bridge design hydraulic study report

October 2016

Bridge Street Bridge Rehabilitation or Replacement Project over Arroyo Grande Creek City of Arroyo Grande, San Luis Obispo County, California Federal Aid Project No. BRLO-5199 (027) Existing Bridge No. 49C-0196 Bridge Design Hydraulic Study Report

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Bridge Design Hydraulic Study Report Federal Aid Project No. BRLO-5199 (027) Bridge Street Bridge Rehabilitation or Replacement Project over Arroyo Grande Creek City of Arroyo Grande, San Luis Obispo County, California Existing Bridge No. 49C-0196

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Table of Contents Executive Summary ........................................................................................................... iv Acronyms .......................................................................................................................... vii 1 General Description ............................................................................................ 1

1.1 Project Description.............................................................................................. 1 1.2 Key Tasks............................................................................................................ 3 1.3 Design Criteria .................................................................................................... 3

1.3.1 Hydrologic Design Criteria ............................................................................... 3 1.3.2 Hydraulic Design Criteria .................................................................................. 3 1.3.3 Scour Design Criteria ........................................................................................ 4

1.4 Vertical Datum .................................................................................................... 4 2 Description of Watershed ................................................................................... 5

2.1 Geographic Location ........................................................................................... 5 2.2 Watershed Size.................................................................................................... 5 2.3 Land Use ............................................................................................................. 6

3 Description of Stream and Site ........................................................................... 7 3.1 Channel Properties .............................................................................................. 7 3.2 Existing Bridge ................................................................................................... 8 3.3 Proposed Bridge .................................................................................................. 9

4 Hydrology ......................................................................................................... 12 4.1 Federal Emergency Management Agency ........................................................ 12 4.2 United States Geological Survey Study ............................................................ 12 4.3 Log-Pearson Type III Statistical Analysis ........................................................ 13 4.4 USGS Regional Flood-Frequency Equations ................................................... 14

4.5 Peak Discharges: Summary and Selection for Hydraulic Analysis .................. 14 4.6 Hydrologic Stability .......................................................................................... 15

5 Hydraulic Analysis ........................................................................................... 16 5.1 Design Tools ..................................................................................................... 16 5.2 Cross Section Data ............................................................................................ 16 5.3 Model Boundary Condition .............................................................................. 17 5.4 Manning’s Roughness Coefficients .................................................................. 18 5.5 Expansion and Contraction Coefficients .......................................................... 18 5.6 Water Surface Elevations .................................................................................. 18

5.7 Freeboard .......................................................................................................... 23 5.8 Flow Velocities ................................................................................................. 24

5.9 Rock Slope Protection for Erosion Protection at Slope Embankments ............ 25 6 Scour Analysis .................................................................................................. 26

6.1 Caltrans Bridge Inspection Reports .................................................................. 26 6.2 Existing Channel Bed ....................................................................................... 26 6.3 Long-Term Bed Elevation Change ................................................................... 26 6.4 Contraction Scour ............................................................................................. 27 6.5 Pier Scour .......................................................................................................... 28

6.6 Abutment Scour ................................................................................................ 29 6.7 Total Scour and Scour Countermeasures .......................................................... 30

6.7.1 Total Scour Depths .......................................................................................... 30


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6.7.2 Scour Countermeasures ................................................................................... 31 6.7.3 Federal Highway Administration RSP Design ................................................ 31 6.7.4 California Bank and Shore RSP Design .......................................................... 32 6.7.5 Selected Class of RSP and Layer Thickness ................................................... 32

7 References ......................................................................................................... 33

Figures Figure 1. Project Location Map .......................................................................................... 1 Figure 2. Project Vicinity Map ........................................................................................... 2 Figure 3. Project Aerial Map .............................................................................................. 2

Figure 4. Project Watershed Map ....................................................................................... 5 Figure 5. Project Land Use Map ......................................................................................... 6 Figure 6. Bridge General Plan for Alternative A (replacement option) ........................... 10 Figure 7. Bridge General Plan for Alternative B (rehabilitation option) .......................... 11 Figure 8. Annual Peak Flows Recorded at Upstream Gaging Station .............................. 13 Figure 9. Location of Lopez Lake and Dam Relative to Project Site ............................... 15 Figure 10. Stream Survey Cross Section Locations.......................................................... 16 Figure 11. Arroyo Grande Creek Stream Profile .............................................................. 17 Figure 12. 100-Year Water Surface Comparison ............................................................. 20 Figure 13. 50-Year Water Surface Comparison ............................................................... 21 Figure 14. Upstream Face of Existing Bridge, Looking Downstream (Southwest) ......... 22 Figure 15. Upstream Face of Alternative A Bridge, Looking Downstream (Southwest) . 22 Figure 16. Upstream Face of Alternative B Bridge, Looking Downstream (Southwest) . 22

Figure 17. Arroyo Grande Creek Stream Measurements at Upstream Face of Bridge Street Bridge ..................................................................................................................... 27

Tables Table 1. FEMA FIS Peak Discharges for Arroyo Grande Creek ..................................... 12 Table 2. Peak Discharges Summary for Arroyo Grande Creek ........................................ 15 Table 3. 100-Year Water Surface Elevations Summary Table ......................................... 18

Table 4. 50-Year Water Surface Elevations Summary Table ........................................... 19 Table 5. Available Freeboard for the 100-year Storm Event ............................................ 23 Table 6. Available Freeboard for the 50-year Storm Event .............................................. 23

Table 7. 100-year Velocities in Vicinity of Project .......................................................... 24 Table 8. 50-year Velocities in Vicinity of Project ............................................................ 24 Table 9. Local Pier Scour Depths for Bridge Street Alternative B Bridge ....................... 29 Table 10. Summary of Scour depths for Alternative A .................................................... 30

Table 11. Summary of Scour Depths for Alternative B .................................................... 30 Table 12. Scour Data Table for Alternative A .................................................................. 30 Table 13. Scour Data Table for Alternative B .................................................................. 30 Table 14. RSP Layers ....................................................................................................... 32


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Photos Photo 1. Channel Downstream (Southwest) of Bridge Facing Downstream...................... 7 Photo 2. Channel Upstream (Northeast) of Bridge Facing Upstream ................................ 8 Photo 3. Existing Bridge ..................................................................................................... 9

Appendices Appendix A HEC-RAS Results for Existing Bridge Appendix B HEC-RAS Results for Alternative A Bridge Appendix C HEC-RAS Results for Alternative B Bridge

Appendix D Scour Calculations Appendix E Rock Slope Protection Calculations Appendix F Grading Plans for Alternatives


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Executive Summary The Bridge Street Bridge Rehabilitation or Replacement Project over Arroyo Grande Creek (Project) is located in the City of Arroyo Grande in the southwestern part of San Luis Obispo County, California. The Project proposes to retrofit or replace the existing bridge. The new bridge was designed by Quincy Engineering, Inc. The purpose of this report is to summarize the hydrology, compare the hydraulics at the existing bridge and proposed bridge alternatives, present the estimated scour depths at the proposed alternatives, and provide recommendations for scour countermeasures. The hydrology adopted for this study was estimated using United States Geological Survey (USGS) data from a nearby gaging station, which is located approximately 0.6 miles (mi) upstream (northeast) of the Project site. A statistical analysis using the Log-Pearson Type III distribution to fit the USGS data was performed to estimate the 100- and 50-year peak discharges at the Project site. The peak discharges at the gaging station were adjusted to obtain the peak discharges at the Project site by applying an adjustment factor based on the watershed areas at the respective locations. The resulting peak discharges at the Project site are listed in the table below: Arroyo Grande Creek Hydrology at Bridge Street Bridge

Return Period Design Discharge (cfs*) 100-year 12,900 50-year 9,030

Note: *cfs = cubic ft per second There are currently two alternatives. Alternative A is a conventional replacement, and Alternative B involves rehabilitating the existing truss to preserve the historic bridge resource. Alternative B is likely the preferred alternative, yet both are included in this study. Separate hydraulic models were generated for each Alternative. The Alternative A clear-span replacement bridge was modeled to be 142.5 feet long by 46 ft wide. The Alternative B bridge was modeled to be 141 feet between abutments, with a 5-ft diameter pier located 104 ft from the northern abutment, and to be 39.5 feet wide. The water surface elevations in the vicinity of the Project are reduced in the proposed conditions upstream of the bridge for both alternatives. The existing bridge does not meet freeboard requirements; both alternatives would meet freeboard requirements per the California Department of Transportation (Caltrans) and Federal Highway Administration’s (FHWA) hydraulic freeboard criteria. The water surface elevations and available freeboard for the existing bridge and proposed bridge alternatives are summarized in the following table. The available freeboard for both alternatives are higher than the existing condition. The existing bridge has one pier, while the Alternative A bridge has no piers, and the Alternative B bridge has one 5-foot diameter pier. The bridge soffit for both alternatives are higher than the soffit for the


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existing bridge. Note that the soffit elevation of the existing bridge and Alternative B is based on the lower chord of the supplemental truss. The soffit elevation of alternative A is much higher than the existing bridge since it does not have a supplemental truss. The soffit elevation of Alternative B is also higher than the existing bridge since the new proposed supplemental truss is shallower than the existing supplemental truss. Existing and Proposed Conditions Hydraulic Summary Return Period

Bridge Condition

Bridge Soffit Elevation1

(ft NGVD 29) 2

Water Surface Elevation (ft NGVD 29)

Freeboard at

Upstream Face of Bridge

(ft)

At Common Upstream Location3

At Upstream

Face of Bridge

100-year Existing 98.5 99.5 98.8 -0.3 Alternative A 104.3 98.9 98.8 5.5 Alternative B 101.3 98.4 98.4 2.9

50-year Existing 98.5 96.5 96.0 2.5 Alternative A 104.3 96.1 96.0 8.3 Alternative B 101.3 95.7 95.7 5.6

