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What is Dewatering?

Dewatering BMP's

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Dewatering
Photo © Team New Orleans US Army Corps of Engineers

What is Dewatering?

Dewatering or construction dewatering are terms used to describe the action of removing groundwater or surface water from a construction site. Normally dewatering process is done by pumping or evaporation and is usually done before excavation for footings or to lower water table that might be causing problems during excavations. Dewatering can also be known as the process of removing water from soil by wet classification.

Why Dewatering?

Construction dewatering is used on most construction sites due to accumulated water in trenches and excavations, places with inadequate slope or due to high water table. In construction projects, this water should be removed to keep working as scheduled or to provide a safe workplace. Normally, builders tend to use water pumps to dewater these areas but are not paying attention to the place where water is discharge, causing erosion and other problems. It is important to follow best management practices when water is being pumped to lakes, wetlands or directly to storm sewer inlets.

Dewatering Precautions

Dewatering activities must be done properly to avoid eroding the soil on the construction site. It is also important to choose the best location for discharge, even when you might be far away from water bodies or catch basins. There are multiple dewatering products that can be used to remove sediment from the pumped water, such as dewatering bags. When choosing discharge areas from a dewatering process remember that:

  • Water should not be pumped directly into slopes.
  • Dewatering activities should be directed to a wooded buffer, if available.
  • Pay special attention and discontinue dewatering if the area shows signs of instability or erosion.
  • Channels used on dewatering must be stable and better if they have been protected with grass or vegetation.
  • Avoid dewatering under heavy rains because the infiltration rate is at a minimum and water will move slower or just the dewatering process will not function.
  • Never discharge water that has been contaminated with oil, grease, chemical products directly. In such instances an oil/water separator may be necessary.
  • Additional permits and requirements might be needed from state, local or federal agencies.

Dewatering Bags

Dewatering bags are made of durable geotextile fabric used to filter water by removing sediments. Dewatering bags are used on dredging operations, construction sites or places with high water table near the shoreline. These bags should be sized accordingly based on the pump flow rate and type of sediment. When using dewatering bags avoid multiple pipe discharges as it may cause the filter bag to fail. Remember to manage water runoff properly by guiding it to the nearest storm inlet.

Dewatering Methods

Construction dewatering from open excavation or trenches can be done by several methods. However, the most simple of all is gravity drain using drainage channels carrying away water from the area to be worked to the discharge point. Other feasible ways for dewatering are by means of water pumping, siphoning and/or using large construction machinery buckets to scoop and dump water from the selected area. Earth channels used for dewatering could also be protected with ditch linings, and additional protection should be placed to reduce water velocities and minimize erosion. It is recommended to build rip rap revetment protection with geotextile to prevent additional erosion at the discharge point.

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