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• <br />� <br />• <br />:7 <br />� <br />� <br />� <br />❑ <br />� <br />CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br />Ucsign Critcria <br />i)esign eriterin for n.�;����v ��ea tilmr strps ure f,c «nnc a� �pec ji�;�l7or hasrc fil��r strfrs. F(mre�cr, tl�.e <br />sizing of a narro��• �uca filtcr strip is b.tiscd on thc Icngth uf flo��path draining to thc liltcr strip and thc <br />longitudinal slope o(the filter strip itself (parallel to die Omvpath). <br />.Srep 1: Detcnninc the lengih o(the flo�vpath fi'om the upstream to the downstream edec ofthe imr�niou� <br />aica draining sheci flow to the strip, Nomiall} this is ,hc same as the «idih of the paved ;.:ea, out if the si�c i. <br />sloped, thc tlo�v path may be longcr ttuvi the width of ihe impervious arca. <br />.Step 1: Calculate thc long�tudin:el slope of the filter strip (along Ihe directior, of unconcennaced flow), <br />aee�:�ged over the totsl �vidth of Nte �iltcr �trip. The ininimum sizing slope is 2 perccnt If ihc slope is Icss <br />than 2 pereenL use 2 percent for sizing purposes. The maumum allo��able filter stnp slope is 20 percen, It <br />ihe slope ezceeds 30 percent, the filter strip must be stepped do�vn the slop, so that the treaiment areas <br />betwccn drop sections do not ha�e a longitudinal slopc t�rcater than 20 percent. Drop sections mu<t h� <br />providcd with erosion proiection at the basc and flow spreaders �o rc-spread flows. Vertical drops a!org the <br />slope must nol exce�d 12 inches in hein,ht. If this is not possihle, � different trcamient faciliry mii�i ;� <br />scfected. <br />.C��p.i Select thc appropria�e filter strip Icng�h tor lhe flowpsth Iength:uid filter sVip longitudin;�: ;;o^e <br />(Sieps I and 2 above) from ihe graph in Figure 4.4. 'fhe fiher strip must Ue desi� icd tr pro�idc ;his <br />minintum Icnglh L along thc cmim strctch of pacemcN draining into ii <br />Tn �ue !he Rrapi�. Find thc lcngth of !he flowpath on onc of the wrves (interpolate bcneccn cun c� ,s <br />necessary). Move along the cuitic io ihc point �ahere the design longitudinal slopc of ihc filtcr strip (�- <br />r.�is) is directly below. Read the 61ter strip len�nh on the y-a.eis �vhich corresponds to the imci:t�c5o❑ <br />����i;�� �Y�1+ �4 Ivu(3'1n tnyt�� OT � <br />� � <br />20.OT---_-._-- —r---_. ._ <br />I50 <br />L <br />� <br />c <br />= 100 <br />� <br />V <br />� i ll <br />4� <br />00 <br />�!��J <br />PloHpaih <br />= 1� �CCI <br />---.� -._--_-- '-- <br />__ _. _�' I : 20 (ecl . _ _ _ <br />i� <br />�� � � <br />/ � i�1 10 tcci <br />� 'i �� I <br />� ��.—f- <br />"j�'�u 5"� 16".1 I i"� <br />Fil�cr Slrip Slo�c <br />,,^;'oie inuunmm ullo�ru6/r fJlrr� sb�p ler�,�th As d fc� � <br />I _ _. <br />Figurc 6.2 Filter Strip Lengths for Narrow Right•of-Way. <br />Vnlurne lll — Ro�v Conhal,�nd Treatment <br />6-4 <br />tn�'n <br />