Notes: 1 The soffit elevation for the existing bridge presented is based on the elevation of the lower brown truss. 2 NGVD 29 = National Geodetic Vertical Datum of 1929 3 The common upstream location is at River Station 1037, which is located 18 ft from the upstream face of the existing bridge, 7 ft from the upstream face of the Alternative A bridge, and 10 ft from the upstream face of the Alternative B bridge. Preliminary scour calculations were performed based on the FHWA’s Hydraulic Engineering Circular No. 18, Evaluating Scour at Bridges (HEC-18). The hydraulic characteristics for the 100-year storm event from the hydraulic analysis and an assumed median grain size diameter of 0.075 mm were used to calculate the potential scour depths for the proposed alternatives. The scour calculations were performed assuming that the channel bed material is erodible. Although clays are present in the soils at this location, it is assumed that cohesive forces do not dominate the erosive process, and therefore cohesionless equations were used to estimate scour. The channel thalweg appears to be relatively stable, based on the two available stream measurements from Caltrans’ Bridge Inspection Reports from the years 1993 and 1999, and compared with the 2012 survey. The total scour depths are summarized in the two tables below for Alternative A and Alternative B, respectively. Summary of Scour depths for Alternative A

Bridge Element Scour Depth (ft) Long-Term Contraction Local Total

Abutment 2 (south) 0.0 0.0 0.0 0.0 Abutment 1 (north) 0.0 0.0 0.0 0.0


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Summary of Scour depths for Alternative B Bridge Element Scour Depth (ft)

Long-Term Contraction Local Total Abutment 3 (south) 0.0 4.1 0.0 4.1

Pier 2 0.0 4.1 11.7 15.8 Abutment 1 (north) 0.0 4.1 0.0 4.1

Rock slope protection (RSP) will be incorporated into the design of the bridge foundation elements, so thalweg migration should be arrested by the RSP, and local scour can be ignored at the abutments for both alternatives. Alternative B should be designed to provide structural support assuming no lateral earth support down to the ground elevation at the pier minus the sum of long-term degradation, contraction scour, and local scour. The minimum elevations for the footing or pile cap are summarized in the tables below. Finished grades were estimated from Figure 6 and Figure 7. Scour Data Table for Alternative A

Bridge Element

Long-Term (Degradation and Contraction) Scour Elevations

(feet)

Short-Term (Local) Scour Depths

(feet) Abutment 2 (south) 102.3 0.0 Abutment 1 (north) 103.6 0.0

Scour Data Table for Alternative B

Bridge Element


(feet)


(feet) Abutment 3 (south) 99.4 0.0

Pier 2 78.9 11.7 Abutment 1 (north) 97.2 0.0

RSP calculations were performed using methodologies published by the FHWA and Caltrans. Based on the RSP calculations, as well as engineering judgment, 1/2 ton class RSP is recommended for both alternatives adjacent to the abutments and at the pier in Alternative B. Per the California Bank and Shore RSP Design, 1/2 ton RSP should also include a backing layer (backing class No. 1) and RSP fabric type B. The RSP fabric should be placed on the bank as the initial filter separator material beneath the backing layer of RSP. The minimum recommended RSP layer thicknesses per the California Bank and Shore RSP Design are presented in the following table. The slope protection should extend from the face of the abutment to the toe of slope. RSP Layers for Both Alternatives Outside Layer Backing Layer Placement Type

Class/Type 1/2 Ton Backing Class No. 1 B Minimum Layer

Thickness 4.3 ft 1.8 ft B


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Acronyms BIR Bridge Inspection Reports Caltrans California Department of Transportation City City of Arroyo Grande D50 median grain size FEMA Federal Emergency Management Agency FHWA Federal Highway Administration FIS Flood Insurance Study ft feet HEC-18 Hydraulic Engineering Circular No. 18 HEC-23 Hydraulic Engineering Circular No. 23 HEC-RAS Hydrologic Engineering Centers River Analysis System mi miles NGVD 29 National Geodetic Vertical Datum of 1929 Project Bridge Street Bridge Rehabilitation Project over Arroyo Grande Creek RSP rock slope protection U.S. 101 United States Highway 101 USACE U.S. Army Corps of Engineers USGS United States Geological Survey


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1 GENERAL DESCRIPTION

1.1 Project Description The City of Arroyo Grande (City) proposes to replace or rehabilitate the existing Arroyo Grande Creek bridge (Bridge No. 49C-0196) at Bridge Street (see Figure 1 for the Project Location Map). Bridge Street follows a north-south corridor approximately 0.2 miles (mi) east of United States Highway 101 (U.S. 101) in the City of Arroyo Grande (see Figure 2 for the Project Vicinity Map and Figure 3 for the Project Aerial Map). Arroyo Grande Creek bridge provides vehicular access over Arroyo Grande Creek, which runs through the City approximately parallel to State Route 227. The surrounding land is generally level and is composed primarily of an urbanized business district.

Figure 1. Project Location Map

Source: United States Geological Survey (USGS) The existing bridge is structurally deficient and was posted for only 3 tons in 1985. This is the lowest posting allowed before bridge closure is required. The Bridge Street Bridge Rehabilitation or Replacement Project over Arroyo Grande Creek (Project) is needed to provide a new or rehabilitated bridge that will increase the load carrying capacity and also improve public safety. If the bridge is not rehabilitated or replaced, the condition will continue to deteriorate and eventually, bridge closure will be required.

PROJECT LOCATION


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Figure 2. Project Vicinity Map

Source: USGS

Figure 3. Project Aerial Map

Source: Google Earth

PROJECT LOCATION

PROJECT LOCATION

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The proposed Project will rehabilitate or replace the bridge on the existing alignment. As a result, road closure is proposed during the construction phase. Depending on the alternative selected, road closure may be required for up to 6 months. There are currently two alternatives. Alternative A is a conventional replacement, and Alternative B involves rehabilitating the existing truss to preserve the historic bridge resource. Alternative B is likely the preferred alternative, yet both are included in this study.

1.2 Key Tasks Key tasks performed in this study included: 1) a review of available hydrologic data, 2) a hydrologic study, 3) a hydraulic analysis to determine design water surface elevations and flow velocities for the existing and proposed Bridge Street bridge over Arroyo Grande Creek, 4) a scour analysis to estimate potential scour depths, and 5) scour countermeasure analyses and recommendations.

1.3 Design Criteria

1.3.1 Hydrologic Design Criteria At least two hydrologic analysis methods were required for the Project per the Local Assistance Program Guidelines from Caltrans. WRECO used:

1. Published design discharges from the Federal Emergency Management Agency’s (FEMA) Flood Insurance Study (FIS) for San Luis Obispo County, California and Incorporated Areas,

2. Published design discharges from the Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006,

3. Statistical distribution analysis of stream flow data from a nearby gaging station, and

4. USGS Regional Regression calculations.

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 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.


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1.3.3 Scour Design Criteria The evaluation of potential scour at the proposed bridge alternatives will follow the criteria described in the FHWA’s Hydraulic Engineering Circular No. 18 (HEC-18), Evaluating Scour at Bridges (Fifth Edition). The evaluation of potential scour shall be based on the 100-year design discharge hydraulic characteristics. 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. The long-term bed elevation change was based on an assumed 75-year design life for a new replacement bridge.

1.4 Vertical Datum The Project references the National Geodetic Vertical Datum of 1929 (NGVD 29).


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2 DESCRIPTION OF WATERSHED

2.1 Geographic Location The Project site is located downstream of Huasna Road and 0.2 mi upstream (northeast) of U.S. 101 within the City of Arroyo Grande. Although the Project site is within an urban area, the upper reaches of the watershed are within a rural area. Arroyo Grande Creek begins in the Santa Lucia Range. The watershed is entirely within the southwestern portion of San Luis Obispo County. The Project site is located approximately 4 mi upstream of Arroyo Grande Creek’s outfall at Pismo State Beach.

2.2 Watershed Size The watershed that drains to the Project site is approximately 106 square mi, and it is shown in Figure 4.

Figure 4. Project Watershed Map

Source: USGS

PROJECT LOCATION


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2.3 Land Use The land use in the immediate Project vicinity is urban within the City of Arroyo Grande (see Figure 5). The land use designations for the watershed that drains to the Project site are a combination of: open space, rural lands, recreation, public facility, residential rural, and residential suburban. The upper reaches of the watershed are predominantly rural and open space, while the lower reaches of the watershed are predominantly urban.

Figure 5. Project Land Use Map

Source: San Luis Obispo County, Google Earth, and Environmental Protection Agency

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NO SCALE

Watershed boundary


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3 DESCRIPTION OF STREAM AND SITE

3.1 Channel Properties Although the area surrounding the Project site is predominantly developed, the channel within the Project vicinity is heavily vegetated with trees and vines. At the time of WRECO’s field visit in August 2012, there was water in the creek. The vegetation and water in the creek can be seen in Photo 1 and Photo 2.

Photo 1. Channel Downstream (Southwest) of Bridge Facing Downstream


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Photo 2. Channel Upstream (Northeast) of Bridge Facing Upstream

3.2 Existing Bridge The existing bridge is a hybrid structure composed of an original steel, Pratt pony through truss that is now supported by a supplemental steel deck truss (see Photo 3). The structure also has a single southern approach span consisting of a reinforced concrete deck supported by steel stringers. The main truss span is approximately 100 ft long, and the approach span is approximately 24 ft long. Two seat type abutments and a single pier support the northern end of the approach span and southern end of the main truss. The pier consists of one approximately 4-ft diameter reinforced concrete pile extension under each truss bearing. The original 24 ft wide pony truss was built in 1908 and carries two traffic lanes with pedestrian sidewalks cantilevered on both sides outside of the truss members. An unknown time later, a large flow event in Arroyo Grande Creek washed out the southern approach embankment. The existing southern abutment was left in the channel and now serves as the intermediate pier. A new steel stringer approach span and concrete abutment were constructed on the newly formed southern embankment. The sidewalks are composed of timber oriented transversely and supported on steel brackets attached to the truss and approach span stringers.


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Photo 3. Existing Bridge

3.3 Proposed Bridge Alternatives Section 1.1 describes the retrofit and replacement alternatives considered for the Project. One hydraulic model was generated for each alternative. The Alternative A clear-span replacement bridge was modeled to be 142.5 ft long by 46 ft wide. The Alternative B bridge was modeled to be 141 ft between abutments, with a 5-ft diameter pier located 104 ft from the northern abutment, and to be 39.5 ft wide. The general plans for these two alternatives are shown in Figure 6 and Figure 7. Each alternative also includes its own grading plan that widens and realigns the stream channel. Please refer to Appendix F for these plans.

