Retention basin is a permanent stormwater management facility designed to capture and store runoff and maintain a standing pool of water, allowing stormwater to be detained for extended periods and treated through natural physical, chemical, and biological processes. Unlike detention basins, which temporarily store water and then release it, retention basins are designed to retain water indefinitely, with outflow occurring primarily through infiltration, evaporation, or controlled discharge.
A retention basin typically consists of a constructed depression with a permanent pool, surrounded by gently sloping side banks and often incorporating vegetated zones along the perimeter. Many designs include forebays to capture coarse sediment, deeper open water areas for storage, and shallow vegetated shelves that support aquatic plants. These features work together to slow incoming runoff, reduce turbulence, and promote pollutant removal.
From a water quality perspective, retention basins are highly effective treatment systems. As stormwater enters the basin, suspended solids settle out through sedimentation. Nutrients and other pollutants are reduced through processes such as filtration, adsorption, microbial activity, and plant uptake. These processes help reduce contaminant loads and mitigate impacts such as eutrophication in downstream receiving waters.
Hydrologically, retention basins reduce peak discharge rates and total runoff volumes by storing stormwater and releasing it slowly or allowing it to be lost through infiltration and evaporation. This helps reduce downstream flooding, erosion, and stress on drainage infrastructure.
Retention basins also provide ecological and aesthetic benefits. The permanent pool can support aquatic habitats, promote biodiversity, and enhance landscape value. However, they require proper design and maintenance, including sediment removal, vegetation management, and attention to safety considerations.
Retention basins are a key component of stormwater management systems, providing long-term storage, water quality treatment, and flow attenuation in a single integrated facility.