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i <br /> Either apermanent control structure or the temporary control structure p ary to e <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 4.22 and 4.23 for details. <br /> Installation <br /> Specifications • If permanent runoff control facilities are part of the project,they <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 /> ' February 2005 Volume II—Construction Stormwater Pollution Prevention 4-103 <br />