Contra Costa County, California Hydrologic Analyses. FEMA Region IX. Hydrologic Analyses Contra Costa County, California

Transcription

1 FEMA Region IX Hydrologic Analyses Contra Costa County, California CONTRACT NUMBER: HSFEHQ 09 D 0368 TASK ORDER HSFE09 09 J 0001 October 2011

2 Document History Document Location Location Z:\Risk MAP Production\REGION 9\CALIFORNIA\CONTRA COSTA COUNTY\ S\Hydrology Revision History Version Number Version Date Summary of Changes Team/Author 01 04/11/ st Draft K. Labuhn 02 10/1/2011 Client Distribution Name Title/Organization Location Eric Simmons FEMA MIP, see Appendix C.

3 Table of Contents 1. TASK SUMMARY INTRODUCTION SCOPE OF WORK UPDATES TO SCOPE OF WORK WATERSHED LOCATIONS AND DESCRIPTIONS BRUSHY CREEK FRISK CREEK KELLOGG CREEK MT. DIABLO CREEK DEM PREPROCESSING HEC-GEOHMS SUBBASIN DELINEATION HEC-HMS MODEL SETUP SOIL MOISTURE ACCOUNTING LOSS METHOD USER SPECIFIED S-CURVE TRANSFORM METHOD BASEFLOW METHOD RESERVOIRS REACHES RAINFALL DATA AND DISTRIBUTION MARSH CREEK RESULTS COMPARISON TO PREVIOUS FLOW CALCULATIONS Appendices Appendix A Appendix B Appendix C Appendix D Hydrologic Analysis Appendices A 1 Landuse and Infiltration Rate Calculations A 2 Lag Time Calculations A 3 Reservoir Information A 4 Reach Information TSDN Documents B 1 Deliverables Checklist B 2 Contact Report List B 3 Hydrologic Analyses Index B 4 Certification of Compliance C 1 Hydrologic Analyses QA/QC Reviews Digital Data on the MIP May 2011 i

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5 1. Task Summary 1.1. Introduction Contra Costa County, California Hydrologic Analyses BakerAECOM has completed the Hydrologic Analyses activities in accordance with Task Order HSFE09 09 J 0001 for Contra Costa County, California under Contract No. HSFEHQ 09 D The project location and a detailed map of the county are shown in Figure Scope of Work Scope: The primary tasks are to conduct detailed hydrologic analyses of four streams (Brushy, Frisk, Kellogg and Mt. Diablo Creeks), update previous model for Marsh Creek and to perform QC of the study currently being conducted by the county for Wildcat and San Pablo Creeks. Specific tasks will include: Compare discharges calculated by the county against results of the HEC HMS model for Brushy, Frisk, Mt. Diablo and Kellogg Creeks Update the HEC HMS model for Marsh Creek to reflect existing landuse conditions (based on the 2008 aerial photos provided by the county) and edit the storage data on Sand Creek to reflect existing conditions Review the hydrologic analysis conducted by the county for Wildcat and San Pablo Creeks as part of the levee certification process Standards: Hydrologic Data Development work shall be performed in accordance with the standards specified in Section 4 Standards. The DCS must be met for this deliverable to be acceptable. Deliverables: BakerAECOM shall make the products available to FEMA and any other deliverables associated with this activity that are defined in the updated Appendix M (Data Capture Standards) by uploading the digital data to the MIP Updates to Scope of Work The hydrologic analysis submitted by Contra Costa County for Wildcat and San Pablo Creeks is not currently included in this document. July

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7 Figure 1. Scoping Map July

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9 2. Watershed Locations and Descriptions 2.1. Brushy Creek Figure 2 below shows the Brushy Creek watershed. The creek begins in the Canada de los Vaqueros hills near the border of Contra Costa and Alameda Counties and flows in a generally north to northeast direction until its confluence with Clifton Court Forebay. The watershed is approximately 16.4 square miles and is composed mostly of public lands/open space, agricultural lands and some low density residential development. The Byron Airport is partially located within the Brushy Creek watershed. Figure 2. Brushy Creek Watershed 2.2. Frisk Creek Figure 3 below shows the Frisk Creek watershed. The creek begins in the Canada de los Vaqueros hills and flows in a generally north to northeast direction until it reach the east side of Byron Highway where it turns and flows due north to its confluence with Discovery Bay. The watershed is approximately 12.2 square miles and is composed mostly of public lands/open space, agricultural lands and some low density residential development. The community of Byron is located in the Frisk Creek watershed. July

10 Figure 3. Frisk Creek Watershed 2.3. Kellogg Creek Figure 4 below shows the Kellogg Creek watershed. The Los Vaqueros Reservoir is a drinking water storage reservoir that also captures the upstream flows from Kellogg Creek. Kellogg Creek flows in a generally northern direction until just south of Marsh Creek Road (Highway 4) where it turns to the south and then flows east through a manmade watercourse to its confluence with Discovery Bay. The watershed is approximately 32.2 square miles and is composed mostly of public lands/open space, agricultural lands and some low density residential development. July

11 Figure 4. Kellogg Creek Watershed 2.4. Mt. Diablo Creek Mt. Diablo Creek starts in the Mount Diablo State Park and flows to the northwest to its confluence with Suisun Bay. The watershed is located in the City of Concord and Contra Costa County and encompasses land that was part of the former Concord Naval Weapons Base. That base has been decommissioned and will eventually become property of the City of Concord and the county. The watershed is 32.7 square miles and consists of residential and commercial lands in the headwaters and open space on the former Concord Naval Weapons Base lands. Mt. Diablo Creek was studied previously by FEMA but that study ended at Bailey Road and did not show any flood hazards on the Naval Weapons Base. This study is intended to determine the flood hazards downstream of Kirker Pass Road to Mt. Diablo Creek s confluence with Suisun Bay. July

12 2.5. DEM Preprocessing Figure 5. Mt. Diablo Creek Watershed Since Brushy, Frisk and Kellogg Creeks are adjacent to one another the tile digital elevation models (DEMs) provided by Contra Costa County were mosaiced together to create one DEM to use for the analysis. There were some areas where the tiles did not meet each other and there were other small areas of missing data. In order to fill in the missing elevation data from some grid cells the raster calculator focalmean function was used. The focalmean function looks at the elevation data in the cells surrounding the empty cell and calculates a mean value for that empty cell. In this case the focalmean statement was written to look at a 3 cell by 3 cell rectangle surrounding the empty cell and calculate the mean. This filled in the necessary missing data in the DEM and could then be used for further analysis. A similar process was used to construct a DEM for the Mt. Diablo Creek watershed. July

