Stormwater retention ponds (also known detention basins) are often incorporated into new housing developments in order to manage flood risk. Typically a pond will be constructed in a low-lying point in a new estate. The ponds can even be situated where vulnerable groundwater is present or above contaminated ground, provided they are securely lined.
Storm drain inlets to the pond can either be enclosed pipes or open gullies. The latter may be vulnerable to being blocked by vandals or fly-tippers. Inlet pipes may be covered by boulders where they enter the ponds, so that they are harder for vandals to block at that point.
The outlet from a pond is often a relatively narrow pipe, leading to a drain. Weirs can also be used. Various devices, such as vortex flow controls or orifice controls, can be incorporated to regulate the flow rate. Under normal conditions, the pond’s surface is at or below the level of the outlet.
A new pond can be planted with suitable native aquatic plants, which will help oxygenate the water, and reduce the levels of pollutants. Shoreline planting can also be carried out, so long as plants that can survive flooding are chosen.
Other benefits of stormwater ponds
- A well-designed pond can take in large quantities of stormwater and thus aid flood risk management .
- The pond can remove pollutants from stormwater, which typically carries a variety of pollutants, picked up from roads, driveways, storm drains, etc. Suspended solids can slowly settle in the pond, and biological processes can help improve the quality of the water by gradually degrading harmful substances. The pond acts as a first line of defence, mitigating the spread of contamination to downstream watercourses.
- A stormwater pond can be a beneficial wildlife habitat.
- A well-kept pond can be a local amenity and enhance the value of nearby housing.
Wildlife in stormwater ponds
Stormwater ponds, even in urbanised areas, can be useful areas for wildlife, such as water plants, fish, birds, insects and amphibians. Researchers studying ponds in Edmonton, Canada found that urban frogs in such ponds tended to be bigger than those in natural ponds. However, this was explained by fewer tadpoles and young frogs being present, thus giving less competition between them.
In some cases ponds can be overrun by invasive non-native species, which may require control measures. Rats can sometimes be a problem, particularly if edible litter is present in and around ponds.
Pollution in stormwater ponds
Pollutants often find their way into stormwater ponds. In residential areas lawn fertilizers and dog waste can add excess nutrients to the ponds. The excess can lead to algal blooms, which can kill fish and other wildlife. Lawn herbicides can kill aquatic plants in ponds. Motor oil, either from engine leaks or deliberate dumping, is another pollutant that may occur. In industrial areas, there are a wide variety of substances that may potentially contaminate ponds.
Litter can accumulate in ponds, and sometimes fly-tipping can occur. This can lead to flooding if inlets or outlets are blocked.
Often homeowners have no idea about the function of the ponds on their estates. Ignorance can lead to pollution and damage to these ponds. Degraded ponds can be unsightly, and may also give rise to unpleasant odours. They can end up lowering the value of adjacent properties.
Brad Buck from the University of Florida has outlined useful actions to reduce the incidence of pond pollution:
- Educate residents about the function of stormwater ponds.
- Remind residents to keep grass cuttings in their gardens and not to let them accumulate in gutters, streets and pavements.
- Dispose of pet waste in bins.
- Ensure contractors working on the estate are aware of the proper management of the storm ponds.
- Obtain professional advice if necessary about the design of a suitable low-maintenance shoreline and the use of aquatic plants.
Long term maintenance of stormwater ponds can be a problem, particularly if the responsibilities of local authorities, housing developers, and residents are not carefully set out. Maintenance problems may occur if a company responsible for pond maintenance goes into liquidation.
Stormwater ponds are useful flood prevention measures; they can also help prevent pollution and can provide valuable habitats for wildlife. However, care needs to be taken to prevent pollution and pond degradation.
Scheffers, B.R. & Paszkowski, C.A., ‘Large body size for metamorphic wood frogs in urban stormwater wetlands’, Urban Ecosystems, 2015. http://link.springer.com/article/10.1007/s11252-015-0495-z?no-access true
Retention Ponds, Susdrain Website. http://www.susdrain.org/delivering-suds/using-suds/suds-components/retention_and_detention/retention_ponds.html