Bridge Design Hydraulic Study Report - Public works … Inf… · · 2017-12-12January 2014 Bon Air Road Bridge Replacement Project City of Larkspur, Marin County, California Bridge

January 2014

Bon Air Road Bridge Replacement Project City of Larkspur, Marin County, California

Bridge Design Hydraulic Study Report

Prepared for:

Prepared by:

Bridge Design Hydraulic Study Report Existing Bridge No. 27C0028 Bon Air Road Bridge Replacement Project Proposed Bridge No. 27C0162 City of Larkspur, Marin County, California

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Table of Contents Executive Summary ............................................................................................................ v Acronyms ........................................................................................................................... vi 1 General Description ............................................................................................ 1

1.1 Project Description .............................................................................................. 1 1.2 Key Tasks............................................................................................................ 3

1.2.1 Existing Conditions ........................................................................................... 4 1.2.2 Proposed Conditions .......................................................................................... 4

1.3 Design Criteria .................................................................................................... 4 1.3.1 Hydrologic Design Methods .............................................................................. 4 1.3.2 Hydraulic Design Criteria .................................................................................. 4 1.3.3 Scour Design Criteria ........................................................................................ 5

1.4 Vertical Datum .................................................................................................... 5 2 Description of Watershed ................................................................................... 8

2.1 Watershed Description ........................................................................................ 8 2.2 Receiving Water Bodies ..................................................................................... 8 2.3 Precipitation ........................................................................................................ 8 2.4 Land Use ............................................................................................................. 8

3 Stream and Site Description ............................................................................. 10 3.1 Channel Properties ............................................................................................ 10 3.2 Existing Bridge ................................................................................................. 10 3.3 Proposed Bridge ................................................................................................ 10

4 Hydrology ......................................................................................................... 13 4.1 Design Discharge Summary ............................................................................. 13

4.1.1 FEMA FIS ....................................................................................................... 13 4.1.2 United States Army Corps of Engineers .......................................................... 14

4.2 Sea Level Rise ................................................................................................... 14 4.3 Hydrologic Stability .......................................................................................... 17

5 Hydraulic Analysis ........................................................................................... 18 5.1 Design Tools ..................................................................................................... 18 5.2 Cross Section Data ............................................................................................ 18 5.3 Modeled Hydraulic Structures .......................................................................... 18 5.4 Model Boundary Condition .............................................................................. 19 5.5 Manning’s Roughness Coefficients .................................................................. 20 5.6 Expansion and Contraction Coefficients .......................................................... 20 5.7 Water Surface Elevations .................................................................................. 20 5.8 Freeboard .......................................................................................................... 23 5.9 Flow Velocities ................................................................................................. 24 5.10 Rock Slope Protection for Erosion Protection at Slope Embankments ............ 25

6 Scour Analysis .................................................................................................. 26 6.1 Caltrans Bridge Inspection Reports .................................................................. 26

6.1.1 Bridge Inspection Report for Inspection Date: July 23, 2003 ......................... 26 6.1.2 Bridge Inspection Report for Inspection Date: August 08, 2001 .................... 27

6.2 Existing Channel Bed ....................................................................................... 27 6.3 Long-Term Bed Elevation Change ................................................................... 27


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6.4 Contraction Scour ............................................................................................. 27 6.5 Pier (Bent) Scour............................................................................................... 28 6.6 Abutment Scour ................................................................................................ 32 6.7 Total Scour and Scour Countermeasures .......................................................... 33

6.7.1 Scour Countermeasures ................................................................................... 35 7 References ......................................................................................................... 36

Photos Photo 1. Upstream Face of the Existing Bridge ................................................................ 10

Figures Figure 1. Location Map ....................................................................................................... 6 Figure 2. Vicinity Map ........................................................................................................ 7 Figure 3. Ross Valley Watershed Land Use ....................................................................... 8 Figure 4. Aerial Map ........................................................................................................... 9 Figure 5. Bird’s Eye View of Existing Bon Air Bridge .................................................... 11 Figure 6. Bon Air Road Proposed Bridge General Plan ................................................... 12 Figure 7. Sea Level Rise ................................................................................................... 16 Figure 8. San Francisco Bay Sea Level Rise at Project Site ............................................. 17 Figure 9. HEC-RAS Cross Section Locations Overlay with Aerial ................................. 19 Figure 10. Existing Bridge Upstream Cross Section (Facing Downstream) .................... 22 Figure 11. Proposed Bridge Upstream Cross Section (Facing Downstream) ................... 22 Figure 12. Typical Section from Bridge General Plan ..................................................... 31 Figure 13. Plan View from Bridge General Plan .............................................................. 31


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Tables Table 1. Peak Discharges for Corte Madera Creek (FEMA) ............................................ 13 Table 2. High Tide Elevations for San Francisco Bay ...................................................... 14 Table 3. Peak Discharges for Corte Madera Creek (USACE/Stetson/WRECO) ............. 14 Table 4. Hydraulic Model Downstream Boundary Condition .......................................... 19 Table 5. Water Surface Elevations for MHHW Condition ............................................... 21 Table 6. Water Surface Elevations for 100-Year Storm MHHW with Sea Level Rise .... 21 Table 7. Water Surface Elevations for 500-Year Storm MHHW with Sea Level Rise .... 21 Table 8. Water Surface Elevations for MLLW Condition ................................................ 21 Table 9. Available Freeboard (Relative to Lowest Part of Arch) for MHHW Condition 23 Table 10. Available Freeboard (Relative to Lowest Part of Arch) for 100-Year Storm

MHHW with Sea Level Rise ............................................................................. 23 Table 11. Available Freeboard (Relative to Lowest Part of Arch) for 500-Year Storm

MHHW with Sea Level Rise ............................................................................. 23 Table 12. Available Freeboard (Relative to Lowest Part of Arch) for MLLW Condition 23 Table 13. Average Channel Velocities for MHHW Condition ......................................... 24 Table 14. Average Channel Velocities for 100-Year Storm MHHW with Sea Level Rise

........................................................................................................................... 24 Table 15. Average Channel Velocities for 500-Year Storm MHHW with Sea Level Rise

........................................................................................................................... 24 Table 16. Average Channel Velocities for MLLW Condition ......................................... 24 Table 17. Contraction Scour Depths ................................................................................. 28 Table 18. Local Pier Scour Depths ................................................................................... 30 Table 19. Local Abutment Scour Depths .......................................................................... 33 Table 20. Scour Depths for 100-Year Storm Draining Against MHHW ......................... 33 Table 21. Scour Depths for 100-Year Storm Draining Against MLLW .......................... 33 Table 22. Scour Depths for 100-Year Storm Draining Against MHHW with Sea Level

Rise Projected to the Year 2100 ........................................................................ 34 Table 23. Scour Depths for 500-Year Storm Draining Against MHHW ......................... 34 Table 24. Scour Depths for 500-Year Storm Draining Against MLLW .......................... 34 Table 25. Scour Depths for 500-Year Storm Draining Against MHHW with Sea Level

Rise Projected to the Year 2100 ........................................................................ 34


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Appendices Appendix A HEC-RAS: 500-Year Mean Higher High Water without Sea Level Rise Appendix B HEC-RAS: 500-Year Mean Higher High Water with Sea Level Rise

Projected to the Year 2050 Appendix C HEC-RAS: 500-Year Mean Higher High Water with Sea Level Rise

Projected to the Year 2100 Appendix D HEC-RAS: 500-Year Mean Lower Low Water without Sea Level Rise Appendix E HEC-RAS: 100-Year Mean Higher High Water without Sea Level Rise Appendix F HEC-RAS: 100-Year Mean Higher High Water with Sea Level Rise

Projected to the Year 2050 Appendix G HEC-RAS: 100-Year Mean Higher High Water with Sea Level Rise

Projected to the Year 2100 Appendix H HEC-RAS: 100-Year Mean Lower Low Water without Sea Level Rise Appendix I HEC-RAS: 50-Year Mean Higher High Water without Sea Level Rise Appendix J Rock Slope Protection for Erosion Protection at Slope Embankments Appendix K Scour Calculations: 500-Year Mean Higher High Water without Sea Level

Rise Appendix L Scour Calculations: 500-Year Mean Lower Low Water without Sea Level

Rise Appendix M Scour Calculations: 500-Year Mean Higher High Water with Sea Level

Rise Projected to the Year 2100 Appendix N Scour Calculations: 100-Year Mean Higher High Water without Sea Level

Rise Appendix O Scour Calculations: 100-Year Mean Lower Low Water without Sea Level

Rise Appendix P Scour Calculations: 100-Year Mean Higher High Water with Sea Level

Rise Projected to the Year 2100 Appendix Q Rock Slope Protection for Scour Countermeasures