13 2.6. HEC-GeoHMS Subbasin Delineation Once the DEMs were constructed the next step was to delineate the subbasins for each creek. In order to do this the ArcGIS extension HEC GeoHMS was used. The standard process for HEC GeoHMS was followed including performing a fill operation to fill in the sink in the DEMs and then the tools were run to define the streams and determine the subbasins. Once the draft streams and subbasins were determined a manual review of the stream locations and subbasins was completed and a number of edits were made to correct the automated process. In particular, the DEMs did not always locate the streams correctly as shown on the provided aerial photos so these were corrected and the subbasins edited manual as needed. Some of the smaller subbasins were combined and a few were split at points were flows were needed HEC-HMS Model Setup A number of discussions were held with Contra Costa County to determine which loss and transform methods would be most appropriate for use in the county. Typically FEMA will use the NRCS curve number and unit hydrograph method but Contra Costa County has a large library of data concerning the hydrology of the county. They have provided guidance regarding methods that have been used previously in the county to calculate flows. A review of these methods (Reference 1) shows that they are appropriate for in this project. The loss method used is the Soil Moisture Accounting method and the transform method is the User Specific S curve. The necessary inputs for these methods are land use and Manning s n data. The only exception to this methodology was for subbasin 1 in the Kellogg Creek watershed. Subbasin 1 is the direct drainage area to the Los Vaqueros Reservoir and for this subbasin the NRCS methods were used. The curve number and lag time were obtained from the HEC 1 model completed for a previous LOMR submittal (case number P). Additionally, Mt. Diablo Creek was previously studied by FEMA and there are an effective discharge values available, with the most downstream available discharge at Bailey Road. In order to determine if this flow was still reasonable two models were created for Mt. Diablo Creek. The first followed the methods described in this section and the section treated the flow upstream of Bailey Road as a constant source input using the effective flow Soil Moisture Accounting Loss Method The Soil Moisture Accounting loss method in HEC HMS allows for a number of inputs including storage in different units such as canopy, surface, soil and groundwater storage. For flood events the only inputs that would have an appreciable impact on flows are the infiltration rate and soil storage amount (or initial infiltration loss). The County provided BakerAECOM with a conversion chart that links landuse with minimum, average and maximum infiltration rates. For the Brushy, Frisk, Kellogg and Mt. Diablo Creek watersheds the landuse shapefile was intersected with the subbasin shapefile to find the landuse by subbasin. Then area weighted average infiltration rate was calculated based on the average values provided by the County. Appendix A 1 provides tables of this data. The initial infiltration loss was set to a standard 0.25 inches which comes from the County s standard. July

14 Contra Costa County provided BakerAECOM with a landuse shapefile names GPLU_Edited. As discussed with the County, this is a shapefile of landuse designations that come from the Contra Costa County general plan and also from information provided by the incorporated cities in the County. This shapefile is of the zoned/planned landuse (i.e., future conditions), not necessarily how the land is currently being used. Since FEMA models existing, not future conditions, the landuse categories were reviewed against the aerial photos provided by the County to determine if the planned landuse compared to the current landuse. In the Frisk and Kellogg Creek watersheds the planned landuse seemed to correspond to the current landuse and no changes were made. The Brushy Creek watershed extends into Alameda County to the south. A review of aerial photos in Alameda County shows no appreciable differences from Contra Costa County so the landuse type associated with subbasins in the Contra Costa County were extended into the subbasins in Alameda County. In the Mt. Diablo Creek watershed the area downstream of Bailey Road was reviewed and it was found that near the Mallard Reservoir there were some areas that are designated as commercial but currently appear to be open space. Additionally, there were a few areas that appeared to be dense residential but were planned as low density residential. Edits were made to the landuse as necessary. Appendix A 1 provides maps and tables showing the breakdown of landuse for each watershed User Specified S-Curve Transform Method Contra Costa County uses an S curve that was developed in a 1971 study of Walnut Creek by the U.S. Army Corps of Engineers. This is now the Contra Costa County Flood Control District standard. This curve was reviewed by BakerAECOM and found to be appropriate for use in this study. The curve is input as a percentage curve in HEC HMS as a table of paired data. The other parameter needed is the lag time. The Flood Control District uses the following formula to calculate lag time: 24.. Where: T lag Elapsed time from the centroid (or 50 percent of volume) of the effective rainfall to the centroid (or 50 percent of volume) of the resulting runoff (hours) N weighted watershed Manning s n coefficient L Length of the longest watercourse (miles) L ca Length along that drainage path from a point opposite the centroid of the watershed to the outlet point (miles) S Overall slope of the main watercourse (feet/mile) The length of the stream reaches was calculated using ArcGIS. ArcGIS was also used to find the centroid of each subbasin and a line was then drawn to intersect with the stream centerline. The distance between that point to the outlet, also calculated using ArcGIS, is L ca. The slope was found from creating a contour map of the DEMs and reading the upstream and downstream elevations. July

15 In addition to infiltration rates the County also provided a conversion table between landuse and Manning s n value. Similar to the infiltration rate calculations the area weighted average Manning s n value was calculated for each subbasin. Appendix A 2 contains maps and tables of the data used for these calculations Baseflow Method Baseflow is typically not included in calculations of flood events so the baseflow method was set to none for the Brushy, Frisk, Kellogg and Mt. Diablo Creek models Reservoirs There are two reservoirs in the studied watersheds. The first is the Los Vaqueros water supply reservoir in the headwaters of Kellogg Creek. The water in this reservoir is delivered and withdrawn by the canal system. This reservoir was previously modeled as part of a Letter of Map Revision case number P. The storage elevation and outlet information was taken from the HEC 1 model and input into the HEC HMS model (see Appendix A 3). The other reservoir of interest is Mallard Reservoir in the Mt. Diablo Creek watershed. Discussions with the water district (Reference 2) revealed that there are no outlet structures for this reservoir to the creek so it is not included in the modeling effort for Mt. Diablo Creek Reaches The Muskingum Cunge method was used to route the flood flows through the watershed. The length and slope were calculated as described in Section The Manning s n used was based on aerial photos of the area and guidance from Contra Costa County. The bottom width and side slopes were estimated from the two foot topographic data. Appendix A 4 contains tables of the input data Rainfall Data and Distribution The SCS method is the rainfall method most commonly used by FEMA. The two parameters necessary are the rainfall distribution curve type (I, IA, II or III) and the storm depth in inches. According to the figures in Appendix B of the NRCS publication Urban Hydrology for Small Watersheds the boundary between the Type I and IA distributions falls approximately through Contra Costa County. The County has reviewed the distributions and compared them to historical analyses done in the county. They have concluded that the Type I distribution is the most appropriate for the County and BakerAECOM agree. The 24 hour rainfall depths were estimated at a midpoint in the watershed and assumed to be an average for the entire watershed. In April 2011 the National Oceanic and Atmospheric Administration (NOAA) released a new analysis of rainfall depths in the western United States that supersedes the data in their publication Atlas 2. Rainfall frequency depths can now be obtained via latitude and longitude from NOAA s website at Table 1 below lists the locations and precipitation frequency estimates obtained from the NOAA website and used in the analysis. July

