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CtTY OF EVERETT STORMWATER MANAGEMENT MANUAL I <br /> The 10-year recurrence interval peak ftow shall be used if the project size,expccted timing�d ' <br /> duration of conshuction,or downstreem conditions warrant a higher level of protection. For <br /> sediment traps having a wntnbuting azea with a time of concentration less than or equal to 60 <br /> minutes,the rational method may be used ro determine the design flow. Ot6crnise,a conunuous <br /> runoff model(WW}IIvi or an approved altemative,see Volume 1,CUapter 5)shall be used to <br /> detertnir.e the design Row. The developed(unmitigated)land use condition s6a11 bc used in the <br /> hydrologic analysis. <br /> VS =The settling velocity of the soil perticle of interest. The 0.02 mm(medium silt)particle <br /> with an azsumed density of 2.65 g/cm3 has becn sclected as the pazticle of intcrest and <br /> has a settling vclocity(VS)of 0.00096 R/sec. <br /> FS=A safety factor of 2 to account for non-ideal scuGng. <br /> The cquation for computmg surface area becomcs: <br /> SA =2 x Q/0.00096 <br /> This is equal to 2,083 square feet per cubic feet per second of inflow, measured at the invert of <br /> the weir. <br /> Note: Even if permanent facilities are used,they must still have a surface arca that is at Icut as <br /> large as that derived �om the above fotmula. [f they do not,the pond must be enlarged or <br /> additional surfece azea must be provided in a temporary sedimeot control (acility. <br /> 4. To aid in detetmining sedimeot dcpth,all sediment traps shall have a staff gauge with a <br /> prominent labeled mazk each I-foot interval above the bottom of the trap. <br /> 5. Scdiment traps may not be feasible on utiliry projccts due to thc limited work space or the short- <br /> term nature of the work. Partable tanks may be used in place of scdiment haps for utility <br /> projects. <br /> Mafnfenance Standards <br /> 1. Sediment shall be removed(rom the trap when it reaches l•(oot in depth <br /> 2. My dainage to the pond embanlanents or slopes shall be repaired. <br /> 3.3.18 BhiF� C:41: Temporary Sediment Pond <br /> Purpose ' <br /> Sediment ponds remove sedimcnt from runo[f originating from disturbcd azeas of the site. Sediment <br /> ponds are typically designcd lo rcmovc sediment no smallcr thao medium silt(0.02 mm). Coascquently, <br /> they usually rcducc turbidity only slightly. <br /> Conditions of Use <br /> • Prior to lcaving a construction sitc,stormwater mnoff must pass Ihrough a scdiment pond or other <br /> appropriate sediment removal BMP. <br /> • A sediment pond shall be used wherc the contributing drainage arca is 3 acres or more. Ponds <br /> must be used in conjunction with erosion control practices to reduce the amount of sediment <br /> flowing into the basin. <br /> Design and fnstallation Speci/ications <br /> L Scdime;it ponds shall not bc installed on sites where failure of the structure would result in loss of <br /> life,damage to homes or buildings,or intettuption of use or service of public roads or utilities. <br /> Volume II— Construction StormwaterPollution Prevention <br /> 3-61 <br />