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HomeMy WebLinkAboutMS 2022-0003; 1098 MAGNOLIA AVE; FINAL DRAINAGE REPORT; 2022-12-05FINAL DRAINAGE REPORT FOR: 1098 Magnolia Avenue Carlsbad, CA JN 3-99 PD2021-0046 MS2022-0003 GR2021-0047 DWG.535-6A PREPARED FOR: MEHRDAD DOKANCHY 1098 MAGNOLIA AVENUE CARLSBAD, CA 92008 (925) 705-6173 PREPARED BY: LANDMARK CONSULTING 9555 GENESEE AVENUE, SUITE 200 SAN DIEGO, CA 92121 858-587-8070 DATE: 12/5/2022 _________________________________________________ DAVID YEH, RCE 62717, EXP. 6-30-24 TABLE OF CONTENTS VICINITY MAP 1 INTRODUCTION 2 METHOD OF ANALYSIS 3-7 HYDROLOGY CALCULATIONS 8 DECLARATION OF RESPONSIBLE CHARGE 9 SUMMARY 10 APPENDICIES APPENDIX A HYDROLOGIC SOILS GROUP MAP APPENDIX B RUNOFF COEFFICIENT APPENDIX C 100-YEAR, 6-HOUR PRECIPITATION APPENDIX D 100-YEAR, 24-HOUR PRECIPITATION APPENDIX E 100-YEAR INTENSITY-DURATION DESIGN CHART APPENDIX F PRE DEVELOPMENT AND POST DEVELOPMENT HYDROLOGY MAPAPPENDIX G HYDRAULIC CALC FOR PROP. SDRSD D-27 SIDEWALK UNDERDRAINS APPENDIX H HYDRAULIC VERIFICATION OF THE CAPACITY OF THE 6” PVC ONSITE . STORM DRAIN SYSTEM AND D-75 BROW DITCH APPENDIX I HYDRAULIC CAPACITY CALC AND SIZING OF THE WS0511B SUBMERSIBLE SUMP PUMPS 1 ` 2 INTRODUCTION The site is located at 1098 Magnolia Avenue, located on the northwest corner of the intersection of Magnolia Avenue and Adams Street in the City of Carlsbad, County of San Diego, State of California. The project site consists of an existing single family residence with related hardscape and landscape located on the eastern half, and the western half of the site consists of a vacant grassy lawn area that has been left undeveloped. The scope of this project proposes the placement of an additional single family residence with a two car garage and concrete driveway on the westerly undeveloped half of the project site. The existing topography of the site consists of moderately westerly sloping terrain. Under the pre-development conditions, the runoff from the project sheet flows across the site from the easterly property line to the westerly property line. The runoff drains off existing rooftops and hardscape on the easterly side of the site and flows out onto the westerly existing grassy lawn. Here, the runoff ponds up along the existing concrete retaining wall at the westerly property line. The project site is underlain with type B soil with a moderate to good infiltration rate. The ponded surface water that does not infiltrate down into the existing soil, eventually discharges out into the street at the southerly corner of the project site. From here, the runoff continues westerly along the northerly gutter of Magnolia Avenue towards the intersection of Magnolia Avenue and Pio Pico Drive. Just before the intersection and along the northerly gutter of Magnolia, the runoff flows into an existing curb inlet. After the project’s runoff enters the inlet, it flows into the existing MS4 storm drain system. This system routes the runoff directly westerly to Interstate – 5, where the existing storm drain network follows I-5 southerly 3,000 feet until the runoff is discharged easterly into Agua Hedionda Lagoon. Under the post-development conditions, the existing overall runoff pattern will be preserved. This project proposes an SB-9 urban lot split, subdividing the existing lot into two separate adjacent parcels. The new easterly parcel of the site denoted as “Basin 1” will have a proposed onsite 6” SD and an SDRSD D-75 type B brow ditch system adjacent to the new dividing lot line which will route runoff to two 0.5 HP sump pumps that will pressure flow runoff to an onsite concrete transfer box. The transition box will then gravity flow runoff out onto Magnolia Avenue via three 3” proposed sidewalk underdrains per SDRSD D-27. This system is a necessary addition to the existing lot to prevent cross lot drainage. The proposed development of the new single family residence will be done on the new westerly parcel of the site, centered in the existing grassy open area. The project proposes grading around the perimeter of the proposed structure to ensure adequate drainage away from the proposed structure’s foundation. The project proposes an on-site storm drain system consisting of 6” PVC storm drain pipes and a series of area drains located at graded low points that carry storm runoff around and away from the proposed structure. All drainage on the new westerly parcel, denoted as “Basin 2”, will be collected in the proposed onsite storm drain network, and will eventually discharge out into the northerly gutter of Magnolia Avenue at the southerly corner of the site via three 3” proposed sidewalk underdrains per SDRSD D-27. At this discharge point, the runoff from Basin 1 and Basin 2 comingle and the combined runoff will follow the exact same drainage pattern as existing conditions, ultimately discharging into Agua Hedionda Lagoon. The overall drainage pattern will not be changed from pre-development conditions. The purpose of this report is to determine the peak discharge rates under both the pre and post-development conditions and to evaluate the adequacy of the proposed and existing drainage system. 