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Chapter 4 - Best Management Practices for Erosion and Sediment Control ' The drainage area of the ditch or swale being protected should not exceed 10 acres (4 ha) . The maximum height of the check dam should be 2 feet (60 cm) . The center of the check dam must be at least 6 inches ( 15 cm) lower than the outer edges. (See Plate 4 . 38a) The ' cross-sections of the dams should be as shown in Plates 4 .38a through 4 . 38d , respectively, for logs , stone , straw bales , and filter fence. The maximum spacing between the dams should be such that the toe of the upstream dam is at the same elevation as the ' top of the downstream dam . (See Plate 4 .38c) . Log check dams should be constructed of 4 to 6 inch (10 to 15 cm) logs salvaged from ' clearing operation site , if possible . The logs should be embedded into the soil at least 18 inches (45 cm) . The 6 inch ( 15 cm) lower height required at the center can be achieved either by careful placement of the logs or by cutting the logs after they are in place . (See Plate 4 . 38a) . Logs and/or brush should be placed on the downstream side of the dam to prevent scour during high flows . Stone check dams should be constructed of FDOT No. 1 Coarse Aggregate (1 . 5 to 3. 5 inch stone) (4 to 9 cm) . The stone should be placed according to the configuration in Plate 4 . 38b . Hand or mechanical placement will be necessary to achieve complete coverage of • the ditch or swale and to insure that the center of the dam is lower than the ends . (See Plates 4 . 38b & 4 .38e) Straw bale and filter fence check dams shall be installed as per STRAW BALE BARRIER - Section 4 .05 (Channel Flow Applications) (See Plates 4 .38c & 4 .38f) and SILT FENCE - Section 4 . 06 (Channel Flow Applications) (See Plate 4 .38d ) . Sediment Removal While this practice is not intended to be used primarily for sediment trapping , some sediment will accumulate behind the check dams . Sediment should be removed from behind the check dams when it has accumulated to half of the original height of the dam. r Removal Check dams must be removed when their useful life has been completed . In temporary ditches and swales , check dams should be removed and the ditch filled in when it is no longer needed . In permanent structures, check dams should be removed when a permanent lining can be installed . In grass-lined ditches, check dams should be removed when the grass has matured sufficiently to protect the ditch or swale . The area beneath the check dams should be seeded and mulched or sodded (depending upon velocity) immediately after they are removed. If stone check dams are used in grass-lined channels which will be mowed , care should be .. taken to remove all the stone from the dam when the dam is removed . This should include any stone which has washed downstream . Since log check dams are embedded in the soil, their removal will result in more disturbance of the soil than will removal of stone check dams. Consequently , extra care should be taken to restabilize the area when log dams are used in permanent ditches or swales . 4-107