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i Chapter 4 - Best Management Practices for Erosion and Sediment Control • 4.40 DEWATERING Definition Lowering the water table by means of pumping . i Purpose To allow the construction of structural and stormwater improvements by removing water i from excavation areas and allowing construction by conventional "dry" methods . Planning Considerations The major planning consideration in dewatering is disposal of removed water. Volume , quality, and topography are the factors governing the method and destination of removed i water. Discharge from well-point dewatering is relatively clear except for the initial discharge after installation or inactivity. Water pumped from a sump hole is thoroughly sediment laden and must always be treated . Turbid water must either be filtered before leaving the site or must be impounded onsite and allowed to settle . In flat terrain it is sometimes more economical to impound relatively clean water rather than pipe it long • distances to a receiving water body. Specifications " The two most common methods of dewatering used in Florida are well-point systems and sump pumps . A well-point system consists of one or more rows of small 2" (5 cm) collector pipes which are jetted vertically into the ground near the proposed excavation . The small " pipes are connected by a larger 6" (15 cm) manifold pipe which is connected to the pump and discharge line . The sump method is simply a hole in the ground with a pump drawing all of the water flowing into the hole . Excess water is conveyed to the sump by open ditches or perforated pipes embedded in sand or gravel. Sumps and Ditches • The water table is lowered by ditching and conveying water to a lowered sump hole . Water pumped from a sump hole is usually heavily laden with sediments. Water flowing over i disturbed and saturated ground detaches and transports all sizes of soil particles into the sump pit to be sucked up by the pump . Saturated liquid soil (mud) is also drawn into the pump . The discharged water must be treated before release into a receiving water body or • stormwater system . Placing haybales around the pump intake or outlet is not sufficient filtration. by itself. Turbid water must either be impounded long enough for effective settling of fines , or filtered through a temporary filter or sediment tank. Initially the water may • percolate freely into the ground , however this will diminish as the fine particles settle and clog the surface layer of soil . In situations which preclude the use of filtration or settlement facilities , and turbid water is discharged directly into a water body, a suitably designed floating turbidity barrier must be used . Note that this method does not remove any sediments , it merely allows for dilution to lower the turbidity level. • 4-113 •