphic study (called a Phase 2 geomorphic assessment) was conducted, which provided, among many issues, information on the ability of the East Branch to access its floodplain during flood events. As described in more detail in Section 8.0 of this report, the East Branch has been incising as the river bed has lowered over time due to a variety of anthropogenic activities (such as gravel mining, straightening of the river, encroachment within the floodplain). The progressive incising of the East Branch channel bed directly impacts flooding in Lyndon due to:

• Increased potential for sediment deposition within the Passumpsic River in Lyndon. Because the East Branch is incising in several locations it is eroding sediment (the channel bed), which is deposited further downstream in the East Branch, or in the slow-moving sections of the project reach in Lyndon. The more sediment that fills the Passumpsic River channel in Lyndon decreases the channel area and reduces the river’s ability to convey flood flow and thus increases the likelihood of overflowing its river banks (Alternative 7 illustrated these findings).

• Increase potential for debris/ice jamming in Lyndon. Some reaches of the East Branch have been

straightened. An incised and straightened channel, as opposed to a naturally meandering channel, is extremely efficient at transporting debris (trees) and ice downstream. This can potentially result in ice and debris jamming at bridges further downstream, including those in Lyndon.

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• Lack of East Branch floodplain storage results in higher flows in Lyndon and no flow

attenuation/storage. Perhaps the most critical physical adjustment on the East Branch-- incision of the channel bed-- is that the river is physically disconnected from its floodplain. This means that flood flows are delivered more rapidly to the Passumpsic River in Lyndon. If the East Branch were connected to its floodplain, it would increase the storage of flood flows and thus reduce flows in Lyndon.

It is unknown if the conditions on the East Branch are also occurring on the West Branch and Miller Run. Similar conditions will add to the flooding problems in Lyndon. Structural and Non-Structural Measures and Recommendations There are a host of structural and non-structural measures that could be implemented to limit flooding in Lyndon; however each has an associated cost and benefit. It is important to recognize that regardless of the structural or non-structural measure, the Passumpsic River that flows through Lyndon has a very mild slope, which is reflected in the meandering pattern of the river. The surrounding topography on the banks of the Passumpsic River is also extremely flat. Thus, if water levels on the Passumpsic River exceed the river banks the water will spread laterally over the floodplain. In addition, there has already been development within the floodplain. In short, regardless of the measure(s) implemented it will not completely eliminate flooding within a mildly sloped river valley, especially those structures already in the floodplain. The following section describes potential structural and non-structural measures. At the end of each description is a recommendation on the merit of a given measure in terms of reducing flooding in Lyndon. Although a given measure may provide flood protection to some structures, consideration must also be given to costs relative to the benefit. It is beyond the scope of this study to estimate the cost of a given flood protection measure; however, we have provided some of the initial steps to evaluate the feasibility of some alternatives. Structural Mitigation Measures Dikes, Levees, Berms In 2006, the Corps evaluated one structural measure, by considering the placement of levees and plastic sheeting in the northern and southern parts of Lyndon to protect floodprone areas. Based on the Corps analysis the cost benefit ratio was well below one, making the project infeasible. Even if levees and plastic sheeting were economically feasible, construction of such structures could result in physical adjustments (changes in slope and planform) to the Passumpsic River within Lyndon and potentially further downstream. Containing river flow to the channel and preventing access to the floodplain will increase the river’s energy that must be resisted by the channel bed and banks (rocks, vegetation, etc). A common occurrence to limiting floodplain access is increased erosion along the channel, which results in scouring sediments. The scoured sediments are then deposited in slow velocity areas further downstream, which can raise the river bed elevation. Over time the scouring of sediments can lower the channel bed profile (incising), making it more difficult for the river to access the floodplain. Also over time, the river will create a new floodplain by eroding it river banks. In short, encroachment on the floodplain or constructing structures that “force” water to stay within the channel may result in physical adjustments to the Passumpsic River. Recommendation: The option of constructing levees may provide “temporary” flood protection to infrastructure investments, however, based on the Corps’ analysis it is not economically justified. We have no way to verify the Corps’ cost estimate, however, even if the cost of floodproofing were reduced by 50%, the Corps’ cost/benefit ratio would still be less than one. Also, assuming cost were not an issue

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relative to floodproofing, Lyndon would have to seriously consider the long-term geomorphic impacts of levees. It has been well documented, that containing a river by the use of berms or levees will cause physical adjustments in the river channel. We do not recommend this option due to costs and potential long term physical adjustments to the river. Dredging There has been considerable debate over dredging a river as a way of protecting property from flood flows. Dredging involves removing sediment from the river channel and increasing its cross-sectional area to pass more flow. Dredging is a justifiable method for protecting property when the dredging will help to restore or maintain the stability of the river. However, dredging a river can also create an unstable river that can lead to greater flood damage in the future. Recommendation: Alternative 7 evaluated the option of removing an arbitrary four feet of sediment from the project reach by lowering the channel cross-section. Before dredging is considered we recommend that Lyndon consider the following incremental steps:

• The stability of the West Branch and Miller Run should be investigated to determine if these rivers are delivering sediment to the project reach, similar to the East Branch. Unless the East Branch, West Branch and Miller Run are stabilized, then sediment will continue to be delivered to the project reach. Thus, any potential benefits of dredging will be temporary, as the dredged areas will likely refill from upstream sediments and would require long term maintenance.

• If the East Branch, West Branch and Miller Run were in a nearly stable condition, then dredging could be considered. However, before any dredging were conducted an analysis is recommended to determine if dredging could cause channel instability within the project reach and downstream.

• If it can be shown that sediment dredging within the project reach does not cause channel instability, then dredging is a potential option to flood mitigation. However, it is recommended that an updated hydraulic model be developed to determine the true flood benefits of dredging.

Sediment deposition in the project reach is truly a function of the stability of upstream tributaries (East Branch, West Branch, Miller Run). In the case of the East Branch, the incising of the river channel is likely delivering sediments to the project reach at a rate faster than it can be transported. In short, we recommend this option if: a) stability of the East Branch is restored b) the stability of West Branch and Miller Run are investigated- and restored, if needed, and c) it can be shown that dredging does not cause channel instability in the project reach and downstream. Dams- Storage Another measure to control flooding is reducing the magnitude of flow entering Lyndon by the constructing flood control reservoirs, which would provide flood protection for communities below the dam. Although most likely viewed unfavorably, flood control reservoirs on the East Branch, West Branch or Miller Run could be constructed to reduce flow entering Lyndon. Recommendation: There is no question that a flood control facility on the East Branch, West Branch or Miller Run would provide flood protection to Lyndon by providing flood storage. However, this option is likely unrealistic due to costs, long term operation and maintenance, environmental impacts, and the potential geomorphic impacts of creating a flood storage reservoir. This option is only provided to provide a complete picture, although it is recognized that it is not likely given the major hurdles that would involved with a project of this nature. In short, we do not consider this option feasible.

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Restoration of Channel/Floodplain Storage As described in greater detail in Section 8.0 of this report, floodplains serve a vital role for storing and attenuating flood flows. In the case of the East Branch it is incised in many locations, which can directly impact flooding in the project reach due to:

• Increased potential for sediment transport in the East Branch (and potentially the West Branch and Miller Run) and subsequent deposition within the Passumpsic River in Lyndon;

• Increased transport of debris and the potential for debris/ice jamming in Lyndon; • Lack of East Branch floodplain storage results in higher flows in Lyndon.

Recommendation: It is believed that floodplain storage on the East Branch (at a minimum) could be improved with the goal of curtailing flooding in Lyndon. Allowing the East Branch to access its floodplain in areas where it currently can not would provide flood attenuation/storage. We believe this is a viable option to providing flood protection in Lyndon; however, the following steps are needed:

• Using the East Branch Phase 2 geomorphic assessment, along with some follow-up field work, estimate the storage capacity on the East Branch. Quantify how much storage capacity could be gained in those areas where the East Branch currently can not access its floodplain. Quantify the overall floodplain storage capacity and the benefit relative to curtailing flooding in Lyndon.

• Work with the Vermont Rivers Management department to conduct geomorphic assessments on the other major tributaries- West Branch and Miller Run- to determine if these rivers can access their floodplains. Conduct the same evaluations described above for the East Branch, including quantifying the floodplain storage capacity that could be made available.

• For all three tributaries develop and implement natural channel design measures with the goal of creating a stable river channel and allowing all three rivers to access their floodplains.

• As described in the non-structural measures section, another component relative to floodplains is to protect these critical lands from future encroachment/development via property acquisition, and/or easement.

Lyndon needs to investigate options for floodproofing within Lyndon itself, but also upstream. This will require a concerted effort and partnership with numerous municipalities including Lyndon, Burke, East Haven, Newark, Sutton, Shefield and Wheelock. In short, restoring floodplain access is a viable option, however parties need to recognize it will take further study and funding to accomplish these goals. Bypass Channel Based on the hydraulic modeling, there is some reduction in the area of inundation with the construction of a bypass channel. There is no question that increasing flow conveyance with a bypass channel will reduce upstream flooding. The question comes down to the cost and benefit. Recommendation: Before it can be concluded that a bypass channel is a feasible option much more information is needed. A feasibility study would be necessary to determine the cost of developing a bypass channel before this alternative can be recommended. Although not a complete list, the feasibility study would include:

• Borings in the area of a proposed bypass channel to determine if ledge (as suspected) is located throughout river right (the right side of the river looking downstream) as there could be blasting costs.

• If the proposed bypass channel ran partially through soils, soil sampling would be needed for contaminant testing.

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• Studies relative to the potential impacts on rare, threatened and endangered species in the proposed location of the bypass and work area would be necessary.

• Archeological studies may be required as development of a bypass channel could impact archeological resources.

• Property issues and access would have to be resolved. • Surveys along river right would be required to develop design plans and cost estimates. • Conceptual and preliminary design plans for the bypass channel are needed to quantify the cost

of potential blasting (if necessary). The bypass channel would have to be long enough and at the proper slope to facilitate upstream and downstream passage of fish- meaning the bypass channel could be relatively long based on the elevation drop between the Vail Dam and downstream.

• Public safety would have to be considered—for example, if a deep cut into bedrock were necessary to construct the bypass channel there could be steep banks.

• Permitting and final design would be required. In summary, as noted above, depending on the size (width, depth) of the bypass channel it would help reduce flooding upstream. We have not provided a final recommendation on this option as the costs associated with constructing the bypass channel have not been determined. However, our initial impression is that the bypass channel could be costly relative to the associated benefits.

Vail Dam Removal Based on the above analysis, there is some reduction in the area of inundation with the Vail Dam removed. The major question that still has not been completely resolved, is whether cross-section 6 or another location within the impoundment would serve as a rigid boundary- in other words is there a hydraulic control within the Vail impoundment that would still cause upstream flooding. Another critical unknown is whether there is bedrock directly beneath the Vail Dam that would serve as a hydraulic control if the dam were removed. If bedrock were present beneath the dam, it is critical to determine the height and geometry of bedrock as it may serve as a hydraulic control. Recommendation: Before it can be concluded that removing the Vail Dam is a feasible option much more information is needed. Similar to the bypass channel, a feasibility study is necessary to determine the merits of removing the Vail Dam relative to reduction in flooding before this alternative can be recommended. Although not a complete list, the feasibility study would include:

• One of the first phases of the project is to determine if there are truly hydraulic controls in the Vail Dam impoundment that would serve as a hydraulic control if the dam were removed. In our opinion this question has not been completed answered from the USGS study. Further probing of sediments is needed and the cross-section data collected by the USGS needs to be tied to a known datum. Also, it is important to have some information on the extent and height of bedrock beneath the Vail Dam.

• Also as part of the initial phases of the project, bathymetric surveys of the impoundment are needed and an up-to-date hydraulic model is needed. Currently, all estimates relative to the reduction in upstream flood levels is based on outdated 1977 cross-sectional information. Probing throughout the impoundment is necessary to determine the geographic extent of bedrock in those areas identified in the USGS study.

• Once all hydraulic controls are surveyed and an up-to-date hydraulic model is developed, quantify the true benefit of potentially removing the dam. If there are minimal flood benefits, we would not recommend removing the dam. If there are substantial flood benefits, then further feasibility work is recommended as described in the following bullets.

• If it can be proven that removing the dam provided reasonable flood protection, then additional feasibility work would be necessary. One of the first steps would be collecting sediment samples

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within the impoundment and conducting contaminant testing. If contaminants are found at high concentrations, then the cost of removing the dam could increase substantially. Removal and disposal of contaminated sediments can be costly.

• The quantity, distribution and characterization of sediments within the impoundment should be quantified.

• The impact of removing the Vail Dam on: a) wetlands, b) recreation, c) rare, threatened and endangered species, d) fire use (withdrawing water from the impoundment), e) infrastructure- for example removing the Vail Dam could result in a headcut that could undermine the Chapel Street Bridge abutments and pier, f) river morphology, and g) archeological/historical resources should be evaluated. In addition, Lyndon would have to consider the financial impact of losing hydropower resources.

• If the project were feasible, then conceptual and preliminary design drawings would be needed to estimate the cost of removal and restoration.

• Permtting and final design would be required. In summary, we have not provided a final recommendation on this option as there are still many unknowns. Main Street Bridge- Relief Bridge VTrans developed a separate hydraulic model that evaluated the merit of replacing the culverts (see inset) that pass beneath Route 5 just south of the Main Street Bridge with a dry bridge. It has been noted that the existing culverts become blocked with debris and do not always function properly during flood conditions. VTrans evaluated the placement of a dry bridge in lieu of the culverts. Comparing the Main Street Bridge without any relief structure (assuming the culverts are completely plugged) to placing a 60-foot dry bridge provided a net reduction in water surface elevations near the Main Street Bridge of 1.5 feet under the 100-yr flood. This is a sizeable reduction and thus further evaluation of this option is likely warranted. Recommendation: Given that the culverts are commonly blocked, the construction of a dry bridge does show some merit. The VTrans study should be expanded to show the reduction in the inundation area with the construction of a dry bridge. If there are a reasonable number of properties that would realize the benefits of a dry bridge, then cost estimates to construct the bridge should be obtained. It is our understanding that in the past a dry bridge was present at the culvert overflow structure and it is important to understand why it was replaced. Non-Structural Mitigation Measures Acquisition, Relocation, and Floodproofing Existing Structures Federal programs that assist in acquiring, relocating, and/or floodproofing existing structures provide one means of reducing flood vulnerability. By encouraging property owners to relocate outside of the floodplain—either by physically moving a structure or by purchasing or building a replacement structure—these programs reduce flood vulnerability and thereby decrease the need for future Federal damage payments. In Vermont there have been a few buy outs of buildings that were damaged in recent

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Vermont floods because they could be not be built in a way that would protect them against future flood damage. Beginning in 1997, FEMA began emphasizing the alternative of acquisition and removal or relocation of structures in high hazard areas. The state of Vermont supports this alternative where it was determined that the cost of providing continued protection from these properties was greater than they were worth. The program is relatively expensive and has resolved only the most high priority and problematic sites. The VDEC supports these acquisition options where:

• The cost effectiveness standard, as noted above, is met; • The building is located within the 100-year floodplain; • The structure has been substantially damaged or flooded two or more times within the last 20

years; and • Buy-out offers are equal to the pre-flood fair market value minus available flood insurance

coverage (Source: Options for State Flood Control, Policies and a Flood Control Program, VANR).

Recommendation: As noted above there have been only a few relocations or acquisitions of structures in Vermont due to the cost. In addition, given the amount of flooding that has occurred in the past within Lyndon, it does not appear reasonable that acquisition or relocation is a feasible option—there are simply too many structures that are impacted by flooding. Develop River Corridor Protection Plans on the East Branch, West Branch, Miller Run and Passumpsic River A river corridor includes lands adjacent to and including the course of a river. Development along many Vermont river corridors continues today – in some areas at a faster pace than others. However, there is still an opportunity to develop river corridor protection areas that would preserve the lands that function as a river’s floodplain. If lands can be protected it would assist in avoiding further degradation (that comes with encroachment), create opportunities for future restoration, and slow the rate of land use conversion in the floodplain. Recommendation: It is recommended that a river corridor protection plan be developed along the East Branch, West Branch, Miller Run and Passumpsic River in the project reach. The purpose of this plan is to identify lands within the river corridor that could serve to attenuate floods. Federal, state, and municipalities, and land use regulators should consider floodplain protection that minimizes erosion hazards to public investments within the river corridor- the corridor is defined by the belt width3 (see inset which defines the zone) requirements of a stream or river. It is recommended that first geomorphic studies be conducted on Miller Run and the West Branch to a) determine the river’s stability, b) identify floodplains that provide key attenuation assets and c) compute the total belt width along each river that would define the river corridor. It is recognized that some floodplains are already occupied by houses or roads, thus emphasis should be placed on floodplains that remain relatively undeveloped. Once these lands are identified it will be up to Lyndon, 3 The width of the corridor is defined by the lateral extent of the river meanders, often referred to as the meander belt width.

Source: VDEC

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other towns, land conservation organizations, town conservation commissions, non-profits, regional organizations and the state of Vermont to consider potential purchase and acquisition on these lands to protect them from future encroachment within the floodplain. It will require a dedicated commitment and coordination among towns within the Passumpsic River watershed Without conservation of floodplains, over time flood conditions in Lyndon will only worsen. If development within the East Branch, West Branch and Miller Run river corridors continues flooding in Lyndon will only increase. We highly recommend the development of river corridor plans.

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Table of Contents Executive Summary ....................................................................................................................................... i List of Figures ............................................................................................................................................xvi List of Tables ............................................................................................................................................xvii Definitions .................................................................................................................................................xix Acronyms..................................................................................................................................................xxii 1.0 Introduction............................................................................................................................................. 1 2.0 Understanding the Causes of Flooding ................................................................................................... 3 3.0 Passumpsic River- Inventory of Project Reach....................................................................................... 7 4.0 Passumpsic River Hydrology.................................................................................................................. 9

4.1 Hydrology........................................................................................................................................... 9 4.2 Tributaries to the Passumpsic River ................................................................................................. 13

5.0 Lyndon Flood Insurance Study ............................................................................................................. 14 5.1 National Flood Insurance Program................................................................................................... 14 5.2 FEMA Flood Insurance Study .......................................................................................................... 14 5.3 Lyndon Flood Study- Hydrologic Analysis...................................................................................... 15 5.4 Lyndon Flood Study-Hydraulic Analysis......................................................................................... 16

6.0 Past Flood Studies (FERC, Corps, VTrans, and USGS)....................................................................... 19 6.1 FERC Flood Study-September 2003 to March 2004........................................................................ 19

6.1.1 FERC Flood Study- Background .............................................................................................. 19 6.1.2 FERC Flood Study- Findings and Independent Review ........................................................... 19

6.2 Corps Flood Study- January 2003 .................................................................................................... 23 6.2.1 Corps Flood Study-Background................................................................................................ 23 6.2.2 Corps Flood Study- Findings and Independent Findings .......................................................... 24

6.3 VTrans Study of Main Street Bridge- February 2004 ...................................................................... 26 6.3.1 VTrans Study of Main Street Bridge- Background................................................................... 26 6.3.2 VTrans Study of Main Street Bridge- Findings and Independent Review................................ 26

6.4 USGS Ground Penetrating Radar Study- November 2004............................................................... 28 6.4.1 USGS Ground Penetrating Radar Study- Background.............................................................. 28 6.4.2 USGS Ground Penetrating Radar-Study Findings and Independent Review............................ 29

6.5 Corps Initial Appraisal Report, Cost Benefit Analysis of Structural Flood Protection, May 2006.. 30 6.5.1 Corps Initial Appraisal Report, Cost Benefit Analysis of Structural Flood Protection- Background ........................................................................................................................................ 30 6.5.2 Corps Initial Appraisal Report, Cost Benefit Analysis of Structural Flood Protection- Study Findings and Independent Review ..................................................................................................... 31

7.0 Independent Hydraulic Analysis........................................................................................................... 33 7.1 Methodology..................................................................................................................................... 33 7.2 Hydraulic Modeling- Description of Alternatives ............................................................................ 34 7.3 Hydraulic Modeling- Results............................................................................................................ 36

8.0 Geomorphology and River Corridor Management ............................................................................... 43 8.1 Introduction ...................................................................................................................................... 43 8.2 Fundamentals of River Systems ....................................................................................................... 43 8.3 East Branch Geomorphic Study........................................................................................................ 45 8.4 Protection of River Corridors ........................................................................................................... 48

9.0 Flood Mitigation Options and Recommendations ................................................................................ 49 9.1 Structural Mitigation Measures ........................................................................................................ 49 9.2 Non-Structural Mitigation Measures ................................................................................................ 54

10.0 References........................................................................................................................................... 56 Appendices Appendix A: Passumpsic River Trip Summary Report (including photographs)

