Matt Hogan, Project Engineer in our Site Engineering [...]
As civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.
As cities grow and land development expands, properly diverting water becomes an important issue. Cities typically require that the developer not increase the amount of water discharging from the site after construction is complete. In order to control excess water and prevent runoff, or the water that flows downhill after a storm, detention or retention ponds are used to prevent any flooding or erosion.
We’ll discuss both detention and retention ponds in this edition of our “Did You Know” series.
What are detention and retention ponds?
A retention pond is an area where water is stored and held on a permanent basis. The only exception to this permanent storage is when the water reaches the overflow elevation and is released slowly downstream. The remaining water is absorbed into the soil or evaporated into the atmosphere.
A detention pond is usually a dry pond that will fill with water during rain events and temporarily store it until it’s able to release the water into the receiving stream.
Are these ponds known by any other names?
Retention ponds can be called wet ponds, while detention ponds are often referred to as dry ponds. A detention pond can be a wet pond, but a retention pond can’t be a dry pond since it’s always storing water.
How do they differ? How are they similar?
Detention and retention ponds are similar in the fact that both are used to control flooding and the quantity of water that’s released into the receiving stream or sewer system.
Detention ponds differ from retention ponds because they only hold the water for a short period of time, generally about 24 hours. Retention ponds have a pool of water at all times and only release water at a predetermined height through evaporation into the atmosphere or into the soil.
Another major difference is that retention ponds can reduce pollutants more than detention ponds. This is because the pollutants and sediments have longer detention time and are able to settle to the bottom of the retention pond. Also, the permanent pool of water helps to prevent the sediments on the bottom of the pond from being stirred up in storm events.
What determines whether you use a detention or retention pond in site development?
In site development, the city ordinances and regulations will often dictate which one you have to use, and the soil in the area is a major factor in the selection process. If you’re in a clay or rocky soil area you have to use a detention pond. If you have sandy soils, you usually select a retention pond.
The Cahaba River Society in Birmingham requires that your ponds provide storage for the first flush of a storm event below the outfall structure. The first flush usually contains the most pollutants and sediments, and containing them onsite reduces impact to adjacent landowners. Most ponds along the Cahaba River are considered wet detention ponds.
How do you decide where each will be placed?
Sites are usually designed with the ponds being at the low areas so that water can continue to be released at the same discharge point. Also, by doing this the storm sewer can follow the existing topography of the land and discharge directly into the pond.
Do you find yourself using one more than the other in your projects?
We usually use detention ponds. It’s rare that we use a retention pond due to the area’s topography.
Can you give local examples of these ponds?
In the Birmingham area, Grants Mill Auto Mall and Church of the Highlands have a wet detention pond. Bessemer Lowe’s, Fultondale Lowe’s, Publix on Montclair Road and Colonial Promenade in Fultondale have detention ponds.