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ICITY <br />OF EVERETT STORMWATER MANAGEMENT MANUAL <br />' <br />8. Keep the center of the check dam at least 12 inches lower than the outer edges at natural ground <br />elevation. <br />' <br />9. Keep the side slopes of the check dam at 2:1 or flatter. <br />10. Key the stone into the ditch banks and extend it beyond the abutments a minimum of 18 inches to <br />avoid washouts from overflow around the dam. <br />11. Use filter fabric foundation under a rock or sand bag check dam. If a blanket ditch liner is used, <br />this is not necessary. A piece of organic or synthetic blanket cut to fit will also work for this <br />purpose. <br />12. Rock check dams shall be constructed of appropriately sized rock. The rock must be placed by <br />hand or by mechanical means (no dumping of rock to form dam) to achieve complete coverage of <br />' <br />the ditch or swale and to ensure that the center of the dam is lower than the edges. The rock used <br />must be large enough to stay in place given the expected design flow through the channel. <br />13. In the case of grass -lined ditches and swales, all check dams and accumulated sediment shall be <br />removed when the grass has matured sufficiently to protect the ditch or swale — unless the slope <br />of the swale is greater than 4 percent. The area beneath the check dams shall be seeded and <br />mulched immediately after dam removal. <br />' <br />14. Ensure that channel appurtenances, such as culvert entrances below check dams, are not subject <br />to damage or blockage from displaced stones. DCSS Drawing #206 depicts a typical rock check <br />dam. <br />Maintenance Standards <br />1. Check dams shall be monitored for performance and sediment accumulation during and after each <br />runoff producing rainfall. Sediment shall be removed when it reaches one half the sump depth. <br />2. Anticipate submergence and deposition above the check dam and erosion from high flows around <br />' <br />the edges of the dam. <br />3. If significant erosion occurs between dams, install a protective riprap liner in that portion of the <br />channel. <br />i3.3.9 <br />BMP C208: Triangular Silt Dike (Geotextile-Encased Check Dam) <br />Purpose <br />Triangular silt dikes may be used as check dams, for perimeter protection, for temporary soil stockpile <br />protection, for drop inlet protection, or as a temporary interceptor dike. See DCSS Drawing # 207. <br />Conditions of Use <br />1. May be used for temporary check dams in ditches of any dimension <br />' <br />2. May be used on soil or pavement with adhesive or staples <br />3. dikes have been build <br />Triangular silt used to temporary: <br />' <br />a. Sediment ponds <br />b. Diversion ditches <br />c. Concrete wash out facilities <br />d. Curbing <br />e. Water bars <br />Volume 11 — Construction Stormwater Pollution Prevention <br />3-47 <br />