Appendix G: Hydrology-Related Reports

Transcription

1 Appendix G: Hydrology-Related Reports (Hydraulic Summary of the Proposed Hawk Street Bridge, Bay Area Hydrology Model Project Report, Preliminary Stormwater Treatment Plan and Details, Evaluation of Potential Hydrological Impacts to Garaventa Wetlands)

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3 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Date: January 9, 212 To: From: Project Subject: Eddie Sieu and Jason White - RJA Han-Bin Liang and Ripen Kaur - WRECO Garaventa Hills Estates Development, City of Livermore Memorandum Hydraulic Summary of the Proposed Hawk Street Bridge over Altamont Creek Introduction The purposes of this memorandum are to 1) present the hydraulic analysis results showing the impact of the proposed Hawk Street bridge to the 1-year flood elevations in Altamont Creek, and 2) predict the extent of floodplains for the 1-year storm event. The proposed bridge site is located in the City of Livermore, Alameda County, California (Figure 1). Proposed Hawk Street Bridge Garaventa Hill Estates Development Site Figure 1. Project Location Map Source: Google Earth Civil Engineering Water Resources 1

4 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Project Description The Hawk Street bridge over Altamont Creek is proposed with the Garaventa Hills Estates Project to connect this project to the existing development south of the channel. The Garaventa Hills Estates residential development is proposed to be constructed in the northeastern part of the city of Livermore. The site is located north of Interstate 58, east of Vasco Road, and west of Laughlin Road in the city of Livermore. Ruggeri-Jensen-Azar & Associates (RJA) is the civil engineer responsible for the site development engineering design. In the existing condition, the project site is an undeveloped parcel consisting of non-native grassland habitat. The site is bordered by open space in the north and west and is adjacent to residential development in the east and the Altamont Creek channel in the south. Altamont Creek originates in the Altamont Hills and flows south and west into the city of Livermore. The project proposes to realign a portion of Altamont Creek in the vicinity of the proposed Hawk Street bridge. Figure 2 shows the realignment plan for the channel. Figure 3 shows the plan and profile for the proposed Hawk Street bridge. The project lies in the jurisdiction of Zone 7, Alameda County Flood Control & Water Conservation District (Zone 7). The Zone 7 basic criterion for the hydraulic design of bridges is that they be designed to pass the one percent (1%) probability of annual exceedance flood (1-year flood or Q 1 ) with 2 feet of freeboard without causing objectionable backwater, causing excessive flow velocities, or encroaching on through-traffic lanes. The scour analysis and geomorphologic assessment were not part of the scope of this study. Hydrology Design discharges were obtained from two different sources: the Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) and Zone 7 (personal communication- April 18, 211) and are listed in the table below: Table 1. Altamont Creek Design Discharges at Hawk Street Bridge Location Source 1-year Flow (cfs) FEMA FIS 72 Zone 7 ACFC&WCD 1,4 The drainage area for Altamont Creek at Vasco Road is 7.39 square miles (Ayres Associates, 1996). The design flow of 1,4 cfs was adopted in the analysis per the recommendation of the Zone 7, which is the local agency that has jurisdiction over the Altamont Creek channel at and near the bridge site. Because the Zone 7 s flow is higher than that of FEMA, it would be conservative to use their number. Hydraulic Analysis The hydraulic analysis of Altamont Creek involved a standard step backwater calculation using the United State Army Corps of Engineers (USACE s) Hydrologic Engineering Center River Analysis System (HEC-RAS), version 4.1, to determine the water surface elevations (WSEs) during the design 1-year storm event. Civil Engineering Water Resources 2

5 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: For the hydraulic analysis, 28 cross sections along a 3,7 ft reach of Altamont Creek within the Project vicinity were cut from the surface that was created in Arc Map based on the channel crosssection survey data provided by RJA and LIDAR survey data provided by the City of Livermore. Figure 4 shows the locations of the cross sections used in the analysis. The pre-processing menu option in Arc Map was used for creating an import file for HEC-RAS. The stream centerline, main channel banks and flow paths were defined in this process using the GeoRAS tool in Arc Map. The geometry information was then imported into HEC-RAS for the hydraulic analysis. The downstream limit of the hydraulic model is approximately 15 ft downstream of the existing North Vasco Road bridge over Altamont Creek. The proposed Hawk Street bridge is approximately 2,25 ft upstream of the existing North Vasco Road bridge. The upstream limit of the hydraulic model is approximately 1,3 ft upstream of the proposed Hawk Street bridge. The design of the proposed Hawk Street bridge in the hydraulic model is based on the conceptual bridge design provided by RJA (September 2, 211). Manning s n values for estimating energy losses in the flow due to friction were selected for the left and right overbanks and the main channel to represent the characteristics of Altamont Creek before and after the installation of the proposed Hawk Street bridge. At the bridge location, Altamont Creek travels in the pasture with tall grass (see Photo 1). For the hydraulic analysis, Manning s n was selected based on FEMA s existing HEC-2 model and a site visit. A Manning s n value of.45 was selected to represent the left and right overbank areas with high grass. Based on aerial photography, the low flow channel of Altamont Creek winds through a pasture area. The low flow channel has minimal vegetation, and there were no signs of rifts or deep pools during the site visit. A Manning s n value of.35 was selected to represent the low flow channel. Photo 1 shows the existing channel condition in the vicinity of the bridge site. Photo 1. Altamont Creek in the Vicinity of the Proposed Bridge Site Civil Engineering Water Resources 3

