PRELIMINARY DRAINAGE STUDY

Transcription

1 PRELIMINARY DRAINAGE STUDY For 34 th & J Residences 3402 J St. San Diego, CA A.P.N Prepared By: Kenneth J. Discenza, P.E. Site Design Associates, Inc Broadway, Suite A El Cajon, CA September 12, 2011 J-1876

2 TABLE OF CONTENTS 1. INTRODUCTION VICINITY MAP RATIONAL METHOD HYDROLOGIC RESULTS DETENTION REQUIREMENTS CONCLUSION DECLARATION OF RESPONSIBLE CHARGE FIGURES FIGURE 1 - OVERLAY OF THE SOIL RUNOFF POTENTIAL MAP ON THE PROJECT SITE... 6 FIGURE 2 - SIX-HOUR PRECIPITATION FOR 100-YEAR STORM FREQUENCY... 7 FIGURE 3-24-HOUR PRECIPITATION FOR 100-YEAR STORM FREQUENCY... 8 TABLES TABLE 1 - RUNOFF COEFFICIENTS, MAXIMUM OVERLAND FLOW LENGTHS & INITIAL T C... 5 TABLE 2: SUMMARY OF PRE-PROJECT PEAK DISCHARGE RATES... 9 TABLE 3: IMPERVIOUS AREA PERCENTAGES TABLE 4: SUMMARY OF POST-PROJECT PEAK DISCHARGE RATES APPENDICES Appendix A: Pre-Project Drainage Map Appendix B: Post-Project Drainage Map

3 1. INTRODUCTION This drainage report presents the pre and post-project condition hydrologic analyses for the proposed 34 th & J Residences project (hereafter referred to as project). The project is located at 3402 J St., in the City of San Diego, County of San Diego. The drainage area associated with the proposed project and adjacent off-site slopes is approximately 0.91 acres. See the Vicinity Map, located in Section 2 of this report, for the approximate location of the project. Currently, there is one home constructed on the western portion of the lot, a large asphalt driveway, and a separate asphalt area on the northern boundary of the site. The remainder of the lot is covered with annual grasses and a few large trees. Stormwater runoff sheet drains in a westerly direction and eventually flows west down J St. From there it enters a curb inlet which empties directly into Chollas Creek, a concrete-lined channel. The land use of the site is currently designated as residential. The proposed project includes the construction of 12 2-story condominium homes, a concrete driveway, 27 parking spaces (including 2 per garage and 3 guest spaces), and associated walks and landscaping. The storm drain improvements for the post-project condition consist of inlets and pipes throughout the site to collect runoff from roofs and landscaped areas. The inlets within the driveway will include filtration devices to treat the water before it gets discharged to the street and then to Chollas Creek. Off-site run-on from the 0.11 acres to the east of the site will be collected in a concrete brow ditch and discharged along with the on-site runoff. Run-on is expected to be minimal as the drainage area is quite small and is heavily vegetated with ice plant. The majority of run-off will flow to an underground detention basin and then be released at the preproject flow rate to a bioretention area for further treatment. Inlets in the northwest and southwest corners of the site discharge directly to the bioretention area (see the Post-Project Drainage Map in Appendix B for details). All runoff will overflow from the bioretention area through a curb outlet onto 34 th street, and will flow down J St to an existing inlet and then to Chollas Creek as it currently does. 34 th & J Residences Page 1 J-1876

4 The western portion of the driveway slopes steeply to 34 th St. The runoff from this portion will be collected by a trench drain at the end of the driveway. The trench drain will slope north and drainage will flow into the bioretention area for treatment before being released through the curb outlet. The City of San Diego Stormwater Standards, dated January 14, 2011 conforms to the County of San Diego Standard Urban Storm Water Mitigation Plan (herein referred to as SUSMP), which requires that all Priority Development Projects must be designed so that runoff rates and durations are controlled to maintain or reduce pre-project downstream erosion conditions and protect stream habitat. All post-project flows will be treated and managed per the SUSMP. Final inlet sizing and hydraulic calculations for the proposed storm drain system will take place during final engineering of this project, and are not included in this report. 34 th & J Residences Page 2 J-1876

5 2. VICINITY MAP 34 th & J Residences Page 3 J-1876

6 3. RATIONAL METHOD The watershed is relatively small; therefore the rational method (RM) is selected to calculate runoff. Q = CIA Where, Q = Peak Rate of Flow (cfs) C = Runoff Coefficient I = Average rainfall intensity (in/hr) A = Drainage Area (acres) The soil runoff potential for this site is determined by overlaying the site on the Soil Runoff Potential Map, published by the County of San Diego, DPW. The Soil Runoff Potential Overlay Map is shown in Figure 1. Soil Group D and undetermined soils are present near this site. Soil group D is conservatively chosen for the purpose of calculations in this report as the map shows the soil type to be unknown. The storm frequency chosen for design purposes this study has a 100-year recurrence interval. The six-hour anticipated precipitation for the project site subject to the design storm frequency, P 6, is shown in Figure 2. The twenty-four hour anticipated precipitation for the project site subjected to the design storm frequency; P 24 is shown in Figure 3. P 6 = 2.5 inches P 24 = 4.0 inches P 6 is in the range of 45% to 65% (62.5%) of P 24 and therefore needs no adjustment. The time of concentration for on-site basin, T c, is determined from the following formula [1]. 1.8(1.1 C) T c = 3 S D Where, T c = Time of Concentration in hours ( T c 5 min.) D = Watercourse Distance (ft.) 34 th & J Residences Page 4 J-1876