16 Table 1. Rainfall Frequency Depth Estimates Stream Brushy Creek Frisk Creek Kellogg Creek Mt. Diablo Creek Marsh Creek Location Latitude/longitude At Vasco Road At Vasco Road At Walnut Boulevard At Bailey Road At Union Pacific Railroad Crossing % 24 Hour Storm Depth (in) 2% 24 Hour Storm Depth (in) 1% 24 Hour Storm Depth (in) 0.2% 24 Hour Storm Depth (in) Marsh Creek Contra Costa County provided Baker/AECOM with a HEC HMS model of Marsh Creek but it was completed using future landuse conditions (expected maximum build out) and expansions to the Sand Creek retention basin (Reference 3). Baker/AECOM compared the planned landuse shapefile discussed above in Section to the aerial photos provided and it was found that there were many locations where the residential development currently built did not match the planned landuse density. For example, there were areas that were planned to be low density residential but the aerial photos showed that high density residential development had already occurred. Numerous changes were made to the land use so that it conformed to the current conditions in the watershed, especially in the residential areas. This resulted in changes to the infiltration rates and Manning s n values as compared to the model provided by the County. Appendices A 1 and A 2 provide further details regarding the changes to the landuse, infiltration rates, and Manning s n values. Baker/AECOM worked closely with the County to modify the elevation storage relationship for the Sand Creek retention basin using the submitted DEM data and GIS software. Additionally, the County provided original construction drawings for the basin to determine the outlet structure geometry and July

17 elevation. Appendix A 3 provides further details regarding the elevation storage calculations and the data regarding the outlets to the basin. Additionally, changes were made to the elevation storage, storage discharge curves for some of the reservoirs in the model, specifically, the Vinyards North and South Reservoirs, Deer Creek Basin, the Fairview Basin, the Freedom Basin, and the Laurel Basin. The changes were made based on a spreadsheet of basin information provided by the county, the DEM data for the Marsh Creek area and best engineering judgment to extrapolate data points from the submitted data. Appendix A 3 has further information regarding the changes made to each reservoir. There is one area in the Marsh Creek watershed, noted as Drainage Area 52D that was included in the submitted HEC HMS model but was ultimately not modeled with HEC HMS due to the complexity of the retention pond hydraulics. This area was modeled using EPASWMM and the County is working to submit the model to BakerAECOM so that that it can be run for the FEMA storm events. Due to the size of the retention ponds it is not expected that these flows will be significant portion of the flow to Marsh Creek but they should be included to document the worst case scenario expected. As soon as the model is received this report will be updated to include those flows. 3. Results The results of the HEC HMS modeling are listed below in Table 2 and Table 3. Table 2. HEC HMS Peak Flows at Junctions for Brushy, Frisk, Kellogg and Mt. Diablo Creeks Flooding Source and Junction Number Drainage Area (square miles) 10% Annual Chance Flood Peak Discharges (cfs) 2% Annual Chance Flood 1% Annual Chance Flood 0.2% Annual Chance Flood Brushy Creek , , , , , , , , , , , , , , , , , , ,097.9 Outlet , , , ,203.4 Frisk Creek , , , , , , , , , ,602.8 Outlet , , ,175.9 July

18 Flooding Source and Junction Number Drainage Area (square miles) 10% Annual Chance Flood Peak Discharges (cfs) 2% Annual Chance Flood 1% Annual Chance Flood 0.2% Annual Chance Flood Kellogg Creek , , , , , , , , , , , , , , , , , , , , , , , ,108.4 Outlet , , , ,083.6 Mt. Diablo Creek Effective Flow , , , , , , , , , , , , , , , , , , , , , , , ,232.8 Outlet , , , ,787.2 Location Table 3. Results of HEC HMS Modeling for Marsh Creek at Effective Locations and Road Crossings Drainage Area (square miles) 10% Annual Chance Flood Peak Discharges (cfs) 2% Annual Chance Flood 1% Annual Chance Flood 0.2% Annual Chance Flood At Concord Boulevard At Balfour Road , , ,117.4 At Central Avenue , , , ,699.6 At Union Pacific , , , ,204.2 Railroad At Delta Road , , , ,610.9 At Santa Fe Railroad , , , , Comparison to Previous Flow Calculations The Contra Costa County Flood Control District provided Baker/AECOM with some flow estimates for Frisk, Brushy and Kellogg Creeks and there are flows published for Marsh Creek in the effective FIS. Table 4 below compares the new HEC-HMS flows to those previously calculated. The comparison locations were not exact for Brushy Creek because the Flood Control District calculated the flows to the Southern Pacific Railroad but the HEC-HMS model was not setup to report the flows at that point, instead the flows from the next upstream junction are reported below. July

19 In 2002, a Letter of Map Revision (LOMR) was submitted that included modeling of Kellogg Creek with and without the Los Vaqueros Reservoir in order to revise the floodplain at the downstream end of Kellogg Creek. That LOMR was approved and the modeling was obtained for review in this study. That LOMR only calculated the 1% annual chance flood discharge and that information is listed below. July

20 Table 4. Comparison of HEC-HMS Discharges to Previously Calculated Discharges Flooding Source and Location Brushy Creek Upstream of Byron Hot Springs Frisk Creek At the Southern Pacific Railway Kellogg Creek At the Southern Pacific Railway 10% Annual Chance Flood HEC HMS Peak Discharges (cfs) 2% Annual Chance Flood 1% Annual Chance Flood 0.2% Annual Chance Flood Location 1,447 3,195 4,017 6,147 At the Southern Pacific Railway 593 1,378 1,741 2,661 At the Southern Pacific Railway 1,097 2,474 3,108 4,736 At the Southern Pacific Railway 10% Annual Chance Flood Previously Calculated Peak Discharges (cfs) 2% Annual Chance Flood 1% Annual Chance Flood 0.2% Annual Chance Flood 1,480 2,580 2,950 N/A 660 1,170 1,310 N/A N/A N/A 1,609 N/A Marsh Creek At Balfour Road 919 1,366 1,568 2,117 At Balfour Road 890 1,900 2,500 5,100 At Union Pacific Railroad At Santa Fe Railroad 1,721 2,751 3,679 6,204 At Union Pacific Railroad 2,476 4,215 5,029 7,209 At Santa Fe Railroad *the effective FIS notes that the 2, 1, and 0.2% annual chance flows are reduced due to overbank spills and non returning flows 2,100 4,200 5,200 8,300 2,300 4,000* 4,000* 4,000* July