6 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Proposed Garaventa Hills Development Figure 2. Altamont Creek Realignment Near Garaventa Hills Estates Development Source: RJA 212 Civil Engineering Water Resources 4

7 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Figure 3. Preliminary Bridge Plan and Profile of the Hawk Street Bridge over Altamont Creek Source: RJA 212. Civil Engineering Water Resources 5

8 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Figure 4. Cross Section Locations Used in the Hydraulic Analysis for Altamont Creek Civil Engineering Water Resources 6

9 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Figure 5 shows the cross section used for the existing and proposed conditions at Station 2375 (see Figure 4), which is 4 ft downstream of the Hawk Street bridge, as used in the hydraulic analysis. Station ft upstream of the Proposed Hawk Road bridge face Existing Condition Proposed Condition Channel Width (ft) 542 Station ft downstream of the Proposed Hawk Road bridge face Existing Condition Proposed Condition Channel Width (ft) Figure 5. Cross Section Comparison between Existing and Proposed Conditions at Model Stations: 2466 and 2375 Civil Engineering Water Resources 7

10 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: The downstream boundary condition was defined as the WSE downstream of the Vasco Road bridge, which is ft North American Vertical Datum (NAVD), as estimated from FEMA s HEC-2 model run for the 1,4 cfs flow. Water Surface Elevations The 1-year WSEs for the existing and proposed conditions are summarized in Table 1. Table 2. Hydraulic Summary HEC-RAS Model Station Distance from Proposed Hawk Street Bridge Faces Water Surface Elevation Existing Proposed (ft) (ft) (ft) ,241 ft upstream ft upstream ft upstream ft upstream ft upstream ft downstream ft downstream Detailed HEC-RAS results are attached with this memorandum in Appendix A. Per the analysis, the proposed bridge would raise the WSE 7 ft upstream of the bridge by approximately.7 ft. This increase is due to the fill proposed in the realigned section of the channel, which reduces the conveyance capacity of the channel. Although the proposed project will increase the WSE under the proposed bridge in the proposed condition, the design WSE would be contained within the banks of the channel (see Figure 6). The soffit elevation of the bridge is.76 ft NAVD88, and the WSE at the cross section 7 ft upstream of the proposed bridge is ft. Therefore, even though there is an increase in the WSE due to the construction of the bridge, the proposed bridge would meet the Zone 7 Water Agency s criterion of passing the 1-year flood with 2 ft of freeboard. Moreover, the proposed realignment of Altamont Creek and installation of Hawk Street bridge would only have a local impact on the design 1-year WSE (see Figure 7). The difference in the 1-year WSE at the cross sections at HEC-RAS model station 2712 (approximately 238 ft upstream of the proposed Hawk Street bridge face) would be less than.1 ft, which would be considered an insignificant elevation difference. WRECO also evaluated the various widths for the low flow channel to determine the width that would provide no increase in the WSE due to the proposed bridge. Based on the calculations, the proposed realigned channel needs to be 3 ft wide in order for the water surface to remain at the existing condition levels. The results from the hydraulic analyses indicate that the hydraulic grade line profile in the proposed condition would be contained within the floodway in the vicinity of the bridge site. There would be a local increase in the water surface elevation at the bridge site due to the proposed bridge construction. Civil Engineering Water Resources 8

