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155 If sheet floN✓ over asqhalt S= 0.008 <br />Total Existing T�= '3.57 min. <br />The storm water runoff rates for the existing basin can be found in Table 2- Existing <br />Peak Storm water Flows and the Stormshed calculations can be found in Appendix A— <br />Basin Summaries. <br />Table 2. Existing Peak Storm water Flows <br />Developed Conditions: <br />The proposed development will consist of the construction of a b k building with a <br />basement and three floo�s above grade with associated parking a drive aisles. The <br />developed site will have approximately 0.866 acres of impervious rface including the <br />building, walkways and paved parking and tra�c aisles. It is prop sed to tightline the <br />rooftop runoff to the existing drainage system in 100T" Street. A�r pump <br />system will be installed to handle flows from the basement footing drains. Discharge <br />from the pum;� system will also be directed to the system in 100T" Street. Runoff from <br />the pollution generating impervious surFaces and sidewalks will be collected in a series <br />of catch basins and pipes and routed to an underground wet tank. Flows will then be <br />routed to a Stormfilter treatment device developed by Stormwater Management, Inc. <br />The Stormfilter is capable of passing up to 1.8 CFS. The 100-year flow rate to the <br />Stormfiiter is approximately 0.57 CFS. Flows will be discharged to the drainage system <br />in 4T" Avenue. See Attachment 3— Developed Conditions. <br />The Stormfilter has been designed with the specific requirements of the City outlined in <br />the pre-application comments. The wet tank must provide 30% of the volume of the 6- <br />month, 24-hour storm runoff volume from the contributing drainage area. The required <br />dead storage volume is 650 cubic feet. The dead storage provided is approximately <br />773 cubic feet. This volume includes storage in 50 feet of 48" pipe and storage in the <br />two manholes. Using the required 15 gpm flow rate per cartridge in the stormfilter, it <br />was determined that five filter cartridges will be required to treat the 6-month, 24-hour <br />flow rate (0.15 cfs). A 6' x 8" precast stormfilter will house the five cartridges. Flows <br />from the stormfilter wil! be routed to the drainage system in 4T" Avenue. <br />The Time of Concentration for the Developed Basin, as shown on Attachment 3, was <br />determined as follows: <br />U <br />M:1Sdg12004L59-SDG-04 - F51 - Horizon Bank in EverettlDRAIN1Dreinage Analysis-259.doc <br />4 <br />