21 Since the 1% annual chance flows for Kellogg Creek have increased significantly from the values calculated in the 2002 LOMR, the inputs for the LOMR HEC 1 model were compared to the inputs to the HEC HMS model to determine potential explanations. The first difference between the two models was the precipitation values used. The HEC 1 model used different precipitation values for different subbasins, starting with 4.8 inches in the headwater subbasin and ending with 3.4 inches at the outlet. The HEC HMS model used a constant precipitation value of 4.53 inches for all subbasins. Additionally, the subbasins in the HEC 1 model used the SCS s curve number and unit hydrograph methods while the HEC HMS model used the methods recommended by Contra Costa County (the soil moisture accounting method and the County derived S Curve). This has resulted in large differences in flows from subbasins of similar size. Table 5 below shows some comparisons between subbasin size and calculated flow rate. These differences appear to explain the change in flows between the LOMR HEC 1 and HEC HMS models. Table 5. Comparisons of 1% Annual Chance Flows at Kellogg Creek Subbasins between HEC 1 and HEC HMS Subbasin Number HEC 1 Subbasin Size (sq miles) Flow (cfs) Subbasin Number HEC HMS Subbasin Size (sq miles) Flow (cfs) The currently effective FIS for Marsh Creek lists flows at 3 locations and those values are also listed in Table 4. The effective flows were calculated using the unit hydrograph method where flood hydrographs were developed for the upper reaches of Marsh Creek, routed through the Marsh Creek flood control reservoir and then summed with hydrographs developed for lower reaches. For the 10% annual chance event the newly calculated flows are comparable to the effective flows. For the 2% annual chance event the flows at Balfour Road have been reduced by about 30%. This is likely due to the five detention ponds and the Marsh Creek Reservoir that store approximately 1,893 acrefeet during the 2% annual chance event. Additionally, as discussed above for Kellogg Creek, the rainfall data has been updated which could result in changes to the flow rates. Similarly, at the Union Pacific Railroad the 2% annual chance flows have been reduced by about 35%. This is likely due to additional detention ponds storing approximately 453 acre feet during the 2% annual chance event. For the 1% annual chance event the flows have been reduced by 48 and 39%, respectively, at Balfour Road and the Union Pacific Railroad. For the 0.2% annual chance event the flows have been reduced by 59 and 25%, respectively, at Balfour Road and the Union Pacific Railroad due to detention pond storage. At the Santa Fe Railroad the effective discharges have been reduced due to overbank spills and non returning flows. Since the new modeling accounts for overbanks spills and nonreturning flows have been removed, the new flows calculated cannot be compared to the effective flows for the 2% annual chance event. July

22 References 1. Contra Costa County Flood Control and Water Conservation District. Verification of the District s Standards. Draft December Personal Communication (phone call). Contra Costa Water District (Mark Seedall). April 19, Contra Costa County Flood Control and Water Conservation District. Marsh Creek Hydrology Report GeoHMS and HEC HMS Analysis. May 10, (and accompanying HEC HMS model) July

23 Appendix A 1 Landuse and Infiltration Rate Calculations July

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25 Table A1 1. Brushy Creek Landuse Area (square miles) by Subbasin Subbasin Agricultural Land Delta Recreation Open Space Parks and Public/Semi Public Water Watershed* Number Recreation * watershed is a landuse type, not the total for the subbasin Table A1 2. Brushy Creek Landuse Fractions by Subbasin Subbasin Agricultural Land Delta Recreation Open Space Parks and Public/Semi Water Watershed Number Recreation Public July

26 Table A1 3. Brushy Creek Area Weighted Infiltration Rate by Subbasin/Landuse Subbasin Agricultural Delta Open Space Parks and Public/Semi Water Watershed Total Number Land Recreation Recreation Public Average Infiltration Rate (in/hr) Please note that the landuse coverage ends at the Alameda/Contra Costa County border. Aerial photos were reviewed to determine if the area of the watershed in Alameda County was being used differently than in Contra Costa County. There did not appear to be a difference in landuse between the two counties so the Contra Costa landuse was applied to the subbasins that fall either partly or entirely in Alameda County. July

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28 Table A1 4. Frisk Creek Landuse Area (square miles) by Subbasin Subbasin Number Agricultural Core Land Agricultural Land Commercial Light Industry Multiple Family Residential Low Multiple Family Residential Medium Open Space Subbasin Number Parks and Recreation Public/Semi Public Single Family Residential High Single Family Residential Medium Single Family Residential Very Low Water Watershed July

29 Table A1 5. Brushy Creek Landuse Fractions by Subbasin Subbasin Number Agricultural Core Land Agricultural Land Commercial Light Industry Multiple Family Residential Low Multiple Family Residential Medium Open Space Subbasin Number Parks and Recreation Public/Semi Public Single Family Residential High Single Family Residential Medium Single Family Residential Very Low Water Watershed July

30 Subbasin Number Agricultural Core Land Table A1 6. Frisk Creek Area Weighted Infiltration Rate by Subbasin/Landuse Agricultural Land Commercial Light Industry Multiple Family Residential Low Multiple Family Residential Medium Open Space Parks and Recreation Average Infiltration Rate (in/hr) July

31 Subbasin Number Table A1 6. Frisk Creek Area Weighted Infiltration Rate by Subbasin/Landuse (cont.) Public/Semi Public Single Family Residential High Single Family Residential Medium Single Family Residential Very Low Water Watershed Total Average Infiltration Rate (in/hr) July

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33 Table A1 7. Kellogg Creek Landuse Area (square miles) by Subbasin Subbasin Number Agricultural Core Agricultural Land Commercial Light Industry Multiple Family Office Park Open Space Land Residential Low Subbasin Number Parks and Recreation Public/Semi Public Single Family Single Family Water Watershed Residential High Residential Medium July

34 Table A1 8. Kellogg Creek Landuse Fractions by Subbasin Subbasin Number Agricultural Core Agricultural Land Commercial Light Industry Multiple Family Office Park Open Space Land Residential Low Subbasin Number Parks and Recreation Public/Semi Public Single Family Single Family Water Watershed Residential High Residential Medium July

