CEQA DRAINAGE STUDY FOR GERMANN TENTATIVE ... CEQA DRAINAGE STUDY FOR GERMANN TENTATIVE SUBDIVISION...

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  • CEQA DRAINAGE STUDY FOR

    GERMANN TENTATIVE SUBDIVISION MAP

    (Tract No. 5520 RPL 1, Project No. PDS2006-3100-5520)

    December 14, 2016

    Wayne W. Chang, MS, PE 46548

    Chang Civil Engineering ◦ Hydrology ◦ Hydraulics ◦ Sedimentation

    P.O. Box 9496 Rancho Santa Fe, CA 92067

    (858) 692-0760

  • TABLE OF CONTENTS

    Introduction ........................................................................................................................................1

    Hydrologic Analyses ..........................................................................................................................2

    Detention Analyses ............................................................................................................................4

    Conclusion .........................................................................................................................................5

    Declaration of Responsible Charge ...................................................................................................5 APPENDIX A. 100-Year Rational Method Data and Results B. 100-Year WSPGW and Detention Results MAP POCKET Existing Condition Rational Method Work Map Proposed Condition Rational Method Work Map

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    INTRODUCTION The Germann Tentative Subdivision Map (Tract No. 5520 RPL 1, Project No. PDS2006-3100- 5520) development will be located along the west side of Westhill Road at the intersection with Hillman Way in the county of San Diego (see Vicinity Map). The tentative subdivision map is being prepared by Farrington Engineering Consultants. The overall project will be located on 5.24 gross acres. The project proposes 14 single-family residential lots, a water quality/detention basin(s), two public streets, and improvements (widening and asphalt berm) along the west side of Westhill Road. The current project area is undeveloped except for a single-family home at the southwest corner and an asphalt roadway/driveway from Westhill Road serving the residence.

    Vicinity Map

    The drainage patterns at and near the site are as follows. The existing site generally slopes northwest and northeast at over 10 percent towards a lower lying area along the center of the site. This lower lying area conveys the runoff over the natural ground surface in a northerly direction to an existing Type F catch basin just beyond the northerly property line. An existing storm drain then conveys the flow away from the site in a northerly direction. There is over 42 acres of off- site area to the south that contributes storm runoff onto the site. The off-site flow enters the center of the southern end of the site and then continues north along the on-site lower lying area. There is also over 7 acres of off-site area to the southeast that contributes storm runoff onto the easterly edge of the site near the northern boundary. This runoff is directed onto the site by a

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    drainage culvert under Westhill Road. The runoff then flows over the natural ground surface towards the on-site lower lying area. The project will continue to direct on-site storm runoff north to the existing storm drain system at the north. In addition, new storm drain lines will be constructed to convey the southerly and southeasterly off-site flows through or along the site to the northerly storm drain. The new storm drain lines will prevent commingling of on- and off- site flows until after the on-site flow is treated. This report contains preliminary existing and proposed condition hydrologic analyses for entitlement purposes. This report analyzes two tentative map alternatives, Alternative A and B. The existing pipe to the north that conveys flows away from the site is a 24-inch HDPE located on private property. The pipe is not sufficient to convey the existing condition tributary flows. An agreement has not been reached with the property owners allowing the pipe to be upsized. Alternative A assumes that an agreement will be reached with the property owners, so that the project will only mitigate for its flow increase. Alternative B assumes that an agreement will not be reached with the property owners, so the project will detain an additional amount to the capacity of the existing 24-inch HDPE. Under Alternative A, a combined water quality/detention basin will be installed at the north end of the site. Under Alternative B, an additional detention basin will be installed at the south end of the site. The southerly off-site flow will be directed to the south basin to meet the additional detention requirements.

