1710 W MARINE VIEW DR 2024-01-03

. 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(Q2Ns) 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 ds 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