As cities grow and land development expands, properly diverting water becomes crucial. Cities typically require the developer not to increase the amount of water discharging from the site after construction is complete. To control excess water after a storm, detention or retention ponds are used to prevent flooding, erosion, and runoff.
As civil engineers and surveyors, many aspects of our daily jobs are second nature to us. However, we hear common questions about our field from the public. Today, let’s explore questions related to detention and retention ponds in terms of site engineering:
What are detention, underground detention, and retention ponds?
- A detention pond is usually a dry pond that fills with water during rain events. It temporarily stores water and slowly releases water into a receiving stream over several days.
- Underground detention systems utilize vaults, pipes, tanks, and other subsurface structures to temporarily store stormwater for water quantity control. An underground detention system can be used to hold water for rainwater harvesting.
- A retention pond is an area where water is stored and held permanently. 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.
Are these ponds known by any other names?
Retention ponds can be called wet ponds, while detention ponds are often called 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. An underground detention system can be designed to be either a wet pond or a dry pond.
How do they differ? How are they similar?
All three types of ponds are similar in that they are used to control flooding and the quantity of water released into the receiving stream or storm sewer system.
Detention ponds differ from retention ponds because they only hold the water for a short time, generally about 24 to 48 hours. Retention ponds always have a pool of water and only release water at a predetermined height through evaporation into the atmosphere or the soil.
Another significant difference is that retention ponds can reduce pollutants more than detention ponds. This is because the pollutants and sediments have longer detention times and can settle to the bottom of the retention pond. Also, the permanent pool of water helps prevent the sediment on the bottom of the pond from being stirred up in storm events.
What determines whether you use a detention, underground detention, or retention pond in site development?
In site development, city ordinances and regulations will often dictate which one you have to use, and the area’s soil is a major factor in the selection process. If you’re in clay or rocky soil, you must use a detention pond. If you have sandy soils, you usually select a retention pond. In urban areas where land is expensive, underground detention systems are used to maximize the developable space.
As municipalities and jurisdictions are required to meet the Municipal Separate Storm Sewer Systems (MS4), ponds must provide storage for the first 1 inch to 1.5 inches of rainfall across the site of the outfall structure. This stormwater usually contains the most pollutants and sediments and having them onsite reduces the impact on adjacent landowners.
How does an engineer decide where each pond will be placed?
Sites are usually designed with ponds in 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 use detention ponds most frequently. We use retention ponds less frequently due to topographical requirements.
Can you give local examples of these ponds?
In Birmingham, Grants Mill Auto Mall and the Church of the Highlands have retention ponds; Amazon’s Lakeshore Facility and Carvana Bessemer have detention ponds; and the Veterans Affairs clinic on Crestwood Boulevard and 2200 Magnolia Avenue South have underground detention ponds.