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List of Figures Figure No. Description 1.0-1 Project Location Map 1.0-2 Passumpsic River Study Area 2.0-1 1939 Topographic Map of Project Reach 2.0-2 1951 Topographic Map of Project Reach 2.0-3 1986 Topographic Map of Project Reach 2.0-4 Subwatersheds within the Passumpsic River Watershed 2.0-5 Topography Map including Photographs of Bridges within the Project Area 4.1-1 Passumpsic River at Passumpsic, VT (Gage No. 01135500), Drainage Area= 436 sq mi Instantaneous Peak Flows for Period of Record: 1927-2006, Trends Analysis 4.1-2 East Branch Passumpsic River near East Haven, VT (Gage No. 01133000), Drainage Area= 53.8 sq mi Instantaneous Peak Flows for Period of Record, Trends Analysis 4.1-3 Passumpsic River at Passumpsic, VT (Gage No. 01135500), Drainage Area= 436 sq mi 5% and 10% Exceedence Flow for Period of Record, Trends Analysis 4.1-4 East Branch Passumpsic River near East Haven, VT (Gage No. 01133000), Drainage Area= 53.8 sq mi, 5% and 10% Exceedence Flow for Period of Record, Trends Analysis 4.1-5 Passumpsic River at Passumpsic, VT (Gage No. 01135500), Drainage Area= 436 sq mi, Monthly Distribution of Instantaneous Peak Flows, Period of Record: 1927-2006 4.1-6 East Branch Passumpsic River near East Haven, VT (Gage No. 01133000), Drainage Area= 53.8 sq mi, Monthly Distribution of Instantaneous Peak Flows, Period of Record 4.1-7 Passumpsic River at Passumpsic, VT (USGS Gage No. 01135500), Drainage Area= 436 sq mi, Log-Pearson Type III Flood Frequency Distribution 4.1-8 East Branch Passumpsic River near East Haven, VT (Gage No. 01133000), Drainage Area= 53.8 sq mi, Log-Pearson Type III Flood Frequency Distribution 5.4-1 Passumpsic River FIS Flood Profiles (Source: FEMA, Lyndon FIS) 5.4-2 Passumpsic River Inundation Map- 100-year Flood (Source: FEMA, Lyndon FIS) 5.4-3 Location of Flood Insurance Study Transects within Study Area 6.1.2-1 Cross-Section of Passumpsic River upstream of Vail Dam used by FERC (assumes the sediment will be transported downstream and the “new” channel will be trapezoidal in shape). (Source: FERC Environmental Assessment for Vail Dam, March 2004) 6.1.2-2 Comparison of dam-in and dam-out 100-year flood water surface elevation for the Passumpsic River (Source: FERC, Environmental Assessment, Vail Dam, March 2004) 6.2.2-1 Passumpsic River Profile with and without Vail Dam (Source: Corps, Jan 2002) 6.2.2-2 Passumpsic River Profile with and without bypass channel (Source: Corps, Jan 2002) 6.4.1-1 Plate 1- Ground Penetrating Radar Profiles Upstream of the Vail Dam, Passumpsic River, Lyndon, VT (Source: USGS, 2004) (3 maps) 6.4.1-2 Plate 2- Ground Penetrating Radar Cross-Sections Upstream of the Vail Dam, Passumpsic River, Lyndon, VT (Source: USGS, 2006) 6.5.1-1 Floodprone Areas in Lyndon (Source: Corps, 2006) 7.2-1 Cross-Section 6 collected by the USGS- included in HEC-RAS model (Source: Gomez and Sullivan, 2006)

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List of Figures (continued) Figure No. Description 7.3-1 Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) 7.3-2 Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and All Bridges Removed (Alt 4) 7.3-3 Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Vail Dam and All Bridges Removed (Alt 5) 7.3-4 Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Bypass Channel (Alt 6) 7.3-5 Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and lowering the river channel by four feet within the project reach (Alt 7) 7.3-6 Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) 7.3-7 Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and All Bridges Removed (Alt 4) 7.3-8 Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam and All Bridges Removed (Alt 5) 7.3-9 Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Bypass Channel (Alt 6) 7.3-10 Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and and lowering the river channel by four feet within the project reach (Alt 7) 7.3-11 Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) in the area around Vail Dam 8.3-1 East Branch Passumpsic River from just below Burke town line to confluence with Passumpsic River- Geomorphic Study Area (Source: Vermont Department of Environmental Conservation). 8.3-2 East Branch Passumpsic River- Physical Adjustment Process (Source: VT Department of Environmental Conservation). 8.3-3 East Branch Passumpsic River- Evolution Stage (Source: VT Department of Environmental Conservation) List of Tables Table No. Description 4.1-1 USGS Gages near Lyndon, VT 4.1-2 Flood Frequency Results for Passumpsic River and East Branch Passumpsic River at USGS Gage Locations 4.1-3 Flood Frequency Results for Passumpsic River just below Great Falls Dam and East Branch Passumpsic River at its confluence with the Passumpsic River. 4.2-1 Drainage Area of the Passumpsic River below Great Falls Dam and Drainage Areas of

Tributaries within the Project Reach (Source: FEMA, Lyndon Flood Study). 5.3-1 Summary of Flood Flow used in FIS (Source: Lyndon FIS, May 1988) 6.1.2-1 Reduction in Water Surface Elevation (feet) at Various Cross-Sections by Removing Vail Dam (Source: FERC Environmental Assessment for Vail Dam, March 2004) 6.2.2-1 Reduction in Water Surface Elevation (feet) at Various Cross-Section by Removing Vail Dam (Source: Corps January 2, 2003 letter report to Lyndon)

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List of Tables (continued) Table No. Description 6.2.2-2 Reduction in Water Surface Elevation (feet) at Various Cross-Section with Bypassing 50% of the Flood Flows around the Vail Dam (Source: Corps January 2, 2003 letter report to Lyndon) 6.3.2-1 Water Surface Elevations at a point about 100 feet upstream of Main Street Bridge (Source: VTrans, February 2004) 6.5.2-1 Planning Level Construction Cost Estimates for both South and North Study Areas (Source: Corps May 2006) 6.5.2-2 Recurring Flood Damages in the South and North Zones (Source: Corps May 2006) 6.5.2-3 Benefit Cost Summary (Source: Corps May 2006) 7.1-1 100-year Flood Flow Water Surface Elevations at Cross-Sections A-K on Passumpsic

River. Comparison of Flood Insurance Study and Gomez and Sullivan Analysis. (Source: Gomez and Sullivan)

7.2-1 Summary of HEC-RAS modeling Alternatives (Source: Gomez and Sullivan, 2006) 7.2-2 Water Surface Elevations under the 100-year flood flow for various Alternatives (Source;

Gomez and Sullivan, 2006)

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Definitions 100-Year Flood- Contrary to popular belief the 100-year flood is not a flood occurring once every 100 years. The 100-year flood is the flood having a 1 percent chance of being equaled or exceeded in magnitude in any given year. The area adjoining a river, stream, or watercourse covered by water under the 100-year flood is referred to as the 100-year floodplain. Aggradation- The process by which a stream’s gradient steepens due to increased deposition of sediment. Bankfull Discharge- The dominant channel forming flow with a recurrence interval seldom outside the 1 to 2 year range. Base Flood Elevation: The term "Base Flood Elevation" refers to the elevation (normally measured in feet above sea level) that the base flood is expected to reach. Base flood elevations can be set at levels other than the 100-year flood. The regulations of the National Flood Insurance Program (NFIP) focus on development in the 100-year floodplain. Base Flow- That part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow; it is usually sustained by groundwater. Channel Morphology- The structure and form of a river channel. Conveyance- Conveyance is a mathematical term applied to the measurement of the carrying capacities of channels and overbank areas. Conveyance is directly proportional to discharge. Cross Section- A cross-section is a graph or plot of ground elevation across a stream valley or a portion of it, usually along a line perpendicular to the stream or direction of flow. Degradation- The process by which a stream’s gradient becomes less steep, due to the erosion of sediment from the stream bed. Such erosion generally follows a sharp reduction in the amount of sediment entering the stream. Drainage Area or Watershed Area- A drainage area is the total surface area, upstream of a point on a stream, where the water from rain, snowmelt, or irrigation (which is not absorbed into the ground) flows over the ground surface, back into the streams to finally reach that point. Encroachment - Construction, placement of fill, or similar alteration of topography in the floodplain that reduces the area available to convey floodwaters. Federal Emergency Management Administration (FEMA)- FEMA is part of the Department of Homeland Security's Emergency Preparedness and Response Directorate. FEMA's continuing mission within the new department is to lead the effort to prepare the nation for all hazards and effectively manage federal response and recovery efforts following any national incident. FEMA manages the National Flood Insurance Program. Flood or Flooding- means a general and temporary condition of partial or complete inundation of normally dry land areas from: (1) The overflow of inland or tidal waters, and/or (2) The unusual and rapid accumulation or runoff of surface waters from any source.

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Floodplain- A floodplain is the land area adjacent to a river, stream, lake, estuary, or other water body that is subject to flooding. This area, if left undisturbed, acts to store excess floodwater. The floodplain is made up of two sections: the floodway and the floodway fringe (see diagram). Floodway- The floodway is one of two main sections that make up the floodplain. Floodways are defined as the channel of a river or stream, and the overbank areas adjacent to the channel, which carries the bulk of the floodwater. The National Floodway Insurance Program (NFIP) regulations require that the floodway be kept open and free from development or other structures that would obstruct or divert flood flows onto other properties. The NFIP floodway definition is "the channel of a river or other watercourse and adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than one foot” (see diagram). Flood Frequency (Stage Frequency)- means the average frequency, statistically determined, for which it is expected that a specific flood stage or discharge may be equaled or exceeded. The frequency of a particular flood stage or discharge is usually expressed as having a probability of occurring on the average of once within a specified number of years. See also "Recurrence Interval." Floodway Fringe- The floodway fringe refers to the outer portions of the floodplain, beginning at the edge of the floodway and continuing outward. The floodway fringe is defined as the area of the 100-year floodplain lying outside of the floodway within interim flood hazard areas. This is the area where development is most likely to occur, and where precautions to protect life and property need to be taken (see diagram). Flood Insurance Rate Map- (FIRM) means an official map on which the Federal Emergency Management Agency has delineated the areas of special flood hazard. Flood Insurance Study- means the official report in which the Federal Emergency Management Agency has provided flood profiles, floodway boundaries, and the water surface elevations of the base flood. Flood Peak- means the highest value of stage or discharge attained during a flood event, i e., peak stage or peak discharge. Flood Profile- means a graph or longitudinal plot of maximum water surface elevations of a flood event versus measured distance along a stream from a fixed point. The zero or beginning point is usually the mouth of the stream and elevations are most commonly expressed as feet above mean sea level. Flood Stage- means the height of the water surface above an arbitrary datum where overflow of the natural banks of a stream results in flood damage. As commonly used by the National Weather Service and others, flood stages are referenced to a particular stream gage that is a representative index of a specific reach of a stream. Floodproofing- means any combination of structural and nonstructural additions, changes, or adjustments primarily for the reduction or elimination of flood damages to real property, water and sanitary facilities, structures, and contents of buildings in flood hazard areas.

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National Flood Insurance Program (NFIP): Managed by FEMA, the NFIP has three components which are: Flood Insurance, Floodplain Management and Flood Hazard Mapping. Nearly 20,000 communities across the United States and its territories participate in the NFIP by adopting and enforcing floodplain management ordinances to reduce future flood damage. In exchange, the NFIP makes federally backed flood insurance available to homeowners, renters, and business owners in these communities. Community participation in the NFIP is voluntary. Flood insurance is designed to provide an alternative to disaster assistance to reduce the escalating costs of repairing damage to buildings and their contents caused by floods. Flood damage is reduced by nearly $1 billion a year through communities implementing sound floodplain management requirements and property owners purchasing of flood insurance. Additionally, buildings constructed in compliance with NFIP building standards suffer approximately 80 percent less damage annually than those not built in compliance. Recurrence Interval- means the average interval of time, based on a statistical analysis of actual or representative streamflow records, which can be expected to elapse between floods equal to or greater than a specified stage or discharge. The recurrence interval is generally expressed in years. See also "Flood Frequency." Sinuosity- The ratio of stream length between two points divided by the valley length between the same two points.

Stage- Stage is the level of the water surface over a datum (often sea level). As river flow increases, stage (or the river water level) increases; however this relationship is not linear. Stage-Frequency Curve- see Flood Frequency. Subwatershed- A subwatershed is a smaller watershed within a larger watershed. For example, the East Branch of the Passumpsic River is a subwatershed with the Passumpsic River watershed.

Watershed- A watershed is a geographic area in which all water flows into a single river. The outer boundaries of watersheds are therefore the highest points of elevation surrounding a water body. Watersheds can be delineated at different scales. For example, the entire Passumpsic River Watershed also includes small watersheds such as Miller Run, South Wheelock Branch, etc. Water Year- A water year extends from October 1 to September 30. For example, water year 2005 would extend from October 1, 2004 through September 30, 2005.

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Acronyms cfs cubic feet per second COE US Army Corps of Engineers EA Environmental Assessment FEMA Federal Emergency Management Agency FERC Federal Energy Regulatory Commission FIS Flood Insurance Study GIS Geographic Information System GPR Ground Penetrating Radar LED Lyndonville Electric Department m meter mi miles mi2 square miles msl mean sea level NFIP National Flood Insurance Program SFHA Special Flood Hazard Areas VANR Vermont Agency of Natural Resources VTrans Vermont Department of Transportation USFWS United States Fish and Wildlife Service USGS United States Geological Survey

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1.0 Introduction The Town of Lyndon and village of Lyndonville, VT (hereafter referred to as the “town”) are located in Caledonian County in the northern Vermont. The town is located in close proximity to the Passumpsic River as shown in Figure 1.0-1. In the northeastern part of Lyndon, the East and West Branches of the Passumpsic River unite, marking the start of the Passumpsic River as shown in the aerial map- see Figure 1.0-2. The Passumpsic River meanders through town, which is located in a relatively wide flat valley floor. After passing through the heart of the town, the river passes over the Vail Dam and then the Great Falls Dam before traveling several miles to its confluence with the Connecticut River in Barnet, VT. For purposes of this study, the Passumpsic River from the East/West Branches to the Vail Dam is hereafter referred to as the “project reach”. Over the last few years, most notably June 2002 and January 2000, the Passumpsic River has overflowed its banks and caused considerable flooding within the town. Other past floods have resulted in similar flooding within the town, including the storms of June 1973, March 1936 and November 1927. The more recent (2000 and 2002) floods resulted in considerable property damage, power interruptions, traffic interruptions due to road closures, and disruption to the residents and businesses located within the town. In addition, repeated flooding can result in environmental and potential public health/safety concerns if unwanted pollutants from underground storage tanks, septic systems, the town wastewater treatment facility, or other sites are exposed to humans. Public health and safety is compromised when access to homes and businesses is unavailable or essential services such as power, telecommunications, water supply, fire, and wastewater collection and treatment are temporarily lost. Many past studies to evaluate flooding in Lyndon have been conducted by the United States Corps of Engineers (COE or Corps), Federal Emergency Management Agency (FEMA), United States Geological Survey (USGS), Federal Energy Regulatory Commission FERC), and the Vermont Department of Transportation (VTrans). To date, a comprehensive and coordinated review of all past flood studies has not been conducted. The purpose of this study is to assist the municipality, residents and businesses in sorting out the varying advice given by prior study efforts. In addition to a review of past findings, an independent analysis was conducted to help understand flooding issues. After reviewing past reports and conducting additional analyses, recommendations are provided to help alleviate flooding, which may include structural (levees, dam removal, bridge modifications, etc) and non-structural measures (limitations on construction within the floodplain, etc). It should be noted that it was beyond the scope of this study to determine the costs for various structural and non structural measures. This study was supported by a grant from the FEMA Hazard Mitigation Program CFDA 83.548 funds under SubGrant Agreement #07710-HM-1428-001. Projects under this program are typically managed by the municipality, as was the case with this project. This report is organized in the following sections:

2.0 Understanding the Causes of Flooding 3.0 Passumpsic River- Inventory of Project Reach 4.0 Passumpsic River Hydrology

Town Garage at Route 114 Bridge, June 2002 flood

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5.0 Lyndon Flood Insurance Study 6.0 Past Flood Studies 7.0 Independent Hydraulic Analysis 8.0 Geomorphology and River Corridor Management 9.0 Flood Mitigation Options and Recommendations 10.0 References

It should be noted that photographs of the June 2002 flood appearing throughout this report were provided by Ken Mason of Lyndonville Electric Department (LED). This report consists of Volume 1 and 2. Volume 1 contains the main report. Volume 2 contains figures, unless the figure is embedded within the main report.

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2.0 Understanding the Causes of Flooding It is important to a have a basic understanding of the various factors that can cause or contribute to flooding, specifically as it pertains to the Passumpsic River watershed4. The purpose of this section is to provide a fundamental overview of the causes of flooding- they are not organized in any specific order. Precipitation The volume (how much), intensity (how fast) and duration (how long) of precipitation in the Passumpsic River watershed will have a direct bearing on the magnitude and volume of runoff- or river flow. A 2-3 day storm with six inches of rain may produce little flooding, whereas three inches of rain in 2-3 hours may result in severe localized flooding. Ground conditions, often called antecedent moisture conditions, prior to a storm event will also influence the runoff volume. For example, the runoff volume from a watershed where soils are saturated prior to a precipitation event will yield higher runoff volumes than a “dry” watershed. Rainfall coupled with a “ripe” snowpack can also lead to some of the worst flooding conditions as snowmelt and rainfall collectively form runoff. In fact, the March 1936 flood, the worst flood in the past century, was the result of rainfall on snowpack. Typically during these spring events the ground remains frozen eliminating infiltration and resulting in direct runoff. Land Use Development in the Passumpsic River Watershed

Land use conditions in the watershed can also have a direct bearing on the magnitude, volume, and attenuation of runoff. Increased urbanization in the form of paved surfaces (parking lots, driveways, roads), and buildings leads to greater degrees of impermeability, which impacts the watershed’s runoff characteristics. Impervious surfaces and impermeable soils with low infiltration rates prevent water from infiltrating into the ground, leading to an increased volume and rate of stormwater runoff. The rate of runoff increases not only due to impervious surfaces, but also the channeling of road and pavement runoff into stormwater collection systems. Stormwater systems deliver flow to a receiving stream much faster than an undeveloped watershed. Increased development within a watershed can also result in the loss of vegetative cover, forest cover, and wetlands. These features of the watershed provide a buffer, allowing water to infiltrate into the ground and eventually appear as base flow5 (groundwater) in a river or slows (attenuates) the runoff process. A watershed that is stripped of vegetation and trees will experience a faster rate and volume of runoff than the same “pristine” or undisturbed watershed. The loss or filling in of wetlands due to

4 Watershed: A watershed is geographic area in which all water flows into a single area. A subwatershed is a smaller watershed within a larger watershed. For example, the East Branch of the Passumpsic River is a subwatershed to the Passumpsic River. 5 Base Flow: Base flow is that part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow; it is usually sustained by groundwater.

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development can have a direct bearing on runoff volumes. Wetlands serve a vital role in reducing the impacts of flood flows by acting as “sponges”, which store and then slowly release flood waters over time. Typically, over time, as a river basin becomes more developed, the volume of runoff increases. For example, the volume of runoff from one inch of rainfall on the Passumpsic River watershed today is most likely greater than the same one inch of rainfall 200 years ago due to increased development in the basin. Later in this document, an analysis of long-term flow trends in the watershed is conducted to determine if flows are increasing, decreasing or remaining relatively stable over time. Land Use Development in the Project Area Floodplain Development within the project reaches floodplain6 has occurred slowly over time. Shown in Figure 2.0-1, 2.0-2, and 2.0-3 are topographic maps of the project reach from 1939, 1951 and 1986, respectively. The maps are at slightly different scales, but the general area of the project reach is shown. In comparing the maps, it is evident that some development has occurred within the floodplain over time. Infrastructure such as homes, hotels, businesses, and transportation routes has been constructed within in the valley floor. Encroachment7 on the floodplain, such as structures and fill, reduces its flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. Increased development within a river’s floodplain contributes to increased flood levels as these structures restrict flow movement causing backwater conditions or a rise in the water level upstream of the restriction. In addition, the structures and fill areas physically occupy space in the floodplain that reduces the area of flow conveyance. Increased development within the floodplain causes the floodplain boundaries to expand. Development in the floodplain includes a manufactured home park, the Miss Lyndonville Diner, LynBurke motel, and several residences north of Lyndonville. A second potential area for flood damage encompasses numerous residences in the southern part of town adjacent to State Route 122 where it joins US Route 5. The commercial-business district along US Route 5, between Lyndonville and Interstate 91, is also in the floodplain (Lyndon FIS). Channel Morphology and Sediment Deposition Channel morphology refers to the structure and form of a river channel. A river depends on the use of its valley to form a stable channel. The shape of a river channel including its dimension (width, depth), its pattern (planform) and its profile (slope), is developed and maintained over time by the action of water, sediment, and debris that drains from the watershed. A stable channel has the ability to carry water and sediment even during high flow conditions, without changing the rivers width, depth, planform or slope. We have provided a brief description of channel morphology here; however, this is expanded upon further in Section 8.0 Geomorphology and River Corridor Management. Figure 2.0-4 is a map of the Passumpsic River watershed showing the main tributaries of the Passumpsic River, and town divides. The East and West Branches of the Passumpsic River mark the beginning of the Passumpsic River just upstream of Lyndon. Both the East and West Branches drain mountainous terrain

6 Floodplain: In general, the floodplain is the flat area located adjacent to the main stream channel. When river banks overflow, the floodplain provides natural storage for excess water. 7 Encroachment: Construction, placement of fill, or similar alteration of topography in the floodplain that reduces the area available to convey floodwaters.