7 S = Slope (%) C = Runoff Coefficient The average rainfall intensity is calculated from the following equation (San Diego County Hydrology Manual, June 2003). I = 7.44 P D Where, I = Rainfall Intensity (in/hr) P 6 = Six hour precipitation (inches) D = Duration (min.) Time of concentration ( T c ) is composed of two components: The initial time of concentration ( T i ), and the travel time ( T t ). The maximum overland flow length ( L M concentration. ) is used from Table 1 in calculating the initial time of Table 1 Runoff Coefficients, Maximum Overland Flow Lengths & Initial T c 34 th & J Residences Page 5 J-1876

8 PROJECT SITE SOIL GROUP Figure 1 - Overlay of the Soil Runoff Potential Map on the Project Site 34 th & J Residences Page 6 J-1876

9 PROJECT SITE P 6 = 2.5 INCHES Figure 2 - Six-Hour Precipitation for 100-Year Storm Frequency 34 th & J Residences Page 7 J-1876

10 PROJECT SITE P 24 = 4.0 INCHES Figure 3-24-Hour Precipitation for 100-Year Storm Frequency 34 th & J Residences Page 8 J-1876

11 4. HYDROLOGIC RESULTS The 100-year, 6-hour peak flow rates for the pre-project can be found in Table 2. Watershed boundaries and drainage areas can be found on the, Pre-Project Drainage Map, located in Appendix A. Area A represents all on-site runoff while Area B represents the small area of off-site run-on. The C-values and initial times of concentration for both areas were taken from Table 1 based on impervious percentage. Area B is entirely pervious and is therefore given a C-value of But due to its steep slope and short travel distance of 38 ft., the T c is shown as the minimum of 5 minutes. Area A has approximately 10,000 sf of impervious area of the total 34,970 sf. This gives an overall impervious percentage of 28.7%, so the values in Table 2 were interpolated from the Table 1 values listed under the 25% and 30% impervious. There is no existing stormwater conveyance system on site, and therefore all existing drainage is sheet flow. So the given T c s are comprised only of T i as T t = 0 Table 2: Summary of Pre-Project Peak Discharge Rates The 100-year, 6-hour peak flow rates for the post-project can be found in Table 4. The outline of Area B remains the same as do its hydraulic characteristics. The overall outline of Area A remains the same but it is broken into three subareas A 1, A 2, and A 3 as shown on the Post-Project Drainage Map in Appendix B. Area A 1 represents the majority of the site which will drain to the proposed detention structure before draining to the bioretention area for treatment. Area A 2 represents the area draining directly to the bioretention area. And Area A 3 represents the areas that will drain directly to 34 th St., the majority of which is part of the expanded right-of-way. The C-values for all three subareas were again interpolated from Table 1 based on impervious 34 th & J Residences Page 9 J-1876

12 percentage. The breakdown of impervious percentages of each subarea is shown in Table 3. Table 3: Impervious Area Percentages In this case, Table 1 does not apply for the initial times of concentration as all of the initial travel distances are shorter then the maximum (L M ) given by the table. Therefore, Formula 1 (given in section 3) was used to calculate T i. In each case the T i was less then the minimum of 5 min. and therefore 5 min. was used in the calculation of the intensity (I) as described in section 3.3 of the San Diego County Hydrology Manual. The T t for area A 1 was calculated from the travel time through 250 ft of pipe length, upstream of the detention structure, at a velocity of 2.5 ft/s. This gives a total travel time off 100 sec. or 1.67 min. Table 4: Summary of Post-Project Peak Discharge Rates 34 th & J Residences Page 10 J-1876

13 5. DETENTION REQUIREMENTS The on-site 100-year pre-project peak flow rate is 2.01 cfs. The on-site 100-year post-project peak flow rate totals approximately 3.31 cfs. Therefore, detention will be required for this storm event. The project proposes an on-site, underground detention facility plus storage within the bioretention area which will detain the post-project flows back to pre-project flow rates. The preliminary storage volume needed to meet this requirement is approximately 1600 sq-ft. Details for the detention basin will be completed with the final design. 34 th & J Residences Page 11 J-1876

14 6. CONCLUSION This drainage report presents the 100-year, 6-hour pre and post-project hydrologic analyses for the 34 th & J Residences project. Off-site flows are conveyed through the project site and are discharged in the same location as they were in the pre-project condition. On-site runoff from the proposed project will be detained, treated as necessary, discharged to the street and ultimately drains into Chollas Creek. The pre-project condition 100-year, 6-hour peak runoff rate on-site is 2.01 cfs. The post-project condition 100-year, 6-hour peak runoff rate on-site is 3.31 cfs. Therefore, detention is required for this storm event. The preliminary storage volume required to detain this event is 1,600 sq-ft. Details for the detention basin will be included with the final grading plan design. Inlet sizing and hydraulic calculations for the proposed storm drain system will take place during final engineering of this project. All post-project flows will be treated per the SUSMP, and will be addressed in a separate report. 34 th & J Residences Page 12 J-1876

15 7. DECLARATION OF RESPONSIBLE CHARGE I, hereby declare that I am the engineer of work for this Drainage Report, that I have exercised responsible charge over the design of the project with respect to this study as defined in section 6703 of the Business and Professions Code, and that the design is consistent with current standards. I understand that the check of project drawings specifications, studies and reports by the City of San Diego is confined to a review only and does not relieve me, as engineer in responsible charge, of my responsibilities for project design. Kenneth J. Discenza RCE Expires 06/30/13 Date 34 th & J Residences Page 13 J-1876

16 APPENDIX A Pre-Project Drainage Map 34 th & J Residences Page 14 J-1876

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18 APPENDIX B Post-Project Drainage Map 34 th & J Residences Page 15 J-1876

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