Bridge Design Hydraulic Study Report Federal Aid Project No. BRLO-5199 (027) Bridge Street Bridge Rehabilitation or Replacement Project over Arroyo Grande Creek Existing Bridge No. 49C-0196 City of Arroyo Grande, San Luis Obispo County, California

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Figure 6. Bridge General Plan for Alternative A (replacement option) Source: Quincy Engineering, Inc.


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Figure 7. Bridge General Plan for Alternative B (rehabilitation option) Source: Quincy Engineering, Inc.


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4 HYDROLOGY The hydrology at the Project site is presented in the following sections. As stated in Section 1.3.1, four methods were used to estimate the design discharges at the Project site: 1) research of FEMA’s FIS, 2) research of a previous USGS study, 3) statistical distribution analysis of stream flow data from a nearby gaging station, and 4) USGS Regional Regression calculations.

4.1 Federal Emergency Management Agency FEMA’s effective FIS (2012) presents the peak discharges for Arroyo Grande Creek, which FEMA determined from an analytical frequency curve that was derived from 28 years of record from USGS gage No. 11141500, at the City of Arroyo Grande. Three flooding locations are listed for Arroyo Grande Creek in Table 1. Table 1. FEMA FIS Peak Discharges for Arroyo Grande Creek

Flooding Location Drainage Area (sq. mi.)

Peak Discharges (cfs) 500-year 100-year 50-year 10-year

Arroyo Grande Avenue 138.6 41,000 15,800 10,000 2,800 U.S. Highway 101 109.3 27,800 10,500 6,700 1,900

Huasna Road 82.5 25,800 8,700 5,100 1,100 The Project site is located between the U.S. Highway 101 and Huasna Road flooding locations.

4.2 United States Geological Survey Study The USGS re-evaluated the magnitude and frequency of floods in California in a study published in 2012 (Gotvald et. al.). Annual peak flow data were analyzed for 771 stream flow gaging stations in California based on data through the water year 2006. Using the Bulletin 17B method of analysis, the study generated flood-frequency estimates for these gaging stations. Gaging station 11141500, which is located 0.6 mi upstream (northeast) of the Project site, was included in this analysis. The flood-frequency estimate for station 11141500, as published in the Gotvald et. al. study, was 11,300 cfs for the 100-year storm and 8,220 cfs for the 50-year storm. The annual peak flows recorded at station 11141500 are presented in Figure 8.


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Figure 8. Annual Peak Flows Recorded at Upstream Gaging Station

Source: USGS Because the gaging station is not located at the Project site, the design discharges were adjusted using a basin transfer method based on the watershed areas at the gaging station and at the Project site. With this adjustment, the 100-year flow was estimated to be 11,670 cfs, and the 50-year flow was estimated to be 8,490 cfs.

4.3 Log-Pearson Type III Statistical Analysis WRECO also performed an independent analysis, and estimated design flows for Arroyo Grande Creek using USGS gaging station 11141500, which is located approximately 0.6 mi upstream (northeast) of the Project site. The gaging station has 47 years of data recorded from the water years 1940 through 1986. The annual peak discharges were analyzed statistically and fit into the Log-Pearson Type III distribution. Based on this analysis, the 100-year flow was estimated to be 12,490 cfs, and the 50-year flow was estimated to be 8,740 cfs. Because the gaging station is not located at the Project site, the design discharges were adjusted using a basin transfer method based on the watershed areas at the gaging station and at the Project site. With this adjustment, the 100-year flow was estimated to be 12,900 cfs, and the 50-year flow was estimated to be 9,030 cfs.


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4.4 USGS Regional Flood-Frequency Equations Flood-frequency equations were developed by the USGS and based on analysis of data from gage stations. California is divided into six regions; the Project site is within the Sierra Nevada region. These flood-frequency equations are generally used to estimate stream flow for ungaged sites that are not affected by substantial urban development and that are natural (unregulated) streams. On July 18, 2012, the USGS released updated regional flood-frequency equations, and revised the boundaries of the six unique regions within California. These equations are based on annual peak-flow data through water year 2006 for 771 streamflow-gaging stations in California having 10 or more years of data. The updated equations were used in support of the Project’s hydrologic analysis. The flood-frequency equations are as follows (Gotvald et. al., 2012):

994.084.0100 0.11 PAQ

15.184.050 32.5 PAQ

Where:

Qx = peak discharge for a storm event with a return period of x years, cfs A = drainage area, square mi P = mean annual precipitation, in

The resulting flows were 12,750 cfs for the 100-year storm and 10,090 cfs for the 50-year storm.

4.5 Peak Discharges: Summary and Selection for Hydraulic Analysis

The peak discharges estimated using the Log-Pearson Type III analysis were selected to be the design discharges for the Project. The USGS regional flood-frequency analysis is generally used for sites that are not affected by substantial urban development that are natural and unregulated. The majority of the watershed is rural, and the immediate Project vicinity is developed. However, Lopez Lake is located within the watershed, approximately 9 mi upstream of the mouth, which drains to the Pacific Ocean (see Figure 9). Lopez Lake is a reservoir that is formed by Lopez Dam. Because the flow is regulated, the regional flood-frequency analysis may not be as appropriate for the Project. The other methods of analysis estimated the flows at the Project site using the upstream USGS gaging station 11141500 (with the exception of the USGS regional flood-frequency analysis). The FEMA study only included 28 years of record.


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Figure 9. Location of Lopez Lake and Dam Relative to Project Site

Source: USGS The flows estimated by USGS in their previous study were comparable to the independent analysis using the Log-Pearson Type III analysis. The slightly larger flows estimated following the Log-Pearson Type III analysis were used to be conservative. See Table 2 for a summary of these flow estimates. Table 2. Peak Discharges Summary for Arroyo Grande Creek

Method/Source 100-year Flow (cfs) 50-year Flow (cfs) FEMA at Upstream U.S. 101* 10,500 6,700

Previous USGS Study with Area Adjustment

11,670 8,490

Log-Pearson Type III with Area Adjustment

12,900 9,030

USGS Regional Flood-Frequency Equations

12,750 10,090

Note: * The flows listed for the FEMA source are not at the Project site and were not adjusted.

4.6 Hydrologic Stability Based on the land use map from San Luis Obispo County, as shown in Figure 5 in Section 2.3, the future land use for the watershed looks consistent with current conditions. The immediate Project vicinity is already developed, and the upper watershed looks to be predominantly rural.

NO

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NO SCALE PROJECT

LOCATION

Lopez Lake


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5 HYDRAULIC ANALYSIS The hydraulic design of the bridge will follow FHWA and Caltrans’ criteria. The following sections describe the development of the hydraulic modeling and summarize the results for the existing bridge and proposed Bridge Street bridge alternatives at Arroyo Grande Creek. The water surface profile plots, hydraulic summary tables, and channel cross sections are included in Appendix A for the existing bridge, A.1.1.1.1.1Appendix B for the Alternative A bridge, and Appendix C for the Alternative B bridge.

5.1 Design Tools The hydraulics at the Project site were evaluated using the Hydrologic Engineering Centers River Analysis System (HEC-RAS) modeling software Version 4.1.0 developed by the U.S. Army Corps of Engineers (USACE).

5.2 Cross Section Data The channel geometry for the hydraulic model was developed using topographic survey data provided by McMillan Land Surveys in 2012. Five stream cross sections were surveyed and included in the hydraulic model (see Figure 10). In addition, four more cross sections, two above and two below the bridge, were included from data provided by Quincy Engineering, Inc., which includes channel grading for the proposed conditions.

Figure 10. Stream Survey Cross Section Locations

Source: Google Earth

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PROJECT LOCATION


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The existing bridge was modeled as described in Section 3.2, and the proposed bridge was modeled as described in Section 3.3. Although the existing bridge superstructure consists of two trusses, the deck was modeled to be solid. The existing bridge and the two proposed alternatives were modeled with a skew of 20 degrees from perpendicular to the channel flow line.

5.3 Model Boundary Condition The hydraulic model was evaluated using the steady state flow analysis and known downstream water surface elevations based on information from the FEMA FIS (see Figure 11).

Figure 11. Arroyo Grande Creek Stream Profile

Source: FEMA The most downstream cross section surveyed for the Project is coincident with the upstream face of the U.S. 101 structure (northbound). Therefore, the water surface elevations at that structure were used as the downstream boundary condition in the hydraulic model. Figure 11 graphically depicts the water surface elevations for the 100-year (long dash followed by two short dashes) and 50-year (long dash followed by one short dash) storm events. The water surface elevations at U.S. 101 northbound are 93 ft NGVD 29 for the 100-year event and 90 ft NGVD 29 for the 50-year event.

100-year =93 ft NGVD 29

50-year =90 ft NGVD 29


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5.4 Manning’s Roughness Coefficients Manning’s roughness coefficients were used in the hydraulic model to represent the frictional energy losses in the flow. The roughness coefficients were selected to best describe the channel and the banks based on aerial imagery and observations made during WRECO’s site visit on August 22, 2012. Based on these observations, the roughness coefficients selected to describe the stream were 0.035 for the low flow channel and between 0.09 and 0.1 for the overbank areas. In the vicinity of the bridge, an average roughness coefficient of 0.04 was selected.

5.5 Expansion and Contraction Coefficients Expansion and contraction coefficients were used in the hydraulic model to estimate hydraulic losses at transitions between cross sections. The expansion and contraction coefficients used in the channel were 0.3 and 0.1, respectively. These values represent a channel with gradual transitions between cross sections. Cross sections adjacent to the bridge were modeled using expansion and contraction coefficients in the channel of 0.5 and 0.3, respectively.

