What is an Underdrain?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 15 August 2019
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An underdrain is a drainage system for removing standing water like stormwater runoff and irrigation water, preventing soil from becoming waterlogged. This can be important for safety, preventing erosion and sinkholes, and it is also beneficial for maintaining healthy soil conditions and eliminating hazards like breeding grounds for mosquitoes and other insects. Underdrain systems typically connect either to a runoff management system, a treatment plant, or a wastepipe to release the water into a waterway.

The underdrain typically consists of perforated tubing laid in a bed of gravel. As water starts to sink into the soil, it percolates through the gravel and into the small perforations for collection. The underdrain can run as part of a larger network bringing water to a central location for handling and disposal. There are variations on this basic design for different settings and companies specializing in installation of storm drains and management of runoff drainage can provide advice on the best options.

The gravel in the underdrain system is an important component. It prevents clogging of the holes in the pipe by trapping particulates. Over time, the gravel will eventually become impacted and it can be necessary to dig up the drainage to clear it out and check the piping to make sure it is in good working condition. Usually, the system is designed to be effective for an extended period of time without maintenance.


Sometimes an underdrain is required, as seen in regions where controls on agricultural runoff mandate the use of drainage systems to trap water instead of allowing it to flow freely. In other cases, it may be done to make ground more usable. Structures on low lying or soggy ground can be prone to problems unless an underdrain is installed to keep the soil more dry, for example, and these devices can also be used in locations like public parks and gardens.

Municipalities typically are responsible for putting in underdrains in public areas and connecting them to systems used for managing rainwater in the streets. Individual property owners can connect to storm sewers if they wish to install underdrains, sometimes simply by equipping a drainage system to shunt water into the gutters of the street. For agriculture and rural applications, options for managing the water collected in the underdrain vary and may include collecting it in drums, pushing it onto unused land, pouring it into waterways, or pushing it through a small wastewater treatment plant before releasing it into the environment.


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