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1 <br />BMP C207: Check Dams <br />Purpose <br />Construction of small dams across a swale or ditch reduces the velocity of concentrated <br />flow and dissipates energy at the check dam. <br />Conditions of Use <br />Where temporary channels or permanent channels are not yet vegetated, channel lining <br />is infeasible, and/or velocity checks are required. <br />• Check dams may not be placed in streams unless approved by the State Depart- <br />ment of Fish and Wildlife. Check dams may not be placed in wetlands without <br />approval from a permitting agency. <br />• Do not place check dams below the expected backwater from any salmonid bear- <br />ing water between October 1 and May 31 to ensure that there is no loss of high <br />flow refuge habitat for overwintering juvenile salmonids and emergent salmonid <br />fry. <br />• Construct rock check dams from appropriately sized rock. The rock used must be <br />large enough to stay in place given the expected design flow through the channel. <br />The rock must be placed by hand or by mechanical means (no dumping of rock to <br />form dam) to achieve complete coverage of the ditch or swale and to ensure that <br />the center of the dam is lower than the edges. <br />• Check dams may also be constructed of either rock or pea -gravel filled bags. <br />Numerous new products are also available for this purpose. They tend to be re- <br />usable, quick and easy to install, effective, and cost efficient. <br />. Place check dams perpendicular to the flow of water. <br />. The dam should form a triangle when viewed from the side. This prevents under- <br />cutting as water flows over the face of the dam rather than falling directly onto the <br />ditch bottom. <br />. Before installing check dams impound and bypass upstream water flow away from <br />the work area. Options for bypassing include pumps, siphons, or temporary chan- <br />nels. <br />• Check dams in association with sumps work more effectively at slowing flow and <br />retaining sediment than just a check dam alone. A deep sump should be provided <br />immediately upstream of the check dam. <br />. In some cases, if carefully located and designed, check dams can remain as per- <br />manent installations with very minor regrading. They may be left as either spill- <br />ways, in which case accumulated sediment would be graded and seeded, or as <br />2014 Stormwater Management Manual for Westem Washington <br />�1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />i <br />1 <br />Volume 11 - Chapter 4 - Page 352 <br />1 <br />