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• Either a permanent control structure or the temporary control structure <br />(described in BMP C241, Temporary Sediment Pond) can be used. If a <br />permanent control structure is used, it may be advisable to partially restrict <br />the lower orifice with gravel to increase residence time while still allowing <br />dewatering of the pond. A shut-off valve may be added to the control <br />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 <br />Local Permitting Authority. <br />Design and • See Figures 4.2.16 and 4.2.17 for details. <br />Installation • If permanent runoff control facilities are part of the project, they <br />Specifications <br />should be used for sediment retention. <br />• To determine the sediment trap geometry, first calculate the design <br />surface area (SA) of the trap, measured at the invert of the weir. Use <br />the following equation: <br />SA = FS(Q2/VS) <br />where <br />Q2 = Design inflow based on the peak discharge from the <br />developed 2-year runoff event from the contributing <br />drainage area as computed in the hydrologic analysis. The <br />10-year peak flow shall be used if the project size, expected <br />timing and duration of construction, or downstream <br />conditions warrant a higher level of protection. If no <br />hydrologic analysis is required, the Rational Method may <br />be used. <br />Vs = The settling velocity of the soil particle of interest. The <br />0.02 mm (medium silt) particle with an assumed density of <br />2.65 g/cm3 has been selected as the particle of interest and <br />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 or <br />2080 square feet per cfs of inflow <br />Note: Even if permanent facilities are used, they must still have a <br />surface area that is at least as large as that derived from the above <br />formula. If they do not, the pond must be enlarged. <br />• To aid in determining sediment depth, all sediment traps shall have a <br />staff gauge with a prominent mark 1-foot above the bottom of the trap. <br />Volume II — Construction Stormwater Pollution Prevention - August 2012 <br />4-102 <br />