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Executive Summary This study presents the hydrologic and hydraulic assessments for the proposed Bon Air Road Bridge Replacement Project (Project) over Corte Madera Creek in the city of Larkspur. The existing bridge is 420 ft long by 44 ft wide and is structurally deficient. The new bridge would be 388 ft long and 62.5 ft wide. The city of Larkspur is proposing to replace the existing bridge (Bridge No. 27C0028) with a new bridge (Bridge No. 27C0162), which would be wider than existing on the northern side. Parsons Brinckerhoff provided the bridge replacement and roadway design. Stetson Engineers provided a hydraulic and hydrologic base model, which was originally prepared by the U.S. Army Corps of Engineers (USACE). This model referenced the National Geodetic Vertical Datum of 1929 (NGVD 29). A vertical datum adjustment (adding 2.52 ft) was applied to the geometry of the model to match the Project datum, which references the North American Vertical Datum of 1988 (NAVD 88) The proposed bridge design was based on public input but limited by the site constraints. The existing roadway profile, limited right-of-way, and high water elevation in the creek largely dictated the bridge configuration and profile. The new bridge would reduce the number of piers, which would improve the conveyance capacity of the creek. Based on the hydraulic analyses, the proposed bridge would not significantly increase or decrease the water surface elevation or the velocities in the channel. The water surface elevation would decrease by an insignificant amount, and the velocity would increase slightly under proposed conditions. The bridge would not meet the freeboard criteria as set forth by the Federal Highway Administration (FHWA) and Caltrans. Because the proposed bridge would not be designed to completely pass this flow, as the minimum bridge design criteria requires, a design exception would be required. However, the proposed bridge would not worsen the hydraulics and the flood flow levels, relative to the current condition. Scour depths were estimated for the 100-year recurrence interval and were checked for the 500-year recurrence interval because the Project site is in a tidal zone. The potential for lateral channel migration, contraction scour, and local pier and abutment scour should be considered in setting the foundation depths. The foundations should be below the estimated scour depths to minimize structural damage and/or undermining. If the structural analysis shows that the foundations can withstand the calculated scour depths, then RSP is not required.


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Acronyms Caltrans California Department of Transportation CIDH cast-in-drilled hole CSU Colorado State University EPA Environmental Protection Agency FEMA Federal Emergency Management Agency FHWA Federal Highway Administration FIRM Flood Insurance Rate Map FIS Flood Insurance Study HEC-18 Hydraulic Engineering Circular No. 18 HEC-23 Hydraulic Engineering Circular No. 23 HEC-25 Hydraulic Engineering Circular No. 25 HEC-HMS Hydrologic Engineering Centers Hydrologic Modeling System HEC-RAS Hydrologic Engineering Centers River Analysis System MHHW mean higher high water MLLW mean lower low water NAVD 88 North American Vertical Datum of 1988 NGVD 29 National Geodetic Vertical Datum of 1929 NPDES National Pollutant Discharge Elimination System RS river station RSP rock slope protection USACE U.S. Army Corps of Engineers USGS United States Geological Survey


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1 GENERAL DESCRIPTION The City of Larkspur (City) proposes to replace the Bon Air Road bridge located in the City of Larkspur within Marin County (County). The purpose of this report is to summarize the hydrology, compare the hydraulic characteristics of the existing and proposed bridges, present the estimated scour depths for the proposed bridge, and provide recommendations for scour countermeasures for the proposed bridge. Figure 1 and Figure 2 show the vicinity of the Bon Air Road Bridge Replacement Project (Project), and Figure 4 shows the location of the existing bridge and immediate surrounding area.

1.1 Project Description The Bon Air Road Bridge Replacement Project (Project) involves replacing the existing 420‐ft long and 44‐ft wide Bon Air Road bridge with a new bridge that is 388 ft long and 62.5 ft wide. The bridge is an additional 18.5 ft wider than the existing bridge to maintain two-way traffic and pedestrians/bicycles during construction. Four precast girders are required for the first stage of construction. In order to maintain bridge symmetry and equal structure stiffness, an additional four precast girders are required for stage 2 on the south side of the bridge. This additional width will also improve accessibility for pedestrians and bicycles. The following Project features are listed in terms of chronological impacts.

Clear and grub vegetation. Construct northern temporary trestle and access ramps. Demolish northern portion of the existing bridge deck. Install northern bridge foundations. Construct northern bridge deck. Construct southern temporary trestle and access ramps. Demolish remainder of existing bridge deck. Install southern bridge foundations. Construct remaining bridge deck. Remove temporary trestles and access ramps. Restore vegetation.

Clear and grub vegetation. Areas within the Project boundary that are required for temporary construction access or within the impact footprint would be cleared of vegetation and debris. Construct northern temporary trestle and access ramps. Bridge demolition and new bridge construction would require the construction of a temporary trestle bridge that spans across Corte Madera Creek along the northern side of the existing bridge. The


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actual design of the trestles would be determined by the contractor. A likely method of constructing the trestle would follow the following steps:

A vibratory or impact pile driver would be used from the banks of the creek or completed portions of the trestle bridge to place approximately 64, 12‐ to 14‐in. steel “H” or pipe piles would be placed into the creek approximately 70 feet deep to support the trestle deck. Vibratory driving would be used to drive each pile/casing to a depth of 50 ft. An impact pile driver would be used for the remaining 20 ft.

Steel beams would be installed that span the piles and wooden plywood sheets or planks would be used to create the final trestle deck.

Temporary clean fill (e.g., gravel or road base) would be used to create 30‐ft wide access ramps to each trestle from both sides of the creek. Geo‐textile fabrics or other barrier fabric would be placed over the wetlands prior to placing the temporary clean fill material needed to create the ramps.

Demolish northern portion of existing bridge deck. The northern deck of the existing bridge would be removed from the temporary trestle. Demolition of the north side of the bridge would involve removal of the bridge railing, pedestrian sidewalk, deck, and a portion of the girders and pier caps. Approximately 10 to 15 ft of the bridge width would initially be removed during this step. The southern bridge deck would be left in place to allow traffic flow to continue during the construction period. The existing bridge piles would be cut 2 ft below the channel bottom. Install northern bridge foundations. Four cast‐in‐drilled‐hole (CIDH) piles would be installed as the foundation for the northerly portion of bridge. Each CIDH pile foundation would be constructed by first installing a 10‐ft diameter steel casing into the creek approximately 70 ft deep using a vibratory and impact pile driver and isolating the foundation construction area. The top 50 ft of each pile would be vibrated in, and the final 20 ft would be installed with an impact hammer. After the 10‐ft diameter casings are in place and deep enough to form a seal at the bottom, an 8‐ft diameter hole would be drilled inside each casing. During the drilling, contractor may install an 8‐ft diameter casing to stabilize the hole. The water and soils removed from the casings would be disposed of offsite following National Pollutant Discharge Elimination System (NPDES) requirements. Each of the drilled foundation holes would receive a 7‐ft diameter steel‐reinforced cage structure that would provide reinforcement for the concrete pier. Concrete would then be pumped into the drilled 8‐ft diameter holes. After the CIDH concrete has reached sufficient strength, the 10‐ft diameter steel casing would be vibrated and removed. If used, the 8‐ft diameter casing would be also removed or the contractor may choose to leave it in place upon the engineer’s approval. Construction of the new bridge abutments would involve first excavating the soil and removing the existing bridge abutment material in the immediate area at the top of the banks to create a shelf for the abutment footings. Eight, 3‐ft diameter CIDH piles would then be installed into each excavated bank, and pile caps would be casted atop the piles to


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provide stable abutment foundations. Structural backfill would then be used to fill in behind the abutments to create the new roadway grade. Construct northern bridge deck. The northern bridge deck would be installed atop the new CIDH piles and abutments. Construction would be performed entirely from the temporary trestle. Construct southern temporary trestle and access ramps. The southern temporary trestle and ramps would be constructed using the same methods described for the northern temporary trestle, using approximately 64 piles, cross beams and a wooden deck. Demolish remainder of existing bridge deck. The remainder of the existing bridge would be removed, and the existing bridge piles would be cut off 2 ft below the channel bottom. Install southern bridge foundations. The southern bridge foundations would be installed using the same methods described for the northern bridge foundations. Four additional CIDH piles would be installed. Construct southern bridge deck. The southern bridge deck would be constructed following the completion of the southern bridge foundations. A closure pour would join the north and south decks to create a single deck. Remove temporary trestles and access ramps. Both trestles and all temporary fill would be removed following completion of the bridge. Restore vegetation. Temporarily affected areas and the area on the south side of the new bridge no longer occupied by the existing bridge would be restored to pre‐construction conditions according to an approved Mitigation and Monitoring Plan to be prepared and approved prior to the start of construction.

1.2 Key Tasks The key tasks for this study included: 1) investigation of previous hydrologic studies of Corte Madera Creek; 2) hydraulic analyses to estimate water surface elevations and flow velocities for the Bon Air Road bridge; and 3) scour analyses to estimate potential scour depths for the bridge replacement. Two hydraulic analyses were conducted for this study:

Current channel conditions with the existing bridge Current channel conditions with the proposed bridge


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1.2.1 Existing Conditions A Hydrologic Engineering Center River Analysis System (HEC-RAS) hydraulic model was provided by Stetson Engineers that represented the current channel conditions of Corte Madera Creek1. This model was based on a model originally developed by the United States Army Corps of Engineers (USACE). It served as the model used for the existing conditions. Elevations in the model originally referenced the National Geodetic Vertical Datum of 1929 (NGVD 29). These elevations were converted to North American Vertical Datum of 1988 (NAVD 88) by adding 2.52 ft to match the Project datum.

1.2.2 Proposed Conditions The model provided by Stetson Engineers was also used for the proposed conditions model for the channel bed configuration. The bridge geometry was updated to reflect the proposed bridge for the Project, as described in Section 3.3. As described in Section 1.4, the elevations in the model were converted to NAVD 88 by adding 2.52 ft, the vertical datum height conversion, which was provided by Meridian Surveying Engineering, Inc.