35 Subbasin Number Agricultural Core Land Table A1 9. Kellogg Creek Area Weighted Infiltration Rate by Subbasin/Landuse Agricultural Land Commercial Light Industry Multiple Family Residential Low Office Park Open Space Parks and Recreation Average Infiltration Rate (in/hr) July

36 Subbasin Number Table A1 9. Kellogg Creek Area Weighted Infiltration Rate by Subbasin/Landuse (cont.) Public/Semi Public Single Family Residential High Single Family Residential Medium Water Watershed Total Average Infiltration Rate (in/hr) July

37 July

38 Table A1 10. Mt. Diablo Creek Landuse Area (square miles) by Subbasin Subbasin Number Agricultural Land Commercial Heavy Industry Landfill Light Industry Multiple Family Residential Low Multiple Family Residential Medium Office Park Subbasin Number Open Space Parks and Recreation Public/Semi Public Single Family Residential High Single Family Residential Low Single Family Residential Medium Single Family Residential Very Low July

39 Table A1 11. Mt. Diablo Creek Landuse Fractions by Subbasin Subbasin Number Agricultural Land Commercial Heavy Industry Landfill Light Industry Multiple Family Residential Low Multiple Family Residential Medium Office Park Subbasin Number Open Space Parks and Recreation Public/Semi Public Single Family Residential High Single Family Residential Low Single Family Residential Medium Single Family Residential Very Low July

40 Subbasin Number Agricultural Land Table A1 12. Mt. Diablo Creek Area Weighted Infiltration Rate by Subbasin/Landuse Commercial Heavy Industry Landfill Light Industry Multiple Family Residential Low Multiple Family Residential Medium Office Park Average Infiltration Rate (in/hr) July

41 Subbasin Number Table A1 12. Mt. Diablo Creek Area Weighted Infiltration Rate by Subbasin/Landuse (cont.) Open Space Parks and Recreation Public/Semi Public Single Family Residential High Single Family Residential Low Single Family Residential Medium Single Family Residential Very Low Average Infiltration Rate (in/hr) Total July

42 Table A1 13 Marsh Creek Original and Revised Landuse Areas (square miles) by Subbasin Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Agricultural Agricultural Business Commercial Agricultural Agricultural Business Commercial Core Land Land Park Core Land Land Park DSFairview 104 DSSpaL 104 LowBasin SpaL 104 UpBasin BtwdLk BtwdLk BtwdLk BtwdLk DeeratMC 105 DeerBasin DeerDam DSBtwdLk DSBtwdLk DSBtwdLk5 105 DSDeerBasin 105 DSDeerDam DryBasin DryDam Basin Basin DS8085Basin DSVYNorthSM DSVYSouthSM VYNorth VYSouth EofMCDam 108 MCDam A 2 30A BrownBas 30A LaurelBas C DSFVBasin C DSLibertyBas C FairviewBas C LibertyBas A A 2 52B C Basin C Basin C Basin C Lowest D Outlet MC MC MC MC MC MC MC MC MC MC MC July

43 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Agricultural Agricultural Business Commercial Agricultural Agricultural Business Commercial Core Land Land Park Core Land Land Park MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC at30alaruelout MC at30coutlet MC at52c MC at52d Outlet MC at52d OutletSM MC at8085basin MC atds7940basin MCatDS7940BasSM MC atvynorth MC atvysouth MC DSECCID MC ECCID MC Outlet MC US30ALaruelOut Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Light MultiFamily MultiFamily Mobile Light MultiFamily MultiFamily Mobile Industry Low Medium Homes Industry Low Medium Homes DSFairview 104 DSSpaL 104 LowBasin SpaL 104 UpBasin 105 BtwdLk BtwdLk BtwdLk BtwdLk5 105 DeeratMC DeerBasin DeerDam 105 DSBtwdLk DSBtwdLk DSBtwdLk5 105 DSDeerBasin 105 DSDeerDam DryBasin July

44 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Light MultiFamily MultiFamily Mobile Light MultiFamily MultiFamily Mobile Industry Low Medium Homes Industry Low Medium Homes 106 DryDam Basin Basin 107 DS8085Basin 107 DSVYNorthSM 107 DSVYSouthSM 107 VYNorth 107 VYSouth 108 EofMCDam 108 MCDam 30A A BrownBas 30A LaurelBas C DSFVBasin C DSLibertyBas C FairviewBas C LibertyBas 52A A B C Basin C Basin C Basin C Lowest 52D Outlet MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MCat30ALaruelOut MC at30coutlet MC at52c July

45 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Light MultiFamily MultiFamily Mobile Light MultiFamily MultiFamily Mobile Industry Low Medium Homes Industry Low Medium Homes MC at52d Outlet MC at52d OutletSM MC at8085basin MC atds7940basin MCatDS7940BasSM MC atvynorth MC atvysouth MC DSECCID MC ECCID MC Outlet MC US30ALaruelOut Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin MultiUse MultiFamily Office Open Space MultiUse MultiFamily Office Open Space Very High Very High DSFairview DSSpaL 104 LowBasin SpaL UpBasin BtwdLk BtwdLk BtwdLk BtwdLk DeeratMC DeerBasin DeerDam DSBtwdLk DSBtwdLk DSBtwdLk5 105 DSDeerBasin DSDeerDam DryBasin DryDam Basin Basin DS8085Basin DSVYNorthSM 107 DSVYSouthSM 107 VYNorth VYSouth 108 EofMCDam 108 MCDam A A BrownBas A LaurelBas 30C DSFVBasin 30C DSLibertyBas 30C FairviewBas C LibertyBas 52A A B C Basin July

46 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin MultiUse MultiFamily Office Open Space MultiUse MultiFamily Office Open Space Very High Very High 52C Basin C Basin C Lowest 52D Outlet MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MCat30ALaruelOut MC at30coutlet MC at52c MC at52d Outlet MC at52d OutletSM MC at8085basin MC atds7940basin MCatDS7940BasSM MC atvynorth MC atvysouth MC DSECCID MC ECCID MC Outlet3 MC US30ALaruelOut Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Parks Public Semi Single Single Parks Public Semi Single Single Recreation Public Family High Family Low Recreation Public Family High Family Low DSFairview DSSpaL July

47 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Parks Public Semi Single Single Parks Public Semi Single Single Recreation Public Family High Family Low Recreation Public Family High Family Low 104 LowBasin SpaL UpBasin BtwdLk BtwdLk BtwdLk BtwdLk DeeratMC DeerBasin DeerDam DSBtwdLk DSBtwdLk DSBtwdLk DSDeerBasin DSDeerDam DryBasin DryDam Basin Basin DS8085Basin DSVYNorthSM DSVYSouthSM 107 VYNorth VYSouth EofMCDam MCDam A A BrownBas A LaurelBas C DSFVBasin C DSLibertyBas C FairviewBas C LibertyBas A A B C Basin C Basin C Basin C Lowest D Outlet MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC July