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in the upper watershed. Both branches are steep gradient, high energy rivers having the ability to transport debris and sediment, before the river gradient drops sharply at the onset of the Passumpsic River mainstem. The East and West Branches have channel bed slopes of 30 feet/mile and 43 feet/mile, respectively (Source: Burke Flood Insurance Study). These channel slopes are considered relatively steep for the size of the watershed. From the East/West confluence to the Vail Dam, the Passumpsic River channel bed slope flattens considerably averaging only 4.3 feet/mile, and is quite sinuous (winding) as it meanders through the valley floor. Because the river bed slope is so much steeper on the East and West Branches there is sufficient energy to move flood flows through the river channel. Alternatively, because the Passumpsic River channel bed slope through the project reach is so mild, there is far less energy to convey flow downstream. Figure 2.0-5 shows the rivers sinuosity (meandering or winding) and the low topographic relief along both river banks through the project reach. The low topographic relief along the river banks allows the flooding to expand considerably within the floodplain when water levels rise above the elevation of the river banks. The combination of a mildly-sloped river, low energy river, coupled with low topographic relief along the river banks, increases the likelihood and extent of flooding in Lyndon. As noted above, the East and West Branches of the Passumpsic River are steep and have the ability to transport sediment and debris. Typically, most sediment is transported during what is termed a “bankfull” event, which occurs every 1.5-2 years. As the sediment from the East and West Branches is transported, it eventually flows into the Passumpsic River, where the river gradient flattens and water velocities slow. Under these conditions, sediment deposition occurs along the river bed. Typically larger sediments (gravel) are deposited at the head of a slow-velocity reach, followed longitudinally by deposition of sand, and then silt. The Vermont Agency of Natural Resources (VANR) has conducted geomorphic studies along the East Branch of the Passumpsic River to determine, among many other concerns, if the river channel is stable or unstable. Stream stability may be defined as: “The ability of a stream, over time and in the present climate, to transport the flow and sediment of its watershed in such a manner that is maintains its dimension, pattern, and profile without aggrading or degrading.” (Rosgen, 1996, Applied Morphology) An unstable channel can result in incising (the channel bed is deepening) or widening, which result in degrading the sediment. Based on geomorphic assessment there are many reaches of the East Branch where the river bed is incising and widening. When the river incises the channel bed lowers and the sediments move further downstream. These sediments have the potential to deposit (aggradation) in the Passumpsic River as water velocities are much slower. It is possible there is an imbalance between the sediment transport capacity (ability to move sediment out) of the Passumpsic River within the project reach compared to the influx of sediment from the East Branch and other tributaries. The point of this discussion is that increased sediment deposition in the project reach could cause the channel bed to fill with sediment. If this occurs, there is less cross-sectional area within the main channel to convey flow, which could result in increasing the frequency of overflowing the riverbanks. Hydraulic Restrictions Another major factor contributing to flooding are hydraulic controls. Hydraulic controls refer to any manmade (such as a bridge or dam) or natural (bedrock/ledge, channel narrowing) controls that will impact the ability of the river to convey flow. For example, if elevated bedrock extends across the Passumpsic River, it will cause a backwater effect- meaning the water level behind the bedrock will increase- hence the bedrock serves as a “hydraulic control”. Other natural controls such as a constriction in the river’s width can also cause a backwater effect. Although described in more detail later, during a

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canoe trip of the project reach, there were no observable natural controls, although it has been reported that some bedrock (or large boulders) is located upstream of the Vail Dam (more on this later). There are several manmade structures along the project reach that may contribute to flooding. These include in upstream to downstream order (from the East/West Branches to Vail Dam) the following:

• Canadian Pacific Railroad Bridge; • Route 114 Bridge; • Route 5 Bridge (northern- Main Street Bridge); • Covered Wooden Bridge (Sanborn Bridge); • Footbridge; • Route 122 Bridge (Center Street Bridge); • Route 5 Bridge (southern- Chapel Street Bridge), and; • Vail Dam.

The area underneath a bridge is used to transport flow; however, bridges can also impede flow if water levels exceed the low chord of the bridge. When this occurs, the bridge is under pressure, and water levels behind the bridge start to increase which can contribute to flooding. Other factors that can also

exacerbate flooding at bridges are ice and debris jams. During high flow events, ice and debris can become lodged at the low chord of the bridge or against the piers, blocking the passage of flow. Again, this will cause water levels behind the bridge to rise. The appended photograph in fact shows debris jamming at the Route 5 (Main Street) Bridge in June 2002, and the attempt to break the jam to allow flow to be conveyed downstream. The photograph also shows water at the low chord of the bridge. In addition to bridges, dams are essentially hydraulic controls as they artificially raise the

water level above the natural channel bed and cause backwater effects. Depending on the type of dam, they can contribute to, or reduce, flooding. The Corps operates numerous flood control dams throughout the Connecticut River watershed, which are designed to store flood flows within their reservoirs, and then discharge flow after the flood event has passed. These facilities are designed to protect downstream communities from flooding. There are no Corps flood control reservoirs or seasonally operated reservoirs8 in the Passumpsic River watershed that would alleviate flooding. Alternatively, non-flood control dams may contribute to flooding. By their nature, dams raise the water level upstream. Dams are typically designed to pass a certain magnitude of flow. Mechanisms to pass flow at a dam include the spillway, gates, turbines (if hydropower is present), and other water conveyance structures. If the magnitude of inflow to a dam exceeds its discharge capacity, water levels behind the dam will rise.

8 In addition to the Corps flood control reservoirs some hydropower reservoirs may also have the ability to store flood flows. Commonly these projects are referred to seasonally operated reservoirs where impoundment water levels are drawn down in the fall and winter and then refilled during the spring runoff.

Route 5 Bridge (Main Street), June 2002 flood

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3.0 Passumpsic River- Inventory of Project Reach To familiarize ourselves with the project reach, representatives of Gomez and Sullivan Engineers canoed the Passumpsic River from the East/West Branch confluence to the Vail Dam. The canoe trip occurred on August 3, 2006 and the flow conditions at the time were higher than normal for August. A detailed summary report of the canoe trip, including photographs, is contained in Appendix A of this document. The purpose of the canoe trip was:

• To observe all of the manmade hydraulic structures on the Passumpsic River, including railroad bridges, highway bridges, pedestrian bridges (footbridge) and the Vail Dam;

• To identify any natural hydraulic controls (channel bed elevation changes such as bedrock controls, or channel width narrowing) that could impact water elevations above the hydraulic control;

• To obtain a better feel for the floodplain, specifically the topography of the valley floor within the project reach and the degree of development and encroachment in the floodplain;

• To identify erosion areas; • To obtain periodic water depth and sediment depth measurements.

There are seven bridges that span the Passumpsic River and the Vail Dam within the project reach as shown in Figure 2.0-5. Observational9 notes made during the trip were the width and height (from the water surface to the low chord on the bridge) of each bridge as well as the number of piers. As noted above, an undersized bridge opening can lead to pressure flow resulting in an increase in water levels upstream of the dam. The number and spacing of piers was noted, and any debris lodged between the piers. Both on-the-water and land photographs of each bridge and the Vail Dam were recorded (see Appendix A). There were no observations of any natural hydraulic controls within the project reach. No visible bedrock controls were detected during the site reconnaissance. However, flows were high and the water was murky due to heavy suspended sediment concentrations making it difficult to observe submerged bedrock formations. In reviewing past reports there is discussion that a bedrock control is located upstream of the Vail Dam. This issue is addressed more fully later in this document. Generally, the river width was relatively uniform, except in areas where the channel was widening on meander bends. There were no sizeable constrictions in the river channel width that would result in significant backwater conditions.

As shown in Figure 2.0-5 the Passumpsic River meanders through the valley floor and includes many bends in the river. It was noted during the survey the river bank was undergoing erosion (see photo) along the outside bend of these meanders, which occurs because river velocities are higher on the outside bend. Typically, sediment deposition occurs along the inner meander bend where the water velocity is slower. This was confirmed from other photographs under low flow conditions. The soils along the Passumpsic River banks appear highly erodable (soft sediments) where the river has carved into the banks and some of this material is deposited in the river channel. In

some locations it was evident that the sediment budget was not in balance. For example, when the river’s

9 It should be noted that no physical measurements of the bridges were made.

Erosion on outside of meander bend

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ability to transport sediment is less than the influx of sediment, sediment deposition occurs and islands within the channel can form. This phenomenon was observed at a few locations where the channel was slowly widening and sediment was filling the channel (aggradation)- see photo.

During the canoe trip periodic measurements of the water depth and sediment depth were made. However, because river flows were so high in many instances the water depth was greater than 6 feet, which was beyond the length of the measuring probe. Generally, the water depth was above 2 feet and closer to 6 feet in many areas. The probed sediments were soft (silt, sand) and relatively easy to penetrate. The depth of penetration in many areas was over two feet, although probing was limited due to the length of the probe. Having looked at pictures of the river under low flow conditions, it appears that the channel is heavily silted and the depth of sediment is likely much

deeper.

Sediment Island

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4.0 Passumpsic River Hydrology 4.1 Hydrology The magnitude of flow has a direct bearing on flooding in the Passumpsic River. As discussed above, land use development in the Passumpsic River watershed, encroachment within tributary corridors, and the inability of major tributaries to access their floodplains could contribute to increased peak flows in Lyndon. To determine if the magnitude of flow has increased over time, an evaluation of peak flow data from two USGS gages in close proximity to the project reach was conducted. The USGS has established river gaging stations that measure the magnitude of flow (in units of cubic feet per second, cfs) and provides mean daily and annual instantaneous peak flow information. Water level data at the USGS gages is recorded every 15 minutes, which is then averaged to compute a mean daily flow. The two USGS gages used in the analysis are described in Table 4.1-1 and located in Figure 2.0-4.

Table 4.1-1: USGS Gages near Lyndon, VT

Gage No. Gage Name Period of Record

Drainage Area Datum Comments

01135500 Passumpsic River at Passumpsic, VT

11/15/1928-8/30/2006

436.0 mi2 500.0 feet above msl10

Between Vail Dam and the gage there are several other hydropower dams that can impact the timing and magnitude of flow recorded at the gage.

01133000 East Branch Passumpsic River near East Haven, VT

7/9/1939-10/31/1945, 10/1/1948-9/30/1979, 10/1/1997-8/30/2006

53.8 mi2 943.88 feet above msl

Upstream of the gaging station there is no regulation from dams or diversions. Generally the river flows reflect or unregulated flow conditions.

The Passumpsic River gage (Gage 01135500) is located well downstream of the Vail Dam and there are several hydropower dams between the gage and Vail Dam (the drainage area at the Vail Dam is 217 mi2). Some hydropower dams have the ability to regulate flow over short time intervals (by either storing flow in the impoundment and/or peaking the discharge through turbines). However, the dams located between the Vail Dam and the Passumpsic gage have no ability to regulate peak or high flows given their limited reservoir storage capacity. The East Branch Passumpsic River gage (Gage 01133000) has no dams or large diversions11 upstream of the gage- thus the flow recorded at this site is considered “unregulated”. Note that the East Branch gage does not have a continuous period of record. Ideally, a very long period of record at the gage is preferred to determine if land use changes in a watershed are having an impact on runoff. For both gages, the USGS records the instantaneous peak flow for each water year12 for the available period of record. Shown in Figure 4.1-1 and 4.1-2 are plots of the instantaneous peak flow (in cfs) for each water year for the Passumpsic and East Branch gages, respectively. The plots include a trend line, showing the trend in the instantaneous peak flow since the gage became operational. The trend line is essentially flat for the Passumpsic gage, while the trend line shows an increase in peak flows on the East Branch.

10 msl- mean sea level. 11 The only known diversion is for the Burke Mountain Ski Area; however, during a flood event the magnitude of the diversion would be minimal compared to the amount of flow. 12 Water Year- A water year extends from October 1 to September 30. For example, water year 2005 would extend from October 1, 2004 through September 30, 2005.

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It should also be noted that these gages have two different periods of record. The June 2002 flood was the 4th highest on record for the Passumpsic gage, while it was the 2nd highest for the East Branch gage. In fact, shortening the period of record of the Passumpsic gage to match that of the East Branch gage the June 2002 flood would be the 2nd highest flood for both gages. In addition to evaluating annual instantaneous peak flows, annual flow duration curves were computed. Flow duration curves show the percentage of time that a specified discharge is equaled or exceeded. For example, the 10% exceedence flow on the East Branch for water year 1940 is 200 cfs. This means that in 1940, 90% of the flow in the East Branch was less than 200 cfs. Flow duration curves were computed for each water year for the available period of record for both the Passumpsic River and East Branch gages. From the flow duration analysis, the 5% exceedence and 10% exceedence flows were plotted for each water year. A trend line was then fit to the data to determine if the 5 and 10% exceedence flows were increasing or decreasing over time. Shown in Figure 4.1-3 and 4.1-4 is the 5 and 10 % exceedences flows for each gages period of record for the Passumpsic River and East Branch, respectively. As Figure 4.1-3 shows, over time the trend line for the 5% and 10% exceedence flows show a decrease in Passumpsic River flows. Alternatively, as Figure 4.1-4 shows, over time the trend line for the 5 and 10% exceedence flows are increasing on the East Branch, similar to the trend observed for the instantaneous peak flows. The increase in East Branch peak flows and 5% and 10% exceedence flows over time, may be a function of natural variability, timber harvesting in the watershed, silviculture relative road construction, or a host of other possibilities. The East Branch watershed above the USGS gage remains relatively undeveloped, thus urbanization is not likely a contributor to increasing peak flows. The East Branch watershed is much smaller than the Passumpsic watershed (53.8 versus 436 square miles). It should be noted that changes in flow characteristics in smaller watersheds are typically more apparent than larger watersheds, which could be the case here. For example, a smaller watershed is more likely to experience flash flooding than a larger watershed. Flooding in June is rare. Shown in Figures 4.1-5 and 4.1-6 are bar charts showing the distribution of annual peak flows by month for the Passumpsic and East Branch gages, respectively. For the Passumpsic River gage, as expected the majority (73%) of peak flow events occurs during the spring runoff months of March, April, and May. Similarly, 69% of the peak flow events occur during March, April and May for the East Branch gage. The timing of the peak flow is important as it typically occurs in the spring months, when ice jamming can potentially occur. A flood-frequency13 analysis was conducted on the Passumpsic and East Branch USGS gages for two different periods of record. The first analysis was based on the full period of available data through 2005. The second analysis was based on the same period of record used in the original FEMA flood insurance studies (FIS)- essentially we were trying to verify the original findings. The FIS study for Lyndon was completed in 1988. The flood-frequency analysis used in the FIS was based on the Log-Pearson Type III distribution—the same analysis was replicated here. The flood frequency analysis utilizes the annual instantaneous peak flows for each water year and computes the probability of flood occurrence. For example, a 100-year flood occurs on average once every 100 years and thus has a 1% chance of occurring in a given year.

13 Flood Frequency- means the average frequency, statistically determined, for which it is expected that a specific flood stage or discharge may be equaled or exceeded. The frequency of a particular flood stage or discharge is usually expressed as having a probability of occurring on the average of once within a specified number of years. The Log-Pearson Type III flood frequency distribution was used (based on Bulletin 17B). The USGS program Peak FQ, Version 5.0 was used to conduct the flood frequency analysis. Weighted skews were used.

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The flood frequency analysis results are shown in Table 4.1-2 and graphically displayed in Figure 4.1-7 (Passumpsic River) and Figure 4.1-8 (East Branch). Table 4.1-2: Flood Frequency Results for Passumpsic River and East Branch Passumpsic River at

USGS Gage Locations Recurrence Interval Gage Name Drainage

Area (mi2)

Period of Record 2-year 10-year 25-year 50-year 100-year

1927-1980 (original FEMA study)

7,110 cfs 13,060 cfs 17,210 cfs 20,900 cfs 25,160 cfs Passumpsic River

436.0

1927-2005 7,505 cfs 13,100 cfs 16,800 cfs 19,990 cfs 23,590 cfs 1940-1945, 1949-1979 (original FEMA study)

1,262 cfs 2,244 cfs 2,853 cfs 3,360 cfs 3,916 cfs East Branch Passumpsic River

53.8

1940-1945, 1949-1979, 1998-2005

1,353 cfs 2,441 cfs 3,110 cfs 3,664 cfs 4,270 cfs

In reviewing Figures 4.1-7 and 4.1-8, the flood frequency curves are relatively close for the two periods of record. The approximate recurrence interval for the June 2002 flood flow is shown on Figures 4.1-7 and 4.1-8; it was close to a 10-year event on the Passumpsic River and close to a 100-year event on the East Branch. However, keep in mind that the period of record used in the flood-frequency analysis for the two gages varies. For example, when the Passumpsic River period of record is set equal to the East Branches period of record (1940-45, 1949-79 and 1998-2005), the June 2002 flood would be closer to a 25-year event. To further evaluate the findings, we compared the flood-frequency results from the above table to those reported in the Lyndon FIS. In the Lyndon FIS, the reported flood flows were provided for locations other than at the Passumpsic River and East Branch USGS gages. In the FIS, the flood flows of the Passumpsic River are provided at a location 0.29 miles below the Great Falls Dam. The drainage area at this location, as reported in the FIS, is 234 mi2. The period of record used in the FIS for the flood-frequency analysis was from 1927 to 1980. Also in the FIS, the flood flows of the East Branch are provided at its confluence with the Passumpsic River. The drainage area at this location, as reported in the FIS, is 80.5 mi2. The period of record used in the FIS for the flood-frequency analysis was from 1940-1945, and 1949-1979. The purpose of these comparisons is to determine if there was close agreement in the flood flows. As noted in the Lyndon FIS, for the Passumpsic River and East Branch, the flood discharges at the USGS gaging stations were transposed to specific points of interest by a proportion method, using the formula: Qp=(Ap/Aq)m x Qq Where Q equals the discharge, A equals the drainage area, p equals the point of interest, q equals the USGS gage, and m equals the transposition coefficient of 0.75. We used the flood frequency results shown in Table 4.1-2 (for the two different periods of record) and transposed the results to two points of interest a) the Passumpsic River approximately 0.29 miles downstream of Great Falls Dam with a drainage area of 234 mi2 and b) the East Branch Passumpsic River

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at its confluence with the Passumpsic River with a drainage area of 80.5 mi2. The results of our analysis were then compared to the flood flows reported in the FIS. Shown in Table 4.1-3 are the findings.

Table 4.1-3: Flood Frequency Results for Passumpsic River just below Great Falls Dam and East Branch Passumpsic River at its confluence with the Passumpsic River.

Recurrence Interval Gage Name Drainage Area (mi2)

Period of Record 10-year 50-year 100-year 500-year

1927-1980 (values reported in the FIS)

7,350 cfs 11,360 cfs 13,460 cfs 19,600 cfs

1927-1980 (computed values)

8,189 cfs +11.4% higher than FIS

13,105 cfs +15.3% higher than FIS

15,776 cfs +17.2% higher than FIS

23,721cfs +21.0% higher than FIS

Passumpsic River (approx 0.29 miles downstream of Great Falls Dam)

234.0

1927-2005 (computed values)

8,214 cfs 12,535 cfs 14,792 cfs 21,257 cfs

1940-1945, 1949-1979 (values reported in the FIS)

3,300 cfs 4,900 cfs 5,700 cfs 7,800 cfs

1940-1945, 1949-1979 (computed values)

*3,036 cfs -8.0% lower than FIS

4,546 cfs -7.2% lower than FIS

5,298 cfs -7.0% lower than FIS

7,346 cfs -5.8% lower than FIS

East Branch (at its confluence with the Passumpsic River)

80.5

1940-1945, 1949-1979, 1998-2005 (computed values)

3,104 cfs 4,666 cfs 5,444 cfs 7,563 cfs

NOTES: *Example calculation: Qp=(Ap/Aq)m x Qq, Qp= (80.5 mi2/53.6 mi2)0.75 x 2,244 cfs = 3,036 cfs See shaded row. We believe there is an error in the original FIS for the 10-, 50-, 100- and 500-year flood flows reported on the East Branch at its confluence with the Passumpsic River (80.5 mi2). The flood flows shown above (shaded row) represent prorated flows at its confluence with the Passumpsic River. In the original FIS, FEMA also reported flood flows on the East Branch at a location approximately 0.23 miles upstream of the confluence of Mountain Brook (drainage area= 72.5 mi2). The flood flows reported in the FIS at the Mountain Brook confluence match extremely closely with the numbers in shading. For example, the 10-year flow on the East Branch upstream of its confluence with Mountain Brook is reported in the FIS as 3,050 cfs, whereas we calculated a flow of 3,036 cfs at the East Branches confluence with the Passumpsic River. Similarly, the 100-year flow on the East Branch upstream of its confluence of Mountain Brook is reported in the FIS as 5,300 cfs, whereas we calculated a flow of 5,298 cfs at the East Branches confluence with the Passumpsic River. The purpose of this exercise was to determine if the flood flows published in the FIS matched our analysis. Shown in Table 4.1-3 is the calculated percent difference between our flood-frequency values and the original FIS. For the Passumpsic River, we predicted higher flows (11.4-21.0%) than the FIS. It should be noted that differences can occur based on the skew value used in the flood frequency analysis. The FIS did not indicate whether a regional skew, weighted skew or generalized skew was used. We used a weighted skew in our analysis. It is also interesting to note that the flood flows are higher when the full period of record is evaluated. On the East Branch, our flood flows are a bit lower (5.8-8.0%) than the FIS, however, as noted above, there may be an error in the transposing of flood flows. It is also interesting to note that our computed flood flows, for the two different periods of record, show that flood flows are higher when using the full period of record.

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4.2 Tributaries to the Passumpsic River As noted above the major tributaries to the Passumpsic River are the East and West Branches of the Passumpsic River. In addition to these, within the project reach, there are several other tributaries that contribute flow to the Passumpsic River. They include: Hawkins Brook, Miller Run and South Wheelock Branch as shown in Figure 2.0-4. The drainage areas of the Passumpsic River at the downstream end of the Lyndon FIS, as well as the drainage areas of tributaries are summarized in Table 4.2-1. The East and West Branches and Miller Run represent the majority of inflow to the project reach, comprising over 194.3 mi2 of the 234 mi2 drainage area or 83%. Also shown in Table 4.2-1 are the approximate river slopes for the Passumpsic River (from East/West Branch) to Vail Dam, and various tributaries to the project reach. Most of the river slopes are steep, with the exception of Miller Run, which has flatter slope, similar to the Passumpsic River in the project reach. Table 4.2-1: Drainage Area of the Passumpsic River below Great Falls Dam and Drainage Areas of Tributaries within the Project Reach (Source: FEMA, Lyndon Flood Study).