5.6 Water Surface Elevations The water surface elevations were estimated for the existing and proposed conditions. The 100- and 50-year water surface profiles comparing the existing and proposed conditions are depicted in Figure 12 and Figure 13, respectively. The water surface elevations in the immediate vicinity of the bridges are compared in Table 3 and Table 4 for the 100-year and 50-year storm events, respectively. Because the proposed bridge alternatives are wider than the existing bridge, the upstream faces of the bridges are not at a common location. The nearest common upstream cross section is at river station 1037, and is presented in the water surface elevations summary tables. The cross sections at the upstream faces of the existing, Alternative A, and Alternative B structures are shown in Figure 14, Figure 15, and Figure 16 respectively. Table 3. 100-Year Water Surface Elevations Summary Table

River Station Description


Existing Alternative

A Alternative

B 1037 Upstream 99.5 98.9 98.4 1030 Upstream face Alternative A N/A 98.8 N/A 1027 Upstream face Alternative B N/A N/A 98.4 1019 Upstream face existing 98.8 N/A N/A 984 Downstream face Alternative A N/A 98.4 N/A 988 Downstream face Alternative B N/A N/A 98.4 995 Downstream face existing 98.3 N/A N/A 977 Downstream 98.3 98.4 98.5


October 2016 19

Table 4. 50-Year Water Surface Elevations Summary Table



Existing Alternative

A Alternative

B 1037 Upstream 96.5 96.1 95.7 1030 Upstream face Alternative A N/A 96.0 N/A 1027 Upstream face Alternative B N/A N/A 95.7 1019 Upstream face existing 96.0 N/A N/A 984 Downstream face Alternative A N/A 95.7 N/A 988 Downstream face Alternative B N/A N/A 95.7 995 Downstream face existing 95.6 N/A N/A 977 Downstream 95.6 95.6 95.8

The hydraulic modeling indicates that the proposed bridge alternatives would result in decreases in water surface elevation. The proposed bridge alternatives will have larger hydraulic openings than the existing bridge. These geometric differences result in the decreases in water surface elevation.


October 2016 20

Figure 12. 100-Year Water Surface Comparison

Proposed Bridge Alternatives

Existing Bridge


October 2016 21

Figure 13. 50-Year Water Surface Comparison

Proposed Bridge Alternatives

Existing Bridge


October 2016 22

-80 -60 -40 -20 0 20 40 60 8075

80

85

90

95

100

105

110

115

Bridge Street Plan: Alt B: Rehab-20160906 9/26/2016 5:39:26 PM RS = 1007.236 BR

Station (ft)

Ele

vatio

n N

GV

D 2

9 (

ft)

Le gend

WS 100yr

WS 50y r

Ground

Bank Sta

Pier Debris

.1 .04 .1

Figure 14. Upstream Face of Existing Bridge, Looking Downstream (Southwest)

-80 -60 -40 -20 0 20 40 60 80

80

90

100

110

Bridge Street Plan: Alt A: Recon-20160906 9/26/2016 5:20:34 PM RS = 1007.236 BR

Station (ft)

Ele

vatio

n N

GV

D 2

9 (

ft)

Le gend

WS 100yr

WS 50y r

Ground

Ineff

Bank Sta

.04

Figure 15. Upstream Face of Alternative A Bridge, Looking Downstream (Southwest)

-80 -60 -40 -20 0 20 40 60 80

80

90

100

110


Station (ft)

Ele

vatio

n N

GV

D 2

9 (

ft)

Le gend

WS 100yr

WS 50y r

Ground

Bank Sta

Pier Debris

.1 .04 .1

Figure 16. Upstream Face of Alternative B Bridge, Looking Downstream (Southwest)


October 2016 23

5.7 Freeboard The freeboard requirements applicable to the Project are discussed in Section 1.3.2. To summarize, the bridge should be designed to pass the 50-year storm event with adequate freeboard to account for debris and bedload, or the 100-year storm event with no freeboard. The available freeboard distances (clearance distance from soffit to the water surface elevation) for the existing bridge and proposed alternative bridge structures are summarized in Table 5 for the 100-year storm event and Table 6 for the 50-year storm event. Table 5. Available Freeboard for the 100-year Storm Event

Bridge Condition

Bridge Soffit Elevation*

(ft NGVD 29)

100-Year Water Surface Elevation (ft NGVD 29)

Freeboard (ft)


At Upstream Face of Bridge

Existing 98.5 99.5 98.8 -0.3 Alternative A 104.3 98.9 98.8 5.5 Alternative B 101.3 98.4 98.4 2.9

Note: * The soffit elevation for the existing bridge presented is based on the elevation of the lower brown truss. Table 6. Available Freeboard for the 50-year Storm Event

Bridge Condition

Bridge Soffit Elevation*

(ft NGVD 29)

50-Year Water Surface Elevation (ft NGVD 29)

Freeboard (ft)


At Upstream Face of Bridge

Existing 98.5 96.5 96.0 2.5 Alternative A 104.3 96.1 96.0 8.3 Alternative B 101.3 95.7 95.7 5.6

Note: * The soffit elevation for the existing bridge presented is based on the elevation of the lower brown truss. The existing bridge does not meet the freeboard requirement of passing the 100-year storm event. The two proposed bridge alternatives would meet the freeboard requirements by passing both the 50-year storm event with at least 2 ft of freeboard and the 100-year storm event. The available freeboard for the both alternatives are higher than the existing condition. The existing bridge has one pier, while the Alternative A bridge has no piers, and the Alternative B bridge has one 5-foot diameter pier. The bridge soffit for both alternatives are higher than the soffit for the existing bridge. Note that the soffit elevation of the existing bridge and Alternative B is based on the lower chord of the supplemental truss.


October 2016 24

The soffit elevation of alternative A is much higher than the existing bridge since it does not have a supplemental truss. The soffit elevation of Alternative B is also higher than the existing bridge since the new proposed supplemental truss is shallower than the existing supplemental truss.

5.8 Flow Velocities Flow velocities were estimated for the existing bridge and proposed alternative bridges. The 100- and 50-year flow velocities for the existing and proposed conditions are presented in Table 7 and Table 8, respectively. The proposed condition results in a very slight increase in velocity upstream of the bridge due to the lowering of the water surface elevation. Table 7. 100-year Velocities in Vicinity of Project


Velocity (ft/s)

Existing Alternative A

Alternative B

1037 Upstream 9.2 9.9 9.5 1030 Upstream face Alternative A N/A 9.9 N/A 1027 Upstream face Alternative B N/A N/A 11.0 1019 Upstream face existing 10.6 N/A N/A 984 Downstream face Alternative A N/A 10.6 N/A 988 Downstream face Alternative B N/A N/A 10.0 995 Downstream face existing 11.4 N/A N/A 977 Downstream 11.2 10.7 9.3

Table 8. 50-year Velocities in Vicinity of Project


Velocity (ft/s)

Existing Alternative A

Alternative B

1037 Upstream 8.0 8.7 8.3 1030 Upstream face Alternative A N/A 8.7 N/A 1027 Upstream face Alternative B N/A N/A 9.5 1019 Upstream face existing 9.2 N/A N/A 984 Downstream face Alternative A N/A 9.4 N/A 988 Downstream face Alternative B N/A N/A 8.7 995 Downstream face existing 10.0 N/A N/A 977 Downstream 9.8 9.5 8.1


October 2016 25

5.9 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 for both alternatives is approximately 9 to 11 ft/s, which are considered to be erosive. Therefore, rock slope protection (RSP) was evaluated as an option to provide erosion protection at the proposed abutment locations. RSP calculations were performed using the methodologies presented in the California Bank and Shore Rock Slope Protection Design (Caltrans, 2000). The RSP is also considered to be a scour countermeasure, and the sizing of the RSP is discussed in Section 6.7.


October 2016 26

6 SCOUR ANALYSIS WRECO evaluated bridge scour per the criteria described in the FHWA HEC-18. The minimum design criterion for bridge scour is the 100-year design storm. WRECO evaluated the scour potential using the results of the steady state flow analysis from HEC-RAS for the proposed bridge alternatives. The scour calculations assume that the channel bed material is erodible. Although clays are present in the soils at this location, it is assumed that cohesive forces do not dominate the erosive process, and therefore cohesionless equations were used to estimate scour. The following sub-sections summarize the results of the analysis, and the detailed calculations are included in Appendix D.

6.1 Caltrans Bridge Inspection Reports The Caltrans Bridge Inspection Reports (BIRs) from the years 1939 through 2015 were reviewed for relevant scour information. A bridge Scour Plan of Action that was prepared in October 2010 indicates that if the bridge is not replaced, “repair of the bridge deck is much more critical than a concern with scour.” The National Bridge Inventory Item 113 Code for the bridge is “U”, which represents a bridge with unknown foundation elevations that has not been evaluated for an appropriate scour rating. Two stream measurements were found in the BIRs from the May 13, 1993 and May 12, 1999 bridge inspections. These stream measurements were used to assess the long-term bed elevation change, as discussed in Section 6.3.

6.2 Existing Channel Bed The Draft Preliminary Foundation Report prepared by Fugro Consultants, Inc. for the Project describes the soils based on three borings that were drilled at the site from a previous study by Earth Systems Pacific. Based on the previous study, the subsurface soils at the Project site are “medium dense to dense silty and clayey sand and stiff clay.” Based on this description, a median grain size (D50) equivalent to a No. 200 sieve (0.075 mm) was assumed for the scour calculations.

6.3 Long-Term Bed Elevation Change Channel bed elevations may fluctuate over time as a result of changes in local sediment transport capacity and availability. In general, channel aggradation occurs when more sediment is supplied by watershed erosion and upstream channel flow than can be transported locally, and channel degradation occurs when sediment transport capacity exceeds supply. Only channel degradation is considered for the purposes of analyzing scour. As stated previously in Section 6.1, the long-term bed elevation change for the Project was determined by reviewing the available stream measurements from the Caltrans BIRs,


October 2016 27

and comparing with the topographic survey for the Project (see Figure 17 for a comparison of the three cross sections).

Figure 17. Arroyo Grande Creek Stream Measurements at Upstream Face of Bridge Street Bridge

Source: McMillan Land Surveys (2012), and Caltrans (1999 and 1993) The two stream cross sections were measured relative to different reference points, which made it difficult to compare the cross sections. The 1993 cross section was measured relative to the top of the rail, while the 1999 stream cross section was measured relative to the top of the sidewalk. A comparison of the cross sections shows some changes in the bed elevation that could be due to variations in the way the data was captured. The thalweg appeared to be relatively stable. Because the upstream face of the replacement bridge will be at a slightly different location, the bridge should continue to be monitored to assess the long-term bed changes.