11 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: However, the requirement of at least 2 ft of freeboard is met under the bridge soffit for the design flow of 1,4 cfs. Figure 6. HEC-RAS Cross Section at the Upstream Face of the Bridge Floodplain Analysis Another objective of the memorandum was to determine the extent of the floodplain for the 1-year storm event using the 1,4 cfs of flood flow, provided by Zone 7. After performing the steady state hydraulic modeling with HEC-RAS, the flood profile was imported into Arc Map. The floodplain was mapped using the post processing tool in Arc Map. The floodplain for the Altamont Creek with the proposed Hawk Street Bridge is shown in Figure 8. The floodplain concluded in this study is not identical to the one provided in the FEMA FIRM (Appendix B). The following could be the causes of the discrepancies in floodplain boundaries between the floodplains presented in the current study and the FEMA FIRM: The floodplain boundaries in the current study are based on more recent survey than the FEMA FIRM. The 1-year flow used in the current study is 1,4 cfs, which is higher than the FEMA FIS flow of 72 cfs. Therefore, the water surface elevations for the base flood are higher compared with the elevations in the FEMA FIRM. WRECO s analysis showed that the adjacent development south of the Altamont Creek between the Vasco Road and Hawk Street will not be in 1-year floodplain as currently shown in the FEMA s FIRM. The current FEMA FIRM is no longer valid and the city or the developer may need to apply for the Letter of Map Revision (LOMR) to update the FEMA FIRM. WRECO s analysis can be used to support the LOMR. Civil Engineering Water Resources 9

12 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Proposed Hawk Street Bridge Local Increase in Water Surface Profile HEC RAS Model Stations Channel Bed Existing W.S. Elev (ft) Proposed W.S. Elev (ft) Bridge Bridge top Bridge-us-face Bridge-ds-face Proposed-channel bed Figure 7. Water Surface Elevation Comparison for the Existing and Proposed Conditions Civil Engineering Water Resources 1

13 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Figure 8. Floodplain for 1-year Event (1,4 cfs) with the Proposed Hawk Street Bridge over Altamont Creek. Civil Engineering Water Resources 11

14 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Appendix A - Hydraulics Analysis Results Civil Engineering Water Resources

15 Existing condition Altamont Creek AC Main Channel Distance (ft) Right Levee in Horiz. = 31 ft 1 in Vert. = 5 ft

16 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 11 ft.45 Levee

17 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 11 ft

18 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 11 ft

19 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 11 ft.45

20 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 11 ft.45

21 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 11 ft Levee Levee

22 Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = Existing condition River = Altamont Creek Reach = AC RS = 15 Culv 52 1 in Horiz. = 11 ft 1 in Vert. = 11 ft

23 Existing condition River = Altamont Creek Reach = AC RS = 15 Culv Existing condition River = Altamont Creek Reach = AC RS = Levee Levee 1 in Horiz. = 11 ft 1 in Vert. = 11 ft

24 HEC-RAS Plan: Exist River: Altamont Creek Reach: AC Profile: 14 cfs-design Reach 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) AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC 15 Culvert AC cfs-design

25 Proposed condition Altamont Creek AC Main Channel Distance (ft) Left Levee Right Levee 15 North Vasco Lan Proposed Hawk Street Bridge in Horiz. = 31 ft 1 in Vert. = 5 ft

26 Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Levee Proposed condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 2 ft

27 Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Levee 1 in Horiz. = 11 ft 1 in Vert. = 2 ft

28 Proposed condition River = Altamont Creek Reach = AC RS = 24 BR Proposed Hawk Street Bridge Levee 535 Pier Debris Proposed condition River = Altamont Creek Reach = AC RS = 24 BR Proposed Hawk Street Bridge Proposed condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 2 ft

29 Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 2 ft

30 Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 2 ft

31 Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = Levee 1 in Horiz. = 11 ft 1 in Vert. = 2 ft

32 Proposed condition River = Altamont Creek Reach = AC RS = Levee Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = in Horiz. = 11 ft 1 in Vert. = 2 ft

33 Proposed condition River = Altamont Creek Reach = AC RS = Proposed condition River = Altamont Creek Reach = AC RS = 15 Culv North Vasco Lane Bridge Proposed condition River = Altamont Creek Reach = AC RS = 15 Culv North Vasco Lane Bridge Levee 1 in Horiz. = 11 ft 1 in Vert. = 2 ft

34 Proposed condition River = Altamont Creek Reach = AC RS = Levee 1 in Horiz. = 11 ft 1 in Vert. = 2 ft

35 HEC-RAS Plan: Prop River: Altamont Creek Reach: AC Profile: 14 cfs-design Reach 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) AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC 24 Bridge AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC cfs-design AC 15 Culvert AC cfs-design

36 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Appendix B- FEMA FIRM Civil Engineering Water Resources

37 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Civil Engineering Water Resources 14

38 1243 Alpine Road, Suite 18 Walnut Creek, CA Phone: Fax: Civil Engineering Water Resources 15