1.3 Design Criteria

1.3.1 Hydrologic Design Methods The hydrology at the Project site was derived from previous hydrologic studies of Corte Madera Creek. This report summarizes two sources that were researched to help establish the peak discharges for the Project.

1.3.2 Hydraulic Design Criteria The hydraulic design of the bridge should follow the Federal Highway Administration (FHWA) and California Department of Transportation’s (Caltrans) criteria. The FHWA criterion for the hydraulic design of bridges is that they be designed to pass the 2% probability of annual exceedance flow (50-year recurrence interval design discharge) with adequate freeboard, where practicable, to account for debris and bedload. The Caltrans criteria for the hydraulic design of bridges is that they be designed to pass the 2% probability of annual exceedance flow (50-year design discharge) or the flood of record, whichever is greater, with adequate freeboard to pass anticipated drift. Two feet (2 ft) of freeboard is commonly used in preliminary bridge designs. The bridge should also be designed to pass the 1% probability of annual exceedance flow (100-year design discharge, or base flood). No freeboard is added to the base flood.

1 The HEC-RAS model for Corte Madera Creek was provided by Stetson Engineers on January 5, 2009. This model was originally created by the United States Army Corps of Engineers and calibrated by Stetson Engineers.


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1.3.3 Scour Design Criteria The evaluation of potential scour at the proposed bridge will follow the criteria described in the FHWA’s Hydraulic Engineering Circular No. 18 (HEC-18), Evaluating Scour at Bridges (Fifth Edition) and the FHWA’s Hydraulic Engineering Circular No. 25 (HEC-25), Tidal Hydrology, Hydraulics and Scour at Bridges (First Edition). Typically, the minimum design criterion for bridge scour is the 100-year design storm. However, because the proposed bridge would be in a tidally influenced area, the hydraulic analysis should also consider the 500-year design storm. The evaluation of potential scour shall be based on hydraulic characteristics of the 100-year and 500-year design discharges. The total scour was estimated based upon the cumulative effects of the long-term bed elevation change, general (contraction) scour, and local scour. The life expectancy of the bridge was considered in determining the long-term bed elevation change of the waterway; it was based on an assumed 75-year design life for a replacement bridge.

1.4 Vertical Datum The Project references the NAVD 88 vertical datum. Other data sources that were used for the Project referenced the NGVD 29 vertical datum. A conversion to adjust the vertical heights was provided by Meridian Surveying Engineering, Inc.: Elevation NGVD 29 + 2.52 ft = Elevation NAVD 88.


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Figure 1. Location Map

Source: United States Geological Survey (USGS), 2001

N

Project Site


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Figure 2. Vicinity Map

Source: USGS, 2001

N

Bon Air Road Bridge

Larkspur

Greenbrae


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2 DESCRIPTION OF WATERSHED

2.1 Watershed Description The city of Larkspur is located in the lower portion of the Ross Valley. Corte Madera Creek runs through the Ross Valley and drains a watershed area of approximately 21.6 square mi at the Bon Air Road bridge (Federal Emergency Management Agency [FEMA] 2009).

2.2 Receiving Water Bodies Corte Madera Creek flows roughly southeast where it drains to Corte Madera Channel and eventually empties to the San Francisco Bay, 9 mi north of the Golden Gate Bridge.

2.3 Precipitation The annual average precipitation in the city of Larkspur is approximately 38 in., which generally occurs between November and April (FEMA 2009). During flood years, this precipitation level is greatly exceeded.

2.4 Land Use The land use in the vicinity of the Project site is predominantly urban with residences located west of the bridge. Creekside Park is located on the northeast bank of Corte Madera Creek and includes a multi-use path along the creek. Marin General Hospital and other medical facilities are also located on the eastern side of the bridge. See Figure 3 for a land use map and Figure 4 for an aerial map of the Project vicinity.

Figure 3. Ross Valley Watershed Land Use

Source: County of Marin Department of Public Works

Bon Air Road Bridge


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Figure 4. Aerial Map

Source: Google Earth

N Creekside

Park

Bon Air Road Bridge

Corte Madera Creek


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3 STREAM AND SITE DESCRIPTION

3.1 Channel Properties The creek, at the bridge site, is naturally lined with marsh or swamp land located on the eastern bank.

3.2 Existing Bridge The existing bridge structure was built in 1958. The bridge has undergone modifications, including widening on the north side, foundation retrofitting, and seismic retrofitting and widening. The widened structure has a 10-span reinforced concrete deck and is 420 ft long and 44 ft wide.

3.3 Proposed Bridge The existing bridge is proposed to be replaced (see Photo 1 for a photograph and Figure 5 for a bird’s eye view of the existing bridge). The bridge would be widened on the northern side. The new bridge would include 11.5-ft wide lanes (one in each direction of travel) with bike lanes and sidewalks on both sides of the bridge; see Figure 6. The new bridge would have five spans, and be 388 ft long and 62.5 ft wide. Based on the alignment from the bridge General Plan, the piers were modeled to have no skew from the 100-year flow.

Photo 1. Upstream Face of the Existing Bridge


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Figure 5. Bird’s Eye View of Existing Bon Air Bridge

Source: Bing Maps

N

Bon Air Road Bridge

Corte Madera Creek


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Figure 6. Bon Air Road Proposed Bridge General Plan

Source: Parsons Brinckerhoff, 2014


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4 HYDROLOGY The FEMA Flood Insurance Study (FIS) and the Flood Insurance Rate Map (FIRM) for the city of Larkspur were reviewed to determine the relevant hydrologic data. The information gathered from these sources is discussed in the sections below.

4.1 Design Discharge Summary FEMA performed a detailed analysis on Corte Madera Creek from San Francisco Bay through the city of Larkspur’s corporate limit. The FIS lists the established peak discharge frequency relationships that were developed through that analysis. Peak discharges for Corte Madera Creek were also estimated by Stetson Engineers for a separate study, the Appraisal-Level Hydraulic Engineering Study, Corte Madera Creek (2007).

4.1.1 FEMA FIS At the Bon Air Road bridge, Corte Madera Creek drains an area of 21.6 square mi, and the peak discharge for the 100-year base flood is 8,300 cfs (peak discharges for other recurrence intervals are summarized in Table 1). This peak flow was estimated using a basin transfer of gage data and was based on the USGS gaging station in the town of Ross, which is located approximately 2.2 mi upstream of the Project site. This stream gage was maintained by USGS between 1951 and 1993. FEMA obtained flow information from the gaging station in the town of Ross and compared the drainage areas between the two sites: at the town of Ross and at U.S. 101. The discharge was estimated by adjusting the site by the drainage area ratio. The drainage area at the Ross gaging station is approximately 18.1 square mi. The water surface elevation of Corte Madera Creek at its confluence with San Francisco Bay is based on the level of the bay, which is discussed further in Section 5.4. The Manning’s roughness value, n, used in the FIS for the Corte Madera Creek, ranged from 0.015 to 0.055 for the main channel and from 0.062 to 0.200 for the overbank areas. Table 1. Peak Discharges for Corte Madera Creek (FEMA)

Location 500-Year Peak Discharge (cfs)

100-Year Peak Discharge (cfs)



Ross Gage 8,400 6,900 6,200 4,060 Bon Air Road 9,000 8,300 7,400 5,000

Source: FEMA, 2009 The Bon Air Road bridge is influenced by tidal fluctuations due to its proximity to the San Francisco Bay. The frequency of occurrence of high tides in the San Francisco Bay was analyzed by the USACE. The data was reviewed by FEMA and summarized in the FIS. These stillwater elevations are due to tidal and wind effects but do not include contributions from wave action. These elevations are summarized in Table 2.


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Table 2. High Tide Elevations for San Francisco Bay Location 500-Year

Elevation (ft NAVD 88)

100-Year Elevation

(ft NAVD 88)

50-Year Elevation

(ft NAVD 88)

10-Year Elevation

(ft NAVD 88) San Francisco Bay at Larkspur

9.4 9.1 8.9 8.3

Source: FEMA, 2009

4.1.2 United States Army Corps of Engineers Peak discharges for Corte Madera Creek at the Ross gaging station were also estimated by Stetson Engineers for a separate study, the Appraisal-Level Hydraulic Engineering Study, Corte Madera Creek (2007). Stetson Engineers also developed a Hydrologic Engineering Centers Hydrologic Modeling System (HEC-HMS) model to derive the flows for Corte Madera Creek. A hydraulic model was provided by Stetson Engineers, which included an analysis conducted by USACE and calibrated by Stetson Engineers. The hydrology in the model was updated by Stetson Engineers to reflect the storms in December 2005 and verified using the December 29, 2003 bankfull2 flow event. The 100-year design discharge used in the model, at the Project site, is 8,620 cfs. WRECO estimated the 500- and 50-year flows for this study using the ratio of the peak discharges for the 500- and 50-year flood events from the flood frequency curve that was developed for the Ross gage by Stetson Engineers. The flows at both the Ross gage and at the Project site are listed in Table 3. Table 3. Peak Discharges for Corte Madera Creek (USACE/Stetson/WRECO)

Location 500-Year Peak Discharge (cfs)



Ross Gage 8,810 7,510 6,880 Bon Air Road 10,150 8,650 7,930

Source: USACE/Stetson/WRECO The flows listed in the FIS are based on older studies from 2009 or earlier. The flows from USACE/Stetson are based on more recent data and were calibrated based on the 2005 storms. These flows, which were based on the flows from the Stetson Engineers’ Appraisal-Level Hydraulic Engineering Study, were adopted for this study.

