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12/13/2982 -;.39 5E32583101 <br />;TORhIWATER MGT INC PPi3E 02 <br />i Y <br />Silver lake Elementary Parking Lot Expansion - StormFilter Sizin & Cost Estimate <br />Everett. Washington <br />information provided by engineer. <br />• Total contributing area = 1.57 acres <br />• Water quality agency and standards = Snohomish County <br />• Detention treatment release rate = 0.02 cfs <br />Assumotions: <br />• SteC=0.95 <br />• Mean annual rainfall = 38' <br />• =vent Mean Concentration of pollutant= 80 mg/ L <br />• Ferlite filter media cartridges <br />• Drop required from inlet to outlet = 2.3' minimum <br />conclusions: <br />6' x 12' precast vault with 10 cartridges requiretl, based on annual mass loading <br />analysis. See attached spreadsheet. vault will require additional interior baffling in <br />order to match detention release rate to cartridge flow rate. <br />Use 7.5 gpm "ow rate through filter cartridges to provide longer residence time and <br />increase TSS loading capacity. <br />Sizing and cos�e2 atest <br />The StormFilter is a flow -based system, and therefore, is typically sized by calculating the <br />peak water quality flow rate associated with the design storm. However, when the <br />StormFilter is placed downstream of detention, both the flow rate through the system and <br />U•,e mass load off the site must be considered. If only the release rate from detention were <br />considered, the StormFilter might not be large enough to accommodate the annual mass <br />load off the site, based on the average rainfall for the site. This could result in the cartridges <br />becoming plugged too quickly, necessitating more frequent maintenance. <br />The enclosed spreadsheet was used to calculate the required number of filter cartridges to <br />acnieve an 80%overall removal rate for suspended solids from 91% of the annual runpff <br />volume, for systems downstream of detention. This spreadsheet is based on the <br />methodology outlined in our new, design manual. <br />The StormFilter for this site was sized based on the above mass loading analysis- To <br />accommodate the required number of cartridges, Stormwater Management recommends <br />usinb a 6' x 12' precast StormFilter with 10 cartridges. The estimated cost of this system is <br />522 000, complete and delivered to the job site. This estimate assumes that the vault is <br />less than SO' feet deep. The final system cost will depend on the actual depth of the units <br />a,id whether extras like doors rather than castings are specified. The contractor is <br />responsible for setting the StormFilter vault and all external plumbing. <br />SMIENGR:BLG <br />J. il/G/02 <br />