March 3, 2011 Final Report

CE 465W Water Resources Capstone Course

Dam Rehabilitation Analysis Lake Somerset, Somerset, Pennsylvania

Prepared For: The Pennsylvania Fish & Boat Commission

Reviewed By: Dr. Norman Folmar, Ph.D., P.E.

Prepared By: Paul Christner, Albert Brulo, & Damian Gaiski-Weitz

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Executive Summary

This is a full hydrological analysis of the Lake Somerset watershed, located in Somerset County, Pennsylvania. The failure of this dam that retains the watershed’s inflow of precipitation presents a threat to vehicular travel along major roadways as well as the town of Somerset itself.

To properly model the watershed and reservoir in the Army Corps of Engineers’ Hydraulic Engineering Center’s (HEC) watershed modeling system HEC-HMS, many watershed, meteorological, and reservoir characteristics needed to be pre-determined. Some of these values were found utilizing online databases from various government agencies. The remaining values were determined using equations related to surface water flow, hydraulic abstractions from the watershed, and precipitation found in government hydrology manuals.

Covered in this study are watershed characteristics such as soil type and land usage. Using these characteristics, the Natural Resource Conservation Service’s (NRCS) curve number for the entire watershed can be determined. Also needed to complete the peak flow analysis is the lag time over the watershed. The curve number and lag time can be used in correlation with the National Weather Service’s intensity-duration-frequency (IDF) and probable maximum precipitation (PMP) documents, to yield the flood events for various precipitation occurrences that take place over the entire watershed and flow into the reservoir.

This reservoir must be able to retain the water already stored in addition to the inflow of water from the watershed. If the dam that is retaining the water of the reservoir is ever overtopped by the precipitation events that occur, then the dam is at great risk of failure and could endanger the population of Somerset downstream. Several reservoir spillway and dam characteristics were used in HEC-HMS to determine peak outflow and water elevations that dictate dam failure. These flows and elevations have been assembled in a table in the results section.

Using this data, a determination can be made on whether or not the dam currently in place is sufficient enough to retain the maximum excess precipitation and protect the population of Somerset. When the model was completed and each meteorological event was computed over the watershed, it was determined that the PMP meteorological event created a probable maximum flood (PMF) that would overtop the dam and result in dam failure.

A recommendation for further analysis upon how to properly rehabilitate the dam is the outcome of this hydrological analysis of the Lake Somerset watershed. The PMF that occurs would cause an elevation rise in the reservoir that would cause overtopping of the dam by 1.3 feet. Due to the risk of damage and life lost from the turnpike and city below the dam, this is unacceptable and the dam should be rehabilitated.

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TABLE OF CONTENTS

Executive Summary 2 Table of Contents 3 List of Tables 4 List of Figures 5 List of Appendices 6 Introduction 7 Watershed Description 8 Curve Number Development 9 Watershed Response Time 10 Precipitation Data 12 Reservoir Characteristics 13 HEC-HMS Model Development 14 Results 15 Summary 16 References 17

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LIST OF TABLES

Table 1: Percentage of Land Use Types 8

Table 2: Percentage of Hydrological Soil Groups over the Watershed 9

Table 3: Curve Numbers Based on Land Use & Soil Type 9

Table 4: Average Curve Numbers for Each Land Use Type 10

Table 5: Precipitation Estimates for Specific Rainfall Events 13

Table 6: Data Input for HEC-HMS 14

Table 7: HEC-HMS Reservoir Flow Statistics 15

Table 8: PA Stream Stats Peak Flow Statistics 15

Table 9: Inflow Percent Difference 15

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LIST OF FIGURES

Figure 1: Locational Map of Lake Somerset, Somerset, PA 7

Figure 2: Storage – Elevation and Outflow graphs for 50-year, 100-year, and PMP Events 16

