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1 <br />area requirement. The permanent pond shall also be divided into two cells as required <br />for sediment ponds. <br />Either a permanent control structure or the temporary control structure (described in BMP <br />C241: Temporary Sediment Pond (p.388)) can be used. If a permanent control structure <br />is used, it may be advisable to partially restrict the lower orifice with gravel to increase <br />residence time while still allowing dewatering of the pond. A shut-off valve may be <br />added to the control structure to allow complete retention of stormwater in emergency <br />situations. In this case, an emergency overflow weir must be added. <br />A skimmer may be used for the sediment trap outlet if approved by the Local Permitting <br />Authority. <br />Design and Installation Specifications <br />. See Figure 11-4.2.16 Cross Section of Sediment Trap (p.386) and Figure II-4.2.17 <br />Sediment Trap Outlet (p.387) for details. <br />If permanent runoff control facilities are part of the project, they should be used for <br />sediment retention. <br />. To determine the sediment trap geometry, first calculate the design surface area <br />(SA) of the trap, measured at the invert of the weir. Use the following equation: <br />SA = FS(Q2Ns) <br />where <br />Q2 = Design inflow based on the peak discharge from the developed 2-year runoff event <br />from the contributing drainage area as computed in the hydrologic analysis. The 10-year <br />peak flow shall be used if the project size, expected timing and duration of construction, <br />or downstream conditions warrant a higher level of protection. If no hydrologic analysis <br />is required, the Rational Method may be used. <br />Vs = The settling velocity of the soil particle of interest. The 0.02 mm (medium silt) <br />particle with an assumed density of 2.65 g/cm3 has been selected as the particle of <br />interest and has a settling velocity (Vs) of 0.00096 ft/sec. <br />FS = A safety factor of 2 to account for non -ideal settling. <br />Therefore, the equation for computing surface area becomes: <br />SA = 2 x Q2/0.00096 <br />n <br />2080 square feet per cfs of inflow <br />2014 Stormwater Management Manual for Western Washington <br />1 <br />1 <br />1 <br />1 <br />Ll <br />r� <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />Volume // - Chapter 4 - Page 384 <br />