Name Drainage Area (mi2)

Percent of Drainage Area flowing into the Passumpsic River (down to

0.29 miles downstream of Great Falls Dam)

Channel Bed Slope (feet/mile)

Passumpsic River – approximately 0.29 miles downstream of Great Falls Dam

234.0 - 4.3 ft/mi

East Branch Passumpsic River at its confluence with the Passumpsic River

80.5 34.4% 30.0 ft/mi

West Branch Passumpsic River at its confluence with the Passumpsic River

66.6 28.5% 43.0 ft/mi

Hawkins Brook at its confluence with the Passumpsic River

11.5 4.9% 74.0 ft/mi

Miller Run at its confluence with the Passumpsic River

47.2 20.2% 3.7 ft/mi

South Wheelock Branch Brook at its confluence with the Passumpsic River

17.2 7.4% 40.1 ft/mi

Indirect Runoff to Passumpsic River within the Project Reach

11.0 4.7% -

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5.0 Lyndon Flood Insurance Study 5.1 National Flood Insurance Program The National Flood Insurance Program (NFIP) is a partnership between a community and the Federal government. In communities that participate in the NFIP, property owners and renters can purchase insurance to protect them against losses from flooding. This insurance is designed to provide an insurance alternative to disaster assistance to meet the escalating costs of repairing damage to buildings and their contents caused by floods. Communities participate by agreeing to adopt and enforce floodplain regulations designed to reduce future flood risks. Participation in the NFIP is based on an agreement between local communities and the Federal government that states if a community will adopt and enforce a floodplain management ordinance to reduce future flood risks to new construction in Special Flood Hazard Areas (SFHA), the Federal government will make flood insurance available within the community as a financial protection against flood losses. The combination of development regulations and insurance means that there is help for everyone already living in a floodplain. In addition, new construction is regulated to ensure that it is not subject to flood damage and does not impede or deflect flood flows. Insurance provides relief for even small floods, unlike disaster relief. The intent of the NFIP is not to prohibit, but to guide development in floodplain areas in a manner consistent with both natures’ need to convey flood waters and a community's land use needs. Over time, homes, businesses, bridges, and transportation routes have been constructed within Lyndon’s floodplain. Repeated flooding of this infrastructure has been occurring for over two centuries. Only in the last couple decades has the responsibility for paying for flood relief and reconstruction shifted to the public sector. It is this concern where the public has paid for inappropriate development that resulted in the implementation, at the federal level, of floodplain management through the NFIP. The NFIP has worked reasonably well with an acceptable compliance record by municipalities in the program. The NFIP has successfully prevented the construction of hundreds of buildings in flood prone areas in Vermont. Out of 272 Vermont municipalities that are eligible 55 do not participate in the program. For those municipalities that participate in the program there is an extremely low rate of policy coverage. Lyndon is enrolled in the NFIP. Anyone looking to fill or build a structure in the floodplain must receive conditional use approval from the Lyndon Development Review Board as well as approval from the Vermont Department of Public Safety (Emergency Management). If allowed to build in the floodplain, any buildings must be either floodproofed or the first floor of the building must be set 1 foot above the 100-flood elevation. Renovations to preexisting buildings in the floodplain must also be approved. It is not uncommon after flood events where public officials hear from frustrated flooded homeowners that they were told they could not buy flood insurance because they were not located in the floodplain. This is incorrect. Anyone living in Lyndon is eligible to buy flood insurance. It should also be noted that if a homeowner is located outside the designated floodplain, it does not mean that the property is not susceptible to flooding. 5.2 FEMA Flood Insurance Study The Federal Emergency Management Agency (FEMA) conducts flood insurance studies (FIS) to determine the extent of the floodplain for the 100-year flood. Once the floodplain is estimated, FEMA designates flood insurance rate zones within the 100-year floodplain. Insurance agents use the zones and

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base flood elevations14 in conjunction with information on structures and their contents to assign premium rates for flood insurance policies (Lyndon FIS). The original Lyndon FIS was completed in 1977. However, the hydrologic and hydraulic analysis, which was used to estimate the 100-year floodplain, was updated in May 1986; the full FIS report was reproduced in May 1988. To conduct a FIS, FEMA focuses on major streams flowing through the community. In the Lyndon FIS, the study area is defined by the Lyndon town line (town lines are referred to as corporate limits in the FIS- see inset). Within the town limits, FEMA estimated the 100-year flow and the corresponding extent of inundation for the East Branch, West Branch, Calendar Brook (a tributary to the West Branch), Hawkins Brook, Miller Run, South Wheelock Branch Brook, and Passumpsic River (as defined by the corporate limits). FIS’s entail two major components- estimating flood flows (hydrology) and predicting the area of inundation for each flood flow (hydraulics). In this section significant detail is provided on how the FIS was conducted. The purpose for providing this information is that studies completed by the Corps, VTrans, and FERC were based on the hydraulic model developed as part of the original FEMA study. Having a general understanding of the hydraulic model will help to interpret and understand the other study results. In addition, the same model was used to independently evaluate the impact of bridges and the Vail Dam on flood levels on the Passumpsic River. 5.3 Lyndon Flood Study- Hydrologic Analysis The Lyndon FIS includes a hydrologic (flow) and hydraulic analysis. The hydrologic analysis included conducting a flood-frequency analysis to compute the 10-, 50-, 100- and 500-year flood flows on the Passumpsic River as well as each of the intervening tributaries listed above. The methods applied in Section 4.0 to compute the flood-frequency analysis were conducted by the FEMA in the original FIS. At those locations with USGS locations, a log-Pearson Type III flood frequency analysis was conducted. If there were no USGS gage flow to conduct the flood-frequency analysis, other methods were used to estimate the 10-, 50-, 100- and 500-year flood flows. Shown in Table 5.3-1 are the peak discharges reported in the May 1988 FIS. 14 Base Flood Elevation- refers to the elevation that the base flood is expected to reach. The regulations of the National Flood Insurance Program (NFIP) focus on development in the 100-year floodplain.

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Table 5.3-1: Summary of Flood Flow used in FIS (Source: Lyndon FIS, May 1988) Peak Discharges (cfs)

Flooding Source and Location Drainage Area

(mi2) 10-year 50-year 100-year 500-year Passumpsic River Approximately 0.29 miles downstream of Great Falls Dam

234.0 7,350 11,360 13,460 19,600

At confluence of South Wheelock Branch Brook

204.4 6,600 10,300 12,200 17,800

At confluence with Miller Run 156.3 5,400 8,400 9,900 14,500 East Branch Passumpsic River At its confluence with Passumpsic River 80.5 3,300 4,900 5,700 7,800 Approximately 0.23 miles upstream of confluence with Mountain Brook

72.5 3,050 4,500 5,300 7,200

West Branch Passumpsic River At confluence with Passumpsic River 66.6 2,900 4,800 5,900 8,900 Calendar Brook At confluence with West Branch 23.5 1,400 2,800 3,600 6,000 Hawkins Brook At confluence with Passumpsic River 11.5 550 1,100 1,280 2,100 Miller Run At confluence with Passumpsic River 47.2 2,200 3,600 4,400 6,800 South Wheelock Branch Brook At confluence with Passumpsic River 17.2 1,300 2,500 3,100 4,600 5.4 Lyndon Flood Study-Hydraulic Analysis The other main component of the FIS is a hydraulic analysis. The purpose of the hydraulic analysis is to determine how high - or to what elevation- the water level will rise on the Passumpsic River for different

flood flows such as the 10-, 50-, 100- and 500-year flood flows. Hydraulic models of river systems are developed to estimate water depths, velocities, and the area inundated along the Passumpsic River under varied flow conditions (10-, 50-, 100-year flood flows). Of particular concern for flood studies is the water surface elevation at a particular point in the river and the corresponding area of inundation.

In order to predict the elevation of the Passumpsic River under flood flow conditions, FEMA used a hydraulic model. The hydraulic model requires data, which includes:

• Cross-sections of the river channel and topography of the valley floor to define the geometry of the channel. FEMA obtained cross-sections of the valley floor from topographic maps at a scale of 1:4,800 feet with a contour interval of 4 feet, which was supplemented by field survey and bridge plans. The below-water cross-section data were obtained by field survey.

• The geometry of all bridges, dams, and culverts were obtained to verify elevation data and structural geometry. The geometry refers to the size of the opening of the bridge, excluding piers. A key input to the hydraulic model is the low chord, or lowest point on the bridge as well as non-conveyance areas such as bridge piers.

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• A total of 67 cross-sections (including river cross-sections, bridge opening cross-sections, the Vail Dam, and Great Falls Dam) were included in the hydraulic model developed in the original FIS.

• Mannings “n”, which is basically a unitless value that describes the roughness of the channel and floodplain are required inputs to the hydraulic model. A high Mannings ”n” means the channel is rough (representative of large boulders), whereas a low Mannings “n” means the channel is smooth (representative of sand or silt).

• Flood flow values, as shown in Table 5.3-1, were included as inputs to the hydraulic model. Using the above inputs, a hydraulic model was developed to predict flood levels. The hydraulic model used in 1988 was called HEC-2. HEC-2 has since been replaced by a new industry-standard called HEC-RAS. Both models use the same equations to balance energy between cross-sections; however, there are some minor differences. FEMA used the HEC-2 model, with the above inputs, to predict the water surface profile of the river for various flood events. Shown in Figure 5.4-1 (3 graphs total) are a series of flood profiles15 of the Passumpsic River which show the channel bed profile along with the predicted water surface elevation for various flood events. Also shown on the flood profiles are the location of select cross-sections (keep in mind that there are over 67 cross-sections in the HEC-2 model, however FEMA has shown only a few on the flood profile), labeled such as cross-section A, cross-section B, and so on. Using the predicted water surface elevation data for the 67 cross-sections, coupled with a topographic map, the area likely to be inundated along the Passumpsic River was also estimated. FEMA publishes what is called a Flood Insurance Rate Map (FIRM) which shows in plan view the areas likely to be inundated under the 100-year flood as shown in Figure 5.4-2 (2 maps total). Also shown in Figure 5.4-3 is a map similar to Figure 5.4-2, however, it shows only those cross sections on the Passumpsic River. Similar to the flood profiles, the plan map shows the same select cross-sections, labeled such as A, B, C etc. It is important to make note of these cross-sections as other studies discussed later refer to these same cross-sections. It should be noted that the hydraulic analysis completed by FEMA was based on unobstructed flow. The flood elevations shown on the profiles are considered valid only if hydraulic structures (bridges, dams) remain unobstructed, operate properly, and do not fail. This is an important assumption, as it has been documented that the bridges do become clogged with debris which can cause upstream water levels to rise further. In addition, the hydraulic analysis assumes the channel cross-sections are rigid, meaning the geometry of the channel and floodplain will not change as a result of flood flows. In reality, the channel geometry continually changes, for various reasons (increased watershed developed, sediment budget out

of balance, etc). Physical adjustments to the river channel geometry (river widening, incising, increased sinuosity, etc) may occur over long periods of time. Thus, the cross-section data collected in 1977 reflect only a snapshot in time. The river has likely undergone physical adjustments over the past 30 years. It should also be noted that the hydraulic model developed by FEMA does not include:

• The footbridge spanning the Passumpsic River located near the park.

15 A flood profile refers to the water surface elevation along the river for different flood events (10-, 50-, 100- and 500-year).

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• The overflow structure at the Main Street Bridge (Route 5)- see inset above. By not having the footbridge in the model, it will not show any backwater impact if water levels in the Passumpsic River were to reach the elevation of the footbridge. Alternatively, by not having the overflow structure in the model, it reduces the ability to convey flow downstream. If the Main Street Bridge restricted flow movement the water levels above the bridge would rise and the overflow structure would pass flow. Without the overflow structure in the model, the predicted water levels above the bridge may be higher. It should be noted that later in this report, the Vermont Agency of Transportation (VTrans) conducted a hydraulic study to determine the effects of replacing the existing overflow structure with a dry relief bridge. As part of VTrans analysis they computed the water surface elevation upstream of the Main Street Bridge with and without the existing overflow structure.

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6.0 Past Flood Studies (FERC, Corps, VTrans, and USGS) As noted earlier, there have been several flood studies conducted in the project reach by the Corps, USGS, VTrans, and FERC. Each of these studies is discussed below and follows a certain format. First, background on the individual studies is provided. This is followed by a summary of each study’s findings. Interwoven in the findings is our comments and independent analysis. To help readers recognize our comments, we have placed them in italics. It is also important to understand that our comments are based on the information provided. In some cases there was supporting data, however, generally there was little to no supporting data available. It is also recognized that many of the studies were conducted with finite budgets and were not designed to fully address flood issues. Thus, although we note in many of the studies that additional information would be helpful to better evaluate flooding, we recognize that budgets were limited and the emphasis of the study may have been different. The studies described below are organized chronological with the older studies discussed first. 6.1 FERC Flood Study-September 2003 to March 2004 6.1.1 FERC Flood Study- Background The FERC became involved with flooding issues on the Passumpsic River due to the relicensing of the Vail Hydroelectric Dam, which is owned and operated by Lyndonville Electric Department (LED). Every 30 -50 years, a licensed hydropower facility with FERC must undergo what is called relicensing. The Vail Dam underwent relicensing in the 1990s and in March 2004 LED received a new 30-year license term. As part of the relicensing process, various studies were conducted to determine if project existence and operations were having an impact on various resources such as fisheries, wildlife, recreation, as well as flooding. The FERC developed a Final Environmental Assessment (EA) that summarized the impacts of the Vail project on various resources, including flooding. As part of the EA one of the alternatives evaluated was decommissioning of the project, which called for removal of the dam. As part of the dam decommissioning alternative, the Vermont Agency of Natural Resources (VANR) also requested FERC to evaluate the effects of removing the Vail Dam on upstream flooding. To evaluate the effects on upstream flooding with and without the Vail Dam in place, the FERC used the same HEC-2 hydraulic model developed by FEMA for Lyndon. The model was used to calculate the water surface profile and floodplain attributable to the 30-year and 100-year flood events with and without the Vail Dam.

6.1.2 FERC Flood Study- Findings and Independent Review FERC used the HEC-2 hydraulic model to calculate and delineate the 30-year and 100-year flood events with and without the Vail dam. FERC first ran the model, computed the water surface elevations for the 100-year flood at various cross-sections and then compared the results to the original FIS. FERC found that the predicted water surface elevations for the cross-sections corresponded closely to the elevations contained in FEMA’s 1988 flood study. Specifically, the predicted water surface elevations for 66 of the 67 cross-sections came within 0.15 feet of FEMA’s predicted 100-year flood, and one case within 0.25 feet. Parties should keep in mind that the cross-sections used in the original FEMA were obtained around 1977. The physical planform and profile of the Passumpsic River may have changed in the last 30 years. Ideally, new cross-sections would be obtained but it is recognized that this was beyond the scope of

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analysis for FERC. It should also be noted that we do not have a copy of the HEC-2 model used by FERC to evaluate the dam-out scenario, thus comments in this section are based solely on the data provided in the Vail Environmental Assessment. Use of hydraulic models is commonly used to evaluate the impacts of dam removal on upstream flood elevations. FERC’s predicted water surface elevations match closely with the original FEMA study; the results were well within the acceptable limits and reflect close agreement. Prior to conducting the hydraulic analysis, FERC estimated the recurrence interval for the June 12, 2002 flood using the computer program PeakFQ (Version 4.1). The input to the computer program included the peak flow of 14,100 cfs and the annual peak flows at the Passumpsic River USGS gage. FERC estimated that the recurrence interval for the June 12, 2002 flood to be about a 30-year event. It states in the FERC report that the USGS gage on the Passumpsic River, located approximately 13.7 miles downstream of Vail Dam, has an estimated 100-year flood flow of 16,750 cfs. It is unclear from FERC’s analysis the exact period of record used to determine that the June 12, 2002 flood was a 30-year event- this is important as the flood frequency results can vary depending on the period of record used. FERC indicated that the 100-year flood flow at the Passumpsic USGS gage was 16,750 cfs- this seems too low, given that FEMA reported the 100-year flood flow to be 13,460 cfs on the Passumpsic River just below the Great Falls Dam. There is over 202 mi2 between the Great Falls Dam and the USGS gage. In reviewing other correspondence, the VANR questioned whether the Passumpsic River USGS gage was the appropriate gage to determine the return frequency of the June 12 flood. The Passumpsic River gage has a drainage area of 436 mi2, whereas the drainage area of the Passumpsic River just below Great Falls Dam is 234 mi2, a difference of 202 mi2. There are several tributaries that enter the Passumpsic River between Great Falls Dam and the USGS gage. We believe that the USGS gage on the East Branch is more representative of flow conditions in Lyndon during the June 12 flood. Although the drainage area at the East Branch USGS gage (53.8 mi2), is small relative to the drainage area at Great Falls Dam (a difference of 180.2 mi2), the East and West Branches collectively represent over 66% of the drainage area at Great Falls Dam. In addition, it is likely that the flow conditions observed on the East Branch were similar to those on the West Branch given the proximity of both branches. Given this, we believe a flood-frequency analysis using the East Branch USGS gage would be a better representation of the recurrence interval of the June 12 flood. Our flood frequency analysis of the East Branch USGS gage in Section 4.0 indicated that the June 12 flood was closer to a 100-year flood. To determine how the Vail Dam impacts upstream water levels, FERC used the HEC-2 hydraulic model to simulate the 30-year and 100-year flood profile for all cross-sections. FERC used the same flood flows as reported in the FIS to estimate flood elevations. To account for removal of the dam, FERC made three changes to the cross-section data from the FEMA model. First, in addition to the existing 67 cross-sections used in the FEMA model, they interpolated a cross-section 300 feet upstream of the Vail Dam to obtain better model resolution for the reach where there would be sediment16 loss due to removal of the dam. Second, FERC removed the dam and powerhouse from the FEMA model. Third, FERC predicted how the sediments behind the dam would scour over time. FERC assumed that the sediments would scour down to ledge upon which the dam is located and assumed that the sediments would scour upstream to the Chapel Street Bridge (US Route 5). Absent any site-specific information, FERC assumed the natural channel would be 60-feet wide and trapezoidal in shape between the Vail Dam and Chapel Street Bridge as shown in Figure 6.1.2-1 below. 16 It should be noted that there is sediment deposition within the Vail Dam impoundment. Upon removal of the dam sediment transport will occur as the existing bed would scour due to increased velocities.

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FERC assumed in the hydraulic modeling that channel bed scour beyond the Chapel Street Bridge would not occur under the dam-out scenario. Figure 6.1.2-1: Cross-Section of Passumpsic River upstream of Vail Dam used by FERC (assumes the sediment will be transported downstream and the “new” channel will be trapezoidal in shape). (Source: FERC Environmental Assessment for Vail Dam, March 2004)

FERC also conducted an analysis to determine how sensitive the base width of the trapezoid would be to estimated upstream water surface elevations. FERC ran scenarios of a 40-foot, and 50-foot base width to determine if it significantly impacted upstream water elevations under the dam-out scenario. The sensitivity analysis determined that a change in the bottom width did not significantly affect the predicted water surface elevation; hence they used 60-feet as a bottom width. Using the above assumptions, the water surface elevations were predicted using the 30- and 100-year flood flows. The hydraulic model was run for the dam-in and dam-out scenarios. Figure 6.1.2-2 shows the water surface elevations under dam-in and dam-out scenarios for the 100-year flood flow. Table 6.1.2-1 lists the reduction in water surface elevations at various FEMA cross-sections (the location of the FEMA cross-sections are shown in Figure 5.4-3) between dam-in and dam-out conditions.

Cross Section 6770

670

675

680

685

690

695

700

705

710

715

720

725

4600 4700 4800 4900 5000 5100 5200

Station (Ft)

Ele

vati

on

(F

t N

GV

D)

Channel Bottom

Removed Sediments

100-yr Dammed WSEL

100-yr Undammed WSEL

4

1

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Table 6.1.2-1: Reduction in Water Surface Elevation (feet) at Various Cross-Sections by Removing Vail Dam (Source: FERC Environmental Assessment for Vail Dam, March 2004) Recurrence Interval

FEMA *Cross-Section D

FEMA *Cross-Section E

FEMA *Cross-Section F

FEMA *Cross-Section G

FEMA *Cross-Section H

30-yr flood 6.1 feet 3.6 feet 2.4 feet 1.9 feet 1.7 feet 100-yr flood 5.9 feet 3.6 feet 2.4 feet 1.9 feet 1.3 feet *The cross-section letters (D, E, F, G and H) are shown in Figure 5.4-3 which is a plan map showing the cross-section locations. Based on the above table, under the dam-out condition, water surface elevations under the 100-year flood would be reduced by 5.9 feet at FEMA cross-section D, and up to 3.6 feet at cross-section E. FERC concluded that the reduction in the water surface elevations resulting from the dam-out scenario diminished as a function of distance upstream of the dam. Relative to the dam-in scenario, the dam-out scenario reduced the water surface elevation by 14.9 feet just upstream of the dam and by 3.92 feet at the Chapel Street Bridge under the 100-year flow. The incremental difference in water surface elevations at each bridge is shown in Figure 6.1.2-2. Figure 6.1.2-2. Comparison of dam-in and dam-out 100-year flood water surface elevation for the Passumpsic River (Source: FERC, Environmental Assessment, Vail Dam, March 2004)

660

670

680

690

700

710

720

6500 11500 16500 21500 26500 31500

Stream Distance in feet above the corporate limits of the Town of Lyndon, Vermont .