39 Bay Area Hydrology Model PROJECT REPORT Project Name: 1194 Site Address: City : Livermore Report Date : 11/8/21 Gage : LIVERMORE Data Start : 1959/1/1 Data End : 24/9/3 Precip Scale: 1. BAHM Version: PREDEVELOPED LAND USE Name : Basin 1 Bypass: No Water: No Pervious Land Use Acres C D,Grass,Flat(-5%) 1.89 C D,Grass,Mod(5-1%) 1.99 C D,Grass,Ste(1-2) 17.8 C D,Grass,Very(>2%) 1.2 Impervious Land Use Acres Element Flows To: Surface Interflow water Name : Basin 1 Bypass: No Water: No Pervious Land Use Acres C D,Urban,Flat(-5%) 6.39 C D,Grass,Very(>2%) 7 C D,Grass,Ste(1-2) 7.19 Impervious Land Use Acres Roads,Flat(-5%) 5.21 Roads,Mod(5-1%).32 Roof Area 5.59 Element Flows To:

40 Surface Interflow water Trapezoidal Pond 1, Trapezoidal Pond 1, Name : Trapezoidal Pond 1 Bottom Length: 12ft. Bottom Width: 12ft. Depth : 4ft. Volume at riser head : ft. Side slope 1: 3 To 1 Side slope 2: 3 To 1 Side slope 3: 3 To 1 Side slope 4: 3 To 1 Discharge Structure Riser Height: 3 ft. Riser Diameter: 18 in. NotchType : Rectangular Notch Width : ft. Notch Height:.17 ft. Orifice 1 Diameter: in. Elevation: ft. Element Flows To: Outlet 1 Outlet 2 Pond Hydraulic Table Stage(ft) Area(acr) Volume(acr-ft) Dschrg(cfs) Infilt(cfs)

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42 MITIGATED LAND USE ANALYSIS RESULTS Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year year year year Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year year year year Yearly Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated

43 Ranked Yearly Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated

44 POC #1 The Facility PASSED The Facility PASSED. Flow(CFS) Predev Dev Percentage Pass/Fail Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass

45 Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass

46 Pass Pass Pass Pass Pass Pass Pass Perlnd and Implnd Changes No changes have been made. This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc, Applied Marine Sciences Incorporated, the Alameda County Flood Control and Water Conservation District, EOA Incorporated, member agencies of the Alameda Countywide Clean Water Program, member agencies of the San Mateo Countywide Water Pollution Prevention Program, member agencies of the Santa Clara Valley Urban Runoff Pollution Prevention Program or any other LOU Participants or authorized representatives of LOU Participants be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc., Applied Marine Sciences Incorporated, the Alameda County Flood Control and Water Conservation District, EOA Incorporated or any member agencies of the LOU Participants or their authorized representatives have been advised of the possibility of such damages. Software Copyright by Clear Creek Solutions, Inc ; All Rights Reserved.

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51 GEOTECHNICAL ENVIRONMENTAL WATER RESOURCES CONSTRUCTION SERVICES March 7, 212 Project No Rebecca Gorton Lamphier-Gregory 1944 Embarcadero Oakland, CA 9466 Subject: Garaventa Hills Livermore, California EVALUATION OF POTENTIAL HYDROLOGIC IMPACTS TO GARAVENTA WETLANDS Reference: Hanes and Stromberg, 1998, Hydrology of Vernal Pools on Non-Volcanic Soils in the Sacramento Valley, Ecology, Conservation, and Management of Vernal Pool Ecosystems Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, CA. Pages Dear Ms. Gorton: The following document evaluates and discusses potential hydrological impacts to the Garaventa Wetlands resulting from the proposed Garaventa Hills development. The purpose is to discuss impacts that may result from the development, evaluate the significance of potential impacts, and offer recommendations for minimizing any significant impacts to the seasonal wetlands. The letter is to be used in the environmental review process. INTRODUCTION The Garaventa Hills project is a proposed 76-lot residential subdivision on 31.7 acres in the City of Livermore, Alameda County, California. The existing site is on a hill that is predominantly grass with sandstone outcroppings. The proposed improvements include the construction of residential lots, roads, utilities, drainage facilities, and grading operations. The project is located on uplands that are immediately adjacent to the Garaventa Wetlands Preserve and Altamont Creek. The Garaventa Wetlands Preserve is administered by the Livermore Area Recreation and Parks District. These wetlands are characterized as seasonal alkali wetlands and vernal pools. Vernal pools are described as seasonal wetlands that are dry during part of the year. This is sensitive habitat to the fairy shrimp and other special status species. The discussion to follow focuses on the potential of the project to impact the vernal pools with respect to hydrology. More specifically, how alterations to existing drainage patterns may affect the quantity and timing of precipitation that enters the wetlands and is needed to maintain a functioning system. 21 Crow Canyon Place, Suite 25 San Ramon, CA (925) Fax (888)