48 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Parks Public Semi Single Single Parks Public Semi Single Single Recreation Public Family High Family Low Recreation Public Family High Family Low MC MC MC MC MC MC MC MC MC MC MC MC MCat30ALaruelOut MC at30coutlet MC at52c MC at52d Outlet MC at52d OutletSM MC at8085basin MC atds7940basin MCatDS7940BasSM MC atvynorth MC atvysouth MC DSECCID MC ECCID MC Outlet MC US30ALaruelOut Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Single Family Single Family Watershed Single Family Single Family Watershed Medium Very Low Medium Very Low DSFairview DSSpaL LowBasin SpaL UpBasin BtwdLk BtwdLk BtwdLk BtwdLk DeeratMC DeerBasin DeerDam 105 DSBtwdLk DSBtwdLk DSBtwdLk5 105 DSDeerBasin DSDeerDam 106 DryBasin DryDam Basin Basin DS8085Basin DSVYNorthSM July

49 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Single Family Single Family Watershed Single Family Single Family Watershed Medium Very Low Medium Very Low 107 DSVYSouthSM 107 VYNorth VYSouth 108 EofMCDam MCDam A A BrownBas A LaurelBas C DSFVBasin C DSLibertyBas C FairviewBas C LibertyBas A A B C Basin C Basin C Basin C Lowest 52D Outlet MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC at30alaruelout MC at30coutlet MC at52c MC at52d Outlet MC at52d OutletSM MC at8085basin MC atds7940basin MC atds7940bassm MC atvynorth July

50 Original Landuse (sq miles) Revised Landuse (sq miles) Subbasin Single Family Single Family Watershed Single Family Single Family Watershed Medium Very Low Medium Very Low MC atvysouth MC DSECCID MC ECCID MC Outlet3 MC US30ALaruelOut Subbasin Table A1 14. Marsh Creek Original versus Revised Infiltration Rates by Subbasin Original Infiltration Rate (in/hr) Revised Infiltration Rate (in/hr) Subbasin Original Infiltration Rate (in/hr) Revised Infiltration Rate (in/hr) MCDam DSFairview A DSSpaL A BrownBas LowBasin A LaurelBas SpaL C DSFVBasin UpBasin C DSLibertyBas BtwdLk C FairviewBas BtwdLk C LibertyBas BtwdLk A BtwdLk A DeeratMC B DeerBasin C Basin DeerDam C Basin DSBtwdLk C Basin DSBtwdLk C Lowest DSBtwdLk D Outlet DSDeerBasin MC DSDeerDam MC DryBasin MC DryDam MC Basin MC Basin MC DS8085Basin MC DSVYNorthSM MC DSVYSouthSM MC VYNorth MC VYSouth MC EofMCDam MC July

51 Table A1 14. Marsh Creek Original versus Revised Infiltration Rates by Subbasin (cont). Subbasin Original Infiltration Rate (in/hr) Revised Infiltration Rate (in/hr) MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC at30alaruelout MC at30coutlet MC at52c MC at52d Outlet MC at52d OutletSM MC at8085basin MC atds7940basin MC atds7940bassm MC atvynorth MC atvysouth MC DSECCID MC ECCID MC Outlet MC US30ALaruelOut July

52 Appendix A 2 Lag Time Calculations July

53 July

54 Table A2-1. Brushy Creek Area-Weighted Manning s N Calculations Subbasin Agricultural Delta Open Space Parks and Public/Semi Water Watershed Total Number Land Recreation Recreation Public Average Manning s n July

55 Table A2-2. Brushy Creek Lag Time Calculations Subbasin Number Stream Length (miles) Lca Length (miles) Upstream Elevation (feet) Downstream Elevation (feet) Slope (ft/mile) Manning s n Lag time (hours) July

56 July

57 Subbasin Number Agricultural Core Land Table A2 3. Frisk Creek Area Weighted Manning s N Calculations Agricultural Land Commercial Light Industry Multiple Family Residential Low Multiple Family Residential Medium Open Space Parks and Recreation Average Manning s N July

58 Subbasin Number Public/Semi Public Table A2 3. Frisk Creek Area Weighted Manning s N Calculations (cont.) Single Family Residential High Single Family Residential Medium Single Family Residential Very Low Water Watershed Total Average Manning s N July

59 Table A2-4. Frisk Creek Lag Time Calculations Subbasin Number Stream Length (miles) Lca Length (miles) Upstream Elevation (feet) Downstream Elevation (feet) Slope (ft/mile) Manning s n Lag time (hours) July

60 July

61 Subbasin Number Agricultural Core Land Table A2 5. Kellogg Creek Area Weighted Manning s N Calculations Agricultural Land Commercial Light Industry Multiple Family Residential Low Office Park Open Space Parks and Recreation Average Manning s N July

62 Subbasin Number Table A2 5. Kellogg Creek Area Weighted Manning s N Calculations (cont.) Public/Semi Public Single Family Residential High Single Family Residential Medium Water Watershed Total Average Manning s N July

63 Table A2-6. Kellogg Creek Lag Time Calculations Subbasin Number Stream Length (miles) Lca Length (miles) Upstream Elevation (feet) Downstream Elevation (feet) Slope (ft/mile) Manning s n Lag time (hours) July

64 July

65 Subbasin Number Agricultural Land Table A2 7. Mt. Diablo Creek Area Weighted Manning s N by Landuse Commercial Heavy Industry Landfill Light Industry Multiple Family Residential Low Multiple Family Residential Medium Office Park Average Manning s N July

66 Subbasin Number Open Space Contra Costa County, California Hydrologic Analyses Table A2 7. Mt. Diablo Creek Area Weighted Manning s N by Landuse (cont.) Parks and Recreation Public/Semi Public Single Family Residential High Single Family Residential Low Single Family Residential Medium Single Family Residential Very Low Average Manning s N Total July

67 Table A2-8. Mt. Diablo Creek Lag Time Calculations Subbasin Number Stream Length (miles) Lca Length (miles) Upstream Elevation (feet) Downstream Elevation (feet) Slope (ft/mile) Manning s n Lag time (hours) July

