3214 BROADWAY BROADWAY KINECT APARTMENTS 2021-08-11

i • CITY OF EVERETT STORMWATER MANAGEMENT MANUAL • Conditions of Use Prior to leaving a construction site, stormwater runoff must pass through a sediment pond or trap or other appropriate sediment removal BMP. Non-engineered sediment traps may be used on site prior to an 4111 engineered sediment trap or sediment pond to provide additional sediment removal capacity. 1111 Sediment traps are intended for use on sites where the tributary drainage area is less than 3 acres,with no • unusual drainage features,and a projected build-out time of 6 months or less. The sediment trap is a • temporary measure (with a design life of approximately 6 months) and shall be maintained until the site area is permanently protected against erosion by vegetation and/or structures. • Sediment traps are only effective in removing sediment down to about the medium silt size fraction. Runoff with sediment of finer grades(fine silt and clay)will pass through untreated, emphasizing the • need to control erosion to the maximum extent first. - Whenever possible, sediment-laden water shall be discharged into onsite,relatively level,vegetated areas (see BMP C234—Vegetated Strip). This is the only way to effectively remove fine particles from runoff • unless chemical treatment or filtration is used. This can be particularly useful after initial treatment in a sediment trap. The areas of release must be evaluated on a site-by-site basis in order to determine • appropriate locations for and methods of releasing runoff. Vegetated wetlands shall not be used for this purpose. Frequently, it may be possible to pump water from the collection point at the downhill end of S the site to an upslope vegetated area. Pumping shall only augment the treatment system, not replace it, • because of the possibility of pump failure or runoff volume in excess of pump capacity. All projects that are constructing permanent facilities for runoff quantity control should use the rough- • graded or final-graded permanent facilities for traps. This includes combined facilities and infiltration facilities. If infiltration facilities are to be used,the sides and bottom of the facility must only be rough S excavated to a minimum of 2 feet above final grade. Final grading of the infiltration facility shall occur • only when all contributing drainage areas are fully stabilized. When permanent facilities are used as temporary sedimentation facilities, the surface area requirement of a sediment trap or pond must be met. 41111 If the surface area requirements are larger than the surface area of the permanent facility,then the trap or S pond shall be enlarged to comply with the surface area requirement. The permanent pond shall also be divided into two cells as required for sediment ponds. • Either a permanent control structure or the temporary control structure (described in BMP C24I, Temporary Sediment Pond)can be used. If a permanent control structure is used, it may be advisable to • partially restrict the lower orifice with gravel to increase residence time while still allowing dewatering of • the pond. A shut-off valve may be added to the control structure to allow complete retention of stormwater in emergency situations. In this case,an emergency overflow weir must be added. A skimmer may be used for the sediment trap outlet if approved by the City. Design and Installation Specifications 11111 1. See DCSS Drawing#s 215 and 216. 41 2. If permanent runoff control facilities are part of the project,they may be used for sediment. 3. To determine the sediment trap geometry, first calculate the design surface area(SA)of the trap, measured at the invert of the weir. Use the following equation: iSA=FS(Q,I VS) • Where: • Q2=Design inflow(cfs) based on the 2-year recurrence interval flow rate. S S Volume 11—Construction Storm water Pollution Prevention 3-60 S •i