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LIST OF APPENDICES

Appendix A

1: USGS PA Stream Stats Map of Watershed

2-3: USGS PA Stream Stats Ungaged Site Report

4: USGS PA Stream Stats Basin Characteristics Report

5-6: NRCS Web Soil Survey

7-9: National Weather Service IDF Curves

10: National Weather Service PMP Map

11: HEC-HMS Output Table for 50-yr Storm Event

12: HEC-HMS Output Graph for 50-yr Storm Event

13: HEC-HMS Output Table for 100-yr Storm Event

14: HEC-HMS Output Graph for 100-yr Storm Event

15: HEC-HMS Output Table for PMP Storm Event

16: HEC-HMS Output Graph for PMP Storm Event

Appendix B

1-2: Curve Number Calculation

2,4: Lag Time Calculations

3-4: Total Time of Concentration Calculation

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Introduction

Lake Somerset is a 253 acre impoundment located in Somerset County, Pennsylvania. The Commonwealth of Pennsylvania owns the lake, and the Pennsylvania Fish and Boat Commission manage the lake. Lake Somerset is one of the 24 state owned dams that are classified as high-hazard. High-hazard dams fail if 50 percent of the probable maximum precipitation (PMP) falls on the corresponding watershed. The lake and dam overlook the town of Somerset and major roadways including the Pennsylvania Turnpike. If the structure were to fail, a high loss of life and property would result from this catastrophic discharge of volumetric water.

As confirmation that Lake Somerset is indeed a high-hazard dam, an in depth analysis of the watershed has been conducted. The Natural Resource Conservation Service’s (NRCS) curve number for the entire watershed can be used in correlation with the calculated lag time and National Weather Service’s intensity-duration-frequency (IDF) curves and PMP data to yield the amount of excess precipitation during a given rainfall event. This data is input into the Hydrologic Modeling System, HEC-HMS, which will analyze the watershed and determine whether or not the dam will fail given the 50-year, 100-year, and PMP events that would occur over a 24-hour period at this location.

Figure 1: Locational Map of Lake Somerset, Somerset, PA

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Watershed Description

The contributing watershed around Lake Somerset has an area of roughly 4.14 square miles. Over the course of this area different types and divisions of land are present. These land use types are important parameters to obtain when dealing with hydrology. Since water moves along the path of least resistance, the less obstruction in its way allows it to move faster. An urbanized area with large quantities of pavement will result in the majority of precipitation to become runoff. On the contrary, a forested area will hinder the travel of water and mainly cause precipitation to become infiltrated into the ground.

Lake Somerset’s land use types consist of forests, wetlands, urban, and meadow regions in which all soil types are defined by the NRCS. This information was obtained through the use of the Basin Characteristics Report on the PA Stream Stats website provided by the U.S. Geological Survey (USGS) agency. These areas have been carefully mapped out, and are updated accordingly. Table 1 summarizes these features from the watershed as seen below.

Table 1: Percentage of Land Use Types

Land Use Type Percentage of Land Use (%)

Forested 36.50 Lakes, Ponds, Reservoirs, and Wetlands 9.03

Urban (1/2 Acre Lots) 6.11 Meadow 48.36

Now that the land use types have been found, the next step is to determine the cumulative hydrologic soil groups of the watershed. The NRCS developed four hydrologic soil groups to categorize areas by how well they are drained. These four groups are the first four letters of the alphabet: A, B, C, D. Soil classification ‘A’ deals with sandy like soils, which are well drained and have high infiltration rates. As you move down groups, soils become more clay like and their rate of infiltration decreases. Soil classification ‘D’ is highly clay like and drains poorly. In some scenarios, this classification may result in no infiltration to occur at all.

Through use of the Web Soil Survey website, these hydrologic soil groups can be determined for the Lake Somerset watershed. The site provided multiple locations with varied soil evaluations. Some areas had split ratings, in which case the lower infiltrated soil group was chosen. This was done to account for the maximum possible runoff from the watershed. The larger percentage of lower hydrologic soil groups present in calculations, results in a higher curve number leading to a higher peak flow. In design calculations, if the dam can withstand an overestimated peak flow, then it will also withstand subsequent lower values. Following is a table summarizing the cumulative percentages of soil groups over the watershed.

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Table 2: Percentage of Hydrological Soil Groups over the Watershed

Curve Number Development

Once the land use types and hydrological soil groups are determined, a weighted curve number can then be calculated. A curve number is a parameter used in hydrologic analysis to determine the amount of direct runoff and infiltration from a rainfall event in a given area. It was developed by the NRCS and is widely used in practice to forecast values.

