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STORMWATC-R SITE PLAN DECEMB[R 19,2005 <br /> NGW CENTURY VILLAGE PAGE I I <br /> RETENTION/DETENTION SYSTEI�1 <br /> I� <br /> DEVGLOPED BASIN 1 <br /> The ilow from the roof is collected and diiccted ro :� intiltration trench. The treuch was <br /> sircd using the WWFIAA2 softwarc.'nc� trcnch �eas also sizcd using the SCS method to <br /> check for which manual required the largest trench. Even with the correction factor, thc <br /> WWHM2 method resulted in a larger trench. The trench was sized by running Ihe i{SPF <br /> ' portion of die software and thcn checking the perccntage of total runoff inflltrated as <br /> directed by DOE until full intiltratiun was achieved. Depth of sforage in the prob�ram was <br /> set at 0.9 feet. This accounts tbr the washed rock Uackfill. Dividing this by 0.3, the void <br /> � ratio, gives the aetual trench d�pth of 3 fcet. <br /> The proposed infiltration trench hns the following dimensiuns: <br /> Length = 70 fect <br /> �� Width = 10 fect <br /> Depth =3.0 fcct <br /> I <br /> The computer screen shots iTom IV�WHNt2 :�re included in rlppendis C. <br /> DEVELOPED BASSN 2 <br /> I'RBATMENT <br /> A lincar Stonn�vatcr Managemcnt cartriJge tiltcr trcatment system �vill Ue used to tilter <br /> nmoff from thc parking areas. Note: thc roof runoff from the existing 6-plex will bypass <br /> thc trcatment systcm to connect direcdy to the infiltration system as it does not require <br /> trcatment. The devcloped basin has a 2 yr flo�v rate of 0.11 cfs. With an impen�ious <br /> percentagc of 71% Table �1.1 of thc DOE Manual gives a multiplicr of 037. Taking <br /> 0.1 I x037 = OA41 efs or 18 gpm. Each storm filter cartridge can trcat 7.5 gmp, so 3 tilters <br /> arc required. The tilter vault is detailed on the construction pla��s. <br /> PRGTREATMENT <br /> The City of Everett required prctreatment prior to the cartridge tilter system. The sire of <br /> thc prctrwtment is 30% of thc 6 month starm. The SCS runoff inethod is used for any <br /> sir.ing that involves 'dcad' storage. The 6 month developed stomi runoff is 1,170 cf. <br /> Thcrefore, the required storage for pmtreatment is 351 cf, or 2,625 gallons. A 3,000 <br /> gallon precast concrete tank will be used to pro��ide this storage. <br /> INF[LTRATION <br /> The tlow trom the trcatment system is directed to an infiltration trench. The trench was <br /> sized using the W WH�12 software as mentioned above. <br /> The propused inYiltration trench has the follo�sing dimensions: <br /> Lcngth= SO fcet <br /> . ,_.,..�-��,,:..�.:,c.�.,�;e.�,rr,i im:z�.g a.+-:se�o,.,.�aq�ssa nrH ce-e:��ooc DWH/srm <br />