6.4 Contraction Scour Contraction (ultimate) 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 contracted 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


October 2016 28

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 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 2012). As described in Section 6.2, the D50 was assumed to be 0.075 mm. Because of the small grain size at the Project site, ultimate scour for cohesive (silt and clay) materials was estimated using a relationship between the median grain size and the critical shear stress. The equation for estimating ultimate scour, as presented in HEC-18, is as follows:

3/111

21

83.194.0gny

K

gyVyy

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

With a density of 2.6 to 3.2 slugs per cubic ft, the ultimate scour was calculated to be 0 feet for Alternative A and 4.1 feet for Alternative B.

6.5 Pier Scour Pier scour is caused by the formation of vortices (known as a horseshoe vortex) at the pier base. 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. The scour depth at the pier is estimated based on the pier design (shape and dimensions), 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. An equation based on the Colorado State University equation was used to estimate local pier scour. The equation predicts maximum scour depths.


October 2016 29

The equation for estimating local pier scour, as presented in HEC-18, is as follows:

7.0

165.021

6.22.2

gVVaKKy 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 65.0

2 )/( aSinLCosK Where:

a = pier width, ft L = length of pier, ft (if L/a is larger than 12, use L/a=12 as a maximum) = angle of attack of the flow

a = pier width, ft 1V = mean velocity of flow directly upstream of the pier, ft/s

CV = critical velocity for initiation of erosion of the cohesive material, ft/s g = gravitational acceleration, 32.2 ft/s2

The local pier scour was estimated assuming that the foundations of the pier will not be exposed, and are summarized in Table 9. Table 9. Local Pier Scour Depths for Bridge Street Alternative B Bridge

Pier Number Location Scour Depth (ft) Pier 2 Center 11.7

6.6 Abutment Scour Abutment scour occurs when the bridge abutments block approaching flow. 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. The Froehlich equation was used for the scour analysis because the ratio of the projected abutment length to the flow depth was less than 25 for the abutment. Both the northern and southern abutments, for both Alternatives A and B, were located above the 100-year water surface elevation, and therefore, local scour was not evaluated at the abutments.


October 2016 30

6.7 Total Scour and Scour Countermeasures

6.7.1 Total Scour Depths The total scour depths are the sum of the long-term bed elevation change, contraction scour, and local abutment scour. The long-term bed elevation change was found to be negligible based on available information. The calculated scour depths for the two Alternatives are presented in Table 10 and Table 11. Table 10. Summary of Scour depths for Alternative A


Abutment 2 (south) 0.0 0.0 0.0 0.0 Abutment 1 (north) 0.0 0.0 0.0 0.0

Table 11. Summary of Scour Depths for Alternative B


Abutment 3 (south) 0.0 4.1 0.0 4.1 Pier 2 0.0 4.1 11.7 15.8

Abutment 1 (north) 0.0 4.1 0.0 4.1 If rock slope protection (RSP) is incorporated into the design of the bridge foundation elements, thalweg migration should be arrested by the RSP. The Alternative B pier should be designed to provide structural support assuming no lateral earth support down to the ground elevation at the pier minus the sum of long-term degradation, contraction scour, and local scour. The minimum elevations for the footing or pile cap are summarized in Table 12 and Table 13 below. Abutment finished grades were estimated from Figure 6 and Figure 7. Scour countermeasures are discussed in Section 6.7.2. Table 12. Scour Data Table for Alternative A

Bridge Element


(feet)


(feet) Abutment 2 (south) 102.3 0.0 Abutment 1 (north) 103.6 0.0

Table 13. Scour Data Table for Alternative B

Bridge Element


(feet)


(feet) Abutment 3 (south) 99.4 0.0

Pier 2 78.9 11.7 Abutment 1 (north) 97.2 0.0


October 2016 31

6.7.2 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. Two procedures for determining RSP design were considered: the FHWA’s Hydraulic Engineering Circular No. 23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance – Third Edition (HEC-23) (2009), and Caltrans’ California Bank and Shore Rock Slope Protection Design – Third Edition (2000). The following sub-sections summarize the results of the analysis, and the detailed calculations are included in Appendix E.

6.7.3 Federal Highway Administration RSP Design The D50 of the RSP at the bridge abutment was calculated using the Ishbash relationship from HEC-23, Design Guideline 14. The following equation was used to determine the median stone diameter required for the proposed riprap erosion control system to protect the channel slope under the bridge: For Froude numbers (V/(gy)0.5) ≤ 0.80 (HEC-23, equation 14.1):

gyV

SK

yD

s

250

1

Where: D50 = median stone diameter, ft V = characteristic average velocity in the contracted section, ft/s Ss = specific gravity of rock riprap (2.65) g = gravitational acceleration (32.2 ft/s2) y = depth of flow in the contracted bridge opening, ft K = 1.02 for a vertical wall abutment 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 banks in the vicinity of the bridge. The minimum RSP for the proposed bridge was calculated to be from 1/2 ton to 1 ton class for Alternative A, and from 3/8 ton to 1 ton class for Alternative B.


October 2016 32

6.7.4 California Bank and Shore RSP Design The third edition of the California Bank and Shore Rock Slope Protection Design (Caltrans 2000) was also used to analyze RSP design criteria:

)(sin)1(00002.0

33

6

arSGSGV

WR

R

Where: V = 2/3 average stream velocity for flows parallel to the bank, ft/s 4/3 average stream velocity for flows which impinge the bank, ft/s RSG = specific gravity of the stones (2.65) r = 70° for broken rock (constant) a = angle of face slope from the horizontal W = RSP weight The Arroyo Grande Creek 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 1.7:1 (horizontal to vertical) for Alternative A and 1.3:1 for Alternative B. 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 minimum RSP for the proposed bridge was calculated to be from Backing No. 3 class to Backing No. 2 class for both Alternative A and Alternative B.

6.7.5 Selected Class of RSP and Layer Thickness RSP calculations were performed using methodologies published by the FHWA and Caltrans. Based on the RSP calculations, as well as engineering judgment, 1/2 ton class RSP is recommended for both alternatives adjacent to the abutments and at the pier in Alternative B. Per the California Bank and Shore RSP Design, 1/2 ton RSP should also include a backing layer (backing class No. 1) and RSP fabric type B. The RSP fabric should be placed on the bank as the initial filter separator material beneath the backing layer of RSP. The minimum recommended RSP layer thicknesses per the California Bank and Shore RSP Design are presented in Table 14. The slope protection should extend from the face of the abutment to the toe of slope. Table 14. RSP Layers for Both Alternatives Outside Layer Backing Layer Placement Type

Class/Type 1/2 Ton Backing Class No. 1 B Minimum Layer

Thickness 4.3 ft 1.8 ft B


October 2016 33

7 REFERENCES California Department of Transportation. (2000). California Bank and Shore Rock Slope

Protection Design – Practitioner’s Guide and Field Evaluations of Riprap Methods. Final Report No. FHWA-CA-TL-95-10. Caltrans Study No. F90TL03. Third Edition – Internet. October 2000.

California Department of Transportation – Division of Maintenance – Structure Maintenance and Investigations. (2012, 1999, 1993). Bridge Inspection Report. Bridge Number: 49C0196. Facility Carried: Bridge St. Location: 0.1 mi S of S.R. 227.

Environmental Protection Agency. (2012).Watershed Assessment, Tracking & Environmental Results System. <https://www.epa.gov/waterdata/waters-watershed-assessment-tracking-environmental-results-system> (Last accessed: August 30, 2012)

Federal Emergency Management Agency. (2012). Flood Insurance Study, San Luis Obispo County, California and Incorporated Areas. Flood Insurance Study Number 06079V001B and 06079V002B.

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

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

Federal Highway Administration. (2004). Code of Federal Regulations. Title 23 – Highways. Sub-chapter G – Engineering and Traffic Operations. Part 650 – Bridges, Structures, and Hydraulics.

Google Earth. (2013). (Last accessed: April 3, 2013). Gotvald, A.J., Barth, N.A., Veilleux, A.G., and Parrett, Charles, 2012, Methods for

determining magnitude and frequency of floods in California, based on data through water year 2006: U.S. Geological Survey Scientific Investigations Report 2012–5113, 38 p., 1 pl., available online only at <http://pubs.usgs.gov/sir/2012/5113/>.

McMillan Land Surveys. (2012). Topographic survey of channel cross sections. San Luis Obispo County. (2013). Planning and Building – Zoning and Maps – Map

Image Download Center – Land Use Maps. <http://www.slocounty.ca.gov/planning/zoning/Map_Image_Download_Center/Land_Use_Maps.htm> (Last accessed: March 27, 2013)

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


October 2016 34

United States Geological Survey. (2012). National Map Viewer. <http://viewer.nationalmap.gov/viewer> (Last accessed: August 30, 2012)

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.


October 2016

Appendix A HEC-RAS Results for Existing Bridge

0 200 400 600 800 1000 1200 140070

80

90

100

110

120

Bridge Street Plan: Exist-20160906 9/27/2016 2:50:36 PM

Main Channel Distance (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

LOB

ROB

269.

885

581.

67

790.

854

917.

236

977.

