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Chapter4- - Best- Management Practices for Erosion andSedirpentControl. - 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 eievation as the top of the downstream dam. (See Plate 4 . 38c) . LooLoa chums 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. cu.I ing .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 placementwiil 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) r 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 . _ Removai 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 filed 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 swate . The area beneath ` the check dams shoutd be ' seeded and mulched . or sodded (depending upon velocity) immedlately after they are removed . ` If.stone check dams are used in grass-lined channels which willbe mowed , care should be taken to remove ail the stone from the dam when the dam is removed . This should include r, any stone which has washed downstream. Since log check dams are embedded in the soil , their removal wilt result in more disturbance of the soil than will removal of stone check dams. Consequently , extra care should be taken to restabilize the areawhen log dams are ,w used in permanent ditches or swales . 4-107