4.2 Sea Level Rise Tidal gages along the California coast have recorded sea level rises over the past several decades that show an increase rate of about 7 to 8 in. per century, which is a similar rate

2 Bankfull refers to the flow stage of a river in which the flow fills the channel to the top of its banks, and the water begins to overflow onto a floodplain (Environmental Protection Agency [EPA] 2010).


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to sea level rise estimates on a global level. Current models predict that by 2050, the sea level will rise approximately 1.0 ft to 1.5 ft from the sea level elevation measured in 2000. By 2100, the sea level is predicted to increase by 2.7 ft to 4.8 ft from the year 2000’s elevation. Global warming could result in a 16-in. sea level rise by 2050 and a 55-in. rise by 2100 in the San Francisco Bay alone (Cayan, et. al. 2009). Figure 7 shows the areas that are affected by the sea level rise; however, the extent of the inundation does not reflect any existing shoreline protection or wave activity. While the Project would not affect the sea level rise, there would be an increase in the flooding potential at the Project location because the Project is susceptible to tidal flooding. An increase of 16 in. in sea level would result in the inundation of the entire Project area (see Figure 7). The sea level rise was also mapped by Noah Knowles of the USGS Water Resources Division. The mapping of the sea level rise at the Project site is shown in Figure 8. Based on this map, the Project site is within an area affected by the sea level rise with a potential increase of 20 in. (50 cm) to 40 in. (100 cm) in sea level.


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Figure 7. Sea Level Rise

Source: San Francisco Bay Conservation and Development Commission

Project Site


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Figure 8. San Francisco Bay Sea Level Rise at Project Site

Source: Noah Knowles and Google Earth

4.3 Hydrologic Stability There have not been significant changes, in recent years, to the basin hydrology. Development in the area is limited due to a lack of available land in the watershed.

Bon Air Road Bridge


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5 HYDRAULIC ANALYSIS The hydraulic analyses were performed for the existing condition and proposed condition using the hydraulic model provided by Stetson Engineers as a base conditions model. This hydraulic model included an analysis conducted by the USACE and calibrated by Stetson Engineers. The hydrology in the model was updated by Stetson Engineers to reflect the storms in December 2005. The proposed HEC-RAS model was developed using the model from Stetson Engineers as a base and incorporating the proposed bridge geometry. The bridge was designed by Parsons Brinckerhoff; see Figure 6 for the bridge General Plan. The results of the hydraulic analysis are included in Appendix A through Appendix I for the modeled scenarios, and it is described in Section 5.7.

5.1 Design Tools The hydraulic analyses were performed for the existing and proposed conditions. The hydraulic analyses were performed using the USACE’s HEC-RAS modeling software Version 4.1.0.

5.2 Cross Section Data The hydraulic model that was provided by Stetson Engineers included three reaches for Corte Madera Creek: Reach 1, Reach 2, and Reach 3. Reach 3 extends from College Avenue Bridge upstream to a point near the Ross Post Office. Reach 2 extends from a point near the Ross Post Office upstream to the Sir Francis Drake Boulevard Bridge, near the Ross gage. Reach 1 extends from the Sir Francis Drake Boulevard Bridge to the San Francisco Bay. The proposed bridge is located at river station (RS) 28128 within Reach 1 of Corte Madera Creek; see Figure 9. No additional surveyed cross sections were included in the analysis. A skew angle of 28 degrees was also applied to the bridge deck and cross sections because the flow direction would not be normal to the bridge; however, the piers would be aligned with the flow direction.

5.3 Modeled Hydraulic Structures The existing bridge was included in the model provided by Stetson Engineers. The proposed bridge was modeled as described in Section 3.3.


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Figure 9. HEC-RAS Cross Section Locations Overlay with Aerial

Source: Google Earth and Stetson Engineers

5.4 Model Boundary Condition The hydraulic model was evaluated using the steady state flow analysis. Water surface elevations were specified for the downstream boundary conditions that correspond to the mean higher high water (MHHW) without sea level rise, MHHW with sea level rise projected to the year 2050, MHHW with sea level rise projected to the year 2100, and the mean lower low water (MLLW) without sea level rise (see Table 4). Table 4. Hydraulic Model Downstream Boundary Condition

Boundary Condition Elevation (ft NAVD 88) MHHW 5.58

MHHW + Sea Level Rise (Projected to the Year 2050)

6.91

MHHW + Sea Level Rise (Projected to the Year 2100)

10.16

MLLW -0.22

Bon Air Road Bridge RS 28128

N


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5.5 Manning’s Roughness Coefficients Manning’s roughness coefficients (n values) were used in the hydraulic model to estimate energy losses in the flow due to friction. The Manning’s n value used in the model for both bridge conditions was 0.035 for the main channel and 0.05 for the left and right banks. These values were established in the HEC-RAS model provided by Stetson Engineers.

5.6 Expansion and Contraction Coefficients Expansion and contraction coefficients are used in the hydraulic model to represent energy losses in the channel that occur due to expansion and contraction. For the existing bridge, an expansion coefficient of 0.3 and a contraction coefficient of 0.1 were used to represent the creek within Reach 1. These values describe gradual transitions between cross sections. These values were established in the HEC-RAS model provided by Stetson Engineers. For the proposed bridge, an expansion coefficient of 0.5 and a contraction coefficient of 0.3 were used to represent the creek in the vicinity of the bridge. These values represent the energy losses at the proposed bridge.

5.7 Water Surface Elevations Water surface elevations were estimated for the existing and proposed bridges for the following cases:

Return Period Tailwater Condition

500-year

MHHW without Sea Level Rise MHHW with Sea Level Rise Projected to the Year 2050 MHHW with Sea Level Rise Projected to the Year 2100

MLLW without Sea Level Rise

100-year

MHHW without Sea Level Rise MHHW with Sea Level Rise Projected to the Year 2050 MHHW with Sea Level Rise Projected to the Year 2100

MLLW without Sea Level Rise 50-year MHHW without Sea Level Rise

The estimated water surface elevations for the existing and proposed conditions are summarized in Table 5, Table 6, and Table 8 for the location immediately upstream of the bridges.


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Table 5. Water Surface Elevations for MHHW Condition Bridge Condition Water Surface Elevation (ft NAVD 88)

500-year 100-year 50-year Proposed 9.53 9.02 8.76 Existing 9.53 9.02 8.76

Table 6. Water Surface Elevations for 100-Year Storm MHHW with Sea Level Rise Bridge Condition Water Surface Elevation (ft NAVD 88)

Projected to the Year 2050 Projected to the Year 2100 Proposed 9.15 10.72 Existing 9.15 10.72

Table 7. Water Surface Elevations for 500-Year Storm MHHW with Sea Level Rise Bridge Condition Water Surface Elevation (ft NAVD 88)


Table 8. Water Surface Elevations for MLLW Condition Bridge Condition Water Surface Elevation (ft NAVD 88)

500-year 100-year Proposed 9.42 8.82 Existing 9.43 8.83

Based on the hydraulic analysis, in general there would be negligible changes to water surface elevation in the vicinity of the bridge. The proposed bridge would result in 0.01 ft decreases (or no change) in elevation for the modeled flow conditions. The greatest difference between the existing and proposed bridges that would affect the hydraulics is that the number of piers would be reduced to less than half the number that currently exists. In addition, the proposed piers would be aligned to be in the direction of flow, whereas the existing piers are skewed to the direction of flow. However, the proposed piers would be wider than the existing piers. These differences in bridge design would result in almost no changes in water surface elevation. The modeled cross sections showing the upstream faces of the existing and proposed bridges are shown in Figure 10 and Figure 11. Only the highest and lowest estimated water surface elevations are shown for each alternative.


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Figure 10. Existing Bridge Upstream Cross Section (Facing Downstream)

Figure 11. Proposed Bridge Upstream Cross Section (Facing Downstream)


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5.8 Freeboard The bridge freeboard criteria are summarized in Section 1.3.2. Per FHWA and Caltrans’ criteria, the bridge should be designed to pass the 50-year design storm with 2 ft of freeboard or the 100-year design storm. The freeboard for a structural arch bridge is measured relative to the lowest part of the arch. The following tables, Table 9 through Table 12, summarize the available freeboard for the evaluated flow conditions for the proposed and existing bridge structures measured relative to the lowest part of the arch. The structures do not provide adequate freeboard to meet the criteria when measured relative to the lowest part of the arch. Because the proposed bridge would not meet these criteria, a design exception would be required. However, although the freeboard criteria would not be met, the proposed bridge would not worsen the hydraulics, relative to the current condition, as described in Section 5.7. In addition, the deck of the bridge would not be overtopped, and the hydraulic modeling shows that there would not be objectionable backwater. Table 9. Available Freeboard (Relative to Lowest Part of Arch) for MHHW Condition Bridge Condition Available Freeboard (ft)

500-year 100-year 50-year Proposed -2.1 -1.6 -1.3 Existing 0 0.5 0.8

Table 10. Available Freeboard (Relative to Lowest Part of Arch) for 100-Year Storm MHHW with Sea Level Rise Bridge Condition Available Freeboard (ft)

Projected to the Year 2050 Projected to the Year 2100 Proposed -1.7 -3.3 Existing 0.4 -1.2

Table 11. Available Freeboard (Relative to Lowest Part of Arch) for 500-Year Storm MHHW with Sea Level Rise Bridge Condition Available Freeboard (ft)

Projected to the Year 2050 Projected to the Year 2100 Proposed -2.2 -3.5 Existing -0.1 -1.4

Table 12. Available Freeboard (Relative to Lowest Part of Arch) for MLLW Condition Bridge Condition Available Freeboard (ft)

500-year 100-year Proposed -2.0 -1.4 Existing 0.1 0.7


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With the 16-in sea level rise (projected to the year 2050), the water surface elevation would be at 9.15 ft, and would still be below the bridge deck. With the 55-in sea level rise (projected to the year 2100), the water surface elevation would be at 10.72 ft, and would still be below the bridge deck. The modeling shows the western approach roadway as being overtopped during these flow conditions, even the lowest modeled condition with the 50-year flow draining against the MHHW.

