Retention & Detention

Small, artificial ponds have gained ubiquity in modern developments, such as neighborhoods and commercial centers. The body might have a fountain in the center, and a gaggle of annoying geese might chill there. They often feature an unnatural quality – geometry too straight or incongruous with the surroundings.

Despite sticking out to the discerning viewer, these ponds continue to arise in new constructions. Why?

To answer this terribly important query, we’ll have to investigate what civil engineers and hydrographers call retention and detention basins.

These basins go by a variety of names:

retention basin
retention pond
detention basin
detention pond
dry pond
wet pond
dry detention basin
wet detention basin
storm water management pond

A few of the designations above refer to the same type of body. The difference between the back portions of these names is relatively meaningless. A retention pond and a retention basin are the same thing. The front halves of the names can imply some functional differences, however. The final term, often abbreviated SWMP, can stand for all of the above.

Before we disambiguate the terms, what is the point of these basins? The catch-all term “storm water management pond” gets us most of the way to the answer. These ponds serve as managers for inundations of rainwater. Before the advent of hardened construction surfaces, most places on Earth with vegetation developed predictable sluices and patterns of stormwater dispersal. Soil and flora soak up a lot of excess or, at the very least, slow the movement of excess water as it rains. Some areas, of course, are prone to natural flooding, especially during heavy rains, such as deserts or natural flood plains. With the advent of roads, parking lots, roofs, gutters, sidewalks, and a host of other hard media, rainwater can rush away quickly from a region. Often, that fast action is the point of those constructions. It can be great for the neighborhood or business, but problems can rise quickly downstream. Without natural soil and vegetation filters, a conglomeration of concrete channels can amplify the amount of water in a river or lake. Funnel all this water into a reservoir with a dam, for example, and a region might have a big problem. SWMPs mitigate these issues by slowing the outflow of rainwater from a zone.

A secondary, but not less important, function of a SWMP is the collection of sediment. The same modern constructions that quicken water surges also allow the concentration of non-water in the water. Highways, for instance, are rife with oil, gasoline, and tiny pieces of tire. During storms, these particles flow toward waterways in abundance. Roads aren’t the only culprit of sediment. Farms and lawns produce myriad materials one would not want to end up in a drinking source. SWMPs allow this sediment to collect in one spot, either partitioning them from entering potable water sources or allowing them to disperse slowly, in naturally digestible levels to the great soil filters of the planet.

The different descriptors describe different roles. Engineers design retention ponds to always have water, while detention ponds are temporary basins for holding water that are typically not filled. “Wet ponds” seem redundant and “dry ponds” seem oxymoronic, but the terms tend to mirror “retention” and “detention,” serving as function signifiers.

The image above displays a dry detention pond with a sand filter. As it rains, the inlet fills. The “plumbing” installed beneath the pond controls the outflow of water. When water nears the lowest points of the pond, it exits through the sand, which is a natural filter of some sediment.

Retention basins, in contrast, also collect rainwater but are not designed to empty in a timely fashion. Many wet ponds lose water through evaporation or permeable soils, though they might be connected to outflow systems for emergencies.

Wet detention basins mix all the jumbled metaphors. They always have water, like a retention/wet pond, but also tout a fillable basin on top of the normal cup. Engineers design these bodies to maintain a certain level; if the detention area fills, outflow will occur until the status quo returns.

Many hypermodern designs attempt to blend into a surrounding environment or appear non-artificial. As with many art forms, certain examples are gorgeous, while others are closer to goose excrement.

With these basins, the function should obviously trump the form, but why not both?

Retention and detention ponds might often stick out like the anti-Waldo but they serve an important purpose in today’s manufactured world. The next time you see an example, think about the flow of water in the area!

Bonus trivia: some experts believe that geese and ducks like to congregate at retention ponds not just because they are new water and food sources but also because the lack of heavy vegetation around the edges of the basins makes it harder for predators, such as coyotes or foxes, to sneak up on them.