Curve numbers range from 0-100, but land values are typically in the range of 30-98. A larger curve number results in lower amounts of infiltration to occur, leading to higher quantities of runoff and peak flow. A smaller curve number results in the opposite occurrence; high infiltration rates causing both low runoff and peak flows. The TR-55 manual provides a table that correlates land use and hydrologic soil groups to curve numbers. Taken from that manual is a specific list showing the land types of interest, which can be viewed below.

Table 3: Curve Numbers Based on Land Use & Soil Type

Land Use Type Soil Type

A B C D

Forested 36 60 73 79

Urban (1/2 Acre Lots) 54 70 80 85

Meadow 30 58 71 78

*Lakes, Ponds, Reservoirs, and Wetlands: CN=100

Now that the corresponding information for all three tables is present, it is now possible to calculate the weighted curve number. This is accomplished first by computing an average curve number for each land use present, based on its respective soil groups. For example, take the average curve number for the forested land use. Multiply each soil type in decimal form from Table 2, excluding water, to its respective curve number in Table 3 and sum these four values. The resulting forested average curve number of 61.26 is then computed. The only exception is with lakes, ponds, reservoirs, and wetlands, as any drop of precipitation that hits these areas is automatically considered runoff. Therefore, their curve number is 100. Following is a table showing the calculated average curve numbers for each land use type.

Soil Group Percentage of Watershed Covered (%)

A 6.3 B 38.2 C 4.4 D 41.6

Water 9.7

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Table 4: Average Curve Numbers for Each Land Use Type

Land Use Type Average Curve Number

Forested 61.26

Lakes, Ponds, Reservoirs, and Wetlands 100.00

Urban (1/2 Acre Lots) 69.02

Meadow 59.62

The last step in determining the weighted curve number will make use of the land use type statistics. This is calculated through multiplying the average curve number for each respected land type from Table 4, to the decimal form that it occupies on the watershed found in Table 1. Upon summing the four values and rounding any decimal point up, a weighted curve number of 65 was determined for the Lake Somerset watershed. Now that this has been determined, it is possible to continue on through the hydrologic analysis and calculate progressive parameters to ensure proper design of the dam.

Watershed Response Time

Now that the curve number has been calculated, the lag time and time of concentration can be solved for. Both variables refer to the watershed response time and are significantly important in design hydrology because they are used in computation as the time parameters of surface runoff. Knowing these values may solely determine whether the area of interest is flooded or not.

The lag time is known as the time between maximum precipitation and peak discharge. As lag time decreases, peak discharge increases which in some instances results in devastating floods. If the lag time is considered large, then peak discharge is at a low, ensuing minimal threat to a flood event.

The lag time is based upon variables of longest hydraulic length (LHL), potential maximum retention (S), and the overall watershed slope. The longest hydraulic length is the distance it takes the furthest drop of water within the watershed to reach the area delineated. This value was determined as 3.73 miles, or 19,694.4 feet, from the PA Stream Stats website. The potential maximum retention is the variable dependent on the curve number. It takes into account how much precipitation will stay where it falls and not contribute to runoff. It was determined that 4.29 inches of precipitation would fall at the longest hydraulic length before flow would begin. The watershed slope was the last variable to consider. For the Lake Somerset watershed, PA Stream Stats gave the slope in degrees. Through geometric computation, the values of rise over run showed a slope of 0.075 foot per foot, or 7.5 percent. Using the lag time equation by plugging in these three values resulted in a lag time of 1.68 hours. Therefore for this watershed, peak discharge will occur 1.68 hours after the maximum precipitation is observed.

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The total time of concentration is the next parameter that can be solved for, using the NRCS segmental approach. The time of concentration is the time it takes the entire watershed to contribute to the overall flow. When peak flow is at its highest, the entire watershed is thus donating. This is therefore another important value to consider, because peak flow can cause flooding if the right conditions are present. It is found by dividing the longest hydraulic length into the three flow regimes of sheet flow, shallow concentrated flow, and channelized flow. Upon proper calculation of each division, the values can be added together resulting in the total time of concentration.

Begin this process by first finding sheet flow, which is a very thin and weak collection of runoff at the start of the longest hydraulic length. Its equation is presented with four variables including Manning’s roughness coefficient (n), flow length (L), the 2-year, 24-hour rainfall depth (P2), and the slope of the flow path.