5.9 Flow Velocities The estimated flow velocities for the existing and proposed conditions are summarized in Table 13 through Table 16 for the evaluated flow conditions. In general, the flow velocities would not change significantly between the existing and proposed conditions. Table 13. Average Channel Velocities for MHHW Condition Bridge Condition Velocity (ft/second)

500-year 100-year 50-year Proposed 4.2 3.8 3.6 Existing 4.1 3.7 3.5

Table 14. Average Channel Velocities for 100-Year Storm MHHW with Sea Level Rise Bridge Condition Velocity (ft/second)


Table 15. Average Channel Velocities for 500-Year Storm MHHW with Sea Level Rise Bridge Condition Velocity (ft/second)


Table 16. Average Channel Velocities for MLLW Condition Bridge Condition Velocity (ft/second)

500-year 100-year Proposed 4.2 3.9 Existing 4.1 3.8

The slight increases in velocity are a result of the differences in bridge geometry. The water surface elevation would drop slightly at the bridge, as indicated in Section 5.7, which would effectively reduce the flow area in the proposed condition. The change is slight but would be enough to increase the flow velocity.


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5.10 Rock Slope Protection for Erosion Protection at Slope Embankments

The average channel flow velocities for the 100-year storm event in the vicinity of the proposed bridge are approximately 3 to 5 ft/s, which are considered to be erosive. Therefore, rock slope protection (RSP) was evaluated as an erosion countermeasure to provide protection at the proposed abutment locations. RSP generally consists of rocks placed along the channel banks to limit the effects of erosion. It is the most common type of erosion countermeasure due to its general availability, ease of installation, and relatively low cost. RSP calculations were performed using the methodologies presented in the California Bank and Shore Rock Slope Protection Design (Caltrans, 2000). The detailed calculations are included in Appendix J. The 100-year flow path in the Project vicinity is roughly parallel to the proposed abutment faces, and therefore, the velocity is considered parallel to the bank. The angle of face slope was assumed to be 1.5:1 (horizontal to vertical). The average channel flow velocities for the 100-year storm event from the hydraulic analysis were used to calculate the minimum required RSP weights. The calculated RSP size for the proposed bridge was Backing No. 3 class. However, due to the large flow volume and the high velocities associated with Corte Madera Creek, the minimum recommended RSP size is ¼ ton class RSP. If the structural analysis shows that the foundations can withstand the calculated scour depths (see Section 6.7), then RSP is not required.


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6 SCOUR ANALYSIS WRECO evaluated bridge scour per the criteria described in the FHWA HEC-18, Evaluating Scour at Bridges (Fifth Edition). The minimum design criterion for bridge scour is typically the 100-year design storm. However, because the proposed bridge would be in a tidally influenced area, the hydraulic analysis should also consider the 500-year design storm. WRECO evaluated the scour potential for the proposed bridge using the results of the steady state flow analysis from HEC-RAS for the 100-year and 500-year storms. Scour analyses were performed for the proposed bridge for the following cases:

Return Period Tailwater Condition

500-year MHHW without Sea Level Rise MLLW without Sea Level Rise

MHHW with Sea Level Rise Projected to the Year 2100

100-year MHHW without Sea Level Rise MLLW without Sea Level Rise

MHHW with Sea Level Rise Projected to the Year 2100 The following scour analysis and scour countermeasure analysis was performed for the proposed bridge replacement. The calculations also considered the channel soil conditions at the Project site based on grain size distributions that were provided by Parikh Consultants, Inc., the Project’s geotechnical engineers.

6.1 Caltrans Bridge Inspection Reports Two Bridge Inspection Reports were reviewed: 1) inspection date July 23, 2003; and 2) inspection date August 08, 2001.

6.1.1 Bridge Inspection Report for Inspection Date: July 23, 2003 The bridge was built in 1958 and was seismically retrofitted and widened in 1994. On September 1, 2000, an inspection of the girders and bearings found that the bearings were at the edge of the bent cap, there were medium-sized vertical cracks on one of the columns, and there was excessive rust on the bearing plates. Spalling and other cracks were also observed. A routine bridge inspection in 2003 noted that paint was peeling and rust was present on all the steel girders. The spalls on the bent caps were patched, but deck spalling was observed at one of the pier joints. Vertical cracks were also present at every original column as well as numerous cracks in the deck surface. This condition was also noted on a previous inspection report from March 17, 1999. Previous work recommendations were also summarized: in February 2000, permit vehicles were to be restricted from using the structure until repairs were made to the spalled bent cap, and in October 1989, erosion protection was recommended at the southwesterly abutment.


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6.1.2 Bridge Inspection Report for Inspection Date: August 08, 2001 Previous bridge reports indicated that the southern abutment had slope erosion beneath the bridge. Light debris and marine growth were also reported. USACE had dredged the channel in 1987. In a previous assessment, it was noted that scour was significant enough to warrant an analysis of the bridge. The Bridge Inspection Report indicated that bridge foundations were determined to be stable for the assessed scour conditions and that the scour was within the limits of the footings or piles. The hydraulic review and stability analysis noted in this Bridge Inspection Report found that the bridge was not scour critical.

6.2 Existing Channel Bed Per the Bridge Inspection Report for the inspection of the Bon Air Road bridge on July 23, 2003, the soil and bed material consists of earth, grass, and silty sand (Caltrans 2003). Four borings were available for the Project: B-1 (located at the eastern abutment), B-2 and B-3 (located within the Corte Madera Creek channel), and B-4 (located at the western abutment). Grain size distributions for the borings were used for the scour calculations. The grain sizes for the borings nearest to each structural element were used for each structural element.

6.3 Long-Term Bed Elevation Change Channel bed elevation may fluctuate over time as a result of changes in local sediment transport capacity and availability. When more sediment is supplied by watershed erosion and upstream channel flow than can be transported locally, the channel bed aggrades. Channel degradation occurs when sediment transport capacity exceeds supply. Only channel degradation is considered for the purposes of analyzing scour. Degradation is not anticipated at the Project site. The Bridge Inspection Report from August 08, 2001 indicated that the USACE dredged the channel. This indicates that aggradation occurs in the channel. In addition, the previous hydraulic review, as noted in Section 6.1.2, indicated that the bridge is not scour critical.

6.4 Contraction Scour Contraction scour occurs when the flow area of a stream is reduced either by: 1) the natural contraction of the stream channel; 2) by a bridge structure; or 3) the overbank flow forced back to the channel by roadway embankments at the roadway approach to a bridge. From the continuity equation, a decrease in flow area results in an increase in average velocity and bed shear stress through the contraction. Hence, there is an increase in erosive forces in the contraction section, and more bed material is removed from the contracted reach than is transported into the reach. This increase in transport of bed material from the reach lowers the natural bed elevation. As the bed elevation is lowered, the flow area increases. Thus, the velocity and shear stress decrease until relative equilibrium is reached; i.e., the quantity of bed material that is transported into the reach is equal to that removed from the reach, or the bed shear stress is decreased to a value


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such that no sediment is transported out of the reach. Contraction scour, in a natural channel or at a bridge crossing, involves removal of material from the bed across all or most of the channel width (FHWA 2001). Ultimate (contraction) scour was calculated for cohesive soils. The equation for estimating ultimate scour, as presented in HEC-18, is as follows:

3/111

21

83.194.0

gny

K

gy

Vyy

cu

ults

Where:

ultsy = scour depth for cohesive soils, ft

1y = average depth in the upstream main channel, ft

2V = average flow velocity in the contracted section, ft/s g = gravitational acceleration, 32.2 ft/s2

uK = 1.486 for U.S. Customary units, and 1.0 for S.I. units

c = critical shear stress, lbs/ft2

= density of sediment, slugs/ft3 n = Manning’s roughness coefficient, unitless

The contraction scour was estimated as summarized in Table 17. The contraction scour calculations are included in Appendix K through Appendix P for the evaluated scenarios. Table 17. Contraction Scour Depths

Return Period

Tailwater Condition

Contraction Scour (ft)

500-year MHHW without SLR 4.1 MLLW without SLR 4.2

MHHW with SLR Projected to the Year 2100 3.9

100-year MHHW without SLR 3.6 MLLW without SLR 3.7

MHHW with SLR Projected to the Year 2100 3.3

6.5 Pier (Bent) Scour Pier scour is caused by vortices (known as a horseshoe vortex) forming at the base of the pier. The horseshoe vortex results from the pileup of water on the upstream surface of the pier and subsequent acceleration of the flow around the base of the pier.


