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1 <br /> BMP C207: Check Dams <br /> ' Purpose Construction of small dams across a swale or ditch reduces the velocity of <br /> concentrated flow and dissipates energy at the check dam. <br /> Conditions of Use Where temporary channels or permanent channels are not yet vegetated, <br /> channel lining is infeasible, and velocity checks are required. <br /> • Check dams may not be placed in streams unless approved by the State <br /> Department of Fish and Wildlife. Check dams may not be placed in <br /> wetlands without approval from a permitting agency. <br /> • Check dams shall not be placed below the expected backwater from <br /> any salmonid bearing water between October 1 and May 31 to ensure <br /> that there is no loss of high flow refuge habitat for overwintering <br /> juvenile salmonids and emergent salmonid fry. <br /> Design and Whatever material is used,the dam should form a triangle when viewed <br /> Installation from the side. This prevents undercutting as water flows over the face of <br /> ,Specifications the dam rather than falling directly onto the ditch bottom. <br /> Check dams in association with sumps work more effectively at slowing <br /> flow and retaining sediment than just a check dam alone. A deep sump <br /> should be provided immediately upstream of the check dam. <br /> • In some cases,if carefully located and designed, check dams can <br /> remain as permanent installations with very minor regrading. They <br /> may be left as either spillways,in which case accumulated sediment <br /> ' would be graded and seeded, or as check dams to prevent further <br /> sediment from leaving the site. <br /> • Check dams can be constructed of either rock or pea-gravel filled bags. <br /> Numerous new products are also available for this purpose. They tend <br /> to be re-usable,quick and easy to install,effective, and cost efficient. <br /> • Check dams should be placed perpendicular to the flow of water. <br /> ' • The maximum spacing between the dams shall be such that the toe of <br /> the upstream dam is at the same elevation as the top of the downstream <br /> dam. <br /> • Keep the maximum height at 2 feet at the center of the dam. <br /> • Keep the center of the check dam at least 12 inches lower than the <br /> outer edges at natural ground elevation. <br /> • Keep the side slopes of the check dam at 2:1 or flatter. <br /> ' * Key the stone into the ditch banks and extend it beyond the abutments <br /> a minimum of 18 inches to avoid washouts from overflow around the <br /> dam. <br /> I <br /> February 2005 Volume!!—Construction Stormwater Pollution Prevention 4-75 <br />