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r <br /> ` i <br /> BMP C241: Temporary Sediment Pond <br /> Purpose Sediment ponds remove sediment from runoff originating from disturbed <br /> areas of the site. Sediment ponds are typically designed to remove <br /> j sediment no smaller than medium silt(0.02 mm). Consequently,they <br /> j - usually reduce turbidity only slightly. <br /> t Conditions of Use Prior to leaving a construction site, stormwater runoff must pass through a <br /> I I. <br /> sediment pond or other appropriate sediment removal best management <br /> i <br /> practice. <br /> i A sediment pond shall be used where the contributing drainage area is 3 <br /> acres or more. Ponds must be used in conjunction with erosion control <br /> practices to reduce the amount of sediment flowing into the basin. <br /> t Design and Sediment basins must be installed only on sites where failure of the <br /> Installation structure would not result in loss of life, damage to homes or <br /> Specifications buildings,or interruption of use or service of public roads or utilities. <br /> Also, sediment traps and ponds are attractive to children and can be <br /> very dangerous. Compliance with local ordinances regarding health <br /> j and safety must be addressed. If fencing of the pond is required,the <br /> type of fence and its location shall be shown on the ESC plan. <br />((((� Structures having a maximum storage capacity at the top of the dam of <br /> j ` t 10 acre-ft(435,600 ft)or more are subject to the Washington Dam <br /> Safety Regulations (Chapter 173-175 WAC). <br /> See Figures 4.2.18,4.2.19,and 4.2.20 for details. <br /> If permanent runoff control facilities are part of the project, they <br /> should be used for sediment retention. The surface area requirements <br /> i of the sediment basin must be met. This may require temporarily <br /> I E enlarging the permanent basin to comply with the surface area <br /> requirements. The permanent control structure must be temporarily <br /> s replaced with a control structure that only allows water to leave the <br /> pond from the surface or by pumping. The permanent control structure <br /> must be installed after the site is fully stabilized. . <br /> l Use of infiltration facilities for sedimentation basins during <br /> construction tends to clog the soils and reduce their capacity to <br /> infiltrate. If infiltration facilities are to be used the sides and bottom of <br /> the facility must only be rough excavated to a minimum of 2 feet <br /> above final grade. Final grading of the infiltration facility shall occur <br /> only when all contributing drainage areas are fully stabilized.The <br /> I: infiltration pretreatment facility should be fully constructed and used <br /> with the sedimentation basin to help prevent clogging. <br /> � a <br /> Determining Pond Geometry <br /> Obtain the discharge from the hydrologic calculations of the peak flow <br /> for the 2-year runoff event(QZ). The 10-year peak flow shall be used if <br /> Votuane II- Construction Stonnii ater Pollution Prevention-August 2012 <br /> j 4-104 <br />! <br /> i <br /> f <br />