HomeMy WebLinkAboutPD 2020-0027; 3481 SEACREST DRIVE SLOPE REHABILITATION; DRAINAGE REPORT; 2020-09-023481 SEACREST DRIVE & 3470 CHARTER OAK DRIVE PREPARED FOR Tim Carroll City of Carlsbad 1635 Faraday Avenue Carlsbad CA, 92008 PROPERTY LOCATION 3481 Seacrest Drive Carlsbad CA, 92008 APN:167-122-13-00 DRAINAGE REPORT 777 Woodside Rd., Suite 2A REDWOOD CITY, CA 94061 Phone (650) 366-0216 Fax (650) 366-0298 Email sf@thainc.com September, 2nd 2020 THA #01-4310 triad/holmes associates civil engineering land surveving mammoth lakes• bishop• redwood city nape • son luis obispo
PURPOSE: The purpose of this report is to determine pre- and post-development runoff and velocity. PROJECT DESCRIPTION: The project consists of two sites. The first site (3481 Seacrest) is 0.28 acre, and the second (3470 Charter Oak Drive) is 0.28 acre. The proposed project is a slope restoration of the lower site (3470 Charter Oak Drive) due to a failed french drain under an excessive rain event. The french drain originally ran parallel between the two sites and will now be removed and replaced with a 4” solid drain. The runoff from the slope will be intercepted by a new concrete swale to direct flow into a new 18”x18” inlet. The existing piping will remain that conveys the stormwater to an inlet in the backyard of 3470 Charter Oak Dr. where the existing 4” drain line will be replaced with a 6” line (due to slope of drain line) in order to convey the runoff out to Charter Oak Drive. The sloped area that was affected by the damage will be replaced with fill and decreased from a 1.6:1 slope to a 2:1 slope max with retaining walls to maintain original site flow. No additional impervious surface will be constructed besides the swale to create excess flow from pre-developed conditions. ASSUMPTIONS: The pre-development area is to provide the same amount of flow directed to the south-west property line of 3481 Seacrest Drive. There is no significant off-site tributary area to the project. The recurrence interval for design purposes is a 100-year storm. Runoff coefficient, rainfall intensity and intensity factor are from San Diego County Hydrology Manual. Rational Method with an additional intensity factor per San Diego County Hydrology Manual Data Figure 3-1, attached appendix A, is used to determine runoff. HYDROLOGIC CALCULATIONS for 3481 Seacrest Drive: DETERMINE PRE-DEVELOPMENT RUNOFF, Qpre Rational Method (modified): Q=CIAF SURFACE TYPE AREA(SF) COEFFICIENT ROOF 1,787 0.95 CONCRETE /DRIVEWAY 3,108 0.9 HARDSCAPE 813 0.9 LANDSCAPE/NATURAL 6,575 0.30 AVERAGE COEFFICIENT (C) 0.76 TOTAL AREA (A) 12,283 SQ. FT. TOTAL AREA CAPTURED BY SOUTH INLETS SURFACE TYPE AREA(SF) COEFFICIENT ROOF 1,018 0.95 CONCRETE /DRIVEWAY 1,191 0.9 HARDSCAPE 0 0.9 LANDSCAPE/NATURAL 4,796 0.30 TOTAL AREA (A) 7,005 SQ. FT. 0.16 ACRE
Time of Concentration = 0-10 minutes (small single family lot) INTENSITY (I) Based on Figure 3-1 (see attached) T=8 min 5.0 IN/HR San Diego Rainfall Runoff Data Q = CIA 0.60 CF Conclusion Based on the data from the Hydraulic Manual from San Diego county, and the calcs produced from Hydraflow Express program, Appendix A, a 6” SDR 35 will be able to handle a 100yr storm event occurring on the site 3481 Seacrest Drive from the drain inlet in the backyard of 3470 Charter Oak to the street outfall. The 4” PVC line from the inlet down the slope will be adequate as shown due to the steepness of the grade the line was installed at. PROJECT LOCATION MAP labile Estates' ~ :uccaneer each Park Q t •. ~ .. <!I Walmart Supercenter 9 9ortho Mattress Target9 SOUTH <I Hosp ,..,,_,,.,•d Grove Park OCEANSIDE~,:.•~ aolive :s nd Oak Interiors ?P~'t,'f>t$J <ft'ip CARLSBAD VILLAGE Carlsbad TamarackQ Surf Beach T \ \ <!I Carlsbad High School 9 3481 Seacrest Drive DORO T College English ... The Home Depot 9 Oceansld l ,J cY Calavera Hills Community Park Costco V DMV~/ LAKE Rancho Carlsbad ft Owners' Association T