1331 MADRONA AVE 2016-01-01 MF Import

CU �L GS -U �(z ROOF RUNOFF INFILTRATION AND DISPERSION SYSTEMS (Section 3-2.6.2 BMP RI.15 of the City of Everett Stormwater Management Manual) General These Base Management Practices (BMPs) are intended to infiltrate rwioff from rooftops only. Because runoff from rooftops is considered relatively ciean, no pretreatment in required prior to discharge to this BMP. Roof runoff infiltration and dispersion systems are usually designed as gravel filled trenches. The pore space between the gravel provides storage for runoff until in�iltration occ�irs. The City may approve other underground storage/infiltration systems provided they meet all applicable criteria given in this manual. Desiqn Criteria 1. Trenches are assumed to have rectangular cross-sections and the infiltration surf3c? 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 trencn. 2. Roof runoff infiltration systems shall be designed to completely infiitrate the 10-year, 24-hour storm event within 24 hours after the occurrence of the storm. Roof runoff dispersion systems shall be designed to completely infiltrate the 2-year, 24-hour storm event within 24 hours after the occurrence of the storm. 3. The trench aggregate material for roof runoff/dispersion systems shall be 3/4 inch to 1-1/2 inches washed rock. Void space for this aggregate material is assumed to be 30 percent. 4. Filter fabric shall be wrapped entirely around the aggregate rock prior to backfilling. 5. Trenches may be located under pavement provided that a small yard drain/catchbasin with a grate cover is placed at the end of the trench pipe such that if the trench infiltration capacity is exceeded, the overflow would occur out of the catchbasin at an elevation at least 1 foot below that of any buildings and in a location wliich can accommodate the overflow. 6. A 25 percent failure rate of roof runoff systems shall be assumed when designing streambank erosion control facilities for residentiai subdivisions. 7. Roof runoff dispersion systems may be located at the edge of Native Growth Protection Areas (NGPA) and wetland and creek buffers, subject to the approval of the City's Environmental Planner. 8. The length of a roof runoff or dispersion system should not exceed 100 feet from the inlet sump. 9. Each roof runoff infiltration or dispersion system shall have an observation well extending to the bottom of the trench and located at a point approximately halfway along the leny:,i of the trench. The observation well shall consist of four to six inch diameter, perforated PVC pipe.