WHAT’S THE DIFFERENCE BETWEEN DETENTION AND RETENTION?


You may remember reading our blog post in April about stormwater systems, which described the benefit of rain gardens to slow stormwater runoff. In this post, we will explore additional ways to control the rate of stormwater runoff. Long ago, ancient civilizations recognized the need to develop practices to control flood waters, remove waste, and collect rain water for domestic and agricultural uses. In the modern age, stormwater management facilities continue to be an important component of urban development to control the increase in the volume of rainwater flow and reduce pollutants in our lakes, rivers, and streams. The construction of buildings, parking lots, sidewalks, and other similar improvements decrease the available area for rainfall to infiltrate into the ground.


Additionally, due to the loss of vegetative surface cover with new developments, the rate of stormwater runoff increases significantly. If the post-development runoff was allowed to freely discharge off-site, it would run off into creeks or drains too quickly and might cause erosion or flooding. The most common structural facility constructed to slow the rate of stormwater runoff is the stormwater pond. Stormwater ponds are essentially classified in two categories depending on their function —detention or retention. Depending on the physical constraints and conditions, they can be wet (permanent pool) or dry.


Detention Generally, detention ponds provide only flood control measures and are known as dry ponds. Detention ponds help control the rate of flow by using a control device that maintains the pre-development rate of flow. The volume of the detention pond is calculated by comparing the pre- and post- development runoff volumes. The difference is the detention volume. Usually, the controlled device is placed at the entrance to the outlet pipe to control the rate of flow to the pre-development rate. The pond is intended to drain the stormwater within a period of time to make the volume available for the next storm event. As shown in the diagram below, the outlet pipe (or control device) is placed at the bottom elevation of the detention volume to allow the pond to drain dry.

Retention

Unlike dry detention ponds, retention ponds hold a permanent pool of water and are referred to as wet ponds. Usually a retention pond is constructed because of a high groundwater table (in other words, the groundwater is near the surface of the earth). The bottom of the pond is excavated below the water table elevation to establish a permanent pool. The outlet of the pond is placed at or above the desired pool elevation. The volume of the permanent pool is set by a desired residence time to allow microbes and vegetation in the water to consume nutrients and to allow suspended pollutants to settle. In general, retention ponds require more area than a detention pond. This is due to constraints in the allowable depth of water to maintain the vegetation on the pond. The level of water in the pond is maintained by setting the outlet structure above the pond bottom at the groundwater elevation.


Detention and Retention

Increasingly, stormwater ponds comprise elements of both retention and detention ponds. This is accomplished similarly to a retention pond where the outlet is elevated above the bottom of the pond at the desired retention elevation. At this elevation, a control device is placed to limit the rate of out-flow to the pre-development rate. The difference in the maximum and retention elevations in the pond is the detention volume. With this method, the detention volume is stacked on the retention volume. Both wet and dry ponds can be configured in this way. The retention volume in a dry pond is recovered using infiltration allowing soil microbes to consume pollutants. Functioning this way, a detention pond is sometimes known as dry retention. Similarly, the difference between the maximum elevation and the permanent pool is the detention volume. In this way, a wet pond may be known as a wet detention pond.


Stormwater facilities are a critical part of protecting our rivers, lakes, and streams from erosion and pollutants from construction and development activities. Dry detention ponds temporarily store a volume stormwater runoff and discharge it at controlled rate to prevent infrastructure and waterbodies from receiving too much water. Wet retention ponds store a permanent volume of water for treatment of runoff to remove pollutants and sediments prior to discharging. Combining the two principles in one facility results in a dry retention or wet detention pond that serves to provide the necessary treatment and flood control.



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