What is a Retention Pond?

A retention pond is a man-made pond or reservoir that is designed to catch runoff water from higher elevation areas. The structures can be either temporary or permanent depending on the circumstances and the volume of water at issue. Most are built to be sloped basins that capitalize on gravity and the natural flow of water down hills and inclines. Keeping runoff away from homes and other structures can keep the ground from growing soggy and causing erosion and foundational problems, and can also drastically reduce the risk of flooding during heavy rains or significant snow melt. Building a pond like this is harder than just digging a hole, though; the structures usually have to be engineered specifically for the circumstances. Most have overflow pipes that can help control water levels and disperse water evenly, for instance, and precautions against contaminants and pollutants are also necessary to protect both the nearby environment and the greater public health.

Though water is essential for the life and growth of nearly all environments, too much can cause serious problems. Floods are one of the biggest concerns, at least where human settlements and agricultural centers are located. When there’s a lot of rain or a lot of melting snow flowing out of mountainous areas, there isn’t always a good place for it to go. It can soak into the ground, but usually only to a certain extent; once the ground becomes too saturated, erosion and rising floodwaters can become serious threats. People build retention ponds to give that water a place to go where it can be concentrated and contained.

Retention ponds are usually designed to be shallow, and most have slow, sloping floors. They rarely take up much land and average less than 1 acre (4046.86 m²) in total size. Most ponds are built in areas that have surrounding land capable of accommodating high water during rainy periods. Allowing for excess surrounding land is usually considered essential for proper function and safety.

Some of the most basic structures are designed for short-term use, often in land areas that are undergoing construction or some other change that causes a temporary erosion issue. It is very common to find them in building zones, for instance, since large areas consisting of impermeable surfaces won’t properly absorb water during rainfall. Most temporary basins are pumped somewhat regularly, and any water that doesn’t evaporate or otherwise dissipate naturally is usually removed and relocated at completion. In most cases, the pond is then filled.

Areas where erosion is a more permanent problem often require more permanent pond structures. Many of these have water in them year-round, though the levels often vary with the seasons and rainfall numbers.

While the ponds are fairly shallow, they usually average 8 to 10 feet (2 to 3 meters) in depth and should by no means be considered an area for recreation or swimming. The banks are typically built to be shallow at first but then drop sharply after about 2 feet (0.5 meter). A gradation allows for more natural water movement and ultimate capture. The shallow banks are sometimes also developed as a safety measure to prevent drowning, but children should still be kept away from the pond and surrounding areas.

Permanent ponds are often built in lower land areas that tend to accumulate excess amounts of water. Retention ponds often actually aid in the removal of pollutants, as trash and debris typically run into the pond after heavy rains. The basins also catch runoff containing petroleum, fertilizers, sediments, bacteria, and other harmful substances that may have negative impacts on overall water quality.

As pollutants enter the pond during a heavy rain, the basin works to slow water movement. The stagnant water allows heavier contaminants, such as solids or metals, to sink to the bottom of the pond and eventually become bottom layer sediments. The retained water naturally filters contaminants and returns clean water to nearby streams or wetlands.

Ponds often need regular maintenance to function properly. Adding features like a waterfall or aerator may help keep mosquitoes and other pests from breeding excessively around the pond. A pest control specialist may also alleviate insect issues by adding a natural larvacide to a pond. Algae overgrowth is another potential concern, and it may be necessary to contact an aquatic management specialist to provide the proper bacteria and microbes.