236

1007

.236

1097

.236

1265

.726

Arroyo Grande Cr Arroyo Grande Cr

-100 -50 0 50 100 15080

85

90

95

100

105

110

115

Bridge Street Plan: Exist-20160906 9/27/2016 2:50:36 PM RS = 1265.726

Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1

.09 .035 .09 .1

0 20 40 60 80 100 120 140 16075

80

85

90

95

100

105

110


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09

-80 -60 -40 -20 0 20 40 60 8075

80

85

90

95

100

105

110

115


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Ineff

Bank Sta

.04 .1

-80 -60 -40 -20 0 20 40 60 8075

80

85

90

95

100

105

110

115

Bridge Street Plan: Exist-20160906 9/27/2016 2:50:36 PM RS = 1007.236 BR

Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Ineff

Bank Sta

Pier Debris

.04 .1

-80 -60 -40 -20 0 20 40 6075

80

85

90

95

100

105

110

115

Bridge Street Plan: Exist-20160906 9/27/2016 2:50:36 PM RS = 1007.236 BR

Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Ineff

Bank Sta

.1 .04

-80 -60 -40 -20 0 20 40 6075

80

85

90

95

100

105

110


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Ineff

Bank Sta

.1 .04

0 20 40 60 80 100 120 140 16075

80

85

90

95

100

105

110


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09 .1

-100 -50 0 50 100 15075

80

85

90

95

100

105

110

115


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09 .1

-150 -100 -50 0 50 100 150 20075

80

85

90

95

100

105

110


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09 .1

-300 -200 -100 0 100 200 300707580859095

100105110115


Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09 .1

-600 -400 -200 0 200 40070

75

80

85

90

95

100

105

110

Bridge Street Plan: Exist-20160906 9/27/2016 2:50:36 PM RS = 0

Station (ft)

Elev

atio

n N

GVD

29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09 .1

HEC-RAS Plan: Exist-20160906 River: Arroyo Grande Cr Reach: Arroyo Grande CrReach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl

(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft) Arroyo Grande Cr 1265.726 50yr-ND 9030.00 81.08 98.41 99.33 0.008483 7.68 1176.25 119.98 0.43Arroyo Grande Cr 1265.726 100yr-ND 12900.00 81.08 101.38 102.47 0.007851 8.36 1542.82 126.96 0.42

Arroyo Grande Cr 1097.236 50yr-ND 9030.00 77.47 96.65 97.72 0.010591 8.31 1086.43 102.02 0.45Arroyo Grande Cr 1097.236 100yr-ND 12900.00 77.47 99.60 100.91 0.010668 9.20 1401.85 111.65 0.46

Arroyo Grande Cr 1037.236 50yr-ND 9030.00 77.18 96.46 90.00 97.47 0.001974 8.06 1120.67 94.37 0.41Arroyo Grande Cr 1037.236 100yr-ND 12900.00 77.18 99.35 92.27 100.67 0.002043 9.23 1396.90 102.31 0.43

Arroyo Grande Cr 1007.236 Bridge






Arroyo Grande Cr 0 50yr-ND 9030.00 72.30 89.13 84.78 90.09 0.007003 7.99 1311.65 317.50 0.46Arroyo Grande Cr 0 100yr-ND 12900.00 72.30 90.82 87.01 91.94 0.007003 8.94 1929.13 407.63 0.48

Plan: Exist-20160906 Arroyo Grande Cr Arroyo Grande Cr RS: 1007.236 Profile: 50yr-ND E.G. US. (ft) 97.47 Element Inside BR US Inside BR DS W.S. US. (ft) 96.46 E.G. Elev (ft) 97.32 97.14 Q Total (cfs) 9030.00 W.S. Elev (ft) 96.00 95.59 Q Bridge (cfs) 9030.00 Crit W.S. (ft) 90.35 90.84 Q Weir (cfs) Max Chl Dpth (ft) 18.82 18.59 Weir Sta Lft (ft) Vel Total (ft/s) 9.22 10.01 Weir Sta Rgt (ft) Flow Area (sq ft) 979.25 901.65 Weir Submerg Froude # Chl 0.48 0.53 Weir Max Depth (ft) Specif Force (cu ft) 9712.18 9112.53 Min El Weir Flow (ft) 109.26 Hydr Depth (ft) 11.65 11.28 Min El Prs (ft) 99.51 W.P. Total (ft) 146.53 102.86 Delta EG (ft) 0.42 Conv. Total (cfs) 129061.2 142395.0 Delta WS (ft) 0.92 Top Width (ft) 84.08 79.96 BR Open Area (sq ft) 1232.50 Frctn Loss (ft) 0.11 0.07 BR Open Vel (ft/s) 10.01 C & E Loss (ft) 0.07 0.03 BR Sluice Coef Shear Total (lb/sq ft) 2.04 2.20 BR Sel Method Energy only Power Total (lb/ft s) 18.83 22.04

Plan: Exist-20160906 Arroyo Grande Cr Arroyo Grande Cr RS: 1007.236 Profile: 100yr-ND E.G. US. (ft) 100.67 Element Inside BR US Inside BR DS W.S. US. (ft) 99.35 E.G. Elev (ft) 100.48 100.25 Q Total (cfs) 12900.00 W.S. Elev (ft) 98.71 98.18 Q Bridge (cfs) 12900.00 Crit W.S. (ft) 92.85 93.28 Q Weir (cfs) Max Chl Dpth (ft) 21.53 21.18 Weir Sta Lft (ft) Vel Total (ft/s) 10.66 11.55 Weir Sta Rgt (ft) Flow Area (sq ft) 1210.02 1116.94 Weir Submerg Froude # Chl 0.41 0.57 Weir Max Depth (ft) Specif Force (cu ft) 14367.89 13544.41 Min El Weir Flow (ft) 109.26 Hydr Depth (ft) 18.83 12.97 Min El Prs (ft) 99.51 W.P. Total (ft) 187.79 116.12 Delta EG (ft) 0.54 Conv. Total (cfs) 155646.0 187658.8 Delta WS (ft) 1.20 Top Width (ft) 64.27 86.15 BR Open Area (sq ft) 1232.50 Frctn Loss (ft) 0.14 0.07 BR Open Vel (ft/s) 11.55 C & E Loss (ft) 0.09 0.04 BR Sluice Coef Shear Total (lb/sq ft) 2.76 2.84 BR Sel Method Energy only Power Total (lb/ft s) 29.46 32.77


October 2016

Appendix B HEC-RAS Results for Alternative A Bridge

0 200 400 600 800 1000 1200 140070

80

90

100

110

120

Bridge Street Plan: Alt A: Recon-20160906 9/27/2016 2:50:47 PM


Elev

atio

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GVD

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(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

LOB

ROB

269.

885

581.

67

790.

854

917.

236

977.

236

1007

.236

1097

.236

1265

.726


-100 -50 0 50 100 15080

85

90

95

100

105

110

115

Bridge Street Plan: Alt A: Recon-20160906 9/27/2016 2:50:47 PM RS = 1265.726

Station (ft)

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Ground

Bank Sta

.1

.09 .035 .09 .1

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-300 -200 -100 0 100 200 300707580859095

100105110115


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.1 .09 .035 .09 .1

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Bridge Street Plan: Alt A: Recon-20160906 9/27/2016 2:50:47 PM RS = 0

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Bank Sta

.1 .09 .035 .09 .1

HEC-RAS Plan: A: Recon-20160906 River: Arroyo Grande Cr Reach: Arroyo Grande CrReach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl











Plan: A: Recon-20160906 Arroyo Grande Cr Arroyo Grande Cr RS: 1007.236 Profile: 50yr-ND E.G. US. (ft) 97.21 Element Inside BR US Inside BR DS W.S. US. (ft) 96.02 E.G. Elev (ft) 97.20 97.01 Q Total (cfs) 9030.00 W.S. Elev (ft) 96.00 95.62 Q Bridge (cfs) 9030.00 Crit W.S. (ft) 90.32 90.61 Q Weir (cfs) Max Chl Dpth (ft) 18.82 18.72 Weir Sta Lft (ft) Vel Total (ft/s) 8.77 9.47 Weir Sta Rgt (ft) Flow Area (sq ft) 1029.61 953.48 Weir Submerg Froude # Chl 0.46 0.51 Weir Max Depth (ft) Specif Force (cu ft) 9789.36 9321.73 Min El Weir Flow (ft) 110.95 Hydr Depth (ft) 11.32 10.84 Min El Prs (ft) 105.60 W.P. Total (ft) 99.80 96.78 Delta EG (ft) 0.23 Conv. Total (cfs) 181262.7 162781.3 Delta WS (ft) 0.44 Top Width (ft) 90.92 87.93 BR Open Area (sq ft) 1956.41 Frctn Loss (ft) 0.13 0.02 BR Open Vel (ft/s) 9.47 C & E Loss (ft) 0.06 0.00 BR Sluice Coef Shear Total (lb/sq ft) 1.60 1.89 BR Sel Method Energy only Power Total (lb/ft s) 14.02 17.93

Plan: A: Recon-20160906 Arroyo Grande Cr Arroyo Grande Cr RS: 1007.236 Profile: 100yr-ND E.G. US. (ft) 100.29 Element Inside BR US Inside BR DS W.S. US. (ft) 98.73 E.G. Elev (ft) 100.27 100.06 Q Total (cfs) 12900.00 W.S. Elev (ft) 98.71 98.25 Q Bridge (cfs) 12900.00 Crit W.S. (ft) 92.67 93.08 Q Weir (cfs) Max Chl Dpth (ft) 21.53 21.35 Weir Sta Lft (ft) Vel Total (ft/s) 10.02 10.77 Weir Sta Rgt (ft) Flow Area (sq ft) 1286.96 1197.77 Weir Submerg Froude # Chl 0.49 0.54 Weir Max Depth (ft) Specif Force (cu ft) 14474.97 13812.55 Min El Weir Flow (ft) 110.95 Hydr Depth (ft) 12.97 12.30 Min El Prs (ft) 105.60 W.P. Total (ft) 109.71 107.65 Delta EG (ft) 0.26 Conv. Total (cfs) 246822.3 221755.4 Delta WS (ft) 0.52 Top Width (ft) 99.22 97.39 BR Open Area (sq ft) 1956.41 Frctn Loss (ft) 0.14 0.02 BR Open Vel (ft/s) 10.77 C & E Loss (ft) 0.07 0.00 BR Sluice Coef Shear Total (lb/sq ft) 2.00 2.35 BR Sel Method Energy only Power Total (lb/ft s) 20.05 25.32


October 2016

Appendix C HEC-RAS Results for Alternative B Bridge

0 200 400 600 800 1000 1200 140070

80

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100

110

120

Bridge Street Plan: Alt B: Rehab-20160906 9/29/2016 2:29:21 PM


Elev

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29

(ft)

Legend

WS 100yr-ND

WS 50yr-ND

Ground

LOB

ROB

269.

885

581.

67

790.

854

917.

236

977.