68 Table A2 9. Marsh Creek Original and Revised Manning s n and Lag Time by Subbasin Subbasin Original Manning s n Revised Manning s n Original lag time (hrs) Revised lag time (hrs) DSFairview DSSpaL LowBasin SpaL UpBasin BtwdLk BtwdLk BtwdLk BtwdLk DeeratMC DeerBasin DeerDam DSBtwdLk DSBtwdLk DSBtwdLk DSDeerBasin DSDeerDam DryBasin DryDam Basin Basin DS8085Basin DSVYNorthSM DSVYSouthSM VYNorth VYSouth EofMCDam MCDam A A BrownBas A LaurelBas C DSFVBasin C DSLibertyBas C FairviewBas C LibertyBas A A B C Basin C Basin July

69 Table A2 9. Marsh Creek Original and Revised Manning s n and Lag Time by Subbasin (cont.) Subbasin Original Manning s n Revised Manning s n Original lag time (hrs) Revised lag time (hrs) 52C Basin C Lowest D Outlet MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MC MCat30ALaruelOut MC at30coutlet MC at52c MC at52d Outlet MC at52d OutletSM MC at8085basin July

70 Table A2 9. Marsh Creek Original and Revised Manning s n and Lag Time by Subbasin (cont.) Subbasin Original Manning s n Revised Manning s n Original lag time (hrs) Revised lag time (hrs) MCatDS7940Basin MCatDS7940BasSM MC atvynorth MC atvysouth MC DSECCID MC ECCID MC Outlet MC US30ALaruelOut July

71 Appendix A 3 Reservoir Information July

72 EDITS TO RESERVOIR ELEMENTS IN THE MARSH CREEK MODEL Mark Boucher from the Contra Costa Flood Control and Water Conservation District provided a spreadsheet with the elevation storage discharge information for all of the reservoirs in the Marsh Creek model. Some of the reservoir curves had to be extended because the elevation computed with the HEC HMS model was above the elevation originally in the model. If data was available from the County then the curves were extended using that information. If there was no additional information then a combination of extrapolation and/or topographic information was used to extend the basin curves. The information below explains where the reservoir data came from and what changes were made. Basin Name in Model: 107 VYSouth Name from County information: Vinyards South Basin Source Information: County reports source as November 2005 Report by Balance Hydrologics for Vinyards at Marsh Creek Notes: the information provided by Contra Costa County was extended using the topographic data. Figure A3 1 below shows the 107 VYSouth basin and from the contour data it appears that the elevation storage discharge data provided by the county can be extrapolated to about elevation At 138 the basin will start to experience weir flow. The elevation storage relationship up to elevation 138 was maintained but at 138 the discharge would start to increase to account for the weir flow. At this point the discharge at 138 was calculated using the standard weir flow equation. For this situation: / C = 2.6 (average coefficient for a broad crested weir) L = 86 feet (measured in GIS) H = 0.5 feet Q = 79.0 cfs July

73 Figure A VY Basin with contours and weir location Figures A3 2 and A3 3 below show the changes to the elevation storage and storage discharge curves. The values are listed in the table A VY South Storage Discharge Storage (ac ft) Discharge (cfs) Modified Storage Discharge Curve Original Storage Discharge Curve Figure A3 2. Reservoir 107 VY South Storage Discharge Curve July

74 107 VY South Elevation Storage Storage (ac ft) Elevation (ft) Modified Elevation Storage Curve Original Elevation Storage Curve Figure A3 3 Reservoir 107 VYSouth Elevation Storage Curve Table A3 1. Reservoir 107 VYSouth Elevation Storage Discharge Data Elevation (ft) Storage (ac ft) Discharge (cfs) The last data point was added to the paired data tables in the model. July

75 Basin Name in Model: 107 VYNorth Name from County information: Vinyards North Basin Source Information: County reports source as November 2005 Report by Balance Hydrologics for Vinyards at Marsh Creek Notes: A review of the aerial photo and topographic data shows an elevation of about 124 to for most of the road that surrounds the basin. It was assumed that water will not overflow the basin at elevation 124 so the two curves were extended slightly to 124 and the corresponding data entered into the model. Figures A3 4 and A3 5 below show the changes to the elevation storage and storagedischarge curves. The values are listed in the table A VYNorth Storage Discharge Discharge (cfs) Storage (ac ft) Modified Storage Discharge Curve Original Storage Discharge Curve Figure A3 4. Reservoir 107 VYNorth Storage Discharge Curve July

76 Storage (ac ft) 107 VYNorth Elevation Storage Elevation (ft) Modified Elevation Storage Curve Original Elevation Storage Curve Figure A3 5. Reservoir 107 VYNorth Elevation Discharge Curve Table A3 2. Reservoir 107 VYNorth Elevation Storage Discharge Data Elevation (ft) Storage (ac ft) Discharge (cfs) The last data point was added to the paired data tables in the model. July

77 Basin Name in Model: 105 Deer Crk Basin Name from County information: None Source Information: County reports source as HYDRO2 Model in the Deer Creek Hydrology Report dated November 10, 1997, File No Notes: the information from the County was extended by using ArcGIS to create 0.5 foot contours from the DEMs provided by the County. The area was calculated at the 95 and 98 contours and additional storage values were calculated. The equation in the County s spreadsheet was used to calculate the corresponding discharge values. A review of the topography and a review of the area in Google StreetView seems to show that water will not start flooding over the roads until the elevation reaches about 99 feet so it was assumed that the discharge equation used by the County would still be accurate at 98 feet. Figures A3 6 and A3 7 below show the changes to the elevation storage and storagedischarge curves. The values are listed in the table A Deer Crk Basin Storage Discharge Discharge (cfs) Storage (ac ft) Modified Storage Discharge Curve Original Storage Discharge Curve Figure A3 6. Reservoir Deer Crk Storage Discharge Curve July

78 Deer Crk Basin Elevation Storage Storage (ac ft) Elevation (ft) Modified Elevation Storage Curve Original Elevation Storage Curve Figure A3 7. Reservoir Deer Crk Basin Elevation Storage Curve Table A3 3. Reservoir Deer Crk Elevation Storage Discharge Data Elevation (ft) Storage (ac ft) Discharge (cfs) The last two data points were added to the paired data tables in the model. July

79 Basin Name in Model: 30C Fairview Name from County information: None Source Information: County reports source as HYDRO2 Model of unknown date Notes: the information from the County was extended by using ArcGIS to create 0.5 foot contours from the DEMs. It appears the basin will not experience weir flow until the water reaches about 98.5so the elevation storage discharges curves were extended slightly up to 98.5 feet. At 99 the basin will start to experience weir flow with water overflowing to the east northeast toward the Union Pacific Railroad, see Figure A3 8 below. The elevation storage relationship up to elevation 98.5 was maintained but at 99 the discharge would start to increase to account for the weir flow. At this point the discharge at 99 was calculated using the standard weir flow equation noted above assuming an effective weir length of 500 feet. Figures A3 9 and A3 10 below show the changes to the elevation storage and storage discharge curves. The values are listed in the table A3 4. Figure A C Fairview Basin with contours and weir location July

