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sediment Pond Operations <br />A 12" CMP will handle 2.1 cfs with no head. Therefore the <br />• calculated 5.4 cfs flox may be to high. The pond contains <br />170,000 gallons within the 1.7 feet (H2-H1). At a flow of <br />2.1 cfs the pond would dewater to the top of the outlet pipe <br />in 3 hours (170,000/(2.1*7.48*60). Use 3 hours for the time <br />it takes to dewater the pond to the top of the CMP. <br />Therefore the water level in the pond can be lowered 1.7 feet in <br />in 3.0 hours. According to the stage diagram, the water storage <br />capacity between elevations 6005.9 and 6004.2 is 0.54 AF. <br />Once the water level in the pond reaches the top of the 12" CMP <br />at elevation 6003.9, the flow though the outlet pipe will reduce. <br />Use the open channel flow equation to dstermino the time it takes <br />to dewater the remaining 0.21 AF (0.75 AF - 0.54 AF). <br />The water level in the pond must be reduced to elevation 6003.5 to <br />provide for 0.75 AF capacity. The invert elevation of the outlet end <br />of the outlet pipe is 6003.2. Therefore use a minimum Plow depth of <br />0.3 feet (6003.5 - 6003.2). The slope or head on the pipe varies <br />from it ((6003.9 - 6003.5)/37) to O4. Use the average slope 0.54. <br />FLOW IN A CIRCULAR CHANNEL <br />DEPTH OF FLOW 3.6 <br />RADI[7S OF PIPE 6 <br />SLOPfi OF PIPE 0.005 <br />• MANNINGS "N" 0.024 <br />ZNCZADED ANGLE <br />AREA <br />WETTED PERIMETER <br />R=AREA/WETTED PERMITER <br />V=(1.49/N)*(R^.667)*(S^ .5)= <br />INCHES <br />INCHES <br />EXPRE33 AS DECIMAL <br />EXPRESS AS DECIMAL <br />132.84 <br />0.198168 FEET^2 <br />1.16 FEET <br />0.17 <br />1.35 FEET/SEC <br />Q A X V 0.2680 CFS <br />Time required to dewater 0.21 AF 9.5 Hours <br />Therefore the total time required to dewater the pond is i2.5 <br />hours (3.0 + 9.5). <br />Sediment Removal <br />The sediment storage for the pond is_provided for between elevations <br />6000.0 and 6002.9. The required sediment capacity is 0.15 AF. <br />According to the stage filling diagram, there is 0.15 AF of sediment <br />storage between elevations 6002.0 and 6002.9. At 604 sediment <br />storage capacity, the elevation of the sediment would be 6002.6. <br />Therefore, the Sediment does not need to ba removed from the pond <br />until it reaches elevation 6002.6 which is 0.3 Peet below the invert <br />of the inlet end of the outlet pipe. <br />• i1-13 Nev August 4, 1993 <br />