7125 EVERGREEN WAY 2018-01-02 MF Import

RUNOFT CONTROL FACILITIES Runoff control facilities will consist of underground detention tanks designed to detain runoff from the developed site and release it to 50% of the existing 2 year runoff, and i00% of the existing runoff for the 10 year and 100 year design storms. The detention tanks aze designed to accom�nodate the stormwater runoff from the entire site, including the parking lot, landscaping and roof tops. Detention volume will be provided by using four 95 inch by 67 inch underground pipe arch detention tanks. Each cank is 100 feet long (access hatch to access hatch). The detention facility will discharge into the existing storm drain system through a proposed cutch basin near the northeast comer of the site. The following parameters were used to design the runoff control facilities (see appendix): Soil Existing impervious area - 2sphalt Existing pervious area - brush and grass Phase 1 Developed impervious area - roof Developed impervious arca - pav�:ment Developed pervious lrea — landscape Rainfall — 2 year Rainfalf —10 ycar Rainfall — 100 year Existing Tc Developed Tc Existing runoff— 2 year Existing runoff— ]0 year �sisting runoff-100 year Aldenvood Typc C 0.15 acres/CN 98 2.85 acres/CN 81 0.21 acres/CN 98 2.45 acres/CN 98 034 acres/CN 86 1.55" 2.25,> 3.25" 30 min 6.3 min (default minimum) 0.11 cfs 0.33 cfs 0.76 cfs Computer analysis for detention tank sizing utilized a vault routine to approximate the non-linear stage-storage relationship of the proposed pipe arches. While this analysis is not 100% accurate it falls within an acceptable margin of error given the variability of the other parameters used (existing and proposed CN values, Tds, rainfall estimates, etc.) to create the design srorms and deterniine allowable de�•eloped site release rates. C��1S2r'F��l �25e) 30