3 METHOD OF ANALYSIS Rational Method 4 5 6 7 8 HYDROLOGIC CALCULATION RESULTS TIME OF CONCENTRATION CALCULATIONS. Time of concentration is calculated by the following formula from figure 3-3 of the San Diego County Hydrology Manual. Time of concentration, Tc: where: C = runoff coefficient (0.40 pre, 0.47 post) D = flow length (244 ft pre, 195 ft post A-1, 182 ft post A-2 ) S = slope (2.9% pre, 2.7% post A-1, 1.0% post A-2 ) Tc=13.8 min pre-project, 11.4 min post project basin-1, 15.29 min post project basin-2. 1.1 PRE-PROJECT HYDROLOGIC CALCULATIONS BASIN-1 – DRAINS TO POC 1 PRE-DEVELOPMENT CONDITIONS AREA (AC) C I (IN/HR) TC (MIN) Q (CFS) 100-YR 0.366 0.401 3.752 13.8 0.55 1. From Table 3-1 for Low Density Residential, Type ‘B’ Soil. 2. Refer to Appendix 2 for intensity calculations. 1.2 POST-PROJECT HYDROLOGIC CALCULATIONS BASIN-1 – DRAINS TO POC 1 POST-DEVELOPMENT CONDITIONS AREA (AC) C I (IN/HR) TC (MIN) Q (CFS) 100-YR 0.185 0.471 4.12 11.4 0.35 1. From Table 3-1 for Low Density Residential (2.7 du/ac), Type ‘B’ Soil 2. Refer to Appendix 2 for intensity calculations. BASIN-2 – DRAINS TO POC 1 POST-DEVELOPMENT CONDITIONS AREA (AC) C I (IN/HR) TC (MIN) Q (CFS) 100-YR 0.182 0.471 3.42 15.29 0.29 1. From Table 3-1 for Medium Density Residential (5.45 du/ac), Type ‘B’ Soil 2. Refer to Appendix 2 for intensity calculations. 1.3 RESULTS SUMMARY POC -1 RUNOFF SUMMARY TABLE PRE-DEV,Q (CFS) POST-DEV,Q (CFS) INCREASE, Q (CFS) 100-YR 0.55 0.64 0.09 9 DECLARATION OF RESPONSIBLE CHARGE I hereby declare that I am the civil Engineer of Work for this project, that I have exercised responsible charge over the design of this project as defined in Section 6703 of the Business and Professions code, and that the design is consistent with current design. I understand that the check of project drawings and specifications by the County of San Diego is confined to a review only and does not relieve me, as Engineer of Work, of my responsibilities for project design. David Yeh, RCE 62717, EXP. 6-30-2024 10 SUMMARY Based on the hydrology calculations performed in this report, no significant increase to the peak 100-year runoff flow rate is anticipated. The peak flow rate increase from pre-project to post-project conditions was calculated from the Rational Method analysis as 0.09 cfs. This increase is minor in magnitude, and can be considered negligible. Overall, no adverse impacts to downstream storm drain facilities or receiving bodies are anticipated as a result of this development. APPENDIX A HYDROLOGIC SOILS GROUP MAP PROJECT SITE APPENDIX B RUNOFF COEFFICIENT APPENDIX C 100-YEAR, 6-HOUR PRECIPITATION APPENDIX D 100-YEAR, 24-HOUR PRECIPITATION APPENDIX E 100-YEAR INTENSITY-DURATION DESIGN CHART APPENDIX F PRE DEVELOPMENT AND POST DEVELOPMENT HYDROLOGY MAP L NDMARKC O N S U L T I N GPlanning Engineering Surveying9555 Genesee Avenue, Suite 200San Diego, CA 92121, (858) 587-8070 UP UP L NDMARKC O N S U L T I N GPlanning Engineering Surveying9555 Genesee Avenue, Suite 200San Diego, CA 92121, (858) 587-8070 APPENDIX G HYDRAULIC CALC FOR PROPOSED SIDEWALK UNDERDRAINS PARCEL 1/BASIN 1 From Hydrology Report: Q100 = 0.35 CFS total. Q100/ 3 = 0.11666 CFS Flow tributary to each individual sidewalk underdrain (3 underdrains). As demonstrated by the hydraulic calc above, the capacities of the prop. 3” sidewalk underdrains are not being exceeded per a normal flow depth of 0.191’ (2.3”). PARCEL 2/BASIN 2 From Hydrology Report: Q100 = 0.29 CFS total. Q100/ 3 = 0.0966 CFS Flow tributary to each individual sidewalk underdrain (3 underdrains). As demonstrated by the hydraulic calc above, the capacities of the prop. 3” sidewalk underdrains are not being exceeded per a normal flow depth of 0.165’ (1.98”). APPENDIX H VERIFICATION OF THE HYDRAULIC CAPACITY OF THE 6” PVC ONSITE STORM DRAIN SYSTEM AND D-75 BROWDITCH PARCEL 1/BASIN 1 As demonstrated by the hydraulic calc below, the capacity of the 6” PVC prop. storm drain system that runs from the northwesterly corner of basin 1 to the sump pump catch basin is sufficient based on calculation of the governing 6” PVC