Retention ponds are used for the purpose of collecting and purifying stormwater. But how do retention ponds work, and why are they so important? This HelpSaveNature post deals with such questions, while telling you about their definition, pros, cons, maintenance, cost, and much more.
Did You Know?
Designing a series of retention ponds connected to one another is more effective at stormwater collection and purification as compared to a single pond.
During a strong spell of rain, the amount of stormwater runoff generated can be enormous. Though, at first, this doesn’t seem like a big deal, but when such a huge amount of water enters a downstream water body, it can cause flooding. This makes it necessary to hold back the runoff generated, especially in urban areas where impervious surfaces like streets, roofs, and sidewalks prevent the water from percolating into the soil. In such places, the stormwater generated has to be collected in man-made ponds. Such ponds can be of two types: dry detention ponds and wet detention, or retention ponds.
The main difference between detention and retention ponds is, while the former store water for temporary periods of time, the latter are always filled with water. This makes retention ponds suitable for more uses rather than plain water-collection. Depending on the region, stormwater runoff may contain pollutants like metals, pesticides, fertilizers, dissolved and suspended solids, oil, grease, etc. Retention ponds are designed to hold the collected runoff for 14 – 21 days, during which they remove between 30 – 80% of most pollutants from the water before it enters nearby streams.
Definition
A retention pond, by definition, is an artificial or man-made pond that serves to collect runoff during storms, and stores it indefinitely, or gradually discharges it into natural streams over the next few days. This prevents any downstream water bodies from flooding. It also serves to purify the collected water by physical and biological processes.
Basic Design
✦ The area of a retention pond should be about 5 – 10% of the total area from which it collects stormwater. Usually, a pond of 12.5 acre-feet capacity is required to collect stormwater from one quarter of land.
✦ The basic design includes a forebay and a main pond. The forebay is a small pool at a higher level than the main pond, and lined with turf grass, which acts like an additional filter before water enters the pond itself. The forebay should be around 10% of the size of the pond.
✦ Of the actual retention pond, 50 – 75% volume should be occupied by water, while in the rest a shallow basin should be constructed at the banks as a precaution against drowning.
✦ The shallow banks should be lined with local water plants, trees, and shrubs. It should be a combination of different plant species, rather than a single one.
✦ The inflow duct should be located at such a height that it should not be submerged when the pond is completely full.
✦ An overflow system is present to discharge excess water downstream during severe storms.
✦ An aeration device should be installed, which provides dissolved oxygen that is vital for aquatic life, by agitation of the surface.
✦ After consulting an expert, beneficial bacterial colonies which help in decomposition of waste can be added artificially.
How Do Retention Ponds Work?
► The stormwater collected by drains, gutters, and other impervious surfaces like parking lots and roofs is directed into the retention pond by pipes.
► Before entering the pond, the plants in the forebay act like a filter, and trap insoluble waste particles from the incoming stream.
► Most of the sediments in the pond are removed from the water as they settle down to the bottom due to their weight, a process called sedimentation.
► Aquatic and other plants surrounding the pond purify the water by absorbing pollutants via their roots, and perform photosynthesis to convert them into less-toxic products.
► A portion of soluble contaminants is removed from the water by getting trapped on the surface of soil particles, a process called adsorption.
► Most of the soluble pollutants are decomposed by microbial colonies, which metabolize them to obtain energy.
Maintenance
☞ A retention pond requires regular maintenance for it to work well. Both, inflow and outlet ducts should be monitored and cleaned of any debris.
☞ The plants around the pond should be mowed regularly, which will keep them in an active growth phase and maintain dense cover.
☞ If accumulated sediments start covering aquatic vegetation, or make the pond eutrophic (oxygen-deprived), it can be removed by hand shovels. According to the US EPA guidelines, this happens once every 20 years.
☞ Invasive aquatic weeds like hyacinth should be treated with herbicides, as they double every 3 weeks, and can rapidly take over the pond.
☞ The forebay should be cleaned whenever 50% of its capacity is clogged by sediments, which happens around once every 10 years.
Cost
The initial cost involved during construction of the pond is high, though it varies considerably. From then on, annual maintenance costs reach about 2 – 5% of the total construction expenses. These costs are expected to reduce with time as beneficial plant species become well-established in and around the pond. When considering the cost, it is important to look at the economic benefits provided by the pond too, as it may also increase property value, since these are regarded as an amenity. This has commonly been observed in countries like the US and Australia.
Pros
▲ It offers a simple and effective technique of purifying the collected water.
▲ The purified water can be reused for non-drinking purposes, like irrigation, landscaping, and industrial uses.
▲ Retention ponds can also recharge underground aquifers.
▲ It improves the biodiversity (variety of organisms) by providing a suitable habitat for several species of plants and animals.
▲ The presence of retention ponds has been observed to increase the property value of an area.
▲ It offers a recreational value to the local community.
▲ Retention ponds can provide an aesthetic value, if plant arrangement is well-planned.
▲ This method is well-suited for areas with high nutritional loading, i.e., with high pollutant concentration.
▲ If well-maintained, such ponds can be operational for a long duration, even more than 20 years.
Cons
▼ The initial cost of construction is high.
▼ If maintenance is not taken care of, performance may be substandard.
▼ It can be a drowning hazard for small children, if not monitored.
▼ Since it requires a large area (at least 5 ha), it is not a good idea for a highly-populated region.
▼ If located in an industrial area, or one with a high water table, a protective liner may be required to avoid groundwater contamination.
▼ Since most of the water entering it is warmer, it can kill temperature-sensitive fish species.
▼ Retention ponds can have standing-water pockets which provide ideal mosquito habitats.
To sum it up, retention ponds are an artificial technique to collect stormwater, thus preventing floods, and at the same time improving water quality. Owing to the simplicity and aesthetic benefits of such ponds, it should come as no surprise that more and more homeowners want one in their neighborhood.