Ele

vati

on

(F

eet

NG

VD

)

Undammed 100-yr

Dammed 100-yr

Stream Bed - SedimentsremovedStream Bed

Vai

l Dam

U.S

. Rou

te 5

(Ch

apel

Str

eet)

U.S

. Rou

te 1

22(C

ente

r S

tree

t)

Cov

ered

Bri

dge

U.S

. Rou

te 5

(Mai

n S

tree

t)

Cro

ss S

ecti

on 6

770

Cro

ss S

ecti

on 1

3835 Cro

ss S

ecti

on 2

0415

Con

flu

ence

of

Wh

eelo

ck B

ran

ch B

rook

Cro

ss S

ecti

on 7

535

Cro

ss S

ecti

on 1

0800

Cro

ss S

ecti

on 1

7990

Cro

ss S

ecti

on 3

1190

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FERC used the modeling results to delineate the 100-year floodplain under dam-in and dam-out conditions (the inundation map was unavailable). They concluded that the total number of acres which are currently subjected to flooding under the 100-year flood (545.4 acres) would be reduced to 449.1 acres with the dam removed, a 21.4% decrease. FERC noted that based on site visits and aerial mapping the area receiving the most benefit by removing the dam are not in areas of concern where numerous residences or commercial structures exist. They did note that two residences on Center Street; two commercial structures on Chapel Street and six residences on Red Village Road and Light Plant Drive would be less subject to less flooding with the dam removed. We have a few concerns with FERC’s hydraulic model analysis for the scenario with the Vail Dam removed. Our comments are offered with an understanding that FERC did not collect any additional field data collection, but relied solely on the original FEMA hydraulic model. When studying dam removal projects typically cross-sections are obtained within the impoundment to define the existing channel geometry. Also, at the same cross-sections sediment probes are conducted by hammering steel rods to refusal. The bottom of the sediment can provide a sense of the new or “original” channel formation. In addition, by collecting sediment depths the volume of impounded sediment can be estimated. Also as part of a dam removal project, information on any hydraulic controls within the impoundment should be obtained. For example, removing the dam may uncover a bedrock ledge a few hundred feet above the dam that spans the channel and is only a few feet below the crest of the dam. Removing the Vail Dam, under this scenario, would yield little flood benefit. It is unclear from FERC’s analysis what cross-section was used (if any) to replace the cross-section at the Vail Dam to simulate the dam-out condition. Again, ideally the new cross-section would reflect the channel geometry directly beneath the Vail Dam- presumably the dam is founded on ledge, which could serve as a hydraulic control. Unquestionably, the Vail Dam contributes to flooding since it artificially raises the water surface elevation of the river. What is unclear is whether there is a natural hydraulic control (bedrock, ledge or large boulders) that extends continuously across the river above the Vail Dam (or directly beneath the dam) that will serve as a new hydraulic control with the dam removed. In FERC’s analysis they assumed there were no hydraulic controls between the Vail Dam and Chapel Street Bridge. As described later, based on USGS mapping there does appear to be some type of hydraulic control approximately 410 feet upstream of the dam. We believe FERC’s analysis, absent any field data is reasonable, although it does not adequately address the question of whether the Vail Dam increases the upstream flood levels significantly above what may occur naturally (no dam). Lastly, FERC assumed that there would be no physical channel adjustments upstream of the Chapel Street Bridge and used the cross-sections from the FEMA model above Chapel Street Bridge without any changes. The VANR noted that the Chapel Street Bridge would not restrict vertical adjustments (headcutting) in the channel. To determine the potential for headcutting above Chapel Street Bridge, grain size analysis (sieve analysis) coupled with velocity data from the hydraulic model would be used in a sediment transport analysis to determine if the sediment could be subjected to a headcut. 6.2 Corps Flood Study- January 2003 6.2.1 Corps Flood Study-Background The Corps of Engineers conducted a study to determine the effects of removing the Vail Dam on flood elevations. The Corps visited the Passumpsic River in September and November 2002 after the June 2002 flood. They also attended a public meeting where members of the public voiced their concerns

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relative to flooding. Following the site visit, the Corps developed a HEC-RAS model (using FEMA’s original HEC-2 model) of the Passumpsic River above the Vail Dam to evaluate a) the hydraulic impacts of removing Vail Dam, and b) constructing a channel to bypass flows around the Vail Dam. The work product, in the form of a letter report, was provided to the town of Lyndon on January 2, 2003. It should be noted that we contacted the Corps and tried to obtain the HEC-RAS input files, but the Corps could not locate them. Similar to FERC, the Corps used the HEC-2 model used in the original FIS, and converted it to HEC-RAS, which is common. The Corps reported that the peak water surface elevations calculated using the HEC-RAS model under the dam-in scenario correlated closely with those from the old HEC-2 model for all flood flows. 6.2.2 Corps Flood Study- Findings and Independent Findings The Corps conducted hydrologic calculations to assess the accuracy of the Passumpsic River flood flows presented in the Lyndon FIS. The Corps conducted a flood frequency analysis using the Passumpsic River USGS gage. They evaluated two different periods of record as follows:

• Water years 1928 (including the November 1927 flood) to 1980, which was the period of record used in the original FIS.

• Water years 1928 to 2002. This was the available period of record at the time the Corps completed their study.

Based on the Corps report, their calculation of the 100-year flow at Vail Dam (prorated to the dam) using the shorter period of record was within 5% of the FIS, indicating close agreement with the original FIS. The prorated computed 100-year flow at Vail Dam for the longer period of record was slightly less than that computed using the shorter period of record. The Corps concluded that their updated frequency analysis did not reveal any major discrepancies when compared to the existing FIS. It is difficult to evaluate the Corps’ hydrologic analysis as no information was provided in the letter report on the actual 100-year flood flow computed using the two different periods of record. Also, the Corps indicated they prorated the 100-year flow at Vail Dam; however, it is unclear what equation was used to prorate the flow. The Corps ran the HEC-RAS model under the dam-out scenario. Water surface elevations calculated using the dam-in and dam-out scenarios were compared. Figure 6.2.2-1 shows the water surface elevations under dam-in and dam-out scenarios for the 100-year flood flows. Table 6.2.2-1 lists the reduction in water surface elevations at various FEMA cross-sections (the location of the FEMA cross-sections are shown in Figure 5.4-3) under dam-in and dam-out scenarios for the 100-year flood. Table 6.2.2-1: Reduction in Water Surface Elevation (feet) at Various Cross-Sections by Removing Vail Dam (Source: Corps January 2, 2003 letter report to Lyndon) Recurrence Interval FEMA *Cross-

Section E FEMA *Cross-Section F

FEMA *Cross-Section G

FEMA *Cross-Section H

10-year flood 1.1 feet 0.8 feet 0.6 feet 0.4 feet 50-year flood 1.1 feet 0.6 feet 0.5 feet 0.4 feet 100-year flood 0.7 feet 0.5 feet 0.4 feet 0.4 feet 500-year flood 0.5 feet 0.4 feet 0.4 feet 0.3 feet *The cross-section letters (E, F, G and H) are shown in Figure 5.4-3 which is a plan map showing the cross-section locations.

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Based on the above table, peak water surface elevations under the 100-year flood would be reduced with the removal of the Vail Dam by 0.7 feet at FEMA cross-section E, located approximately 0.8 miles upstream of the dam, and by 0.4 feet at FEMA cross-section H, located approximately 740 feet upstream of the Center Street Bridge. It is interesting to note is that both the Corps and FERC studies used the same 100-year flood flow. However, the reduction in water surface elevation at the same cross-section E for the 100-year flow was 0.7 feet based on the Corps study, and 3.6 feet based on the FERC study. This is a large difference, but without more supporting information on the Corps study we can not explain the difference. The Corps concluded that “given the small magnitude of decrease in peak water surface elevations associated with removal of Vail Dam, the fact that there is a hydropower-generating facility at the dam, and other complicating issues (sediment removal, ice jam impacts, etc) it appears unlikely that Vail Dam would be removed for flood damage reduction purposes”. It is difficult to comment on the analysis, since the Corps’ letter report did not contain the cross-sections used within the impoundment and beneath the Vail Dam under the dam-out scenario. Our comments are similar to those provided in the FERC analysis. There needs to be additional cross-section survey beneath the Vail Dam and within the impoundment (up to the Chapel Street Bridge) to determine if there are any hydraulic controls that span the river. It is also unclear why the reduction in water level for cross-section E, which is the closest to the Vail Dam was not reported in the Corps findings. The dam-in HEC-RAS model was also modified to reflect a diversion around the dam of an arbitrarily-chosen 50% of the total flood flow (for all flood flows) for the reach extending from FEMA cross-section D, located approximately 0.5 miles upstream of the dam, to 230 feet downstream of the dam. Water surface elevations calculated with the diversion in-place were compared to existing conditions (without a bypass channel and Vail Dam in-place). Figure 6.2.2-2 shows the peak water surface elevations for the bypass flow alternative under the 100-year flood. Table 6.2.2-2 lists the reduction in peak water surface elevations at various FEMA cross-sections upstream of Vail Dam assuming there was a bypass channel Table 6.2.2-2: Reduction in Water Surface Elevation (feet) at Various Cross-Section with Bypassing 50% of the Flood Flows around the Vail Dam (Source: Corps January 2, 2003 letter report to Lyndon) Recurrence Interval FEMA *Cross-

Section E FEMA *Cross-Section F

FEMA *Cross-Section G

FEMA *Cross-Section H

10-year flood 2.0 feet 1.4 feet 1.0 feet 0.6 feet 50-year flood 2.4 feet 1.7 feet 1.3 feet 1.0 feet 100-year flood 2.6 feet 1.8 feet 1.5 feet 1.2 feet 500-year flood 2.7 feet 1.9 feet 1.7 feet 1.3 feet *The cross-section letters (E, F, G and H) are shown in Figure 5.4-3 which is a plan map showing the cross-section locations. Based on the above table, the Corps noted that with a bypass channel, the water surface elevation under the 100-year flood would be reduced by 2.6 feet at FEMA cross-section E. The Corps hydraulic modeling found that dam removal caused only a minor reduction of the peak water surface elevation upstream of the dam (0.5 feet at Chapel Street Bridge at the 100-year recurrence interval) and even lesser at locations further upstream. The Corps also found that bypassing of flood flows around the dam had significant potential of reducing peak water surface elevations, depending on the capacity of the bypass channel. The Corps indicated that bypassing 50% of the flood flows around the dam would cause a reduction in water surface elevations of 2-3 feet at Chapel Street Bridge under the 100-year recurrence interval and approximately one foot at the Route 122/Center Street.

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In reviewing the correspondence there has been discussion regarding construction of a bypass channel around the Vail Dam to help alleviate upstream flooding and for environmental purposes. Based on the Corps analysis a bypass channel could alleviate flooding upstream; however, it would be valuable to have a plan map showing the current 100-year floodplain in the FIS as compared to the 100-year floodplain with a bypass channel. The 100-year floodplain with a bypass channel would be smaller than the existing floodplain; however, the areas where the floodplain is reduced would have to be identified to determine the true benefits. In addition, it is unclear what size channel (conveyance area) would be needed to pass 50% of the 100-year flood flows. Using the values in the FIS, the 100-year flood flow just below the Great Falls Dam in the FIS was 13,460 cfs, 50% of this flow is 6,730 cfs. A large bypass channel would be needed to convey 6,730 cfs. As a point of reference the mean annual flow near the Great Falls Dam is approximately 465 cfs (straight drainage area proration from East Branch gage). 6.3 VTrans Study of Main Street Bridge- February 2004 6.3.1 VTrans Study of Main Street Bridge- Background The Vermont Agency of Transportation (VTrans) conducted a preliminary hydraulic study of the Main Street Bridge (Route 5) and the overflow structure. The overflow structure (see inset), includes four culverts to convey flow beneath Route 5 and into an open field. VTrans evaluated options to replacing the culverts with a dry bridge of varying lengths. The Hydraulics Unit of VTrans reported their findings in a memo dated February 13, 2004 to the town of Lyndon. The Main Street Bridge is a 3 span bridge. It has a 124 feet clear span between abutments with two 5-foot wide piers, resulting in a 114 feet clear span opening. The bridge is skewed 30 degrees to the road, and has about an 11 feet clear height. The bridge was constructed in 1930 and reconstructed and widened in 1969. Bridge inspection reports indicate there is often a lot of large debris (trees) caught on the upstream ends of the piers. The overflow culverts are located about 350 feet south of the Main Street Bridge. It includes four 72-inch diameter corrugated metal pipes that were installed in 1970, and apparently replaced a dry flood relief bridge. As part of an effort to reduce flood damage that has occurred upstream of the Main Street Bridge, VTrans studied the effects of replacing four culverts with a larger single opening relief structure. The new structure would be located closer to the Main Street Bridge. 6.3.2 VTrans Study of Main Street Bridge- Findings and Independent Review VTrans used the flood flows as reported in the FIS. Their hydraulic analysis was limited to the Main Street Bridge area and extended about 200 feet upstream and downstream of the bridge. The hydraulic analysis consisted for six HEC-RAS hydraulic modeling scenarios and yielded the following results:

Passumpsic River Flood Study Gomez and Sullivan Page-27

Table 6.3.2-1 Water Surface Elevations at a point about 100 feet upstream of Main Street Bridge (Source: VTrans, February 2004)

Water Surface Elevations (feet) at a point about 100 feet upstream of Main Street Bridge

Conditions 10-yr flood 25-yr flood 50-yr flood 100-yr flood Natural channel without Main Street Bridge 703.8 ft 704.2 ft 704.6 ft 705.5 ft Main Street Bridge without any relief structure 704.5 ft 705.3 ft 706.2 ft 707.6 ft Existing Condition- Main Street Bridge with the existing culverts (assumes no plugging of culverts)

704.1 ft 704.7 ft 705.3 ft 706.5 ft

Proposed 20-foot relief bridge 704.0 ft 704.5 ft 705.1 ft 706.3 ft Proposed 40-foot relief bridge 704.0 ft 704.5 ft 705.1 ft 706.2 ft Proposed 60-foot relief bridge (net decrease in water surface elevation relative to existing conditions)

703.9 ft (0.2 feet)

704.4 ft (0.3 feet)

705.0 ft (0.3 feet)

706.1 ft (0.4 feet)

VTrans first evaluated flood levels absent the Main Street Bridge as this would provide the lowest possible water surface elevations that could be achieved. VTrans reported that the water surface elevations shown in the above table are based on unobstructed flow through the structures. Both the existing bridge and the existing relief pipes are very prone to blockage by debris. Water surface elevations above the bridge would be higher if debris was present. If the bridge waterway area is reduced due to debris on the piers, as is common, much more water could flow through the relief bridge than the existing culverts. VTrans noted that actual reductions in upstream flooding may be more than shown in the above table if debris blockage were considered. VTrans concluded that replacing the relief pipes with a larger, single opening relief structure will increase the total flow capacity under the Main Street Bridge. Upstream water surface elevations and flooding will be reduced. The benefit of a single opening relief bridge is it would not plug with debris so easily. VTrans also noted that “While the US 5 fill and structures may increase upstream flooding somewhat, it appears that most flooding in this area is due to the fact this is a natural floodplain area”. VTrans also suggested that a cost benefit analysis be conducted to determine if the potential flood reduction is justified. VTrans suggested that further evaluation of the potential effects of allowing more overbank flow through a relief structure, on surrounding property and stream stability, should also be considered. Scour at the relief bridge and through fields upstream and downstream could be an issue. We generally concur with VTrans analysis and conclusions. Replacing the culverts with a dry bridge would reduce the upstream water as the above table shows. However, as VTrans notes, if the Main Street Bridge becomes clogged a dry bridge would be effective for conveying flow downstream and thus the net decrease in water levels could be higher. It has been reported that the existing culverts provide little flow relief as they are commonly plugged. It is assumed that debris and vegetation would plug the culverts in the summer, while during the winter, ice blockage could be an issue. Assuming the existing culverts were completely plugged, the corresponding water surface elevations would be representative of the second row of values shown in the Table 6.3.2-1 entitled- Main Street Bridge without any relief structure. Comparing the Main Street Bridge without any relief structure (assuming the culverts are completely plugged) to placing a 60-foot relief structure provides a net reduction in water surface elevations of 0.6 feet (10-yr flood), 0.9 feet (25-yr flood), 1.2 feet (50-yr flood), and 1.5 feet (100-yr flood). This is a sizeable reduction and thus further evaluation of this option is likely warranted.

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6.4 USGS Ground Penetrating Radar Study- November 2004 6.4.1 USGS Ground Penetrating Radar Study- Background

The Corps contracted with the USGS to further evaluate the feasibility of removing the Vail Dam. At the request of the Corps, ground penetrating radar (GPR) surveys were conducted by the USGS to collect data on the distribution and thickness of streambed materials in the vicinity of the dam. The GPR surveys were designed to provide information on 1) presence of bedrock or coarse materials that could limit bed scouring upstream of Vail Dam; 2) cross-sections and profiles of the streambed to support future hydraulic analyses and 3) the potential for sediment transport from the three-mile reach above Vail Dam following dam removal. It is assumed that the investigation was also conducted to determine if bedrock extended across the Passumpsic River above the Vail Dam, and if so, would it form a hydraulic control that could limit the flood benefits of removing Vail Dam. Based on conversations with the USGS, the investigation was limited in scope and was meant as a preliminary survey. It included the GPR and Global Positioning Surveys (GPS) surveys with minimal interpretation. Though the information in these surveys can be used to help interpret the location of bedrock within the channel, there are several other techniques that could also be used. At the time of the survey there was insufficient funds to collect sediment cores and probe the sediments for ground-truthing (verifying) the GPR surveys, or to conduct other geophysical methods that pick up where GPR does not work. The USGS reported that overburden at this site consists of thick deposits of course-grained stratified glacial drift. Streambed sediment grain size analyses for neighboring stream reaches on Hawkins Brook and the East Branch of the Passumpsic River identify sand to boulder and sand to cobble ranges with a median grain size of 46.6 millimeters (1.83 inch) and 56.2 millimeters (2.21 inch). Drilling logs from wells indicate a bedrock depth of 6 to 40 meters (20 to 130 feet) within the river valley (Agency of Natural Resources Internet Mapping, 2004). Continuous GPR surveys were conducted to image streambed surface characteristics and subsurface sediment interfaces. Material types were indicated based on the pattern of the reflection. Results were described in reference to the water surface or thickness of layers. Depths and thickness were measured using separate scales for water and sediment (provided with each section).

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GPR surveys were located, and distances normalized with precise concurrent GPS surveys. The USGS flagged control points at the edge of water during their survey to allow the surveys to be tied to a known datum. GPR surveys included 8 profiles (see Plate 1-Figure 6.4.1-1) and 18 cross sections (see Plate 2-Figure 6.4.1-2) upstream of the dam. Thirteen of the cross-sections were located at sites where cross-sections were surveyed as part of a previous Lyndon FIS, starting at 31 feet upstream of the dam. Five additional cross-sections (cross-sections 2-6) were surveyed with GPR immediately upstream of the Vail Dam (see photo inset above). GPR profiles of the river were started at approximately 190 feet upstream of the dam (Profile 1) and extended upstream approximately 3 miles from the dam. Per discussions with USGS, they strived to collect the channel bed profiles along the thalweg17 of the river. 6.4.2 USGS Ground Penetrating Radar-Study Findings and Independent Review The analysis conducted by the USGS displayed the findings in meters. For reference 1 meter equals approximately 3.3 feet. The depth to bedrock and/or boulders, immediately (10-30 meter) upstream of the dam were 2.6 meters (8.5 feet) or less, below the water surface (see cross-sections 1-3, Plate 2). It should be noted that the cross-sections were plotted in a downstream view. Bedrock was also exposed in places along the river banks near the dam. The USGS reported that cross-section 6 may represent the shallowest depth to bedrock or boulder stream bed which could serve as a natural hydraulic control if the Vail Dam were removed (see cross-section 6 below). The erosion-resistant bed at cross-section 6 ranged in depth from 0.8 to 1.4 meters (2.6 to 4.6 feet) below the waters surface. A shallower erosion-resistant bed (0.6 meters or 1.2 feet below the water surface) was found at a distance of 80 meters (260 feet) along profile 1 (Plate

1), but the USGS reported it was not known if this continued across the channel or was a local point anomaly, such as the rock cribbing used for historical logging operations in this reach. Bedrock was

17 The thalweg represent the deepest points along the length of a river bed.

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visually observed approximately 1 meter (3.3 feet) below the water surface at a distance of 17 meters (56 feet) above the Vail Dam (cross-section 5, Plate 2). Fine and medium grained sediments were apparent in all profiles (Plate 1). The thicknesses of fine and medium-grained sediments were measured in the upstream portions of the profiles where they comprise the top layer. The USGS noted that deeper penetration of the GPR signal can be blocked by boulders which scatter the signal (240-300 meters or 790-980 feet, Profile 3, Plate 1). For example, a sediment thickness of 2.5 meters, below 1.2 meters of water, can be observed in Profile 8 at a distance of 360 meters (Plate 1). The information collected by the USGS would be extremely beneficial to include in an updated hydraulic model to better examine the flood impacts, if any, of removing Vail Dam. Cross-section 6 in particular, could be added to the existing FIS hydraulic model. However, a critical piece of information was unavailable from the study. As noted above, the vertical control for the cross-section plots were relative to the water surface elevation at the time of the survey. If the cross-sections were tied to a known datum, they could be used in the hydraulic model. In order to conduct the hydraulic analysis, all cross-sections--those collected by the Corps, and those collected in the original FEMA study-- must be on the same datum. We contacted the USGS to determine a) if pictures were taken during the survey, particularly at cross-sections 1-6, and at the Vail Dam spillway and b) the date of the survey. The purpose for requesting pictures was to determine if the Vail Dam created a flat pool through cross-section 6 and also to determine the amount of spill over the Vail Dam. It was thought that perhaps the USGS cross-sections could be tied to the Vail Dam spillway crest elevation since this datum is known from the original FIS. No pictures were available. The USGS cross-section survey was conducted on November 19, 2004, while the profiles were conducted on November 17. The flow at the East Branch USGS gage was 54 cfs on November 17 and 62 cfs on November 19. The flow at the Passumpsic USGS gage was 311 cfs on November 17 and 333 cfs on November 19. We checked the variability in the river’s water surface elevation at the Passumpsic and East Branch USGS gages during November 17 and 19 and the water surface elevation changed less than 1/10 of a foot. In the end, as described in Section 7.0, we incorporated cross-section 6 in the original FEMA hydraulic model to determine if there are any flood benefits to removing Vail Dam. As discussed in Section 7, although the hydraulic modeling was conducted it has some limitations. 6.5 Corps Initial Appraisal Report, Cost Benefit Analysis of Structural Flood Protection, May 2006 6.5.1 Corps Initial Appraisal Report, Cost Benefit Analysis of Structural Flood Protection- Background The village of Lyndonville requested the assistance of the Corps under Section 205 authority to study flooding problems in two areas of town, the north and south study areas as shown in Figure 6.5.1-1. This study was prompted by the recent flooding in 2000 and 2002. The south study area extends approximately from the Chapel Street Bridge (Route 5) to South Street. The south area has approximately 12 buildings that are affected by the flooding. The north study area extends approximately from Nick’s Gas’n’go (Main Street) until the Route 114 Bridge. Approximately 40 buildings, including a manufactured home park are affected by flooding.

