3900 BROADWAY BASE FILE 2016-01-01 MF Import

CITY OF EVERETT STORMWATER MANAGEMENT MANUAL l ' Conditions of Use Prior to leaving a couswction site,stormwater runoff must pass tivaugh a scdiment pond or trap or other appropriate sediment removal BiVIP. Non-engineered sediment traps may be used on site prior lo an engincercd sediment hap or sediment pond to provide additional sediment remaval capaciry. Sediment traps are intended for use on sites where the tributary drainage area is less than 3 acres,with no unusual drainage featutes,and a projected build-out time of 6 months or less. Thc scdiment trap is a tcmporary measure(with a design life of approximately 6 months)and shall be maintained until the sitc area is permanently pmtccted against erosion by vegetation and/or sWctures. Sediment traps arc only eKective in removing sediment down to about the mcdium silt size fraction. Runotf with sediment of finer gradcs(fine silt and clay)will pass through untreated, emphasizing the need to control erosion lo the maximur�i extent first. Whenever possible,sediment-laden water shall be discharged into oasite,reletively level,vegetatcd areas (see BMP C234—Vcgetated Strip). This is the only way ro effectively remove fine particles (rom runoff unless chemical treatmer,t ar filhation is i�sed. This can be pacticularly usefut after initial treahnent in a sediment trap. The areas of release must be evaluated on a site-by-site basis in order to determine appropriatc locations for end met6ods of releasing runofT. Vegetated wetlands shall not be used For this putpose. Frequently, it may be possible to purop water Gom the collection point et the do�whill end of the site to an upslope vegetated area. Pumping shall only augment the treatment system,not replece it, because oCthe possibility of pump failure or�unoff volume in exccss of pump capaciry. All projects that are constructing permanent facilities for mnoff quantiry control should use the rough- graded or final-graded pertnanent facili6es far haps. This includes combined facilities and infiltration \ facilitics. If infiltration facilitics aze to be used,the sides and bottom af the facility must only be rough � excavated to a minimum of 2 feet above final grade. Final grading oi thc infilhation faciliry shall occur � only when all contributing drainage areas aze fiilly stabilized. Wheo permanent facilities arc used as tempotary sedimentation facilities,the surface area requirement of a sediment trap or pond must be met. IClhc surfacc arca requirements are lazger than thc surface azea oCthe pertnanent(acility,then the trap or pond shall bc enlarged lo comply with the surface atea requiremcnt. The pertnanent pond shall also be dividrd into two cells as required for sediment ponds. Either a pecmanent control swchue or the temporary control sWchue(described in H�iP C241, Temporary Sediment Pond)can be used. If a pertnanent control shvchuc is used, it may be advisablc to partially restrict the lower orifice with gravel to increase residence time while still allowing dewatering o( the pond. A shut-off valvc may be added to the control shucturc to allow complete retcntion of stormwatcr in emcrgency situations. In this casc, an cmergcncy ovcrflow wcir must be added. A skimmcr mey be uscd tor thc scdimenl trap outlet if approved by the City. Deslgn and Instaflation Specifieatlons l. See DCSS Drawing Ns 215 and 216. 2. If permanent cuuoff control facilities arc part oC the project,they may be used for sediment. 3. To determine the sediment trap gcometry, first calculatc the design surtacc area(SA)ot thc trap, measured at the iuvcrt of the weir. Use the following equalion: SA = FS(Q=I�S) Where: Q==Design inllow(cFs)based on the 2-yeaz recurrence intcrval flow rale. Volume II—Construction Stormwater Pollufion Prevention 3-60