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The scour depth at the pier is estimated based on pier design (shape and dimension), flow characteristics (flow rate, local flow velocity at the pier, and local flow depth at the pier), and sediment particle size distribution. Scour depths at the piers were estimated assuming the pile caps and footings will not be exposed. For piers in cohesionless materials, the HEC-18 manual recommends the Colorado State University (CSU) equation to determine pier scour. For this analysis, the live-bed equation was selected to estimate pier scour. The equation used to estimate pier scour was:

1

11

43.01

65.0

132110.2

gy

VFr

Fry

aKKKyys

Where: sy scour depth, ft

1y flow depth directly upstream of the pier, ft 1K correction factor for pier nose shape; 1.1 for square nose, 1.0 for round

nose, circular cylinder and group of cylinders, and 0.9 for sharp nose 2K correction factor for angle of attack; 1.0 when angle is 0 degrees

3K correction factor for bed condition; 1.1 for clear-water scour and small

dunes a pier width, ft 1Fr Froude Number directly upstream of the pier 1V mean velocity of flow directly upstream of the pier, ft/s g gravitational acceleration, ft/s2

For piers in cohesive materials, pier scour is more dependent on soil properties, and the HEC-18 recommends an equation presented by Briaud et. al. (2011):

7.0

165.021

6.22.2

g

VVaKKy c

s

Where: sy scour depth, ft

1K correction factor for pier nose shape; 1.1 for square nose, 1.0 for round nose, circular cylinder and group of cylinders, and 0.9 for sharp nose

2K correction factor for angle of attack; 1.0 when angle is 0 degrees a pier width, ft 1V mean velocity of flow directly upstream of the pier, ft/s

Vc = critical velocity for initiation of erosion of the cohesive material, ft/s


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g = acceleration due to gravity, ft/s2 For all piers, the local velocity of the pier was obtained from the HEC-RAS model using a velocity distribution. Each pier bent is designed to consist of two columns over CIDH piles; see Figure 12. Thus, the pier nose shape correction factor used for the calculations was 1.0 for a “group of cylinders.” The scour depths were also estimated with an angle of attack of 0 degrees because the flow appears to be aligned with the angle of the columns and pile groups; see Figure 13. The local pier scour depths are summarized in Table 18. The local pier scour calculations are included in Appendix K through Appendix P for the evaluated scenarios. The scour depths for the piers farther away from the banks are greater because the channel is deeper in those areas; thus, the water depth is greater. The foundations of the piles should be set considering that there could be fluctuations in the channel geometry due to lateral channel migration. Table 18. Local Pier Scour Depths

Return Period

Tailwater Condition

Local Scour (ft) at Pier Number 2 3 4 5

500-year

MHHW without SLR 12.9 15.3 9.3 5.4

MLLW without SLR 13.2 15.5 9.3 5.3

MHHW with SLR Projected to the

Year 2100 11.8 13.7 9.0 6.0

100-year

MHHW without SLR 10.8 14.5 9.0 4.7

MLLW without SLR 10.9 14.7 9.0 4.5

MHHW with SLR Projected to the

Year 2100 9.8 12.5 8.7 5.6


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Figure 12. Typical Section from Bridge General Plan

Source: Parsons Brinckerhoff

Figure 13. Plan View from Bridge General Plan

Source: Parsons Brinckerhoff


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6.6 Abutment Scour Abutment scour occurs when the bridge abutments and/or embankments block approaching flow. The obstruction to flow forms a horizontal vortex that starts at the upstream end of the abutment and runs along the toe of the abutment and a vertical vortex at the downstream end of the abutment. Abutment scour is commonly evaluated using either the Froehlich or HIRE live-bed scour equation. The HIRE equation is applicable when the ratio of the projected abutment length (the L parameter) to the flow depth (the y1 parameter) is greater than 25. Both the HIRE and Froehlich equations were used for the scour analysis depending on the ratio of the projected abutment length to the flow depth at each of the abutments. The Froehlich equation is given below:

1

'27.2 61.0

21 Fry

LKKyy

aas

Where:

sy scour depth, ft

1K abutment shape coefficient (from Table 8.1 of HEC-18) 2K coefficient for skew angle of abutment to flow

'L length of active flow obstructed by the embankment, ft Fr Froude number, based on the velocity and depth adjacent to and upstream

of the abutment ay average depth of flow at the abutment = Ae/L, ft

L length of embankment projected normal to the flow, ft eA flow area of the approach cross-section obstructed by the

embankment, ft2 The HIRE abutment scour equation is given below:

2133.0

1 55.04 K

KFr

y

ys

Where: sy = Scour depth, ft

1y = Depth of flow at the abutment on the overbank or in the main channel, ft

rF = Froude Number based on the velocity and depth adjacent to and upstream of the abutment

1K = Abutment shape coefficient (from Table 8.1 of HEC-18) 2K = Coefficient for skew angle of abutment to flow


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The velocities at the abutments were obtained from the HEC-RAS model. The results of the scour analysis are summarized in Table 19 and Appendix K through Appendix P for the evaluated scenarios. Table 19. Local Abutment Scour Depths

Return Period

Tailwater Condition Local Scour (ft) at Abutment Number

1 (West) 6 (East)

500-year

MHHW without SLR 5.6 9.8 MLLW without SLR 5.1 9.8

MHHW with SLR Projected to the Year 2100 8.5 15.2

100-year

MHHW without SLR 2.9 8.9 MLLW without SLR 1.8 8.9

MHHW with SLR Projected to the Year 2100 7.7 14.1

6.7 Total Scour and Scour Countermeasures Total scour is the sum of long-term bed elevation change (degradation), general (contraction) scour, and local (pier and/or abutment) scour. Table 20 through Table 25 summarize the total scour depths for each of the evaluated conditions. Table 20. Scour Depths for 100-Year Storm Draining Against MHHW Abutment/Pier

Number Location Scour Depth (ft)

Local Contraction Long-Term Total 1 West, right 2.9 3.6 4.4 10.9 2 10.8 3.6 4.4 18.8 3 14.5 3.6 4.4 22.5 4 9.0 3.6 4.4 17.0 5 4.7 3.6 4.4 12.7 6 East, left 8.9 3.6 4.4 16.9

Table 21. Scour Depths for 100-Year Storm Draining Against MLLW Abutment/Pier




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Table 22. Scour Depths for 100-Year Storm Draining Against MHHW with Sea Level Rise Projected to the Year 2100 Abutment/Pier



Table 23. Scour Depths for 500-Year Storm Draining Against MHHW Abutment/Pier



Table 24. Scour Depths for 500-Year Storm Draining Against MLLW Abutment/Pier



Table 25. Scour Depths for 500-Year Storm Draining Against MHHW with Sea Level Rise Projected to the Year 2100 Abutment/Pier




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6.7.1 Scour Countermeasures RSP generally consists of rocks on channel and structure boundaries to limit the effects of erosion. It is the most common type of scour countermeasure due to its general availability, ease of installation, and relatively low cost. RSP sizing was calculated following Caltrans’ California Bank and Shore RSP Design manual (Caltrans 2000) and checked using the FHWA’s Hydraulic Engineering Circular No. 23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance – Third Edition (HEC-23) (2009). The detailed calculations are included in Appendix Q. The median grain size of the RSP for the bridge abutments was calculated using the Ishbash relationship and Equation 14.2 from HEC-23, Design Guideline 14. The median stone diameter is a function of velocity and depth. The average channel flow velocities and flow depths for the 100-year storm event from the hydraulic analysis were used to calculate the minimum required median stone diameter of the RSP to protect the embankments in the vicinity of the bridge. The results of the HEC-23 calculations showed an RSP size of Backing No. 2 class with a median particle size of 6 in. However, due to the large flow volume and the high velocities associated with Corte Madera Creek, the minimum recommended RSP size is ¼ ton class RSP, which has a median particle diameter of 1.5 ft. The RSP layer thicknesses should be designed per the California Bank and Shore RSP Design manual (Caltrans 2000). A minimum layer thickness of 3.3 ft is recommended for ¼ ton class RSP. A backing layer and RSP fabric are also recommended. The RSP should be installed down to the naturally stable material. If the structural analysis shows that the foundations can withstand the calculated scour depths (see Section 6.7), then RSP is not required.


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7 REFERENCES California Department of Transportation. Bridge Memo to Designers.

<http://www.dot.ca.gov/hq/esc/techpubs/manual/bridgemanuals/bridge-memo-to-designer/bmd.html> (Last Accessed: 6/6/2013)

California Department of Transportation. California Bank and Shore Rock Slope Protection Design. Final Report No. FHWA-CA-TL-95-10. Caltrans Study No. F90TL03. Third Edition. October 2000.

California Department of Transportation. (2003, 2001). Bridge Inspection Report. Bridge Number: 27C0028. Facility Carried: Bon Air Rd. Location:.5 mi SW SR Francis Drk. Inspection Type: Routine.

Cayan, D., Tyree, M., Dettinger, M., Hidalgo, H., Das, T., Maurer, E., Bromirski, P., Graham, N., Flick, R. (March 2009). Climate Change Scenarios and Sea Level Rise Estimates for the California 2008 Climate Change Scenarios Assessment. Publication Number CEC-500-2009-014-D.

