2106 120TH PL SE 2016-01-01 MF Import

ROOF RUNOFF INFILTRATION AND DISPERSION SYSTEMS (Section 3-2.6.2 BMP RI.15 oi the City ot Everett Stormwater Management Manual) �enerai These Base Management Practices (BMPs) are intended to infiltrate runoff from roof�ops only. Because runoff from rooftops is considered reiatively clean, no pretreaiment in required prior to discharge to this BMP. Roof runofi infiltration and dispersion systems are usually designed as gravel filled trcnches. The pore space between the gravel provides storage (or runoff until iniiltration occurs. The Ciry may approve other underground storage/infiltration systems provided ihey meet all applicable criteria given in this manual. Desiqn Criteria 1. Trenches are assumed to have rectangular cross-sections and the infiltration surtace area includes the sides and bottom of the trench. The storage volume of the trench is determined using the void ratio of the stone aggregate used to backfill the trench. 2. Roof runoff inliltration systems shall be designed to completely infiltrate the 10-year, 24-hour storm event within 24 hours after the occurrence of the storm. Rooi runoff dispersion systems shall be designed to completely infiltrate the 2-year, 24-hour storm event within 24 hours after the occurrence oi ihe storm. 3. The trench aggregate material for roof runoH/dispersion systems shal; be 3/4 inch to 1-1/2 inches washed rock. Void space tor this agg�egate meterial is assumed to be 30 percent. 4. Filter tabric shall be wrapped entirely around the aggregate rock prior to backlilling. 5. Trenches may be located under pavement provided that a smali yard drain/catchbasin ��ith a yrate cover is placed at the end ot the trench pipe such that if the trench infiltration capacity is exceeded, the overftow v.ould occur out of ihe catchbasin at an elevation at laast 1 foot below that of any buildings and in a location which can accommodate the overflow. 6. A 25 percent tail�rF ra!e of roo( runotf systems shall be assumed when designing streambank erosion conlrol facilities tor residential subdivisions. 7. Rcoi runoff dispersion systems may be located at the edge of Native: Growlh Protection Areas (NGPA) and �vetland and creek buffers, subject to the approval of the City's Environmental Planner. 8. 1 he ler �th of a roof runoif or dispersion system sliould not exceed 100 feet Irom the inlet sump. 9. Each roof runofi infiltration or dispersion system shall have an observation well extending to the bottom of the trench and located et a point approximately halfway along the length of the trench. The observation well shali consist o} four to six inch diameter, pertorated PVC pipe.