Dewatering, Dewatering Bags, and Other Filter Devices

Stormwater can create many challenges to the construction world including issues with ponding and flooding water. Water flows downhill to low-lying areas, and if it gets to the low lying areas, it will likely stay there for a long period of time. If that low lying area is in a construction zone, then the water will likely be pumped out towards a nearby stream or anything else that may be downhill. Pumping the water from areas that have exposed sediment can potentially be a big problem because the sediment -laden water will be transferred off-site and can clog up the waterways downstream. In this month’s article in the Development Digest, we will dive into a best management practice (BMP) that we all can use to effectively pump water away from the site while also ensuring that all the sediment does not leave with it.
 
Dewatering is a common practice used during projects if ponding water hinders the ability for a crew to work. It is important to use the right BMPs during this removal process to ensure that water is moved offsite, and sediment is kept onsite. Dewatering bags are one of the most common BMPs used to achieve the goal of removing the water without letting the sediment go with it. Dewatering bags are simple devices that use pipes/hoses to move water out of an area and into a bag. In the bag, the sediment in the water can settle to the bottom, and water is able to flow out the other end of the bag cleaner than it came in.
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Dewatering bags do require maintenance.  Over time, the bags will fill up with sediment that has filtered out. If the bag becomes full of sediment, water will not be able to enter the bag. Suspended solids in the water will flow off-site into the nearby waterway, and because of this the bags need to be replaced when full. Dewatering bags should ideally be placed on a flat surface and combined with some other filtration method such as a dense vegetation buffer for the bag outlet. When used properly, dewatering bags can be a very useful tool to dewater from a site and can be especially effective when using other BMPs alongside the bags.

Another commonly used BMP when removing water from a certain area is a sediment trap. A sediment trap basically serves the purpose of a stormwater basin, except the water is not meant to leave the pit that has been dug out. As a result of this, sediment can settle downwards and get out of the water column. This dewatering BMP is most feasible when a future stormwater basin is already planned to be at that location. Water can be pumped out of desired areas of a site and into the sediment trap that has been created to hold the water.

A sediment trap should be designed to hold all the pumped water for a minimum of 24 hours to allow sediment to settle out (per Ohio Rainwater and Land Development Manual). The size of the sediment trap should be calculated during the design phase of a project, and once the project has begun and the area cleared of trees and vegetation, the sediment trap should be constructed immediately. This BMP does not need much maintenance during the project. However, if the sediment trap is meant to be converted into a stormwater basin, the excess sediment will need to be removed, and the permanent basin reconstructed.

Sediment and erosion control is a required practice for all projects over 1 acre here in Warren County. It makes sense to use some sort of sediment control BMP when dewatering a site to efficiently and safely move water out of an area, while ensuring the sediment is not leaving with the water. Sediment is one of the largest pollutants to the natural water bodies here in Ohio. Using erosion control BMPs like the dewatering bags and sediment traps listed above can go a long way to preventing this pollutant from continuing to get into and affect the rivers and streams that flow through our back yards!
 
References:
Rainwater and Land Development Manual: https://epa.ohio.gov/divisions-and-offices/surface-water/guides-manuals/rainwater-and-land-development

Article Written by Seth Byerly, District Technician