While detention and retention may sound the same, the terms cannot be used interchangeably. Detention and retention both refer to storing rain water on-site during a storm event, when the risk of flooding is highest. The difference is that when water is detained, it is slowly released into a stream or river and leaves the site. When water is retained, it is not intended to leave the site. Instead, it is infiltrated into the ground or evapo-transpired into the air.
Detention is provided in detention ponds, which are man-made ponds designed to fill up, store water during heavy rains, and drain within 48 hours. Detention ponds may be quite large in size. They typically serve multi-acre developments. Retention is achieved in green-infrastructure such as rain gardens or bio-swales. Bio-retention areas are generally small compared to detention ponds. They are best suited for residential development, site development, or roadside ditches.
Retention is often considered a component of green or low-impact development (LID). The philosophy of LID is for post-project drainage conditions to mimic pre-project conditions. Before development occurs, rainwater infiltrates into the ground in the spaces between soil particles. The ground acts like a sponge, soaking up water and reducing runoff into streams and rivers. With development, grassy fields are covered by concrete and buildings. Water that used to soak into the soil now travels swiftly from rooftop to gutter to storm sewer. If the capacity of the storm sewer is exceeded, urban flooding can occur, as was seen in Houston in May of 2015.
Retention interrupts the rooftop to gutter to storm sewer scenario. Rather than entering the storm sewer system, rainwater is directed to infiltration basins like rain gardens. The soils in these areas are specially designed, and may be referred to as “engineered media.” The design of rain gardens or bioswales often uses layers of different sized rocks, gravel, and soil. The areas are often seeded with native plants adapted to both wet and dry soil conditions. A diagram of a bioswale designed to provide storm water management for an adjacent parking lot is shown in the image in the top right.
In conclusion, the terms detention and retention are similar but not the same. Hopefully this blog will help you understand the subtle difference. To remember, just think back to your Latin! The root word for detention means delay; the root word for retention means keep.
Detention = water is slowly released
Retention = water is infiltrated or evapo-transpired