pipe @ 3.4% slope which carries a maximum of a quarter (Majority of Basin 1 sheet flows into the proposed brow ditch, there is only one inlet that captures runoff for the 6” SD line at the northwest corner of Basin 1) of the basin’s runoff into the proposed sump pump catch basin system prior to the sump pumps pressure flowing into the transfer box at the southerly property line, which then empties runoff out into existing gutter of Magnolia Avenue via the three (3) proposed sidewalk underdrains. For a conservative capacity check of the 6” pvc storm drain system within this drainage basin, one 6” PVC storm drain pipe at 3.4% slope was checked to carry a quarter of the Q100 from basin 1. Q100 (Governing 6” PVC @ 3.4% slope) = C I A (Rational Method) From Hydrology Report: Q100 = 0.35 CFS / 4 = 0.087 CFS Depth of flow = 0.098’ < 0.5’. Capacity not exceeded. PARCEL 1/BASIN 1 The capacity of the D-75 Type B Brow Ditch system that runs just easterly of the proposed lot split line to the sump pump catch basin is sufficient based on the calculation below of the ditch at its critical section. The critical section of the ditch, is at a point just before the sump pump catch basin, where the ditch is conveying the largest amount of tributary runoff. Under real storm event conditions, the tributary runoff flow to the ditch running down the adjacent 2:1 slope to the east will be split up between the two sides of the ditch on either side of the sump pump catch basin. From an extremely conservative analytical standpoint, the D-75 Type B Brow Ditch was checked to carry the entire Q100 at its critical section. As seen in the calculation below, the depth of flow within the circular channel of the ditch does not exceed the full depth of the ditch at its critical section (12”). The Q100 depth within this ditch will actually be far less than this, as Basin 1’s total tributary flow is carried to the sump pump catch basin by both side of the ditch, as well as the 6” PVC storm drain system running from the northwesterly corner of this easterly parcel (analyzed in the previous example). Q100 (D-75 Type B Brow Ditch (2’ wide, 1’ max. depth, @ 2% slope) = C I A (Rational Method) From Hydrology Report: Q100 = 0.35 CFS Depth of flow = 0.147’ < 1’. Capacity not exceeded. Although the ditch seems to be oversized at the critical point, the depth of the ditch decreases as you move along the ditch away from the critical point (Sump Pump Box) in order to maintain a 2% longitudinal slope against adjacent level grades. The depth of the ditch at the beginning of the ditch on the north end is only 4” deep and only 6” at the south end. If the ditch was to be modified to lessen the depth at the critical point, the end depths would need to also be adjusted up by the same amount to hold a 2% longitudinal slope. As the North end of the ditch is already only 4” deep, it is not constructible nor feasible to try to install a lesser depth at the critical section. As the ditch also cuts into a proposed slope, utilizing a D-75 Type B Brow Ditch (J-Ditch) fits the best with the given site conditions. PARCEL 2/BASIN 2 As demonstrated by the hydraulic calc below, the capacity of the 6” PVC prop. storm drain system is sufficient based on calculation of the governing 6” PVC pipe @ 0.5% slope which carries all of basin 2’s runoff prior to discharging into the prop. catch basin that empties runoff into the three (3) proposed sidewalk underdrains that flow out into the existing gutter on Magnolia Avenue. Q100 (Governing 6” PVC @ 0.5% slope) = C I A (Rational Method) From Hydrology Report: Q100 = 0.29 CFS Depth of flow = 0.317’ <0.5’. Capacity not exceeded. APPENDIX I VERIFICATION OF THE HYDRAULIC CAPACITY OF THE TWO GOULDS WS0718B 0.75 HP SUBMERSIBLE SUMP PUMPS (MAX FLOW = 166 GPM) Total Dynamic Head Needed = 5.97’ Pumping rate minimum required per pump = Q100 = 0.35 cfs Factor of Safety of 1.1 applied to TDH (Accounting for minor losses) = 6.6 ft, round up to 7 ft Per Pump Curve below, the WS07B series pump can provide 160 GPM (0.356 cfs) at 7’ of total dynamic head. A single WS0718B series pump can convey the needed Q100 flow rate to the gravity concrete transition box. As a conservative measure, two (2) WS0711B submersible pumps (or equal) shall be installed within the 24”x24” sump pump catch basin. If one pump gets jammed or fails, the secondary redundancy pump will turn on, which is also sized to convey the needed Q100 flow rate. This will ensure that in the event of one pump failure, the secondary pump will turn on and function properly so that the sump pump catch basin and D-75 brow ditch will not overflow.