It’s easy to confuse retention ponds for detention ponds or basins when you’re exploring storm water management options. The Environmental Protection Agency (EPA) provides guidelines on designing both types of water management systems, but they’re not interchangeable. If you build a detention basin when you really need a retention pond, you’ll experience more issues with water pollution and erosion in the drainage area. Explore the differences between both types of ponds to ensure you’re building the right kind.
Period of Water Storage
Retention ponds are designed to stay at least partially filled at all times throughout the year. Water levels may fluctuate between the seasons, but the pond won’t completely dry out unless there are unusual conditions like a severe drought. In contrast, detention ponds dry out regularly between seasonal patterns of heavy rainfall. These basins are built to only hold a limited amount of water for a short period. Drying out doesn’t cause the bottom of the detention pond to erode or wash away thanks to constant vegetative cover that can handle occasional flooding. Retention ponds require lining with a material like a geomembrane to stabilize the sides and bottom of the structure. In contrast, detention ponds hold water for a maximum period of 24 to 48 hours.
Amount of Flow
Detention ponds can handle hundreds or thousands of gallons per day, but the largest retention ponds can hold up a million gallons or more for months at a time. Yet a detention pond is better equipped to handle the sudden influx of rapid water flow that could damage a retention basin. Both types of structures may be used in conjunction to deal with both flash flooding conditions and long-term water storage.
Since detention ponds dry out regularly and then experience sudden flooding, they require either a tough living plant cover or a geomembrane liner to prevent erosion. Running water suddenly flowing over dry soil results in destabilization and silt movement that carves deep gouges into the surface of the soil. Retention ponds don’t experience such severe erosion forces, but they can still collapse if the soil becomes saturated with water soaking through the ground. Lining both retention and detention ponds is the best option to control erosion.
The holding time of a body of water determines how much silt and suspended solids will settle out of the liquid and onto the bottom of the pond. Since retention ponds hold water for so much longer than detention basins, they naturally fill up with silt far more rapidly too. Detention ponds rarely need cleaning out and can wait for cleaning until the dry season, while retention ponds need cleaning whenever it’s necessary to keep the depth at its intended measurement.
In areas where flash floods are common but overall rainfall amounts are low when averaged for the entire year, detention ponds make more sense than retention basins. Yet retention ponds also play an important role in flood protection in wet climates where a few extra inches of rainfall can push systems to the breaking point.
The use of both types of ponds is separated largely by climate conditions like average rainfall and seasonal patterns. Retention ponds are more commonly installed in temperate and wet climates with higher annual rainfall totals, while detention basins are widely seen in arid prairie and desert environments in the Western and Midwestern states of the U.S. Areas that experience heavy snow melts in the spring also tend to install both detention and retention ponds for short and long-term storage of the water. When detention ponds are installed in the East or Northwest, they tend to be paired with retention ponds for long-term water holding.
Whether you’re designing a retention pond on its own or with a set of detention basins to direct water to it, we can help. Reach out to us here at BTL Liners for more information on protecting both types of storm water management ponds from the risks of erosion and seepage.