Manning’s roughness coefficient accounts for the coarseness of the base of the flow path. Using the TR-55 manual, a value of 0.4 was determined for woods. The flow length for sheet flow is usually less than 100 feet, so for calculation purposes it is assumed this value. The 2-yr, 24-hr rainfall depth was determined to be 2.46 inches. This value was found through use of the National Weather Service’s website, by obtaining the IDF curves for the Lake Somerset region. The slope of the flow path was then determined through use of PA Stream Stats delineated watershed graph. Computing the average elevation over distance, or rise versus run, for the specific portion of the graph, the slope of 0.024 foot per foot was determined. The resulting sheet flow was then computed as 0.38 hours.

Next, the shallow concentrated flow must be evaluated. Shallow concentrated flow is the flow following that of sheet flow. At this point, flow has become more concentrated and therefore contains a greater amount of volume and control. Its equation is simple, as it only requires the flow length (L) and velocity (v).

In order to find the value of velocity for this flow, the National Engineering Handbook (NEH-4) from the NRCS must be available. In chapter 15, there is a log based graph relating velocity versus slope. Compute the slope and flow length with the aid of the same PA Stream Stats graph that was used to find the sheet flow slope. Make sure to use the portion of the graph that correlates to shallow concentrated flow this time however. A flow length of 0.58 miles, 3.062.4 feet, was determined along with a slope of 0.049 foot per foot. Now, using the NEH-4 graph, the velocity that intersects with the corresponding slope and the line denoted as forestry, resulted in a velocity of 0.55 feet per second. From here, the shallow concentrated flow was determined as 1.55 hours.

Finally, the channelized flow equation can be solved for. Channelized flow is the continuation of shallow concentrated flow in the form of an authentic channel. At this point the flow is similar to that of a stream or river, resulting in a fairly large quantity of water.

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The equation for channelized flow is identical to that of shallow concentrated flow, with the exception of using Manning’s velocity equation to solve for the channels velocity. Manning’s equation involves the channels roughness coefficient (n), the hydraulic radius (Rh), and the slope of the channel. For this particular channel, a roughness coefficient of 0.04 was used. The hydraulic radius is the cross sectional area divided by the wetted perimeter. Using assumed channel dimensions of eight inches wide by half an inch deep, the hydraulic radius was determined as 0.4 (unitless). Again through use of the same PA Stream Stats graph, a slope of 0.000636 foot per foot was determined using the channelized portion of the graph. Also using this graph, the flow length was found to be 0.7 miles, or 3,696 feet. Manning’s velocity was determined to be 0.55 feet per second, which then resulted in a channelized flow of 1.88 hours.

The total time of concentration is now ready to be solved for. Adding up the divisions of sheet, shallow concentrated and channelized flows lead to a total time of concentration of 3.8 hours. Using this value, the lag time was then computed a second time. Multiplying the total time of concentration by a factor of 0.6 led to an alternative lag time of 2.28 hours.

Concluding, the lag time turned out to be 1.68 hours in one scenario and 2.28 hours in another. Although these values differ, the individual methods they were solved for were correct. Later in the analysis, they will both be used to determine which value is more representative of the actual peak flow statistics given by PA Stream Stats through use of the computer modeling program HEC-HMS. This will be discussed more thoroughly later in the analysis.

Precipitation Data

Quantities of precipitation vary dramatically due to factors such as location and time of year. There are three main types of precipitation events based upon their mechanisms: convective, orographic, and cyclonic. The majority of rainfall events that occur over the Pennsylvania region are cyclonic, defined as movement of air masses.

The precipitation data required to perform a successful hydrologic analysis is the most important parameter to consider. Without the correct information, the study is flawed. Therefore, it must be obtained from respected sources. For this report, the statistics were pulled from the National Weather Service’s website. IDF curves showing the estimated rainfall depths for several return periods are shown graphically and in tabular format. Here, 24-hour values for the 50-year and 100-year precipitation events are revealed. The same website also includes two Hydrometeorological Reports, HMR-51 and HMR-52, in which the PMP can be estimated for a given region in the United States. The PMP has been thoroughly researched to serve as a basis for the maximum amount precipitation that a region can see over a 24-hour time period. Following is a table summarizing the three precipitation values that have been obtained for the design storm event.

