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Chapter 4 - Best Management Practices for Erosion and Sediment Control <br /> 4. 26 TEMPORARY SEDIMENT BASIN <br /> ( ES BMP 1 . 26) <br /> Definition <br /> A temporary basin with a controlled stormwater release structure , formed by constructing <br /> an embankment of compacted soil across a drainageway. <br /> _ Purpose <br /> To detain sediment-laden runoff from disturbed areas long enough for most of the <br /> sediment to settle out. <br /> r <br /> Conditions Where Practice Applies <br /> Below disturbed areas greater than 5 acres (2 ha) . There must be sufficient space and <br /> appropriate topography for the construction of a temporary impoundment. These structures <br /> are limited to a useful life of 18 months unless they are designed as permanent ponds by a <br /> qualified professional engineer. <br /> Planning Considerations <br /> Effectiveness <br /> r Sediment basins are at best only 70-80% effective in trapping sediment which flows into <br /> them . Therefore , they should be used together with erosion control practices such as <br /> temporary seeding , mulching , diversion dikes, etc. to reduce the amount of sediment <br /> + flowing into the basin . <br /> Location <br /> To improve the effectiveness of the basin , it should be located so as to intercept the largest <br /> possible amount of runoff from the disturbed area . The best locations are generally low <br /> ■. areas and natural drainageways below disturbed areas . Drainage into the basin can be <br /> improved by the use of diversion dikes and ditches . The basin must not be located in a live <br /> stream but should be located to trap sediment-laden runoff before it enters the stream . <br />• The basin should not be located where its failure would result in the loss of life , damage to <br /> adjacent properties , or interruption of the use of public utilities or roads . <br />.. Multiple Use <br /> Sediment basins may be designed as permanent structures to remain in place after <br /> construction is completed . The Stormwater Rule (Ch . 62-25 , F .A. C . ) makes the use of <br />• these structures desirable for stormwater detention purposes . Always leave the bottom of <br /> the sediment basin 6" - 12" higher than the eventual bottom of a retention basin . This will <br /> ensure removal of accumulated fine sediments which could prematurely clog the retention <br />• basin . Wherever these structures are to become permanent, or if they exceed the size <br /> limitations of the design criteria , they must be designed as permanent ponds by a qualified <br /> professional engineer. Permanent ponds are beyond the scope of this BMP . <br />• <br /> 4-79 <br />