1502 WALL ST IMAGINE CHILDRENS MUSEUM 2022-08-25

. A floating pond skimmer may be used for the sediment trap outlet if approved by the Local Per- mitting Authority. . Sediment traps may not be feasible on utility projects due to the limited work space or the short-term nature of the work. Portable tanks may be used in place of sediment traps for utility projects. Design and Installation Specifications • See Figure II-3.26:Cross Section of Sediment Trap and Figure II-3.27:Sediment Trap Outlet for details. . 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: SA= FS(Q2/Vs) where Q2= o Option 1 -Single Event Hydrograph Method: Q2= Peak volumetric flow rate calculated using a 10-minute time step from a Type 1 A, 2-year,24-hour frequency storm for the developed condition.The 10-year peak volu- metric flow rate shall be used if the project size,expected timing and duration of con- struction,or downstream conditions warrant a higher level of protection. Option 2-For construction sites that are less than 1 acre,the Rational Method may be used to determine Q2. Vs=The settling velocity of the soil particle of interest.The 0.02 mm(medium silt) particle with an assumed density of 2.65 g/cm3 has been selected as the particle of interest and has a set- tling velocity(Vs)of 0.00096 ft/sec. FS=A safety factor of 2 to account for non-ideal settling. Therefore,the equation for computing sediment trap surface area becomes: SA=2 x Q2/0.00096 or 2080 square feet per cfs of inflow . Sediment trap depth shall be 3.5 feet minimum from the bottom of the trap to the top of the overflow weir. . To aid in determining sediment depth,all sediment traps shall have a staff gauge with a prom- inent mark 1-foot above the bottom of the trap. 2019 Stormwater Management Manual for Western Washington Volume ll-Chapter 3-Page 384