City of Larkspur Public Works, Engineering Division. (2008). Bridge Rehabilitation – Bon Air Bridge. <http://www.ci.larkspur.ca.us/3041-BonAirBridgeFactSheet.pdf> (October 16, 2008).

County of Marin Department of Public Works. Ross Valley Watershed – Land Use. <http://www.marinwatersheds.org/pdf_maps/Watershed_pdfs/W_RossValley_LandUse.pdf> (December 20, 2013)

Environmental Protection Agency (EPA). (2010). Hydrologic Processes: Bankfull Discharge. <http://www.epa.gov/warsss/sedsource/bankfull.htm> (June 16, 2010)

Federal Emergency Management Agency. (2009). Flood Insurance Study. Marin County, California and Incorporated Areas. Flood Insurance Study Number 06041CV001A.

Federal Highway Administration. (April 2012). Hydraulic Engineering Circular No. 18, Evaluating Scour at Bridges (Fifth Edition)

Federal Highway Administration. (September 2009). Hydraulic Engineering Circular No. 23, Bridge Scour and Stream Instability Countermeasures – Experience, Selection, and Design Guidance, Third Edition.

Federal Highway Administration. Tidal Hydrology, Hydraulics, and Scour at Bridges. Hydraulic Engineering Circular 25. <http://www.fhwa.dot.gov/engineering/hydraulics/hydrology/hec25c5.cfm> (October 16, 2008)

Federal Highway Administration. (1994). Federal-Aid Policy Guide. Title 23, Code of Federal Regulations, Part 650, Subpart A (23 CFR 650A) “Location and Hydraulic Design of Encroachment on Flood Plains.” December 7, 1994, Transmittal 12. <http://www.fhwa.dot.gov/legsregs/directives/fapg/cfr0650a.htm>


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Google. (2010). Google Earth. (Last accessed June 15, 2010)

PRISM Group, Oregon State University. (2008). Annual Precipitation Map: 1971-2000. <http://www.ocs.orst.edu/prism> (October 16, 2008)

Stetson Engineers. (2007). Appraisal-Level Hydraulic Engineering Study, Corte Madera Creek. Prepared for County of Marin. December 2007.

United States Army Corps of Engineers Hydrologic Engineering Center. HEC-RAS River Analysis System Manual. Version 3.1.3 May 2005.

United States Army Corps of Engineers - Hydrologic Engineering Center. (2010). River Analysis System. HEC-RAS. (Version 4.1.0) [Computer software]. January 2010. Available from: http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html.

United States Geological Survey. (2001). California: Seamless USGS Topographic Maps (CDROM, Version 2.6.8, 2001, Part Number: 113-100-004). National Geographic Holdings, Inc

Waananen, A.O. and Crippen, J.R. (1977). Magnitude and Frequency of Floods in California. United States Geological Survey Water-Resources Investigations 77-21. June 1977.


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Appendix A HEC-RAS: 500-Year Mean Higher High Water without Sea Level Rise


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Appendix B HEC-RAS: 500-Year Mean Higher High Water with Sea Level Rise Projected to the Year 2050


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Appendix C HEC-RAS: 500-Year Mean Higher High Water with Sea Level Rise Projected to the Year 2100


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Appendix D HEC-RAS: 500-Year Mean Lower Low Water without Sea Level Rise


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Appendix E HEC-RAS: 100-Year Mean Higher High Water without Sea Level Rise


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Appendix F HEC-RAS: 100-Year Mean Higher High Water with Sea Level Rise Projected to the Year 2050


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Appendix G HEC-RAS: 100-Year Mean Higher High Water with Sea Level Rise Projected to the Year 2100


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Appendix H HEC-RAS: 100-Year Mean Lower Low Water without Sea Level Rise


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Appendix I HEC-RAS: 50-Year Mean Higher High Water without Sea Level Rise


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Appendix J Rock Slope Protection for Erosion Protection at Slope Embankments


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Bon Air Road Bridge Rehabilitation Project

City of Larkspur, Marin County, CaliforniaRock Slope Protection Calculations for Banks

Calculation guideline from California Bank and Shore Rock Slope Protection Des

Input from HEC‐RAS for Proposed Bridge

100‐year Flow MHHW

Location Upstream Upstream Face Downstream Face DownstreamStorm Event 100-year 100-year 100-year 100-yearVM (ft/s) 3.8 4.5 4.5 3.7Flow Condition Impinging Impinging Impinging ImpingingSG 2.7 2.7 2.7 2.7r (degrees) 70 70 70 70a (degrees) 34 34 34 34

Impinging Flow ConditionV (ft/s) 5.0 5.9 6.0 4.9W (lb) 0.9 2.4 2.5 0.8RSP Class Backing No. 3 Backing No. 3 Backing No. 3 Backing No. 3

)()1(

00002.033

6

arSINSG

SGVW


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100‐year Flow MHHW with Sea Level Rise Projected to 2050



)()1(

00002.033

6

arSINSG

SGVW


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)()1(

00002.033

6

arSINSG

SGVW


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100‐year Flow MLLW



)()1(

00002.033

6

arSINSG

SGVW


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Appendix K Scour Calculations: 500-Year Mean Higher High Water without Sea Level Rise


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City of Larkspur Marin County, CaliforniaUltimate (Contraction) Scour


Calculation guideline from HEC‐18 5th Edition

Input from HEC‐RAS for Proposed, P19, Q50, 100, 500 w/MHHW to MLLW

Page 6.15, Page 151 / 340 , Section 6.7 Contraction Scour in Cohesive Materials (6.7.1 Ultimate Scour)

y1 8.2 ft 2.5 m Upstream depth

V2 4.9 ft/s 1.5 m/s Average velocity in contracted section

tc Critical shear stress

n 0.035 0.035 Manning's n

Ku 1.486 1 1.486 fot U.S. Customary, and 1.0 for S.I.

r slugs/ft3 Density 1000 kg/m3 = 1.94 slugs/ft3

g 32.2 ft/s^2 9.81 m/s^2

D50 0.075 mm

Density, rhohttp://www.mo10.nrcs.usda.gov/references/guides/properties/moistbulkdensity.html

Material Density

Clay 1250 kg/m3 2.43 slugs/ft3

Fine sand 1700 kg/m3 3.30 slugs/ft3

Water, sea 1026 kg/m3 1.99 slugs/ft3

Water, pure 1000 kg/m3 1.94 slugs/ft3

Critical Shear Stress Tc Tc (N/m2) Tc (lb/ft2)

Tc=0.05(D50)^‐0.4 0.1

Tc=0.006(D50)^‐2 1.1

Clay ,Tc=0.05(D50)^‐0.4 Fine sand ,Tc=0.05(D50)^‐0.4

1.25 m 4.1 ft ys 1.26 m 4.1 ft ys


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesive500‐year Flow MHHW



Page 7.38, Page 204 / 340 , Section 7.12 Pier Scour In Cohesive Materials

ys = 2.2 K1 K2 a^0.65 [ (2.6V1 ‐ Vc) / sqrt (g) ] ^0.7

Bent 3 Bent 2

L 16 16 ft Pier length

a 8 8 ft Pier width

L/a 2 2 If L/a is larger than 12, then use 12 as a maximum

Ө 0 0 degrees Angle of attack of flow

Group of cylinders Group of cylinders Pier shape

K1 1 1 Correction factor for pier shape

K2 1.0 1.0 Correction factor for angle of attack

V1 5.2 4.1 ft/s Approach velocity

Vc 0.1 0.1 m/s From Figure 4.7:

Vc 0.3 0.3 ft/s using an erosion rate of 0.1 mm/hr

g 32.2 32.2 ft/s^2

ys 15.3 12.9 ft


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesionless




Units = (SI or English) = English English

Complex Pier Scour? (Yes or No) No No

Pier Scour component (Facing downstream, from left to right) Bent 5 Bent 4

Water Surface Elevation 9.5 9.5 ft

f = distance between front edge of cap/footing and pier = (enter n/a n/a ft

T = thickness of pile cap or footing (enter n/a for no pile cap) = n/a n/a ft

Top of Pile Cap Elevation (enter n/a for no pile cap) n/a n/a ft

Bottom of Pile Cap Elevation (enter n/a for no pile cap) n/a n/a ft

Ground Elevation at Pier 6.6 0.9 ft

Contraction Scour Depth ft

ho = height of pile cap above bed at beginning of computation

(negative indicates partially or entirely submerged pile cap) = n/a n/a ft

a = pier width = 8.0 8.0 ft

S = spacing between columns of piles, center to center = n/a n/a ft

h1 = ho+T=height of the pier stem above the bed before scour

(negative indicates top of pile cap is below bed elevation) = n/a n/a ft

y1 = Approach flow depth at the beginning of computations = 2.9 8.6 ft

V1 = Approach velocity used at the beginning of computations = 1.4 3.6 ft/s

Khpier = coefficient to account for the height of the pier stem above

the bed and the shielding effect by the pile cap overhang distance

"f" in front of the pier stem 1.00 1.00

Ө = angle of attack of flow = 0 0 degrees

Pier shape Group of cylinders Group of cyl inders

K1 = correction factor for pier nose shape = 1.0 1.0

L = length of pier = 16.0 16.0 ft

L/a (if L/a is larger than 12, then use 12 as a maximum) 2 2

K2 = correction factor for angle of attack = (cosӨ+(L/a)*sinӨ)^0.65 1.0 1.0

K3 = correction factor for bed condition = 1.1 1.1

Soil Sample Avg B‐2, B‐1 B‐2

D50 = grain size for which 50% of bed material is finer = 0.0042 0.0082 ft


Ku = constant = 6.19 (SI) or 11.17 (English) 11.17 11.17

VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 2.2 3.2 ft/s

VcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 3.9 5.9 ft/s

g = acceleration due to gravity = 32.2 32.2 ft/s^2

Yspier = scour component for the pier stem in the flow =

y1*(Khpier*(2*K1*K2*K3*((a/y1)^0.65)*((V1/((g*y1)^0.5))^0.43)) = 5.4 9.3 ft


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Abutments ‐ Froehlich or HIRE