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Table 5: Precipitation Estimates for Specific Rainfall Events

Rainfall Event 24-Hour Precipitation (inches)

PMP 34.00

100-Year 5.86

50-Year 5.08

Depending on the design characteristics of the dam, these values will be tested to see if in fact the dam is capable of holding back the flow from each event. If the water elevation proves to be greater than the elevation of the dam, then it faces the possibility of failure if outflow is not released in a timely manner.

Reservoir Characteristics

The construction of the Somerset Dam commenced in 1935 and was completed in 1959. Due to World War II, work was suspended until 1948 which caused the lengthy time duration. Flow from Somerset Dam travels into the Wivenhoe Dam which is then released and strengthens the flow of the Brisbane River.

The dam was built with 666,010.5 cubic feet of concrete with a maximum wall thickness of 134.5 feet. The weight of the dam itself is enough to hold back the enormous quantity of water it retains. At full capacity, the dam withholds 307,948.6 acre-feet of water, which when exceeded will result in overtopping of the dam.

The spillway at the dam is a broad crested weir type. The characteristic flow equation for this type of spillway is equal to the length of the spillway multiplied by the weir coefficient and elevation of the water over the spillway to the three halves power. This flow equation yields the outflow of the reservoir before overtopping. The length of this spillway is 100 feet long, the weir coefficient is 3.5, and the outflow from the spillway occurs when the water in the reservoir reaches an elevation of 2,114.5 feet.

This study was used not only to find the peak inflow and outflow for given precipitation events, but to also discover whether or not the peak maximum flood would cause overtopping of the dam that retains the volume of water in the reservoir. To determine this, several dam characteristics were needed. The elevation used for this dam was 2,120.5 feet. If the water level in the reservoir were to rise above an elevation of 2,120.5 feet, then the dam would begin overtopping. The length of the dam is 1,500 feet long, and when overtopping occurs, the dam begins to behave as a weir with a weir coefficient of 2.65.

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HEC-HMS Model Development

The Hydrologic Modeling System, HEC-HMS, is a computer program that simulates the precipitation to runoff processes of specific watershed systems. For this model analysis, the 24-hour, 50-year, 100-year, and PMP events are the desired input parameters of precipitation. Meteorological events were created for each of the desired storms. These events were then distributed according to the NRCS distribution. The NRCS provides four storm distributions, in which the Type II distribution was used for the Lake Somerset region model. It also required the input of a correctly determined curve number and lag time. These meteorological events needed to be linked to some sub-basin in order for the model to run.

The model created for this watershed dealt with one basin, appropriately named Somerset. A sub-basin was also required to properly model the peak inflow to the reservoir from the precipitation events analyzed. Only one sub-basin was required to properly model this watershed however, so it was also labeled Somerset. The area used for the Somerset sub-basin was 4.1 square miles. A transform method was required to properly model the inflow over time for the three meteorological events. The unit hydrograph method was used, which is the peak flow that occurs from one inch of precipitation over the watershed. This sub-basin required a method to determine the losses over the sub-basin, which was chosen as the NRCS curve number method using the curve number and lag time previously determined. The following table illustrates the parameters that were input into HEC-HMS, to effectively provide the results necessary to determine if the dam would overtop or not.

Table 6: Data Input for HEC-HMS

Parameter Input Value

Lag Time (Tl) 136.8 Minutes (2.28 Hours) Curve Number (CN) Initial Abstraction

65 0.86 Inches

24-Hour, PMP Storm Event 34 Inches

24-Hour, 50-Year Storm Event 5.86 Inches

24-Hour, 100-Year Storm Event 5.08 Inches

*Other non-specific values were also input/altered

This sub-basin was then connected to the reservoir, in which the characteristics have been described in the previous section. Initially, the inflow was set to equal the outflow before the precipitation event. The peak inflows for each of the meteorological events were used to model the storms’ influence on the reservoir and calculate peak storage, elevation, and outflow for the reservoir for each meteorological event. The reservoir’s storage method was set as storage-elevation since peak elevation for the Probable Maximum Flood (PMF) is essentially what was to be determined.

The data was set to be taken at 10 minute time intervals over a two day period. This allowed the total inflow to occur from the 24-hour event and provides enough data to make educated conclusions on the peak flows and elevation.

