31 SW EVERETT MALL WAY 2018-01-02 MF Import

Labco Auto Body Repair Facility Drainage Report, February 21, 1993 Page 2 The proposed drainage facilities for this development include curbing to transport water to with orifices will catch basin, a stone water collection and detention system. Outlet structures meter the collected stone water to match the pre -development flow rates off the site. p The Everett Drainage Ordinance No 670-80 is used when evaluating the sites runoff characteristics and when sizing the sites storm water detention facilities. A 5-year, 24 hour stone frequency is used to estimate the allowable release rate for the site with a 25-year, 24 hour stone frequency for sizing the detention system. The Everett Drainage Calculation Submission Form was used to estimate storm water runoff, required stone water detention and orifice sizing for the storm water control structure. It is proposed that the detention system's flow control structure be constructed with two orifices. This approach removes the peaks off the smaller frequency stones and provides additional protection to the downstream conveyance system. In addition to using the 25-year and 5-year return stones for sizing the storm water control system , a 2-year, 24-hour return stone is used for calculating an allowable storm water release rate to a grass -lined swale. Stone water runoff from the building roof and paved areas will be conveyed through a series of catch basins and 12 inch diameter conveyance piping to a 36-inch diameter underground detention tank located north of the proposed building. Stone water discharged from the detention facilities at a rate equivalent the 2-year pre developed Now rate will be directed to an oil -water separator. Discharge from the oil -water separator will be conveyed to the grass -lined swage for final polishing. For storms with a discharge greater than the 2-year event, the flow will be directed by the control stricture to the existing ditch line along the east edge of the site. The portions of the site disturbed by construction and not covered by surface parking or building foot print will be landscaped or hydroseeded to protect against erosion. A temporary erosion plan is provided for the control of stone water construction activity. nRTEMP.WPS