236

1007

.236

1097

.236

1265

.726


-100 -50 0 50 100 15080

85

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95

100

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110

115

Bridge Street Plan: Alt B: Rehab-20160906 9/29/2016 2:29:21 PM RS = 1265.726

Station (ft)

Elev

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WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1

.09 .035 .09 .1

0 20 40 60 80 100 120 140 16075

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.1 .09 .035 .09

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.1 .04 .1

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Pier Debris

.1 .04 .1

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.04 .1

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.1 .09 .035 .09 .1

-300 -200 -100 0 100 200 300707580859095

100105110115


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.1 .09 .035 .09 .1

-600 -400 -200 0 200 40070

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Bridge Street Plan: Alt B: Rehab-20160906 9/29/2016 2:29:21 PM RS = 0

Station (ft)

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WS 100yr-ND

WS 50yr-ND

Ground

Bank Sta

.1 .09 .035 .09 .1

HEC-RAS Plan: B:Rehab-20160906 River: Arroyo Grande Cr Reach: Arroyo Grande CrReach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl











Plan: B:Rehab-20160906 Arroyo Grande Cr Arroyo Grande Cr RS: 1007.236 Profile: 50yr-ND E.G. US. (ft) 97.22 Element Inside BR US Inside BR DS W.S. US. (ft) 96.15 E.G. Elev (ft) 97.10 96.84 Q Total (cfs) 9030.00 W.S. Elev (ft) 95.70 95.66 Q Bridge (cfs) 9030.00 Crit W.S. (ft) 89.71 89.56 Q Weir (cfs) Max Chl Dpth (ft) 18.70 18.86 Weir Sta Lft (ft) Vel Total (ft/s) 9.49 8.71 Weir Sta Rgt (ft) Flow Area (sq ft) 951.05 1036.75 Weir Submerg Froude # Chl 0.48 0.44 Weir Max Depth (ft) Specif Force (cu ft) 9802.15 10006.69 Min El Weir Flow (ft) 110.95 Hydr Depth (ft) 12.34 12.00 Min El Prs (ft) 104.44 W.P. Total (ft) 125.81 124.99 Delta EG (ft) 0.49 Conv. Total (cfs) 136081.7 157813.3 Delta WS (ft) 0.44 Top Width (ft) 77.05 86.41 BR Open Area (sq ft) 1550.08 Frctn Loss (ft) 0.15 0.03 BR Open Vel (ft/s) 9.49 C & E Loss (ft) 0.11 0.08 BR Sluice Coef Shear Total (lb/sq ft) 2.08 1.70 BR Sel Method Energy only Power Total (lb/ft s) 19.73 14.77

Plan: B:Rehab-20160906 Arroyo Grande Cr Arroyo Grande Cr RS: 1007.236 Profile: 100yr-ND E.G. US. (ft) 100.39 Element Inside BR US Inside BR DS W.S. US. (ft) 98.96 E.G. Elev (ft) 100.21 99.87 Q Total (cfs) 12900.00 W.S. Elev (ft) 98.29 98.27 Q Bridge (cfs) 12900.00 Crit W.S. (ft) 92.33 91.92 Q Weir (cfs) Max Chl Dpth (ft) 21.29 21.47 Weir Sta Lft (ft) Vel Total (ft/s) 11.12 10.14 Weir Sta Rgt (ft) Flow Area (sq ft) 1159.91 1271.85 Weir Submerg Froude # Chl 0.53 0.48 Weir Max Depth (ft) Specif Force (cu ft) 14331.05 14641.55 Min El Weir Flow (ft) 110.95 Hydr Depth (ft) 13.80 13.61 Min El Prs (ft) 104.44 W.P. Total (ft) 142.31 139.03 Delta EG (ft) 0.66 Conv. Total (cfs) 174507.2 206662.5 Delta WS (ft) 0.61 Top Width (ft) 84.04 93.47 BR Open Area (sq ft) 1550.08 Frctn Loss (ft) 0.18 0.03 BR Open Vel (ft/s) 11.12 C & E Loss (ft) 0.16 0.11 BR Sluice Coef Shear Total (lb/sq ft) 2.78 2.23 BR Sel Method Energy only Power Total (lb/ft s) 30.92 22.57


October 2016

Appendix D Scour Calculations

1243 Alpine Road, Suite 108

Walnut Creek, CA 94596

Phone: 925.941.0017

Fax: 925.941.0018

www.wreco.com

Bridge Street Bridge Replacement Project

San Luis Obispo, CaliforniaContraction Scour100-year FlowCalculation guideline from HEC-18 5th EditionInput from HEC-RAS for Proposed Alternative A: Reconstruction

Units = (SI or English) EnglishKu = constant = 6.19 (SI) or 11.17 (English) 11.17g = acceleration due to gravity = 32.2 ft/s^2

ChannelVchannel = Mean velocity of flow in main channel just upstream of

bridge = 9.9 ft/sD50channel = grain size in channel for which 50% of bed material is

finer = 0.0002 ftYochannel = existing depth in the contracted channel section before

scour = 13.1 ftYchannel = depth of flow just upstream of bridge in channel = 13.1 ftVcD50channel = Ku*(Ychannel^(1/6))*(D50channel^(1/3)) 1.1 ft/sContraction scour equation for channel = Live Bed Equation

Live Bed Equation

Q1 channel = Flow in the upstream channel transporting sediment = 12900 ft^3/sQ2 channel = Flow in the contracted channel = transporting sediment

= 12900 ft^3/sW1 channel = top width of the upstream channel that is transporting

bed material = 100 ftW2 channel = top width of the contracted channel section less pier

widths = 100 ftω channel = fall velocity of bed material based on D50 = 0.03 ft/sS channel = slope of energy grade line in main channel = 0.003 ft/ftV* channel = shear velocity in the upstream channel section =

(Ychannel*g*S channel)^.5 = 1.1 ft/sV* channel/ω channel = 37.8k1 channel = (if V*/ω <0.5, 0.59, if(0.5<=V*/ω<=2,0.64,0.69)) = 0.69Y2channel = average depth in contracted section after scour =

Ychannel*((Q2 channel/Q1 channel)^(6/7))*((W1 channel/W2

channel)^k1 channel) = 13.1 ftYs channel = Y2 channel - Yo channel = 0.0 ft

Clear Water EquationKu = constant = 0.0077 (English) or 0.025 (SI) = 0.0077Q = Discharge through bridge associated with the width W = 12900 ft^3/sDm = Diameter of the smallest non transportable particle in the bed

material in contracted section = 1.25*d50 = 0.000308 ftW = Bottom width of contracted section less pier widths = ftY2channel = average depth in contracted section after scour =

((Ku*(Q^2))/((Dm^(2/3))*(W^2)))^(3/7) = n/a ftYs channel = Y2 channel - Yo channel = n/a ft

Bridge St Scour Analysis-AltA - Contraction-coarse grain 9/29/2016



Phone: 925.941.0017

Fax: 925.941.0018

www.wreco.com


San Luis Obispo, CaliforniaContraction Scour100-year FlowCalculation guideline from HEC-18 5th EditionInput from HEC-RAS for Proposed Alternative B: Rehabilitation

Units = (SI or English) EnglishKu = constant = 6.19 (SI) or 11.17 (English) 11.17g = acceleration due to gravity = 32.2 ft/s^2

ChannelVchannel = Mean velocity of flow in main channel just upstream of

bridge = 9.5 ft/sD50channel = grain size in channel for which 50% of bed material is

finer = 0.0002 ftYochannel = existing depth in the contracted channel section before

scour = 13.9 ftYchannel = depth of flow just upstream of bridge in channel = 14.2 ftVcD50channel = Ku*(Ychannel^(1/6))*(D50channel^(1/3)) 1.1 ft/sContraction scour equation for channel = Live Bed Equation

Live Bed Equation

Q1 channel = Flow in the upstream channel transporting sediment = 12900 ft^3/sQ2 channel = Flow in the contracted channel = transporting sediment

= 12900 ft^3/sW1 channel = top width of the upstream channel that is transporting

bed material = 111 ftW2 channel = top width of the contracted channel section less pier

widths = 79 ftω channel = fall velocity of bed material based on D50 = 0.03 ft/sS channel = slope of energy grade line in main channel = 0.002 ft/ftV* channel = shear velocity in the upstream channel section =

(Ychannel*g*S channel)^.5 = 1.0 ft/sV* channel/ω channel = 36.7k1 channel = (if V*/ω <0.5, 0.59, if(0.5<=V*/ω<=2,0.64,0.69)) = 0.69Y2channel = average depth in contracted section after scour =

Ychannel*((Q2 channel/Q1 channel)^(6/7))*((W1 channel/W2

channel)^k1 channel) = 18.0 ftYs channel = Y2 channel - Yo channel = 4.1 ft

Clear Water EquationKu = constant = 0.0077 (English) or 0.025 (SI) = 0.0077Q = Discharge through bridge associated with the width W = 12900 ft^3/sDm = Diameter of the smallest non transportable particle in the bed

material in contracted section = 1.25*d50 = 0.000308 ftW = Bottom width of contracted section less pier widths = ftY2channel = average depth in contracted section after scour =

((Ku*(Q^2))/((Dm^(2/3))*(W^2)))^(3/7) = n/a ftYs channel = Y2 channel - Yo channel = n/a ft

Bridge St Scour Analysis-AltB - Contraction-coarse grain 9/29/2016



Phone: 925.941.0017

Fax: 925.941.0018

www.wreco.com


San Luis Obispo, CaliforniaLocal Scour at Piers - Cohesionless

100-year Flow

Calculation guideline from HEC-18 5th EditionInput from HEC-RAS for Proposed Alternative B: Rehabilitation

Units = (SI or English) = EnglishComplex Pier Scour? (Yes or No) No

Pier Scour component Bent 2Water Surface Elevation 98.4 ftGround Elevation at Pier 83.0 ftContraction Scour Depth 4.1 ftho = height of pile cap above bed at beginning of computation

(negative indicates partially or entirely submerged pile cap) = n/a fta = pier width = 5.0 fth1 = ho+T=height of the pier stem above the bed before scour

(negative indicates top of pile cap is below bed elevation) = n/a fty1 = Approach flow depth at the beginning of computations = 15.4 ftV1 = Approach velocity used at the beginning of computations = 8.2 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 1.00Ө = angle of attack of flow = 0 degreesPier shape Square noseK1 = correction factor for pier nose shape = 1.1L = length of pier = 5.0 ftL/a (if L/a is larger than 12, then use 12 as a maximum) 1K2 = correction factor for angle of attack = (cosӨ+(L/a)*sinӨ)^0.65 1.0K3 = correction factor for bed condition = 1.1D50 = grain size for which 50% of bed material is finer = 0.0002 ftD95 = grain size for which 95% of bed material is finer = 0.0066 ftKu = constant = 6.19 (SI) or 11.17 (English) 11.17VcD50 = Ku*(y1^(1/6))*(D50^(1/3)) 1.1 ft/sVcD95 = Ku*(y1^(1/6))*(D95^(1/3)) 3.3 ft/sg = acceleration due to gravity = 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)) = 11.7 ft

Bridge St Scour Analysis-AltB - Pier-Cohesionless 9/29/2016


October 2016

Appendix E Rock Slope Protection Calculations

Bridge Street Bridge Rehabilitation Project over Arroyo Grande Creek, Proposed Alternative A

Rock Slope Protection Calculations for Banks (California Bank and Shore Protection)

Location Upstream Upstream Face Downstream Face Downstream

W (lb) 4 4 6 6

RSP Class Backing No. 3 Backing No. 3 Backing No. 2 Backing No. 2

HEC-23

D50 23 23 26 26 median stone diameter, inches

RSP Class 1/2 ton 1/2 ton 1 ton 1 ton rock class

Selected RSP Size

Outer layer 1/2 ton 1/2 ton 1/2 ton 1/2 ton

Fabric Type B B B B

Thickness Per HEC-11

Thickness should not be less than the larger of 1.5 times D50 or D100.

