10901 27TH AVE SE 2025-07-29 - Laserfiche WebLink

I I I provides storage within the voids. Runoff from the detention system will release to the existing on-site catch basin that conveys site runoff across 27th Avenue SE to the unnamed stream channel to the west. IRunoff from building roofs,non-pollution-generating surfaces, and pervious landscaping and existing vegetation will be collected and conveyed to the detention system by a series of catch basins,yard drains, and storm drain pipe. This runoff will bypass any water quality facility I before discharging to the detention system. The underdrain system for the existing athletic field will remain intact but discharge to the new storm drainage system that bypasses the water quality measures and discharges to the proposed detention system. IStormwater detention for the site will be provided by an underground detention system of StormTech Chambers. The detention system volumes are calculated using the Santa Barbara I Urban Hydrograph method and the StormSHEDTM computer program. A correction factor(from Figure 1-3.1,COE correction factor chart) of 1.30 is included in the detention design for the impervious surfacing,which covers 47 percent of the site. Existing conditions used in the Idetention design release rate are based on site conditions as of 1996 according to City of Everett. Summary Table: Storm Return Release Rate Release Rate Detention Detention Detention Frequency Required Provided Volume Volume Volume I (Years) (CFS) (CFS) Required F.S. 1.3 Provided (CF) (CF) (CF) I2 ' 0.76 0.75 3,390 4,407 4,834 10 2.44 2.38 4,687 6,093 6,685 I100 3.82 3.74 7,026 9,134 10,115 ICalculations are provided in Appendix B. SUMMARY OF DESIGN PARAMETERS & RESULTS ITotal Site Area 9.16 ac Total Existing Impervious Area 4.46 ac ITotal Existing Pervious Area 4.70 ac Total Proposed Impervious 4.26 ac I Total Pervious Area 4.90 ac • • I IJames Monroe Elementary School 5 Reid iddleton Drainage Report I