Units = (SI or English) English

g = acceleration due to gravity = 32.2 ft/s^2

Left Overbank = Abutment #6 (East)

y1 = depth of flow at abutment on the overbank or in the main

channel = 2.7 ft

L = length of embankment projected normal to flow = 46.2 ft

Ratio of projected embankment length to flow depth = L/y1 = 1.712E+01

Abutment scour equation to be used = Froehlich

Froehlich's Live Bed Abutment Scour Equation

L' = length of active flow obstructed by the embankment = 26.3 ft

Ae = flow area of the approach cross section obstructed by the

embankment = 204.1 ft^2

ya = average depth of flow on the flood plain = Ae/L' 4.42 ft

Qe = flow obstructed by the abutment and approach

embankment = 663 ft^3/s

Ve = flow velocity = Qe/Ae = 3.2 ft/s

Fr = Froude Number of approach flow upstream of the abutment 0.27

Ө = abutment skew = 90 degrees

K1 = coefficient for abutment shape = 1

K2 = coefficient for angle of embankment shape = (Ө/90)^0.13 = 1.0

Ys = abutment scour =

ya*(2.27*k1*k2*((L'/ya)^0.43)*(Fr^0.61)+1) = 9.8 ft

Right Overbank = Abutment #1 (West)


channel = 0.9 ft


Ratio of projected embankment length to flow depth = 7.376E+01

Abutment scour equation to be used = HIRE

HIRE Live Bed Abutment Scour Equation

V = velocity of flow at upstream face of abutment = 3.0 ft/s

Fr = Froude Number = V/((g*y1)^.5) = 0.6



K2 = coefficient for angle of embankment shape = (Ө/90)^0.13 = 1

Ys = abutment scour = y1*(4*(Fr^0.33)*(K1/0.55)*K2) = 5.6 ft


January 2014

Appendix L Scour Calculations: 500-Year Mean Lower Low Water without Sea Level Rise


January 2014













g 32.2 ft/s^2 9.81 m/s^2

D50 0.075 mm


Material Density






Tc=0.05(D50)^‐0.4 0.1

Tc=0.006(D50)^‐2 1.1


1.26 m 4.1 ft ys 1.27 m 4.2 ft ys


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesive500‐year Flow MLLW




ys = 2.2 K1 K2 a^0.65 [ (2.6V1 ‐ Vc) / sqrt (g) ] ^0.7

Bent 3 Bent 2


a 8 8 ft Pier width









g 32.2 32.2 ft/s^2

ys 15.5 13.2 ft


January 2014






































VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 2.1 3.2 ft/s

VcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 3.9 5.8 ft/s





January 2014










channel = 2.6 ft

















ya*(2.27*k1*k2*((L'/ya)^0.43)*(Fr^0.61)+1) = 9.8 ft



channel = 0.8 ft












January 2014

Appendix M Scour Calculations: 500-Year Mean Higher High Water with Sea Level Rise Projected to the Year 2100


January 2014













g 32.2 ft/s^2 9.81 m/s^2

D50 0.075 mm


Material Density






Tc=0.05(D50)^‐0.4 0.1

Tc=0.006(D50)^‐2 1.1


1.18 m 3.9 ft ys 1.18 m 3.9 ft ys


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesive500‐year Flow MHHW with Sea Level Rise Projected to 2100




ys = 2.2 K1 K2 a^0.65 [ (2.6V1 ‐ Vc) / sqrt (g) ] ^0.7

Bent 3 Bent 2


a 8 8 ft Pier width









g 32.2 32.2 ft/s^2

ys 13.7 11.8 ft


January 2014






































VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 2.3 3.3 ft/s

VcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 4.1 6.0 ft/s





January 2014










channel = 4.1 ft













channel = 2.4 ft












January 2014

Appendix N Scour Calculations: 100-Year Mean Higher High Water without Sea Level Rise


January 2014













g 32.2 ft/s^2 9.81 m/s^2

D50 0.075 mm


Material Density






Tc=0.05(D50)^‐0.4 0.1

Tc=0.006(D50)^‐2 1.1


1.10 m 3.6 ft ys 1.11 m 3.6 ft ys


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesive100‐year Flow MHHW




ys = 2.2 K1 K2 a^0.65 [ (2.6V1 ‐ Vc) / sqrt (g) ] ^0.7

Bent 3 Bent 2


a 8 8 ft Pier width









g 32.2 32.2 ft/s^2

ys 14.5 10.8 ft


January 2014






































VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 2.1 3.2 ft/s

VcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 3.8 5.8 ft/s





January 2014










channel = 2.2 ft

















ya*(2.27*k1*k2*((L'/ya)^0.43)*(Fr^0.61)+1) = 8.9 ft



channel = 0.5 ft












January 2014

Appendix O Scour Calculations: 100-Year Mean Lower Low Water without Sea Level Rise


January 2014













g 32.2 ft/s^2 9.81 m/s^2

D50 0.075 mm


Material Density






Tc=0.05(D50)^‐0.4 0.1

Tc=0.006(D50)^‐2 1.1


1.12 m 3.7 ft ys 1.13 m 3.7 ft ys


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesive100‐year Flow MLLW




ys = 2.2 K1 K2 a^0.65 [ (2.6V1 ‐ Vc) / sqrt (g) ] ^0.7

Bent 3 Bent 2


a 8 8 ft Pier width









g 32.2 32.2 ft/s^2

ys 14.7 10.9 ft


January 2014






































VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 2.1 3.2 ft/s

VcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 3.7 5.8 ft/s





January 2014










channel = 2.0 ft

















ya*(2.27*k1*k2*((L'/ya)^0.43)*(Fr^0.61)+1) = 8.9 ft



channel = 0.3 ft












January 2014

Appendix P Scour Calculations: 100-Year Mean Higher High Water with Sea Level Rise Projected to the Year 2100


January 2014













g 32.2 ft/s^2 9.81 m/s^2

D50 0.075 mm


Material Density






Tc=0.05(D50)^‐0.4 0.1

Tc=0.006(D50)^‐2 1.1


1.00 m 3.3 ft ys 1.00 m 3.3 ft ys


January 2014


City of Larkspur Marin County, CaliforniaLocal Scour at Piers ‐ Cohesive100‐year Flow MHHW with Sea Level Rise Projected to 2100




ys = 2.2 K1 K2 a^0.65 [ (2.6V1 ‐ Vc) / sqrt (g) ] ^0.7

Bent 3 Bent 2


a 8 8 ft Pier width









g 32.2 32.2 ft/s^2

ys 12.5 9.8 ft


January 2014






































VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 2.3 3.3 ft/s

VcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 4.1 6.0 ft/s





January 2014










channel = 4.0 ft













channel = 2.2 ft












January 2014

Appendix Q Rock Slope Protection for Scour Countermeasures


January 2014


City of Larkspur, Marin County, CaliforniaRock Slope Protection Calculations for Abutments

Calculation guideline from HEC‐23 3rd Edition



Location Upstream Upstream Face Downstream Face DownstreamV 3.8 4.5 4.5 3.7 ft/s

g 32.2 32.2 32.2 32.2 ft/s2

y 7.7 8.7 8.6 8.4 ftFr 0.24 0.27 0.27 0.22

Isbash Isbash Isbash Isbash from HEC-23

For Froude Numbers (V/(gy)1/2

)<=0.80, Isbash relationshipy 7.7 8.7 8.6 8.4 depth of flow in the contracted bridge opening, ftK 1.0 1.0 1.0 1.0 (1.02 for vertical wall abutment, 0.89 or for spill-through abutment)

Ss 2.7 2.7 2.7 2.7 specific gravity of rockV 3.8 4.5 4.5 3.7 average velocity in contracted section, ft/s

g 32.2 32.2 32.2 32.2 gravitational acceleration, ft/s2

D50 0.3 0.4 0.4 0.3 median stone diameter, ft

D50 3.3 4.6 4.6 3.1 median stone diameter, inches

Backing No. 2 Backing No. 2 Backing No. 2 Backing No. 2 rock class


January 2014







g 32.2 32.2 32.2 32.2 ft/s2

y 8.9 8.4 8.4 6.7 ftFr 0.26 0.22 0.22 0.16










January 2014







g 32.2 32.2 32.2 32.2 ft/s2

y 8.5 14.2 14.1 8.4 ftFr 0.19 0.18 0.16 0.17










January 2014







g 32.2 32.2 32.2 32.2 ft/s2

y 7.5 8.2 8.1 8.3 ftFr 0.25 0.28 0.29 0.23









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Bridge Design Hydraulic Study Report - Public works … Inf… · · 2017-12-12January 2014 Bon Air Road Bridge Replacement Project City of Larkspur, Marin County, California Bridge