    HYDROLOGIC ANALYSES The County of San Diego’s 2003 Hydrology Manual rational method procedure was used for the 100-year hydrologic analyses. The existing and proposed condition rational method input parameters are summarized as follows:  Precipitation: The 100-year, 6- and 24-hour precipitation values are 2.6 and 5.6 inches,

    respectively. The isopluvials are included in Appendix A.  Drainage areas: The existing condition drainage areas were delineated from the 2-foot

    contour interval topographic mapping prepared for the project and the County of San Diego’s 5-foot contour interval topographic mapping for the off-site areas. A site visit was performed to verify the drainage area delineation. The proposed condition on-site drainage areas were delineated from the tentative map. The Existing Condition Rational Method Work Map and Proposed Condition Rational Method Work Maps are included in the map pocket.

     Hydrologic soil groups: The hydrologic soil groups were determined from the National Resources Conservation Service’s “Web Soil Survey.” Nearly the entire drainage area contains the type B soil group. There is a small sliver of the upstream-most drainage subbasin that contains the type D soil group. For simplicity, the entire study area was assumed to consist of the type B soil group since the small amount of type D will not appreciably alter the results for entitlement purposes.

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     Runoff coefficients: Runoff coefficients were established for each drainage basin based on the underlying land use or estimated impervious percentage (and soil type B). The existing condition land uses primarily include undisturbed natural terrain or low-density residential development. In addition, there is a lower drainage subbasin that contains a higher impervious percentage. This subbasin was assigned a high-density residential land use to reflect a 65 percent impervious cover. The proposed condition on-site land use was generally assumed to be low-density residential development (2.9 du/ac or less) within the subdivision area – the project proposes 14 lots in a 5.20 acre drainage area, which is 2.7 du/ac, so the assigned land use is slightly conservative. The other drainage areas were assigned land uses based on the underlying land use or the estimated percentage of impervious cover.

     Flow lengths and elevations: The flow lengths and elevations were digitized and obtained

    from the topographic mapping and tentative map. The 100-year existing and proposed condition rational method results are in Appendix A. The analyses were performed using CivilDesign’s San Diego County Rational Hydrology Program. The overall existing condition drainage area was set equal to the overall proposed condition drainage area to allow a comparison of the existing and proposed condition results. Table 1 summarizes the 100-year results. For proposed conditions, the undetained flow rate is given and is the essentially same for Alternative A and B. Table 1 shows that the project will increase 100- year runoff from the on- and off-site areas by 6 cubic feet per second (cfs). Detention will be provided to mitigate for the increase and is discussed in the next section.

    Analysis Overall Tributary Area, ac Overall 100-Year

    Flow Rate, cfs Existing Conditions 55.91 73

    Proposed Conditions1 55.91 79 1The proposed condition undetained flow rates are similar for Alternatives A and B.

    Table 1. Rational Method Results

    Rational Method Nodes

    Alternative A Pipe Size, in

    Alternative B Pipe Size, in

    36-26 (Alt A) 37-26 (Alt B) 30 24

    26-38 36 24 38-52 36 24 52-58 36 24 58-60 36 24 (existing)

    All others 18 18

    Table 2. Preliminary Pipe Sizing (Proposed Sizes Except as Noted

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    The hydrologic analyses provide pipeflow normal depth routines to estimate the required pipe sizes. Table 2 summarizes the results assuming 18-inch is the minimum diameter. The Alternative B sizing reflects additional on-site detention that will reduce the flow rate in the rational method results. During final engineering detailed hydrologic and hydraulic analyses shall be performed to verify the sizing. DETENTION ANALYSES The County of San Diego can require the proposed condition 100-year runoff from the project to be no greater than the existing condition 100-year runoff. In order to meet this requirement, the project proposes to construct a detention facility within the northerly portion of the site. The detention basin will also address water quality and hydromodification requirements. A 100-year detention analysis was performed to estimate the storage volume needed to detain the on-site proposed condition flow by 6 cfs. The proposed condition rational method results from the 5.20 acre area tributary to the basin was converted to a hydrograph using the Rational Method Hydrograph Procedure from the County’s Hydrol