Permanently Filled vs Temporary Drainage Basin

Retention and detention ponds are commonly confused, and the similarities in their names don’t help. While they’re both commonly installed in similar locations, they serve slightly different purposes and require drastically different designs. Trying to build a detention pond like a retention basin will only result in wasted costs and unsatisfactory performance. Understand the differences between holding ponds built for permanent storage and short-term drainage basins to understand how they can be used together for one large storm-water management system.

Detention vs Retention

While they may only differ by one letter, detention and retention ponds are less similar than they sound. Detention ponds are designed to only hold storm water for a very short period and then discharge it as quickly as possible into the ground or a nearby waterway. In contrast, retention ponds must retain the water permanently or for long periods of time. Retention ponds can handle far more polluted runoff because they isolate the water from the environment. Detention ponds must only accept relatively clean storm water from natural surfaces so there’s a lower concentration of chemicals like fuel, oil, and paint. Retention ponds will have outlets for slowly releasing water or allowing safe overflow, but they aren’t specifically designed to empty out. Detention ponds should be mostly empty between storms. Both types of ponds can require equal amounts of maintenance when you consider that detention ponds are more likely to accumulate weed growth and debris.

Discharge Outlets

The detention pond itself is often designated as the primary discharge method for water. The most informal types of these ponds will just be unlined depressions and low points where the soil can absorb the water quick enough to handle it during a storm. This recharges local underground water supplies while relying on the filtering method of the soil to manage small amounts of nutrients and runoff chemicals. More advanced detention ponds may feature controlled discharge grates and wells that ensure the water is absorbed deep underground where it can’t cause erosion issues. Retention ponds also require discharge outlets, but these features are generally only used for overflows and flooding control. They’re far more limited and generally won’t connect to a waterway since these ponds have high concentrations of potentially damaging runoff.

Encouraging Evaporation

There’s little to no need to encourage evaporation in the detention pond, since it’s only designed to hold water for a short period. In contrast, retention ponds are generally reliant on this method of water loss to keep them from overflowing over time. This means that retention ponds tend to be larger and shallower to help sun exposure and wind speed up evaporation rates. However, retention ponds can also cover large amounts of ground in order to maximize contact with the soil. If no wells or grates are used to increase soil infiltration, large surface area over bare soil may be necessary for a successful detention pond. Either way, the size and shape of the pond are generally dictated by the desired amount of water loss, whether it’s through the air or the ground.

Lining the Retention and Detention Ponds

While it’s obvious that retention ponds will require durable lining, it’s a common misconception that detention ponds never need the material. Some detention designs connect to dry wells and similar infiltration devices and therefore shouldn’t lose water to seepage during the short period of holding in the pond structure. Other detention ponds only need part of their bottom exposed to control how much water is absorbed at any time and to protect the edges from destabilization.

Both types of ponds are quite similar despite their differences in use and design. Whether you need a liner for the entire bottom of a retention pond or just part of a detention system, contact us here at BTL Liners for help in selecting the right product.

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