D50 = 2.25 2.25 2.25 2.25 ft

1.5 * D50 = 3.4 3.4 3.4 3.4 ft

D100= 2.85 2.85 2.85 2.85 ft

Thickness Per CABS for outer layer

Thickness 4.3 4.3 4.3 4.3 ft

Placement B B B B

Inner layer

Inner layer not required for 1/2 ton

Backing

Backing class no. 1

Backing 1 has a thickness of 1.8 ft

Rock Slope Protection Calculations for AbutmentsBridge Street Bridge Rehabilitation Project over Arroyo Grande Creek, Proposed Alternative A

Location Upstream Upstream Face Downstream Face DownstreamV 9.9 9.9 10.6 10.7 ft/sg 32.2 32.2 32.2 32.2 ft/s2

y 13.1 13.1 12.4 12.4 ftFr 0.48 0.48 0.53 0.53

Isbash Isbash Isbash Isbash from HEC-23

For Froude Numbers (V/(gy)1/2)<=0.80, Isbash relationshipy 13.1 13.1 12.4 12.4 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 9.9 9.9 10.6 10.7 average velocity in contracted section, ft/sg 32.2 32.2 32.2 32.2 gravitational acceleration, ft/s2

D50 1.9 1.9 2.2 2.2 median stone diameter, ftD50 22.6 22.7 26.1 26.2 median stone diameter, inches

1/2 ton 1/2 ton 1 ton 1 ton rock class

For Froude Numbers (V/(gy)1/2)>0.80, Equation 14.2y 13.1 13.1 12.4 12.4 depth of flow in the contracted bridge opening, ftK 0.7 0.7 0.7 0.7 (0.61 for spill-through abutment, 0.69 or for vertical wall abutment)Ss 2.7 2.7 2.7 2.7 specific gravity of rockV 9.9 9.9 10.6 10.7 average velocity in contracted section, ft/sg 32.2 32.2 32.2 32.2 gravitational acceleration, ft/s2

D50 N/A N/A N/A N/A median stone diameter, ftD50 N/A N/A N/A N/A median stone diameter, inches

gyV

SyKDs

2

50 )1(

Rock Slope Protection Calculations for Banks (California Bank and Shore Protection)Bridge Street Bridge Rehabilitation Project over Arroyo Grande Creek, Proposed Alternative A

Location Upstream Upstream Face Downstream Face DownstreamStorm Event 100-year 100-year 100-year 100-yearVM (ft/s) 9.9 9.9 10.6 10.7Flow Condition Impinging Impinging Impinging ImpingingV (ft/s) 13.2 13.2 14.2 14.2SG 2.7 2.7 2.7 2.7r (degrees) 70 70 70 70a (degrees) 30 30 30 30W (lb) 239.8 241.3 367.4 373.6RSP Class 1/4 ton 1/4 ton 1/4 ton 1/4 ton

Location Upstream Upstream Face Downstream Face DownstreamStorm Event 100-year 100-year 100-year 100-yearVM (ft/s) 9.9 9.9 10.6 10.7Flow Condition Parallel Parallel Parallel ParallelV (ft/s) 6.6 6.6 7.1 7.1SG 2.7 2.7 2.7 2.7r (degrees) 70 70 70 70a (degrees) 30 30 30 30W (lb) 3.9 3.9 6.0 6.1RSP Class Backing No. 3 Backing No. 3 Backing No. 2 Backing No. 2

W = minimum rock mass, poundsVM = average channel velocity, ft/sV = velocity to which bank is exposed, ft/s

(for impinging flow, multiply VM by 1.33)SG = specific gravity of rockr = 70 degrees (for randomly placed rubble, a constant)a = outside slope face angle with horizontal, degrees

x yD (ft)= 1.7 1q (a) = 30.47

)()1(00002.0

33

6

arSINSGSGVW

Bridge Street Bridge Rehabilitation Project over Arroyo Grande Creek, Proposed Alternative B

Rock Slope Protection Calculations for Banks (California Bank and Shore Protection)

Location Upstream Upstream Face Downstream Face Downstream

W (lb) 5 12 7 5

RSP Class Backing No. 2 Backing No. 3 Backing No. 3 Backing No. 3

HEC-23

D50 21 28 23 20 median stone diameter, inches

RSP Class 3/8 ton 1 ton 1/2 ton 3/8 ton rock class

Selected RSP Size

Outer layer 1/2 ton 1/2 ton 1/2 ton 1/2 ton

Fabric Type B B B B

Thickness Per HEC-11

Thickness should not be less than the larger of 1.5 times D50 or D100.

D50 = 2.25 2.25 2.25 2.25 ft

1.5 * D50 = 3.4 3.4 3.4 3.4 ft

D100= 2.85 2.85 2.85 2.85 ft

Thickness Per CABS for outer layer

Thickness 4.3 4.3 4.3 4.3 ft

Placement B B B B

Inner layer

Inner layer not required for 1/2 ton

Backing

Backing class no. 1

Backing 1 has a thickness of 1.8 ft

Rock Slope Protection Calculations for AbutmentsBridge Street Bridge Rehabilitation Project over Arroyo Grande Creek, Proposed Alternative B

Location Upstream Upstream Face Downstream Face DownstreamV 9.5 11.0 10.0 9.3 ft/sg 32.2 32.2 32.2 32.2 ft/s2

y 14.2 13.9 13.7 13.9 ftFr 0.45 0.52 0.48 0.44

Isbash Isbash Isbash Isbash from HEC-23

For Froude Numbers (V/(gy)1/2)<=0.80, Isbash relationshipy 14.2 13.9 13.7 13.9 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 9.5 11.0 10.0 9.3 average velocity in contracted section, ft/sg 32.2 32.2 32.2 32.2 gravitational acceleration, ft/s2

D50 1.7 2.3 1.9 1.7 median stone diameter, ftD50 21.0 27.9 23.2 20.1 median stone diameter, inches

3/8 ton 1 ton 1/2 ton 3/8 ton rock class

For Froude Numbers (V/(gy)1/2)>0.80, Equation 14.2y 14.2 13.9 13.7 13.9 depth of flow in the contracted bridge opening, ftK 0.7 0.7 0.7 0.7 (0.61 for spill-through abutment, 0.69 or for vertical wall abutment)Ss 2.7 2.7 2.7 2.7 specific gravity of rockV 9.5 11.0 10.0 9.3 average velocity in contracted section, ft/sg 32.2 32.2 32.2 32.2 gravitational acceleration, ft/s2

D50 N/A N/A N/A N/A median stone diameter, ftD50 N/A N/A N/A N/A median stone diameter, inches

gyV

SyKDs

2

50 )1(

Rock Slope Protection Calculations for Banks (California Bank and Shore Protection)Bridge Street Bridge Rehabilitation Project over Arroyo Grande Creek, Proposed Alternative B

Location Upstream Upstream Face Downstream Face DownstreamStorm Event 100-year 100-year 100-year 100-yearVM (ft/s) 9.5 11.0 10.0 9.3Flow Condition Impinging Impinging Impinging ImpingingV (ft/s) 12.7 14.6 13.3 12.4SG 2.7 2.7 2.7 2.7r (degrees) 70 70 70 70a (degrees) 38 38 38 38W (lb) 319.2 750.1 431.1 281.1RSP Class 1/2 ton 1 ton 1/2 ton 1/2 ton

Location Upstream Upstream Face Downstream Face DownstreamStorm Event 100-year 100-year 100-year 100-yearVM (ft/s) 9.5 11.0 10.0 9.3Flow Condition Parallel Parallel Parallel ParallelV (ft/s) 6.4 7.4 6.7 6.3SG 2.7 2.7 2.7 2.7r (degrees) 70 70 70 70a (degrees) 38 38 38 38W (lb) 5.2 12.3 7.0 4.6RSP Class Backing No. 2 Backing No. 3 Backing No. 3 Backing No. 3

W = minimum rock mass, poundsVM = average channel velocity, ft/sV = velocity to which bank is exposed, ft/s

(for impinging flow, multiply VM by 1.33)SG = specific gravity of rockr = 70 degrees (for randomly placed rubble, a constant)a = outside slope face angle with horizontal, degrees

x yD (ft)= 1.3 1q (a) = 37.57

)()1(00002.0

33

6

arSINSGSGVW


October 2016

Appendix F Grading Plans for Alternatives

BRIDGE STREET

SECTION B-B

SECTION A-A

AT BRIDGE STREET

ARROYO GRANDE CREEK BRIDGE REPLACEMENT

65% PLANS

CONTOUR GRADING AND RSP PLAN

G-1

BRIDGE STREET

SECTION A-A

SECTION B-B

AT BRIDGE STREET

ARROYO GRANDE CREEK BRIDGE REHABILITATION

65% PLANS

CONTOUR GRADING AND RSP PLAN

G-1

bridge design hydraulic study report

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