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Results

Through analyzing Lake Somerset’s watershed via HEC-HMS, it has been determined that the dam will withstand the 24-hour, 50-year and 100-year events. However, if the PMP event were to occur, it will overtop the dam by 1.3 feet. By comparing the peak flows given by PA Stream Stats to the peak flows output through the analysis of the watershed in HEC-HMS, there is less than a ten percent difference for each respective storm event. This therefore confirms that the lag time, calculated from the time of concentration, and weighted curve number are both acceptable values and within sufficient reasoning.

Although the PMP is a significantly high rainfall event, there is a possibility of its occurrence. Because the resulting storage will overtop the elevation of the dam, this leads to the conclusion that the Lake Somerset dam should be considered a hazard. It is impossible to foresee the future, but if this event were to occur, there most likely will be devastating effects to the town of Somerset and the citizens that reside there. The resultant reservoir statistics from HEC-HMS, peak flow statistics from PA Stream Stats, along with the inflow percent differences can be viewed in the following three tables, respectively. Also, figure 2 shows the storage, elevation, and outflow versus time graphs for all events.

Table 7: HEC-HMS Reservoir Flow Statistics

Event Inflow (CFS) Outflow (CFS) Elevation (FT) Storage (AC-FT) Overtopping

50-Year 934.80 310.20 2,115.40 1,306.90 No

100-Year 1,233.40 439.2 2,115.70 1,361.10 No

PMP 14,959.30 12,470.20 2,121.80 3,039.80 1.3 FT

*Elevation of Dam = 2120.5 FT

Table 8: PA Stream Stats Peak Flow Statistics

Event Inflow (CFS)

50-Year 946.00

100-Year 1160.00

Table 9: Inflow Percent Difference

Event PA Stream Stats (CFS) HEC-HMS (CFS) % Difference

50-Year 946.00 934.80 1.20

100-Year 1,160.00 1,233.40 5.95

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Figure 2: Storage – Elevation and Outflow graphs for 50-year, 100-year, and PMP Events

Concluding Recommendations

Upon extensive analysis of Somerset Lake and its watershed for the Pennsylvania Fish and Boat Commission, it is evident that the dam is insufficient to withstand the PMP event. Through the design storms mentioned, HEC-HMS has determined the PMP storm event will overtop the Somerset Lake dam. Therefore, the citizens downstream of the dam are at a risk to lose their lives and property, should the dam fail. It is recommended that analysis begins on how the dam can be properly rehabilitated to withstand the PMP event, in case this was to occur. It is important to explore various options in order to ensure the protection of the downstream population and property.

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References

HEC, 2011a, HEC-HMS User's Manual, Version 3.5.0, CPD-74A, U.S. Army Corps of Engineers, Hydrologic Engineering Center, Davis, CA, November 2011.

"Hydrometeorological Design Studies Center."NOAA's National Weather Service.N.p., n.d. Web. 3 Mar. 2011. <hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html>.

"Hydrometeorological Report NO.51." Probable Maximum Precipitation Estimates, United States East of the 105th Meridian.N.p., n.d. Web. 3 Mar. 2011. <www.nws.noaa.gov/oh/hdsc/studies/pmp.html#PMP_documents>.

"Rebuilding Pennsylvania."Pennsylvania Fish and Boat Commission Home Page.N.p., n.d. Web. 3 Mar. 2011. <http://www.fish.state.pa.us/dams/index.htm>.

"Somerset Dam | Seqwater."Home | Seqwater.N.p., n.d. Web. 3 Mar. 2011. <http://www.seqwater.com.au/public/catch-store-treat/dams/somerset-dam>.

"StreamStats in Pennsylvania."USGS Water Resources of the United States.N.p., n.d. Web. 3 Mar. 2011. <http://water.usgs.gov/osw/streamstats/pennsylvania.html>.

"Urban Hydrology for Small Watersheds."National Resources Conservation Service.N.p., n.d. Web. 3 Mar. 2011. <www.wsi.nrcs.usda.gov/products/w2q/H&H/docs/other/TR55_documentation.pdf>.

"Web Soil Survey - Home."Web Soil Survey - Home.N.p., n.d. Web. 3 Mar. 2011. <http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm>.