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6.5.2 Corps Initial Appraisal Report, Cost Benefit Analysis of Structural Flood Protection- Study Findings and Independent Review The Corps developed stage frequencies for both study areas in Lyndon using the existing FIS as well as the previous Corps study concerning the affects of Vail Dam. The Corps conducted a flood frequency analysis using the USGS gage flow data and compared the findings with flood flows reported in the FIS. The Corps concluded that there was close agreement between their analysis and the FIS, and thus adopted the flood flows reported in the FIS. The Corps then developed a stage-frequency18 curve at a location approximately 400 feet upstream of Chapel Street Bridge (Route 5) – since the profiles were very flat for the south study area, the Corps developed one stage-frequency curve. Similarly, a second stage-frequency curve was developed for the north study area. These are existing condition curves and could be used to screen any structural or non-structural alternatives. The Corps examined two alternatives to limit flooding as follows:

1. Building a combination of levees and plastic sheeting 2. Plastic sheeting throughout

The Corps concluded that “A third alternative involving either removal of the Vail Dam or a bypass channel around the dam was ruled out as being ineffective and expensive, since the ground penetrating radar (GPR) analysis tests determined there is a ledge in the river bed approximately 100 feet upstream of the Vail Dam which would act as a natural dam even if the Vail dam is removed”. It should be noted that to our knowledge no hydraulic modeling was conducted with ledge to confirm that removal of the Vail Dam yielded no flood benefits. For each alternative, an economic analysis was conducted of the potential flood damage reduction benefit in the Passumpsic River floodplain. Expected annual damages were calculated for both the natural and modified conditions. The difference between these amounts is a measure of flood damage reduction. For each alternative, the annual benefit was divided by the annual cost to determine a benefit cost ratio. This ratio must be equal to or greater than one for Federal participation in water resource improvement projects. Shown in Table 6.5.2-1 is the Corps’ planning level construction cost estimates for both the north and south study areas. Table 6.5.2-1. Planning Level Construction Cost Estimates for both South and North Study Areas (Source: Corps May 2006) Item Cost Construction Contract $1,620,000 Feasibility Study $200,000 Plans and Specifications $180,000 Total $2,000,000

18 Stage-Frequency Curve: A stage-frequency curve provides information on how often the stage (water surface elevation) in a section of river becomes flooded.

Manufactured Home Park & LynBurke Motel Flooded- June 2002

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Recurring flood losses are those potential damages that are estimated to occur at various flood stages. Recurring damages by elevation for both the south and north damage areas are shown in Table 6.5.2-2. For the 100-year flood, floodwater would reach an elevation of approximately 704 feet in the south zone and approximately 707 feet in the north zone. Depth-damage relationships developed by the Institute for Water Resources were used by the Corps to develop stage (elevation) versus damage relationships. Table 6.5.2-2 Recurring Flood Damages in the South and North Zones (Source: Corps May 2006) Recurring flood damages in South Zone Recurring flood damages in North Zone Elevation (feet) Damage ($000) Elevation (feet) Damage ($000) 700 701 702 703 704 705 706 707

0.0 108.0 186.0 392.7 613.9 855.5 1,177.0 1,583.8

703 704 705 706 707 708 709 710

0.0 16.3 115.4 207.9 342.4 479.1 607.2 692.0

Expected annual damages were determined by weighting the damages associated with different events with their probability of occurrence. The Corps summed these products over the range of all possible events. Expected annual damage in the south and north zones were reported as $13,100 and $17,500, respectively. Shown in Table 6.5.2-3 is the benefit cost summary for the north and south zones. Table 6.5.2-3. Benefit Cost Summary (Source: Corps May 2006) Southern Zone Northern Zone Annual Benefit $13,100 $17,500 Annual Cost $55,800 $50,300 Net Annual Benefits -$42,800 -$32,800 Benefit-to-Cost Ratio 0.2 0.3 The Corps evaluated other alternatives for floodproofing sections within the north and south zones. In all, three alternatives were evaluated in the north zone and two alternatives in the south zone. However, the benefit to cost ratio for all alternatives were less than 0.5. The Corps reported that “in order to continue the development of a feasibility report, at least one alternative has to have a cost to benefit ratio of one or more”. None of the alternative plans, or the full plans for the southern and northern zones meets the criterion. Given this, the Corps concluded that the feasibility study should be discontinued. It is difficult to fully assess the Corps’ study as there is limited background information available on costs for the various alternatives evaluated. However, even if the costs were half that projected by the Corps, it the cost-benefit ratio would still be less than one. As described in Section 9.0 even if the Corps analysis had a cost-to-benefit ratio greater than one, Lyndon would have to seriously consider how installing levees could impact the Passumpsic River over time. Limiting the Passumpsic River’s ability to access its existing floodplain could result in additional erosion and flooding downstream as described later.

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7.0 Independent Hydraulic Analysis As noted above several studies have been conducted to determine if there are any flood benefits to removing the Vail Dam using the original hydraulic model developed by FEMA. In this section the original FIS hydraulic model was used, with some modifications, to determine the contribution of flooding due to the bridges and Vail Dam. As described later an incremental analysis was conducted by removing all structures (Vail Dam, and all bridges) from the hydraulic model. By removing all structures it provides information on baseline flooding, which can then be compared to flood elevations when the bridges and Vail Dam are added to the hydraulic model. After removing all structures from the hydraulic model, the 100-year floodplain will be developed. The bridges and Vail Dam were then incrementally added to determine which structures contribute to the greatest impact on the floodplain. Two types of output is provided in our analysis- a water surface profile for the 100-year flood, and a plan map showing the area of inundation. Described next are the steps taken to conduct the analysis. 7.1 Methodology For purposes of consistency with the original FIS, we used the same 100-year flood flow as the FIS for the analysis, recognizing that our calculations of the 100-year flood flow differed from FEMA’s. We obtained the original HEC-2 hydraulic model from the FERC and converted it to HEC-RAS. Before running the HEC-RAS model the input was reviewed and we noticed that one of the input variables did not make sense. In the hydraulic model, the distances (along the left, center and right banks) between cross-sections are entered into the model. In the original model, for cross-section D the reach lengths on the left, center and right banks were reported as 1000, 990, and 9701 feet, respectively. The right bank reach length of 9701 feet is obviously an error. Based on looking at the topographic maps, the 9701 foot reach length was changed to 970 feet. The correction was made and the HEC-RAS model was rerun. Model results were compared to the 100-year flood elevations at cross-sections A-K from the FIS as shown in Table 7.1-1. Table 7.1-1: 100-year Flood Flow Water Surface Elevations at Cross-Sections A-K on Passumpsic River. Comparison of Flood Insurance Study and Gomez and Sullivan Analysis. (Source: Gomez and Sullivan)

Passumpsic River

Cross-Section

100-year flood flow Water Surface Elevation (FIS)

100-year flood flow Water Surface Elevation (Gomez and Sullivan)

Difference in Water Surface

Elevation A (downstream) 676.8 ft 676.8 ft +0.2 ft

B 680.7 ft 680.7 ft 0.0 ft C 682.8 ft 683.1 ft +0.3 ft

Vail Dam D 700.7 ft 700.6 ft -0.1 ft

Chapel Street Bridge (Route 5 south) E 702.6 ft 702.7 ft +0.1 ft F 703.7 ft 703.7 ft 0.0 ft G 704.1 ft 704.0 ft -0.1 ft

Center Street Bridge (Route 122) H 704.7 ft 704.5 ft -0.2 ft I 705.4 ft 705.3 ft -0.1 ft J 705.7 ft 705.6 ft -0.1 ft

Covered Bridge Main Street Bridge (Route 5 north) Route 114 Bridge

K (upstream) 709.9 ft 710.0 ft +0.1 ft

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The difference in water surface elevations between the two models is minimal. It is general engineering practice to accept the accuracy of the HEC-2 model to be within +/- 0.5 feet and because the differences above are within this acceptable limit, it was assumed that our use of the HEC-RAS model was acceptable. 7.2 Hydraulic Modeling- Description of Alternatives Before running the hydraulic model cross-section 6, from the USGS study, was added to the hydraulic model. However, parties should understand that several assumptions were made relative to including cross-section 6 as follows:

• It was assumed that cross-section 6 was representative of bedrock or large boulders that spanned the river and would act as a natural hydraulic control if the Vail Dam were removed. In short, it was assumed that the channel geometry at cross-section 6 would be stable even with the Vail Dam removed- no scour would occur. This is a gross assumption because based on the GPR data for cross-section 6 the bedrock or boulder substrate does not extend along the entire right side of the channel.

• As noted earlier, the USGS did not tie the cross-sections to a known datum. However, cross-section 6 was indirectly tied to a known datum- the Vail dam spillway crest elevation of 688.7 feet. It was assumed that cross-section 6 was located in the impounded reach- meaning the water surface elevation of the dam was equal to the water surface elevation at cross-section 6. This is a gross assumption since the depth of water over the spillway at the time of the USGS survey is unknown. If the depth of water over the spillway were 0.5 feet, then cross-section 6’s bed elevation should be raised 0.5 feet. Thus, our analysis placed cross-section 6 perhaps lower in the streambed than actual. It was confirmed with the USGS that water was spilling over the spillway crest during the survey although it is unknown if flashboards were in place.

• Because only channel geometry was collected by the USGS, cross-section 6 was extended upland to simulate flood flows. Using the 20 feet contour intervals on the topographic map, a full cross-section was developed as shown in Figure 7.2-1 (below). Based on scaling from the topographic map, cross-section 6 is located approximately 125 meters (410 feet) upstream of the Vail Dam.

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Figure 7.2-1: Cross-Section 6 collected by the USGS- included in HEC-RAS model (Source: Gomez and Sullivan, 2006) Given the above assumptions cross-section 6 was added into the model. The following runs were then made to compute the predicted water surface elevation under the 100-year flood (Table 7.2-1 is a summary of the various Alternatives):

• Alternative 1-Existing Conditions- cross-section 6 was not added to the model, such that the FIS results could be compared to other alternatives.

• Alternative 2-Existing Conditions Modified- same as Alternative 1, but cross-section 6 was added to the model.

• Alternative 3- same as Alternative 2, but the Vail Dam was removed. To simulate the Vail Dam removed, the entire Vail Dam cross-section (spillway, powerhouse, upland topography, etc) was removed from the HEC-RAS model.

• Alternative 4- cross-section 6 was added to the model, Vail Dam remained, and all bridges19 were removed.

• Alternative 5- Natural Conditions- cross-section 6 was added to the model, Vail Dam was removed and all bridges were removed. This alternative represents the lowest possible flood elevations.

• Alternative 6- Bypass channel. Approximately 50% of the 100-year flood was bypassed around Vail Dam.

• Alternative 7- Lowering river channel. This alternative was added to illustrate how channel cross-section area can impact flooding. It is recognized that lowering the entire channel within the project reach is not practicable. For purposes of this analysis, the channel cross-sections from

19 It should be noted that initially individual bridges were removed on an incremental basis in the hydraulic model to determine the flood impacts of individual bridges. However, because the net effect of removing all bridges was minimal, only the model results for the removal of all bridges is provided.

680

685

690

695

700

705

0 100 200 300 400 500 600

Station (feet)

Elev

atio

n (fe

et)

Upland elevations obtained from topographic map

Channel bed elevations obtained from scaling of cross-section 6 provided by USGS

Passumpsic River Flood Study Gomez and Sullivan Page-36

the beginning of the Passumpsic River to just upstream of cross-section 6 were lowered an arbitrary 4 feet.

Table 7.2-1: Summary of HEC-RAS modeling Alternatives (Source: Gomez and Sullivan, 2006)

Options Alt. 1 Alt. 2 Alt. 3 Alt. 4 Alt. 5 Alt. 6 Alt. 7 Cross-Section 6 added - X X X X X X Vail Dam removed - - X - X - - All Bridges removed - - - X X - - Bypass 50% of 100-year flood

- - - - - X -

Lowering river channel in project reach

- - - - - - X

7.3 Hydraulic Modeling- Results Shown in the Table 7.3-1 are the water surface elevations at different stations, under each alternative, for the 100-year flood flow. It should be noted that station 6739 is located approximately 9 feet upstream of the Vail Dam. The next upstream station is 6770. To have a sense of where station 6770 resides relative to Vail Dam (station 6739), subtract the two station values- thus station 6770 is located 31 feet upstream of the Vail Dam. Shown on the top row of Table 7.3-1 is the station number, and feet upstream of Vail Dam. Also, some stations are called out as X-sec D, which means cross-section D from the FIS or station number 9245. Shown in Table 7.3-1 is the computed water surface elevation at various cross-sections under the 100-year flood. In addition, Alternative 2 was used to reflect baseline conditions- those conditions that are currently present in the river system. To determine incremental changes in the water surface elevation, Alternatives 3-7 were compared to Alternative 2- the incremental difference in the water surface elevation is shown in the table.

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Table 7.3-1: Water Surface Elevations under the 100-year flood flow for various Alternatives (Source: Gomez and Sullivan, 2006) Condition

St 6739 Just upstream of Vail Dam

St 6770 (31 ft)

St 7144 USGS X-sec 6 (405 ft)

St 7535 (796 ft)

St 8255 (1,516 ft)

St 9245 X-sec D (2506 ft)

St 10990 X-sec E (4251 ft)

St 13835 X-sec F (7096 ft)

St 17990 X-sec G (11251 ft)

St 22120 X-sec H (15381 ft)

St 26020 X-sec I (19281 ft)

St 27850 X-sec J (21111 ft)

St 32075 X-sec K (25336 ft)

Alternative 1-Existing Conditions

698.41 ft 698.39 ft - 698.80 ft 699.70 ft 700.61 ft 702.68 ft 703.74 ft 704.00 ft 704.47 ft 705.27 ft 705.60 ft 710.05 ft

Alternative 2- Modified Existing Conditions- cross-section 6 added

698.41 ft 698.39 ft 698.77 ft 698.74 ft 699.66 ft 700.58 ft 702.66 ft 703.73 ft 703.99 ft 704.46 ft 705.26 ft 705.59 ft 710.05 ft

Alternative 2- change in water surface elevation relative to Alternative 1

0.00 ft 0.00 ft - -0.14 ft -0.04 ft -0.03 ft -0.02 ft -0.01 ft -0.01 ft -0.01 ft -0.01 ft -0.01 ft 0.00 ft

Alternative 3- cross-section 6 added, Vail Dam removed

- 690.81 ft 695.51 ft 695.67 ft 697.75 ft 699.22 ft

701.90 ft

703.15 ft

703.46 ft

704.01 ft

704.94 ft

705.33 ft

710.00 ft

Alternative 3- change in water surface elevation relative to Alternative 2

- -7.58 ft -3.26 ft -3.07 ft -1.91 ft -1.36 ft -0.76 ft -0.58 ft -0.53 ft -0.46 ft -0.32 ft -0.26 ft -0.05 ft

Alternative 4- cross-section 6 added, Vail Dam remained, all bridges removed

698.41 ft 698.39 ft 698.77 ft 698.74 ft 699.66 ft 700.58 ft 702.46 ft 703.57 ft 703.84 ft 704.16 ft 705.04 ft 705.42 ft 709.45 ft

Alternative 4- change in water surface elevation relative to Alternative 2

0.00 ft 0.00 ft 0.00 ft 0.00 ft 0.00 ft 0.00 ft -0.20 ft -0.16 ft -0.15 ft -0.30 ft -0.22 ft -0.17 ft -0.60 ft

Alternative 5- Natural Conditions- cross-section 6 added, Vail Dam removed, all bridges removed.

- 690.80 ft 695.52 ft 695.67 ft 697.75 ft 699.22 ft

701.68 ft

702.98 ft

703.31 ft

703.70 ft

704.74 ft

705.18 ft

709.44 ft

Alternative 5- change in water surface elevation relative to Alternative 2

- -7.59 ft -3.25 ft -3.07 ft -1.91 ft -1.36 ft -0.98 ft -0.75 ft -0.68 ft -0.76 ft -0.79 ft -0.41 ft -0.61 ft

Alternative 6- cross-section 6 added, bypass channel.

- 695.55 ft 696.58 ft 696.64 ft 698.24 ft 699.54 ft 702.07 ft 703.27 ft 703.58 ft 704.11 ft 705.01 ft 705.39 ft 710.00 ft

Alternative 6- change in water surface elevation relative to Alternative 2

- -2.84 ft -2.19 ft -2.10 ft -1.42 ft -1.04 ft -0.59 ft -0.46 ft -0.41 ft -0.35 ft -0.25 ft -0.20 ft -0.05 ft

Passumpsic River Flood Study Gomez and Sullivan Page-38

Condition

St 6739 Just upstream of Vail Dam

St 6770 (31 ft)

St 7144 USGS X-sec 6 (405 ft)

St 7535 (796 ft)

St 8255 (1,516 ft)

St 9245 X-sec D (2506 ft)

St 10990 X-sec E (4251 ft)

St 13835 X-sec F (7096 ft)

St 17990 X-sec G (11251 ft)

St 22120 X-sec H (15381 ft)

St 26020 X-sec I (19281 ft)

St 27850 X-sec J (21111 ft)

St 32075 X-sec K (25336 ft)

Alternative 7- remove 4 feet of sediment

698.41 ft 698.39 ft 698.77 ft 698.74 ft 699.31 ft 699.71 ft 700.90 ft 701.60 ft 701.75 ft 702.00 ft 702.47 ft 702.67 ft 705.98 ft

Alternative 7- change in water surface elevation relative to Alternative 2

0.00 ft 0.00 ft 0.00 ft +0.27 ft -0.35 ft -0.87 ft -1.76 ft -2.13 ft -2.24 ft -2.46 ft -2.79 ft -2.92 ft -4.07 ft

Passumpsic River Flood Study Gomez and Sullivan Page-39

The water surface profile along the Passumpsic River was plotted for each of the alternatives. On each plot the water surface profile for existing conditions (Alt 2) is shown along with one alternative for comparison purposes and to illustrate the difference in water levels. The following figures were developed: Figure 7.3-1: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) Figure 7.3-2: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and All Bridges Removed (Alt 4) Figure 7.3-3: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Vail Dam and All Bridges Removed (Alt 5) Figure 7.3-4: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and Bypass Channel (Alt 6) Figure 7.3-5: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and lowering the river channel by four feet within the project reach (Alt 7) Figure 7.3-6: Comparison of Water Surface Profile (100-year flood) for Existing Conditions (Alt 2) and construction of dry bridge just south of Main Street Bridge (Alt 8) The above figures provide information only on the vertical change in water surface elevation- it does not show the corresponding horizontal change in the area of inundation. The question is whether a reduction in the area of inundation reduces flooding of infrastructure (roads, houses, businesses, etc). To determine the area of inundation for the various alternatives, several steps were conducted as described next. The maps of the original flood insurance study were rectified to current mapping. Using the flood maps, a centerline was established which was consistent with the flood profile. The centerline was used to place the cross-sections from the hydraulic model along the river. The cross- sections were assigned the appropriate water surface elevation from the hydraulic modeling. Using a digital elevation model and the water surface elevations from the hydraulic model (for the 100-year flood), new inundation areas were determined for each of the alternatives. The area of inundation for the 100-year flood was plotted for each of the alternatives. On each plot the area of inundation for existing conditions (Alt 2) is shown along with one alternative for comparison purposes and to illustrate the difference in the inundation area. The following figures were developed: Figure 7.3-7: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) Figure 7.3-8: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and All Bridges Removed (Alt 4) Figure 7.3-9: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam and All Bridges Removed (Alt 5) Figure 7.3-10: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Bypass Channel (Alt 6) Figure 7.3-11: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and and lowering the river channel by four feet within the project reach (Alt 7) Figure 7.3-12: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and construction of dry bridge just south of Main Street Bridge (Alt 8) Figure 7.3-13: Comparison of Inundation Areas (100-year flood) for Existing Conditions (Alt 2) and Vail Dam Removed (Alt 3) in the area around Vail Dam

Passumpsic River Flood Study Gomez and Sullivan Page-40

Discussion of Results Alternative 1 At the Vail Dam, the HEC-RAS model computed critical depth at the Vail Dam because of the steep drop in the water surface elevation. As noted earlier, because the same 100-year flood flow was used, the computed water surface elevations matched closely with FEMA’s original study. Alternative 2 USGS cross-section 6 (coded as station 7144) was added to the HEC-RAS model. As Table 7.3-1 shows, there are negligible differences in the water surface elevations between Alternative 1 and 2. Because Alternative 2 included cross-section 6 it was used as a point of comparison for the other Alternatives. Alternative 3 Alternative 3 includes cross-section 6 and the full removal of the Vail Dam cross-section (dam, powerhouse, and upland topography) from the hydraulic model. Between Vail Dam (station 6739) and USGS cross-section 6 (station 7144), there is one more cross-section (station 6770). Parties should recognize that this cross-section does not represent what the future condition of the stream channel would be if the Vail Dam were ever removed. The cross-section for station 6770 reflects existing conditions, which includes several feet of deposited sediment that would likely be scoured if the Vail Dam were ever removed. Thus, the “new” cross-section, after sediment scouring, would be lower than current. Based on Figure 7.3-1, removal of the Vail Dam reduces the water surface elevation by approximately 7.6 feet at station 6770, located 31 feet upstream of the dam. The water surface elevation drops by approximately 3.3 feet at station 7144 located 410 feet above the dam. The water surface elevation drops by approximately 1.4 feet at cross-section D (station 9245), which is located between Vail Dam and Chapel Street Bridge. Above Chapel Street Bridge, the water surface elevation at the other cross-sections dropped on the order of 0.3 to 0.8 feet as shown in Figure 7.3-1. It is also important to note that the water surface profile through most of Lyndon is very flat, which illustrates the very mild slope of the Passumpsic River. As shown in Figure 7.3-7 removal of the Vail Dam does reduce the area of inundation primarily in the area just upstream of the dam, and just upstream of the Chapel Street Bridge. Shown in Figure 7.3-12 is a closer plan view of the inundation area with and without the Vail Dam. As the figure shows, there are a few buildings (houses) on Town Highway 66 and Red Village Road as well as on Back Center Road (above Chapel Street Bridge) that would benefit from removal of the Vail Dam. In terms of flooding above the Chapel Street Bridge, removal of the Vail Dam does not show significant flood benefits. Alternative 4 Alternative 4 includes the removal of all bridges (include encroachments due to roadway fill leading up to the bridges), but leaving the Vail Dam in place. As shown in Figure 7.3-2 the water surface profile for Alternative 2 closely matched existing conditions until the area of the Main Street Bridge. This would indicate that the Chapel Street Bridge and Center Street Bridge have minimal impact on flooding. With the Main Street Bridge removed, the water levels decreased around 0.6 feet upstream. Although the hydraulic model showed that the Main Street Bridge contributes minimally to flooding; the model does not account for debris jamming. Experience has shown that in fact debris jamming at the bridges has occurred and exacerbates flooding, thus the modeling effort does not reflect worst case conditions. As shown in Figure 7.3-8 there is some differences in the inundation area between Alternative 4 and existing conditions near the Main Street Bridge and along Miller Run.

