By Matthew Trygg—Regional Manager // 2022-10-10
Dewatering is one of the riskiest aspects of following the Construction General Permit. Issues with dewatering typically occur when pumping water without a proper control measure allowing water to flow without the chance to drop sediment out.
The most common way to dewater is to pump water into a sediment bag which is located on a flat staMatthew Trygg—Regional Managerbilized surface (usually a vegetated area or placed on filter fabric and stone). The sediment bag will then act as a filter to catch the suspended solids before releasing the water. The water is then allowed to flow through vegetation and make it to its ultimate destination. This can be an effective way of dewatering; however, this method of dewatering in not always enough. Most construction site surfaces are predominantly clay because the topsoil is stripped at the beginning of the construction process. Clay particles may be small enough to slip through sediment bags and lead to dirty water making it to basins or outfalls. It is important to constantly monitor or test the water to ensure the sediment bag is functioning properly. When the process does not perform correctly there are additional measures to consider.
Flumes are channels made for dewatering when the standard dewatering procedure is not enough. There are many ways to construct a flume, but the process is generally the same. First the flume is constructed by calculating out the length of flume necessary to allow the dropping out of sediment. Then that length is placed down in a constructed channel. Next, a series of check dams (wattles, rock, high flow silt fence, or silt logs) are placed in the flume to slow the water down. Then jute net is placed in between the check dams to add more surface area to trap the sediment. Flocculent is used to chemically bind to the fine sediment particle and capture the sediment before leaving the flume. Lastly, a dewatering bag is used at the start of the channel pump to capture as much sediment as possible at the beginning of the flume.
An additional bmp can be used in conjunction with the flume called a sediment retention barrier (SRB). To construct the SRB, high flow silt fence is installed in front of safety fence to create an area to place either woodchips or straw. Flocculent is then mixed in with the woodchips or straw to help bind to the sediment. This works by pumping the water toward the high flow silt fence, which slows the water down. The woodchips or straw increase the surface area working to slow the flow of water and helps bind the sediment with the flocculant. The safety fence is to help hold the woodchips or straw in place.
Flumes are an effective way of dewatering when the standard method of dewatering is not effective or when there is not enough vegetated area to use.
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