80 30C Fairview Storage Discharge 500 Discharge (cfs) Storage (ac ft) Modified Storage Discharge Curve Original Storage Discharge Curve Figure A3 9. Reservoir 30C Fairview Storage Discharge 30C Fairview Elevation Storage Storage (ac ft) Elevation (ft) Modified Elevation Storage Curve Original Elevation Storage Curve Figure A3 10. Reservoir 30C Fairview Elevation Storage Curve July

81 Table A3 4. Reservoir 30C Fairview Elevation Storage Discharge Data Elevation (ft) Storage (ac ft) Discharge (cfs) The last two data points were added to the paired data tables in the model. July

82 Basin Name in Model: 30C Freedom (basin shapefile names this as 30C LibertyBas) Name from County information: None Source Information: County reports source as HYDRO2 Model of unknown date Notes: the information from the County was extended by using ArcGIS to create 0.5 foot contours from the DEMs. It appears the basin will not experience weir flow until the water reaches about 80.5 so the original elevation storage discharges curves were maintained up to 80 feet. At 80.5 it was assumed that the basin will start to experience weir flow with water overflowing to the east toward O Hara Street, see Figure A3 11 below. The elevation storage relationship up to elevation 80.5 was maintained but at 80.5 the discharge would start to increase to account for the weir flow. At this point the discharge at 80.5 was calculated using the standard weir flow equation noted above assuming an effective weir length of 150 feet. Figures A3 9 and A3 10 below show the changes to the elevation storage and storage discharge curves. The values are listed in the table A3 5. Figure A CFreedom Basin with contours and weir location July

83 30C Freedom Storage Discharge Discharge (cfs) Storage (ac ft) Modified Storage Discharge Curve Original Storage Discharge Curve Figure A3 12. Reservoir 30C Freedom Storage Discharge Curve 30C Freedom Elevation Storage 100 Storage (ac ft) Elevation (ft) Modified Elevation Storage Curve Original Elevation Storage Curve Figure A3 13. Reservoir 30C Freedom Elevation Storage Curve July

84 Table A3 5. Reservoir 30C Freedom Elevation Storage Discharge Data Elevation (ft) Storage (ac ft) Discharge (cfs) The last data point was added to the paired data tables in the model. July

85 Basin Name in Model: 30A Laurel Basin Name from County information: None Source Information: County reports source as HYDRO2 Model of unknown date Notes: the information from the County was extended by using ArcGIS to create 0.5 foot contours from the DEMs. Based on the topographic data it appears that the basin will be overflowing at 33 feet but due to the heavy vegetation to the northwest of the basin the LIDAR may be slightly off. It was assumed that the basin will not start overflowing until it reached 33.5 feet and then it will overflow to the northwest. Figure A3 14 below shows the 33 contour location and the assumed direction of overflow. The elevation storage relationship up to elevation 33.5 was maintained but at 33.5 the discharge would start to increase to account for the weir flow. At this point the discharge at 33.5 was calculated using the standard weir flow equation noted above assuming an effective weir length of 235 feet, figure A3 15 shows the location of the weir. Figures A3 16 and A3 17 below show the changes to the elevationstorage and storage discharge curves. The values are listed in the table A3 6. Figure A ALaurel Basin with contours and assumed direction of overflow July

86 Figure A A Laurel Basin with contours and weir location 30A Laurel Storage Discharge 250 Discharge (cfs) Storage (ac ft) Modified Storage Discharge Curve Original Storage Discharge Curve Figure A3 16. Reservoir 30A Laurel Bas Storage Discharge Curve July

87 30A Laurel Elevation Storage Storage (ac ft) Elevation (ft) Modified Elevation Storage Curve Original Elevation Storage Curve Figure A3 17. Reservoir 30A Laurel Bas Elevation Storage Curve Table A3 5. Reservoir 30A Laurel Bas Elevation Storage Discharge Data Elevation (ft) Storage (ac ft) Discharge (cfs) The last data point was added to the paired data tables in the model. July

88 Basin Name in Model: 104 UpSCBasin Source Information: topographic information and Sand Creek Storm Drain Improvements construction drawings dated July 1, 1994 Issue: The elevation storage discharge information in the model provided by the County reflected planned improvements to the Sand Creek basin. Since FEMA models existing conditions, the current elevation storage discharge relationships had to be calculated. To calculate the elevation storage relationships the submitted DEM was contoured at 0.5 feet intervals and the area of the basin at each 0.5 feet increment was totaled. To calculate the volume at each 0.5 foot increment the following formula was used: 6 Where: H = height between three successive 0.5 foot elevation increments B 1 = surface area of the base elevation B 2 = surface area of the middle elevation B 3 = surface area of the top elevation Total the cumulative storage volume at each 0.5 foot elevation yields the following elevation storage relationship, see Figure A3 17. The data used in the model is in Table A3 6. Elevation (ft) Sand Creek Elevation Storage Storage (acre ft) Figure A3 17. Sand Creek Elevation Storage Curve July

89 Table A3 5. Sand Creek Reservoir Elevation Storage Data Elevation (ft) Storage (ac ft) To calculate the discharge from the basin the outlet structures option was used. Contra Costa County provided a set of drawings titled Sand Creek Storm Drain Improvements dated July 1, 1994 that included information regarding the outlet from the basin. The major outlet is 42.4 feet of corrugated metal pipe that measures 54. The following information was used for this outlet, entrance and exit coefficients and Manning s n were obtained from the HEC RAS hydraulic reference manual. The scale was based on aerial photos and the construction drawings of the outlet. Table A3 6. Outlet Structure Information Shape Circular Chart 2: corrugated metal pipe Scale 2: mitered to conform to slope Length (ft) 42.4 Diameter (ft) 4.5 Inlet Elevation (ft) Entrance Coefficient 0.7 Outlet Elevation (ft) 171 Exit Coefficient 1 Mannings n July

90 The construction drawings and photos also show two risers that act as emergency outlets but according to the drawings the elevation of the tops of the risers is 188, which is above the overtopping elevation for the basin so these risers were not included in the model. Based on the topographic data is appears that the basin will start to overtop to the east at an elevation just above 186 feet, flowing directly to Marsh Creek. To account for this the dam over tops function was also used. Figure A3 18 shows the basin with contours and the assumed weir location and length. Figure A3 18. Sand Creek basin with contours and weir location It was assumed that weir flow would begin at 186 feet and the length of the weir was estimated at 530 feet with a typical broad crested weir coefficient of 2.6. July