Passumpsic River Flood Study Gomez and Sullivan Page-41

Alternative 5 Alternative 5 includes the removal of all bridges and the Vail Dam., which basically mimics “natural” conditions. As shown in Figure 7.2-3 the water surface profile for Alternative 5 shows a reduction in water levels immediately above the Vail Dam until the Chapel Street Bridge. From the Chapel Street Bridge to the Main Street Bridge, the reduction in water levels is minimal relative to existing conditions. Upstream of the Main Street Bridge the water levels decreased by approximately 0.6 feet. As shown in Figure 7.3-9, there is some reduction in the 100-year flood inundation area near the Vail Dam and at a few locations upstream. Alternative 6 Alternative 6 includes the addition of a bypass channel that would divert flow from upstream of the Vail dam to a location below the dam. In the analysis, it was assumed that 50% of the 100-year flood flow would be conveyed into the bypass channel. Fifty percent of the 100-year flood flow at the Vail Dam is approximately 6,730 cfs. As shown in Figure 7.3-4 the water surface profile for the effect of the bypass channel shows a reduction in water levels above the Vail Dam to the Chapel Street Bridge and diminishes thereafter. As shown in Figure 7.3-10, there is a reduction in the 100-year flood inundation area particularly in the area of Vail Dam and Chapel Street Bridge. The concept of a bypass channel would require considerably more investigation such as a) the physical location of the bypass channel, b) the physical dimensions of the channel relative to width and slope, c) the need for inlet control to the channel, d) the need for the channel to serve as a conduit for upstream and downstream fish passage and e) Lyndonville Electric Department would obviously want to determine the impact of maintaining a bypass flow on hydropower generation (bypass flow would be unavailable for generation). Relative to location, it does not appear feasible to create a bypass channel on river left (the left side of the river looking downstream) as it would prohibit access to the Vail Dam and powerhouse unless a bridge were constructed over the bypass channel. On river right, the topography is steep and it is appears that bedrock is located throughout the area. Construction of a bypass channel on river right may require blasting. The physical dimensions of the bypass channel would be sized to pass both flood flows and lower flows to allow fish passage. The inlet elevation to the bypass channel would have to be configured to maintain flow in the bypass channel presumably year-round. During non-flood periods, 20 5/8-inch-high wooden flashboards are affixed to the spillway to increase the head available for generation. During flood periods, the flashboards would fail. Thus, the inlet elevation to the bypass channel would have to be designed to accommodate a range of flow and headpond elevations. The vertical drop from the Vail impoundment to the just below the dam is approximately 8-15 feet depending on flow conditions. To create a bypass channel that would also allow the upstream and downstream movement of fish, the channel bed would require a reasonable slope. For example, using a 1% river bed slope would equate to a bypass channel length between 800 feet (8 feet drop) to 1,500 feet (15 foot drop). This would require a relatively long bypass channel and there may be physical constraints to construct the channel. In lieu of a bypass channel, another option to consider at the Vail Dam is increasing its flow capacity. Currently, there are two ways to convey below the Vail Dam- over the spillway crest of the dam, and through the turbines. When flows in the Passumpsic River are in excess of the station turbine flow capacity (285 cfs) the station operates at full generation and all inflow above 285 cfs is spilled over the spillway. The length of the spillway is approximately 96 feet long. The addition of a gate at the dam could increase the overall capacity of the project. However, it is recognized that installation of a gate(s) will likely have limited benefits of reducing upstream water levels under a 100-year flood flow (a gate would increase flow conveyance, but would probably not reduce flooding under a 100-year flood, but could under a 10-year flood.

Passumpsic River Flood Study Gomez and Sullivan Page-42

Alternative 7 Alternative 7 includes the arbitrary lowering of the channel bed by four feet throughout the entire project reach up to cross-section 6. This was accomplished in the model by reducing the full channel cross-sections by 4 feet. As shown in Figure 7.3-5 the water surface profile for Alternative 7 shows a reduction in water levels on the order of 1.8 feet near the Chapel Street Bridge and slowly reducing further upstream where the water levels drop as much as 4 feet in the upper reach (FEMA cross-section K). As shown in Figure 7.3-11, there is a reduction in the 100-year flood inundation area throughout the entire project reach, with greater reductions occurring further upstream. This alternative illustrates that the channel cross-sectional area has a direct relationship to the level of flooding. For example, consider the concept of adding four feet of sediment to each cross-section as opposed to removing it as in this alternative. Four of sediment added to the cross-sections would raise the water surface elevations much higher. Keep in mind that the original cross-sections used in this model were obtained in 1977, which reflects only a snapshot in time. As described later, it is likely that the cross-sections surveyed in 1977 are different than today as there has likely been sediment deposition within the project reach resulting in “filling” the channel. The sediment deposition is likely the result of channel adjustments that are on-going on the East Branch and perhaps other major tributaries. The channel adjustments have resulted in delivering sediments to the project reach as described more fully in the next section.

Passumpsic River Flood Study Gomez and Sullivan Page-43

8.0 Geomorphology and River Corridor Management 8.1 Introduction Similar to most towns throughout the United States, towns in Vermont were developed along river corridors. Rivers provided various benefits including a source of transportation, powering sawmills, gristmills, and during the industrial age rivers received discharges from manufacturing plants. Most of

the early development centered along rivers. Over the last few centuries population growth and continued development has occurred within the river corridor. As encroachment within the river corridor continued so have the costs associated with flood damage. In the past, the philosophy to reducing flood damage was to “control” flood flows through various structural means such as berms/floodwalls, or straightening the river channel. In fact, many Vermont rivers were straightened or channelized after some of the early 1900 (’27, ’36, ’38) floods. The thinking was that by straightening the channel it will move it away from investments such as a farmers’ crops or

infrastructure. However, over time, we are learning more about the dynamic nature of rivers. Over time a straightened rivers will eventually erode its banks and move back to a meandering channel and again jeopardize critical investments that were constructed after channel straightening. In this section, there is considerable background on river geomorphology that may appear irrelevant to flooding in Lyndon. However, the goal is to provide readers with a better understanding of fluvial processes in rivers and how these processes can be directly linked to flooding in Lyndon. 8.2 Fundamentals of River Systems A stable river is one that has sufficient depth, width and length to move water and sediment produced in the watershed. The stable river will erode its banks and change course only minimally even under flood conditions. However, when a river becomes unbalanced or unstable, then it will change either its planform (course, meander pattern), profile (slope), dimension (width and depth), or all of the above, until it becomes balanced again. What throws a river’s stability out of balance? It is a balance between water

input from the watershed and sediment input from the watershed as shown in the inset. There are many potential causes of a river becoming unbalanced ranging from the watershed scale to the local scale. Some potential causes are summarized below:

Stable Channel Equilibrium, Lane 1955

1884

Passumpsic River Flood Study Gomez and Sullivan Page-44

• Changes in land use, past forest management practices, and increased development within a watershed may impact the hydrology or flow runoff in the watershed. Most land use changes – the conversion of forested land to development-- results in increased runoff which can throw the stability of a river channel out of balance.

• In the past, gravel was mined from river beds, which throws the sediment regime out of balance by degrading the channel.

• As noted above, many rivers have been straightened and channelized with the goal of eliminating flooding in sensitive areas. Although rivers were straightened and channelized in the past, over time the same river will revert back to its former meandering pattern by eroding its river banks.

• Many river channels have been armored (rip-rapped) with the goal of preventing erosion on the river banks.

• Along the banks of rivers berms and floodwalls have been constructed with the goal of “containing” flood waters.

• One of the major causes of channel instability is encroachment within the floodplain. Roads are commonly placed along river systems, and increased development within the floodplain has occurred.

• Dams and undersized bridges or culverts can impact stream stability by decreasing sediment transport (retaining sediment).

Human land uses, particularly within river corridors, that significantly alter the runoff pattern of water and sediment will trigger a channel adjustment process. For example, the construction of a dam will result in eliminating sediment transport to the river reach below the dam. When this occurs, the downstream river becomes “sediment starved” which commonly results in a downcutting (incising) of the channel. Incision of a river channel can result in a disconnection with the river’s floodplain that is needed to store and attenuate flood flows. Another example is after the 1927 flood many Vermont rivers were straightened and channelized. After straightening occurred development (roads, housing) ensued within the river corridor. However, over time the channel will start to widen back to its historic planform (meander) by eroding the river banks and jeopardizing those investments that were constructed in the river corridor after the channels were straightened. Shown in the schematic below is a summary of the channel evolution process and how changes occur in the historic river corridor- this is for illustration purposes only. The point of this illustration is that in all but some of the steepest rivers in Vermont, rivers naturally meander and if straightened will slowly revert back to a meandering river.

Passumpsic River Flood Study Gomez and Sullivan Page-45

Source: Adapted from VDEC 8.3 East Branch Geomorphic Study The Vermont Department of Environmental Conservation has established a Rivers Management Program that studies the geomorphic processes of rivers throughout the state. To define and minimize lands under risk to future erosion and flood damage, the Rivers Management Program has created a physical, science-based approach for defining river corridors and floodways necessary to accommodate river meanders. As part of this program, geomorphic studies are conducted which entails the collection of field data on rivers to determine the current state of the river (stable or unstable)--- and if unstable, identification of potential causes and solutions. The East Branch is a major tributary to the Passumpsic River and its river corridor is of critical importance to providing flood attenuation and storage. Attenuation and storage means that flood flows passing through the East Branch can be temporarily stored in the East Branch floodplain, which will reduce the peak flows in Lyndon. In 2003, the VDEC conducted a geomorphic assessment of the East Branch from just below the Burke town line to the confluence with the Passumpsic River – approximately 3.2 miles, as shown in Figure 8.3-1. The East Branch river corridor has already been encroached upon from roads (primarily Route 114 which follows the East Branch), structures, East Burke Dam, straightening, and bridges. In addition, as noted above gravel mining has historically occurred on the East Branch. These past activities have affected the stability of the East Branch and the river is slowly reacting to those changes by undergoing physical adjustments.

House

Historic River Corridor

Channel

Stable Channel with full access to floodplain

After 1927 house and road are constructed within the historic river corridor

PlanformCross-Section

Historic River Corridor

Historic River Corridor

Channel dredged, and straightened after 1927 flood

Floodplain

Road

River

River

River

RoadHouse

House

Historic River Corridor

Road

RoadHouse

Over time channelreadjusts to its formerplanform jeopardizinginfrastructure in the historic floodplain

Passumpsic River Flood Study Gomez and Sullivan Page-46

As part of the Phase 2 geomorphic study, a field crew mapped the East Branches channel widths, depths, sediment aggradation areas, sediment degradation areas, and channel cross-sections. The Phase 2 assessment concluded that the East Branch is currently unstable and is slowly undergoing physical adjustments (widening and incising). Field data is used to determine the physical adjustment process, which can consist of:

• Aggradation – aggradation is the progressive accumulation of in-channel sediment resulting in an increased channel bed. Aggradation is a response to a channel system changes that reduces the channel’s capacity to transport the sediment delivered to it. Generally, this occurs as result of either increased sediment supply (load or gradation) or diminished stream power (transport capacity).

• Degradation- degradation is the inverse of

aggradation and involves the progressive lowering of the channel bed relative to its floodplain elevation. Degraded channels (also called entrenched, eroded, or incised channels) occur when stream power exceeds the channel bed’s resistance, or when sediment transport capacity exceeds the sediment supply. The process of degradation begins when stream power exceeds a threshold condition of bed stability.

• Widening- the river channel is widening.

• Planform- planform is best seen in an aerial view

and generally describes how much the river meanders. For example, the Passumpsic River in Lyndon meanders. Channel migration is the progressive movement of a channel across a valley and involves bank erosion and transport of eroded materials. Lateral migration may also occur in response to disturbance or external changes in input variables.

Based on the East Branch geomorphic assessment, the VDEC developed a map depicting the physical adjustment process on-going in the East Branch as shown in Figure 8.3-2. In addition, shown in Figure 8.3-3 is another VDEC map illustrating where the East Branch is relative to its evolution stage- either incising or widening. As Figure 8.3-3 shows, the East Branch channel is incising (lowering of the river channel bed) in several locations. As the above photograph the East Branch has incised so much that the floodplain is almost 20 feet above the channel. The physical adjustments on-going on the East Branch can affect flooding in Lyndon in various ways as summarized below.

East Branch Passumpsic River, Channel degradation causes disconnect with floodplain, Source: VDEC

~20+ feet

East Branch Passumpsic River, Channel widening and sediment aggradation, Source: VDEC

East Branch Passumpsic River, Channel planform, Source: VDEC

Passumpsic River Flood Study Gomez and Sullivan Page-47

• Increased potential for sediment deposition within the Passumpsic River in Lyndon. Because the East is incising in several locations it is eroding sediment (the channel bed). These sediments are either deposited further downstream in the East Branch, or most likely settle out in the slow-moving sections of the Passumpsic River in Lyndon. The more sediment that fills the Passumpsic River channel in Lyndon decreases the channel area and reduces the river’s ability to convey flood flow and thus increases the likelihood of overflowing its river banks. The Passumpsic River in Lyndon has a mild gradient and thus is efficient at trapping sediments. The ability of the Passumpsic River to transport sediment downstream is likely less than the amount of incoming sediment resulting in aggradation.

• Increase potential for debris/ice jamming in Lyndon. In some locations the East Branch has been

straightened. An incised and straightened channel, as opposed to a naturally meandering channel, is extremely efficient at transporting debris (trees) and ice downstream. This can potentially result in ice and debris jamming at bridges further downstream, including those in Lyndon.

• Lack of East Branch floodplain storage results in higher flows in Lyndon and no flow

attenuation/storage. Perhaps the most critical physical adjustment on the East Branch-- incision of the channel bed-- is that the river is physically disconnected from its floodplain. This means that flood flows are delivered more rapidly to the Passumpsic River in Lyndon. If the East Branch were connected to its floodplain, it would increase the storage of flood flows and thus reduce flows in Lyndon. Straightened and incised channels are excellent at delivering flow much more rapidly than a meandering channel with floodplain access

Development (roads, buildings) within the East Branch floodplain will continue in the future unless measures are taken to identify critical floodplains that should be preserved. Continued encroachment and loss of future floodplain along the East Branch will only exacerbate the physical adjustments of the river and contribute to increased flows and sediment delivery to Lyndon. Helping the East Branch approach a more stable channel and restoring floodplain access will assist in reducing flooding in Lyndon. There are natural channel design options that could be employed to help the East Branch return to a stable, fully-functional river channel with access to its floodplain. In addition to natural channel design options, river corridor plans should be developed with the goal of protecting and preserving currently undeveloped floodplain lands from future development. In the recommendations section is a suggestion to develop river corridor plans to help reduce flooding in Lyndon. The plan would include various options such as – a) employing natural stream channel design measures with the goal of having upstream tributaries access to their floodplains and b) the protection of existing floodplain lands from future development through land acquisition. It should be noted that no geomorphic assessments have been conducted on the other two main tributaries to the Passumpsic River in Lyndon- the West Branch and Miller Run. Thus, it is unknown if these rivers are in a similar state as the East Branch. As noted in the recommendations section, geomorphic assessments of these tributaries should also be considered to determine potential options to promote flood flow attenuation.

East Branch, Channel incision causes undermining of the Randall Covered Bridge abutments, Source VDEC

Passumpsic River Flood Study Gomez and Sullivan Page-48

The “take away” message here is that flooding in Lyndon is directly related to activities further upstream in the watershed. Although towns commonly seek a flood control “fix” within the town itself, they need to look at the upstream watershed and determine what activities are occurring in the upper watershed that are impacting conditions in Lyndon. For Lyndon, the town should be considering potential options to attenuate and store flood flows in the watershed above Lyndon. As further background on the need for floodplain access in 1993 a new policy framework was developed that recognized the critical role wetlands and floodplain access play in improving water quality, recharging groundwater supplies, and providing natural flood control. Heeding the lessons of past flood events, the policy statement underscored the critical link between rivers and natural flood protection areas. The policy statement reads:

Many scientists have concluded that past manipulation of the rivers in the Midwest has contributed to the current level of devastation by separating river channels for their natural floodplain and eliminating millions of acres of additional flood storage capacity. Wetlands within the floodplain and higher in the watershed reduce floods by absorbing rain, snow melt, and floodwater and releasing it slowly, thereby reducing the severity of downstream flooding.

8.4 Protection of River Corridors One method to prevent rivers from becoming unbalanced (unstable), or to allow them to re-establish a stable channel, is to protect their “river corridors.” River corridors consist of the river channel, the overflow banks, and those areas in close proximity to the river that carries flood flow and accommodate the meander pattern of the river. Unbalanced rivers increase the risk of damage from flooding to Vermont communities. From 1995 through 1998 alone flash flooding damage in Vermont approached $60,000,000 (Source: VDEC, River Corridor Protection and Management Fact Sheet). Much of this damage occurred where rivers were separated from their floodplains by some kind of development, or where rivers were adjusting their length, depth or width because activities in the river, on the banks, or in the floodplains have caused a river to become unbalanced. To assist landowners and towns, the State of Vermont River Management Program is providing technical assistance and funding incentives for landowners to manage their lands to:

• Avoid any future structural encroachments within river corridors; • Conduct only those channel management activities within river corridors that accommodate

natural meander and floodplain processes; • Provide for a wooded buffer to help stabilize natural meanders and support healthy aquatic and

riparian ecosystems. (Source: VDEC, River Corridor Protection and Management Fact Sheet) The Vermont Rivers Management Program conducts state-sponsored stream geomorphic assessments to explain a stream’s reference and departure from equilibrium conditions and identify “key attenuation assets” within a watershed. This is the type of geomorphic assessment that was conducted for the East Branch as discussed above.