52 Lamphier-Gregory Garaventa Hills, Livermore, California March 7, 212 EVALUATION OF POTENTIAL HYDROLOGIC Page 2 IMPACTS TO GARAVENTA WETLANDS The geology found at the site is predominantly classified by the NRCS as being Briones or Cierbo Sandstone. The watershed consists of shallow soils on top of sandstone creating relatively impervious hydrologic surface conditions, which facilitates a rapid response time and minimal infiltration. Precipitation falling on the site drains quickly in the form of surface flows and is not stored onsite for any significant amount of time. POTENTIAL IMPACTS During the environmental review process, two primary issues of concern were identified that may cause negative impacts to the wetlands. First, since the development is in close proximity to the vernal pools, pollutants from the project may be transported to the wetlands by surface runoff and subsurface flows. A second potential impact of concern is the decrease of flows entering the wetlands due to the alteration of drainage patterns. The project proposes to treat onsite stormwater runoff with a single detention basin located at the south edge of the project. In order to accomplish this, proposed adjustments to the onsite drainage patterns will convey runoff from impervious areas to the basin. Approximately 4.6 acres of onsite tributary area that contributes to the wetlands from north side of the project will be redirected southward to the detention basin, as will roughly 2.6 acres that currently drain to vernal pools from the western edge of the parcel be re-routed to the detention basin. The concern is that by diverting flows from these tributary areas away from the wetlands, that water entering the pools may be reduced enough to significantly impact the hydrologic function of the wetlands. CONCLUSIONS The developer of Garaventa Hills is proposing two site plan features to address the issue of pollutants leaving the project and entering the wetlands. The first is the implementation of natural areas along the west and north edges of the parcel to act as a buffer between the development and the wetlands. These areas will act as a barrier to sediments and other pollutants from leaving the project and entering the vernal pools and also maintains the interface between the upland areas and the wetlands. In addition, all stormwater from disturbed areas will be routed to the detention/bioretention basin for treatment prior to being discharged into Altamont Creek. We concur that the combination of buffer areas and the storm drain layout with a bioretention pond reduces the potential of the project to significantly increase pollutant loads to the wetlands. Hanes and Stromberg (1998) researched the hydrology of vernal pools in the Sacramento Valley that are similar in nature to the Garaventa Wetlands. They found that direct precipitation is by far the most important source of water to the vernal pools. They also concluded that significant watershed contributions to the wetlands rarely occurs and generally occurs during wet times when the pools are already full. This does not influence the ability of the pools to fill with water. Direct precipitation events in early winter are the primary source of water for the vernal pools.

53 Lamphier-Gregory Garaventa Hills, Livermore, California March 7, 212 EVALUATION OF POTENTIAL HYDROLOGIC Page 3 IMPACTS TO GARAVENTA WETLANDS For this reason, the contribution of surface water to the wetlands is not significantly impacted by the alterations to watershed drainage patterns that are proposed as part of the project. Furthermore, since the watershed characteristics facilitate rapid stormwater discharge from the site with minimal infiltration and limited capacity to store groundwater, the subsurface hydrology of wetlands is also not significantly affected by the development. The natural buffer area proposed between the development and the wetlands will help maintain the function of the water exchange between the pools and the adjacent uplands and is essential for the integrity of the natural system. RECOMMENDATIONS Even though the completed project will not result in significant hydrologic impacts to the wetlands, we recommend the following measures be implemented to minimize potential negative impacts. Careful attention should be given to the implementation of storm water Best Management Practices (BMP s) during construction to avoid the potential for sediments and other pollutants to enter the wetlands. Fencing and signage should be installed that identifies the limits of the wetlands and provides a physical barrier so that construction equipment does not disturb the wetlands. It is advised to limit grading activities in close proximity to the pools to dry times of the year to minimize the possibility of sedimentation impacting the wetlands. It is further recommended that the natural vegetated buffer area at the perimeter of the project be fully stabilized during the early phases of construction to act as a protective barrier for the vernal pools during the course of construction. If you have any questions or concerns regarding the contents of this letter, please call and we will be glad to discuss them with you. Sincerely, ENGEO Incorporated Sean Cleary, PE, QSD sc/jem/cjn:eval James E. Moore, Jr., PE

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