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COME RAIN, WE sHiNe:' STORNiWATE�� <br /> MANAOEMEN7 INC. <br /> Sile+er Lake Medical- Stormwater Treatment Svstem <br /> Size and Cost Estimate <br /> E.verett, Washington <br /> Information orovided: <br /> • Total contributing area = 1.81 acres <br /> • Impervious area = �.49 acres <br /> • Volume of the settling basin = 5,376 cubic feel <br /> • Volume of runof(_ �,�77 cubic feet <br /> • Uepth of water in basin = 1.56 ft <br /> • Delention release rate = 0.03 c(s <br /> • Presiding agency= City of Everett, Wasliington <br /> Assumolions: <br /> • Media = Periite cartridges <br /> • Drop required from inlet to ouUet = 2.3' minimum <br /> • Cartridge flow rate = 7.5 gpm <br /> Size and cost estimales: <br /> The StormFilter is a flow-based syctem, and therefore, is sized by calculating the peak water <br /> quality fiow rate associaled with the design storm. However, when the StormFilte� is placed <br /> downstream of detention the flow rate generated at the water qualiry storm is not always <br /> representative of the total volume of water Uiat wiil go through the system or type of <br /> pollutant-loading the system may experience in one year. <br /> Taking into consideration the setUing that will occur within detention systems prior to the <br /> StomFiiter vau!t, a pretreatmen[efficiency, or "credit" is awarded. Generally a credit of 50°/a <br /> is the maximum that is awarded, but due to die fact that this StormFilter is downstream of <br /> two prelreatment syslems, an assumption can be made that the pretreatment removal <br /> efficiency is greater than 50%. If a pretreatment removal credit of 60% is aw�arded the <br /> estimated annual mass captured by the filters is 220.94 Ib. Assuming a mass load per <br /> cartridge of 36 Ib, Uie number of cartridges required to treat the mass load off the sile �s 6.1 <br /> Please see the atlached calculations worksheet. <br /> Stormwater Management, Inc. (SMI) recommends using a 6' x 8' precast StormFilter with 6 <br /> filter r,art�idges. From a mass load standpoint, it was estimated tha[6.1 cartridges will <br /> remove the annual sediment loading. For a flow based system we typicaily round up to the I <br /> next whoie cartridge to assure that we're treating the fuil hydrauiic flow rate. Because the <br /> mass loading design is based on an estimated annual loading, SMI feels it is reasonab!?to <br /> round down when rounding up would influence the size of the vaulL <br /> 3 II <br /> O 'I <br /> SMI ENGR: SPH 9/2/04 I <br />