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9.0 Flood Mitigation Options and Recommendations There are a host of structural and non-structural measures that could be implemented to limit flooding in Lyndon; however each has an associated cost and benefit. As the name implies, structural techniques seek to build something in order to change or "control" the physical environment such as levees, berms or floodwalls. The Corps is the Federal agency charged with the construction and maintenance of major structural flood mitigation projects. Nonstructural approaches are designed to change the way humans interact with the flood problem and to remove structures from the floodplain or floodway. The goal of nonstructural flood mitigation is to take the people away from the river. The most prevalent nonstructural activities include: acquisition, relocation, or elevation of flood-prone structures. In cooperation with each State, the FEMA funds several nonstructural flood mitigation programs. It is important to recognize that regardless of any potential structural or non-structural measure, the Passumpsic River that flows through town has a very mild slope, which is reflected in the meandering pattern of the river. The surrounding topography on the banks of the Passumpsic River is also extremely flat. Thus, if water levels on the Passumpsic River exceed the river banks the water will spread laterally over the floodplain. The Passumpsic River floodplain is expansive and there has already been development within the floodplain. The point of this discussion is that regardless of any measure to limit flooding it will not be possible to completely eliminate flooding within a mildly sloped river valley, especially in those areas already in the floodplain. The following section describes potential structural and non-structural measures. At the end of each description is a recommendation on the merit of a given measure in terms of reducing flooding in Lyndon. Keep in mind that in some cases, a given measure will undoubtedly provide flood protection to some structures; however, consideration must also be given to costs relative to the true benefit. For example, a structural measure may result in protecting 5 structures, but the costs could be exorbitant. Thus, a cost/benefit analysis would be needed. It is beyond the scope of this study to estimate the cost of a given flood protection alternative and conduct a cost/benefit analysis, however, we have provided some of the initial steps to evaluate the feasibility of some options. The recommendations are shown in italics. 9.1 Structural Mitigation Measures Dikes, Levees, Berms Dikes, Levees, Berms In 2006, the Corps evaluated one structural measure, by considering the placement of levees and plastic sheeting in the northern and southern parts of Lyndon to protect floodprone areas. Based on the Corps analysis the cost benefit ratio was well below one, making the project infeasible. Even if levees and plastic sheeting were economically feasible, construction of such structures could result in physical adjustments (changes in slope and planform) to the Passumpsic River within Lyndon and potentially further downstream. Containing river flow to the channel and preventing access to the floodplain will increase the river’s energy that must be resisted by the channel bed and banks (rocks, vegetation, etc). A common occurrence to limiting floodplain access is increased erosion along the channel, which results in scouring sediments. The scoured sediments are then deposited in slow velocity areas further downstream, which can raise the river bed elevation. Over time the scouring of sediments can lower the channel bed profile (incising), making it more difficult for the river to access the floodplain. Also over time, the river will create a new floodplain by eroding it river banks. In short, encroachment on the floodplain or constructing structures that “force” water to stay within the channel may result in physical adjustments to the Passumpsic River.

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Recommendation: The option of constructing levees may provide “temporary” flood protection to infrastructure investments, however, based on the Corps’ analysis it is not economically justified. We have no way to verify the Corps’ cost estimate, however, even if the cost of floodproofing were reduced by 50%, the Corps’ cost/benefit ratio would still be less than one. Also, assuming cost were not an issue relative to floodproofing, Lyndon would have to seriously consider the long-term geomorphic impacts of levees. It has been well documented, that containing a river by the use of berms or levees will cause physical adjustments in the river channel. We do not recommend this option due to costs and potential long term physical adjustments to the river. Dredging There has been considerable debate over dredging a river as a way of protecting property from flood flows. Dredging involves removing sediment from the river channel and increasing its cross-sectional area to pass more flow. Dredging is a justifiable method for protecting property when the dredging will help to restore or maintain the stability of the river. However, dredging a river can also create an unstable river that can lead to greater flood damage in the future. As discussed earlier dredging (gravel mining) on the East Branch was historically conducted. The gravel mining, development (encroachment) in the floodplain, channelization, and other anthropogenic influences has resulted in an unstable East Branch, where the planform, profile and dimension continues to adjust. As further background on gravel mining, experience from the 1970s and 1980s in Vermont has demonstrated the destabilization of river systems and excessive damages to private property and municipal roads and bridges resulting from gravel mining. Damage occurs from stream channel dredging where such practice is not accompanied by restoration of channel dimensions (width, depth), pattern (sinuosity), and profile (channel slope) appropriate to the geographic location and other physical attributes of the stream and its valley setting. Damage is also associated with removing volumes of gravel that exceeds the rivers rate of gravel replenishment being transported upstream. The gravel mining conducted on the East Branch was not accompanied by stream restoration efforts. Recommendation: Alternative 7 evaluated the option of removing an arbitrary four feet of sediment from the project reach by lowering the channel cross-section. Before dredging is considered we recommend that Lyndon consider the following incremental steps:

• The stability of the West Branch and Miller Run should be investigated to determine if these rivers are delivering sediment to the project reach, similar to the East Branch. Unless the East Branch, West Branch and Miller Run are stabilized, then sediment will continue to be delivered to the project reach. Thus, any potential benefits of dredging will be temporary, as the dredged areas will likely refill from upstream sediments and would require long term maintenance.

• If the East Branch, West Branch and Miller Run were in a nearly stable condition, then dredging could be considered. However, before any dredging were conducted an analysis is recommended to determine if dredging could cause channel instability within the project reach and downstream.

• If it can be shown that sediment dredging within the project reach does not cause channel instability, then dredging is a potential option to flood mitigation. However, it is recommended that an updated hydraulic model be developed to determine the true flood benefits of dredging.

Sediment deposition in the project reach is truly a function of the stability of upstream tributaries (East Branch, West Branch, Miller Run). In the case of the East Branch, the incising of the river channel is likely delivering sediments to the project reach at a rate faster than it can be transported. In short, we recommend this option if a) stability of the East Branch is restored b) the stability of West Branch and

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Miller Run are investigated- and restored, if needed, c) it can be shown that dredging does not cause channel instability in the project reach and downstream. Dams- Storage Another measure to control flooding is reducing the magnitude of flow entering Lyndon. As noted earlier there are several Army Corps flood control projects throughout the Connecticut River watershed; however none are located in the Passumpsic River Watershed. Typically, flood control projects are constructed relatively low in the watershed where the drainage area is larger. These reservoirs provide flood protection only for communities on or along the mainstem of the river downstream. However, the reservoirs do not provide flood protection in upland tributaries where the majority of flash flooding may occur. As noted above there are three major tributaries contributing flow to the Passumpsic River in Lyndon including the East Branch, West Branch and Miller Run. Although most likely viewed unfavorably, flood control reservoirs above Lyndon could be constructed to reduce flow entering Lyndon during floods. It is recognized that there would be considerable capital cost, property acquisition cost, long-term operation and maintenance cost, and significant environmental issues that would have to be addressed if a flood control facility were considered. This option is only provided to provide a complete picture, although it is recognized that it is not likely given the major hurdles that would involved with a project of this nature. Recommendation: There is no question that a flood control facility on the East Branch, West Branch or Miller Run would provide flood protection to Lyndon by providing flood storage. However, this option is likely unrealistic due to costs, long term operation and maintenance, environmental impacts, and the potential geomorphic impacts of creating a flood storage reservoir. This option is only provided to provide a complete picture, although it is recognized that it is not likely given the major hurdles that would involved with a project of this nature. In short, we do not consider this option feasible. Restoration of Channel/Floodplain Storage As described in greater detail in Section 8.0 of this report, floodplains serve a vital role for storing and attenuating flood flows. In the case of the East Branch it is incised in many locations, which can directly impact flooding in the project reach due to:

• Increased potential for sediment transport in the East Branch (and potentially the West Branch and Miller Run) and subsequent deposition within the Passumpsic River in Lyndon;

• Increased transport of debris and the potential for debris/ice jamming in Lyndon; • Lack of East Branch floodplain storage results in higher flows in Lyndon.

Recommendation: It is believed that floodplain storage on the East Branch (at a minimum) could be improved with the goal of curtailing flooding in Lyndon. Allowing the East Branch to access its floodplain in areas where it currently can not would provide flood attenuation/storage. We believe this is a viable option to providing flood protection in Lyndon; however, the following steps are needed:

• Using the East Branch Phase 2 geomorphic assessment, along with some follow-up field work, estimate the storage capacity on the East Branch. Quantify how much storage capacity could be gained in those areas where the East Branch currently can not access its floodplain. Quantify the overall floodplain storage capacity and the benefit relative to curtailing flooding in Lyndon.

• Work with the Vermont Rivers Management department to conduct geomorphic assessments on the other major tributaries- West Branch and Miller Run- to determine if these rivers can access their floodplains. Conduct the same evaluations described above for the East Branch, including quantifying the floodplain storage capacity that could be made available.

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• For all three tributaries develop and implement natural channel design measures with the goal of creating a stable river channel and allowing all three rivers to access their floodplains.

• As described in the non-structural measures section, another component relative to floodplains is to protect these critical lands from future encroachment/development via property acquisition, and/or easement.

Lyndon needs to investigate options for floodproofing within Lyndon itself, but also upstream. This will require a concerted effort and partnership with numerous municipalities including Lyndon, Burke, East Haven, Newark, Sutton, Shefield and Wheelock. In short, restoring floodplain access is a viable option, however parties need to recognize it will take further study and funding to accomplish these goals. Bypass Channel Based on the above analysis, there is some reduction in the area of inundation with a bypass channel as summarized above. There is no question that constructing a bypass channel just upstream of the Vail Dam will increase flow conveyance and will help to reduce upstream flooding. The question comes down to the cost, relative to the benefit. Based on the hydraulic modeling, there is some reduction in the area of inundation with the construction of a bypass channel. There is no question that constructing that increasing flow conveyance with a bypass channel will reduce upstream flooding. The question comes down to the cost and benefit. Recommendation: Before it can be concluded that a bypass channel is a feasible option much more information is needed. A feasibility study would be necessary to determine the cost of developing a bypass channel before this alternative can be recommended. Although not a complete list, the feasibility study would include:

• Borings in the area of a proposed bypass channel to determine if ledge (as suspected) is located throughout river right (the right side of the river looking downstream) as there could be blasting costs.

• If the proposed bypass channel ran partially through soils, soil sampling would be needed for contaminant testing.

• Studies relative to the potential impacts on rare, threatened and endangered species in the proposed location of the bypass and work area would be necessary.

• Archeological studies may be required as development of a bypass channel could impact archeological resources.

• Property issues and access would have to be resolved. • Surveys along river right would be required to develop design plans and cost estimates. • Conceptual and preliminary design plans for the bypass channel are needed to quantify the cost

of potential blasting (if necessary). The bypass channel would have to be long enough and at the proper slope to facilitate upstream and downstream passage of fish- meaning the bypass channel could be relatively long based on the elevation drop between the Vail Dam and downstream.

• Public safety would have to be considered—for example, if a deep cut into bedrock were necessary to construct the bypass channel there could be steep banks.

• Permitting and final design would be required. In summary, as noted above, depending on the size (width, depth) of the bypass channel it would help reduce flooding upstream. We have not provided a final recommendation on this option as the costs associated with constructing the bypass channel have not been determined. However, our initial impression is that the bypass channel could be costly relative to the associated benefits.

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Vail Dam Removal Based on the above analysis, there is some reduction in the area of inundation with the Vail Dam removed. The major question, that still has not been completely resolved, is whether cross-section 6 or another location within the impoundment would serve as a rigid boundary- in other words is there another hydraulic control within the Vail impoundment that would still cause upstream flooding. Another critical unknown is whether there is bedrock directly beneath the Vail Dam that would serve as a hydraulic control if the dam were removed. If bedrock were present beneath the dam, it is critical to determine the height and geometry of bedrock. For example, if bedrock were present and spanned across the river, and if the bedrock was only a few feet below the Vail Dam spillway crest elevation, it may serve as a hydraulic control. Thus, there may not be as much flood benefits relative to removing the Vail Dam. Similar to the bypass channel, there is no question that removing the Vail Dam will provide some reduction in upstream flood levels. The question again comes down to the cost relative to the benefit. Another major factor is the potential loss of hydropower production. Recommendation: Before it can be concluded that removing the Vail Dam is a feasible option much more information is needed. Similar to the bypass channel, a feasibility study would be necessary to determine the cost of removing the Vail Dam before this alternative can be recommended. Although not a complete list, the feasibility study would include: Recommendation: Before it can be concluded that removing the Vail Dam is a feasible option much more information is needed. Similar to the bypass channel, a feasibility study is necessary to determine the merits of removing the Vail Dam relative to reduction in flooding before this alternative can be recommended. Although not a complete list, the feasibility study would include:

• One of the first phases of the project is to determine if there are truly hydraulic controls in the Vail Dam impoundment that would serve as a hydraulic control if the dam were removed. In our opinion this question has not been completed answered from the USGS study. Further probing of sediments is needed and the cross-section data collected by the USGS needs to be tied to a known datum. Also, it is important to have some information on the extent and height of bedrock beneath the Vail Dam.

• Also as part of the initial phases of the project, bathymetric surveys of the impoundment are needed and an up-to-date hydraulic model is needed. Currently, all estimates relative to the reduction in upstream flood levels is based on outdated 1977 cross-sectional information. Probing throughout the impoundment is necessary to determine the geographic extent of bedrock in those areas identified in the USGS study.

• Once all hydraulic controls are surveyed and an up-to-date hydraulic model is developed, quantify the true benefit of potentially removing the dam. If there are minimal flood benefits, we would not recommend removing the dam. If there are substantial flood benefits, then further feasibility work is recommended as described in the following bullets.

• If it can be proven that removing the dam provided reasonable flood protection, then additional feasibility work would be necessary. One of the first steps would be collecting sediment samples within the impoundment and conducting contaminant testing. If contaminants are found at high concentrations, then the cost of removing the dam could increase substantially. Removal and disposal of contaminated sediments can be costly.

• The quantity, distribution and characterization of sediments within the impoundment should be quantified.

• The impact of removing the Vail Dam on: a) wetlands, b) recreation, c) rare, threatened and endangered species, d) fire use (withdrawing water from the impoundment), e) infrastructure- for example removing the Vail Dam could result in a headcut that could undermine the Chapel Street

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Bridge abutments and pier, f) river morphology, and g) archeological/historical resources should be evaluated. In addition, Lyndon would have to consider the financial impact of losing hydropower resources.

• If the project were feasible, then conceptual and preliminary design drawings would be needed to estimate the cost of removal and restoration.

• Permtting and final design would be required. In summary, as noted above, removal of the Vail Dam will reduce flooding upstream. We have not provided a final recommendation on this option as there are still many unknowns. The major unknowns are whether bedrock upstream of the Vail Dam truly spans the entire river channel and acts as a rigid hydraulic control. The height and depth of the bedrock spanning the channel would also have to be known. If bedrock spans the river and is a few feet below the Vail Dam, then there are minimal flood benefits. In this case, removal of the Vail Dam would not be recommended. If there were no bedrock upstream or directly beneath the dam, then removal of the Vail Dam relative to flooding would have more merit. Main Street Bridge- Relief Bridge VTrans developed a separate hydraulic model that evaluated the merit of replacing the culverts (see inset) that pass beneath Route 5 just south of the Main Street Bridge with a dry bridge. It has been noted that the existing culverts become blocked with debris and do not always function properly during flood conditions. VTrans evaluated the placement of a dry bridge in lieu of the culverts. Comparing the Main Street Bridge without any relief structure (assuming the culverts are completely plugged) to placing a 60-foot dry bridge provided a net reduction in water surface elevations near the Main Street Bridge of 1.5 feet under the 100-yr flood. This is a sizeable reduction and thus further evaluation of this option is likely warranted. Recommendation: Given that the culverts are commonly blocked, the construction of a dry bridge does show some merit. The VTrans study should be expanded to show the reduction in the inundation area with the construction of a dry bridge. If there are a reasonable number of properties that would realize the benefits of a dry bridge, then cost estimates to construct the bridge should be obtained. It is our understanding that in the past a dry bridge was present at the culvert overflow structure and it is important to understand why it was replaced. 9.2 Non-Structural Mitigation Measures Acquisition, Relocation, and Floodproofing Existing Structures Federal programs that assist in acquiring, relocating, and/or floodproofing existing structures provide one means of reducing flood vulnerability. By encouraging property owners to relocate outside of the floodplain—either by physically moving a structure or by purchasing or building a replacement structure—these programs reduce flood vulnerability and thereby decrease the need for future Federal damage payments. In Vermont there have been a few buy outs of buildings that were damaged in recent Vermont floods because they could be not be built in a way that would protect them against future flood damage. Beginning in 1997, FEMA began emphasizing the alternative of acquisition and removal or relocation of structures in high hazard areas. The state of Vermont supports this alternative where it was determined that the cost of providing continued protection from these properties was greater than they were worth. The program is relatively expensive and has resolved only the most high priority and problematic sites. The VDEC supports these acquisition options where:

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• The cost effectiveness standard, as noted above, is met; • The building is located within the 100-year floodplain; • The structure has been substantially damaged or flooded two or more times within the last 20

years; and • Buy-out offers are equal to the pre-flood fair market value minus available flood insurance

coverage (Source: Options for State Flood Control, Policies and a Flood Control Program, VANR).

Recommendation: As noted above there have been only a few relocations or acquisitions of structures in Vermont due to the cost. In addition, given the amount of flooding that has occurred in the past within Lyndon, it does not appear reasonable that acquisition or relocation is a feasible option—there are simply too many structures that are impacted by flooding. Develop River Corridor Protection Plans on the East Branch, West Branch, Miller Run and Passumpsic River A river corridor includes lands adjacent to and including the course of a river. Development along many Vermont river corridors continues today – in some areas at a faster pace than others. However, there is still an opportunity to develop river corridor protection areas that would preserve the lands that function as a river’s floodplain. If lands can be protected it would assist in avoiding further degradation (that comes with encroachment), create opportunities for future restoration, and slow the rate of land use conversion in the floodplain. Recommendation: It is recommended that a river corridor protection plan be developed along the East Branch, West Branch, Miller Run and Passumpsic River in the project reach. The purpose of this plan is to identify lands within the river corridor that could serve to attenuate floods. Federal, state, and municipalities, and land use regulators should consider floodplain protection that minimizes erosion hazards to public investments within the river corridor- the corridor is defined by the belt width20 (see inset which defines the zone) requirements of a stream or river. It is recommended that first geomorphic studies be conducted on Miller Run and the West Branch to a) determine the river’s stability, b) identify floodplains that provide key attenuation assets and c) compute the total belt width along each river that would define the river corridor. It is recognized that some floodplains are already occupied by houses or roads, thus emphasis should be placed on floodplains that remain relatively undeveloped. Once these lands are identified it will be up to Lyndon, other towns, land conservation organizations, town conservation commissions, non-profits, regional organizations and the state of Vermont to consider potential purchase and acquisition on these lands to protect them from future encroachment within the floodplain. It will require a dedicated commitment and coordination among towns within the Passumpsic River watershed Without conservation of floodplains, over time flood conditions in Lyndon will only worsen. If development within the East Branch, West Branch and Miller Run river corridors continues flooding in Lyndon will only increase. We highly recommend the development of river corridor plans. 20 The width of the corridor is defined by the lateral extent of the river meanders, often referred to as the meander belt width.

Source: VDEC

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10.0 References Federal Emergency Management Agency, Town of Burke, Vermont, Caledonia County, Flood Insurance Study, December 1979 Federal Emergency Management Agency, Town of Lyndon, Vermont, Caledonia County, Flood Insurance Study, May 17, 1988. Federal Energy Regulatory Commission, Village of Lyndonville Electric Department, Vermont, Project No. 3090-008, Notice of Availability of Final Environmental Assessment (includes Appendix B: Final Flood Analysis), March 11, 2004. Federal Energy Regulatory Commission, Village of Lyndonville Electric Department, Vermont, Project No. 3090-008, Draft Flood Analysis- Removal of Vail Dam, August 11, 2003 Federal Energy Regulatory Commission, Draft Flood Analysis- Removal of Vail Dam, Meeting Minutes of FERC, VANR, LED, USFWS, September 3, 2003. Kleinschmidt Associates, Village of Lyndonville Electric Department, Lyndon, Vermont, Vail Dam (FERC No. 3090), Dam Removal Plan, September 1998. Kleinschmidt Associates (on behalf of Lyndonville Electric Department), Letter to the Federal Energy Regulatory Commission regarding FERC’s Draft Flood Analysis – Removal of Vail Dam, September 10, 2003. Lane, E.W. 1955. The Importance of Fluvial Morphology in Hydraulic Engineering. Proceedings of the American Society of Civil Engineers, Journal of the Hydraulics Division, vol. 81, paper no. 745. Mason, Ken, Photographs of the June 2002 flood. Rosgen, David, Applied River Morphology, 1996. Vermont Agency of Natural Resources, Letter to the Federal Energy Regulatory Commission regarding FERC’s Draft Flood Analysis – Removal of Vail Dam, September 11, 2003. Vermont Agency of Natural Resources, Options for State Flood Control Policies and a Flood Control Program, Prepared for the Vermont General Assembly, February 1999. Vermont Department of Transportation, Hydraulics Unit - memo summarizing preliminary hydraulic study for US 5 Bridge 144 and Bridge 143 (Feb 2004). United States Army Corps of Engineers, Initial Appraisal Report, Continuing Authorities Fact Sheet, Section 205, May 27, 2006. United States Army Corps of Engineers, Letter to Mr. Kenneth Burchesky, Town of Lyndon Energy Coordinator regarding Corps flood study, January 3, 2003. United States Fish and Wildlife Service, Letter to the Federal Energy Regulatory Commission regarding FERC’s Draft Flood Analysis – Removal of Vail Dam, September 9, 2003.

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United States Geological Survey, Flow Data for USGS gages on the East Branch Passumpsic River and the Passumpsic River near Passumpsic, VT. United States Geological Survey, Ground Penetrating Radar Study, November 2004. Vermont Agency of Natural Resources, Options for State Flood Control Policies and Flood Control Program, Vermont DEC River Management Program, February 1999. Vermont Agency of Natural Resources, River Corridor Protection as a Part of River Restoration, A Note to Restoration Ecologists, Planners and Engineers, Vermont DEC River Management Program, October 2006. Vermont Agency of Natural Resources, Vermont Fluvial Erosion Hazard Mitigation Program, Vermont DEC River Management Program, May 2005. Vermont Agency of Natural Resources. Erosion Hazards and the Benefits of River Corridor Protection, Vermont DEC River Management Program. Undated. Vermont Agency of Natural Resources. Stream Geomorphic Assessment Protocol Handbook. Waterbury, VT, 2004.