Despite the lack of aesthetic qualities, detention ponds are much better at providing flood protection to their locality, than retention ponds. For this reason, they are often installed in locations with a high rate of rainfall or with topography that is prone to flooding.
With the rapid rate of urbanization in the modern world, and haphazard construction of buildings without any regard to the environment or nature, flooding is very common even after a spell of moderate rain. The most common way of managing storm water is creating man-made storm water basins. They collect the water and slowly release it at a controlled flow to avoid flooding and erosion of soil from the surrounding area.
Let us take a closer look at the differences between detention ponds and retention ponds, about their purpose, effectiveness in controlling flood water, construction, and maintenance costs.
Dry ponds, or detention ponds, are man-made basins which hold water temporarily and are designed to help floating pollutants and particles to settle, while gradually emptying themselves in a controlled flow. It is one of the most popular flood control options available. They are usually built for large areas of land and are not suitable for small individual homes. They reduce the amount of pollutants that enter the ground water, and at the same time improve the quality of water and protect water channels.
Water that accumulates between the forebay and the embankment is released at a slow, controlled rate through the stone riprap to a bigger destination water body. During this process, heavy pollutants settle at the bottom of the sediment forebay. However, in extremely rare cases where the water level gets to the top of the concrete riser, the excess runoff flows directly through the barrel increasing its outflow. The grass on the forebay helps prevent erosion from constant inflows of water.
- Detention ponds are best suited for areas that are around 5-10 acres in size. Using them for smaller areas can make it a little difficult to provide channels and outlets for water flows.
- Detention ponds are best suited for areas with poorly draining soil. However, you can use them in soils such as sand, if appropriate modifications are made to stop groundwater pollution. The slope around the basin should ideally be no more than 15 degrees.
- Construction should be done so that ground water does not seep out into the dry pond. Failure in such construction can create a mosquito breeding ground.
- Perennial plants should be grown around the pond to act as a buffer.
- Although they make good flood control facilities, they are not completely effective to filter pollutants, especially soluble ones.
- Improperly maintained detention ponds can become breeding grounds for pests
- Dry ponds are not very attractive; hence, they may decrease the value of the property.
Construction and Maintenance Cost
Regular checking for erosion, and damage to the slopes and outlet devices has to be checked. The setup also has to be checked for accumulation and blockage by sediment or debris.
The construction of a detention pond can range from USD 40,000, to USD 1,000,000 depending on the size and modifications made to the pond. Maintaining these ponds can cost 7-8 percent of the construction cost annually, which is actually slightly higher than that of retention ponds.
Wet ponds, or retention ponds, are man-made basins which have a permanent pool of water. Like detention ponds, they too can be used to control heavy water runoffs and settle pollutants in the water. These ponds, however, do not empty water from their holding area unless it crosses a fixed level. If done well, these fixtures also provide aesthetic beauty to the area, which may increase the value of the property. They can be used in smaller areas as compared to dry ponds, i.e., homes with large outdoor areas are suitable. An advantage of wet ponds is that the algae, bacteria, and other biological beings in the water, consume the pollutants, eventually purifying the water to a large extent.
Unlike detention dry ponds, retention ponds do not release water at a controlled rate through stone riprap. Instead, water accumulates till it reaches the top of the riser, during which time, pollutants settle to the bottom and are consumed by biological beings in the water such as algae. Only the excess runoff is discharged through the barrel to an external water body. This means that there is always water in the pond, unless it dries up in drought-like conditions.
Retention tanks can be made in any space where at least 2-3 acres of space is available. The exceptions to this are places that have arid climates and a scarcity of water, which is detrimental to the pond's sustainability. Landscaping around these ponds can greatly benefit the area. Besides enhancing the beauty of the area, these ponds help in stopping erosion. Also, with some extra expense, you can use large-sized retention ponds for irrigation purposes. Most other considerations are similar to that of detention ponds.
- Improperly planned large-sized retention ponds can cause significant loss to forest and wetland ecosystems.
- These ponds are not suitable for arid areas and urban areas, due to the lack of space and water supply.
- The temperature of the water inside the retention pond generally rises by a significant amount. Therefore, its outlet cannot be directed towards cold water streams.
- Wet ponds can pose danger to unwary people, especially small children.
Construction and Maintenance Costs
Regular checking for invasive vegetation, buildup of hydrocarbons, and repair of outlet devices and eroded areas has to be undertaken. The setup also has to be checked for accumulation and blockage by sediment or debris.
The construction of a retention pond can range from USD 35,000, to USD 1,000,000 depending on the size and modifications made to the pond. Maintaining these ponds can cost 3-5 percent of the construction cost annually, which is compensated by a 10-20% increase in the property's value.
Now that you have learned about the major differences between detention and retention ponds, you can take an appropriate decision as to which one you would like to construct, and this can be made better by making it a community project, by building one for your entire locality. However, make sure you fulfill all legal requirements before you go ahead with the project. Best of luck!