Regional Controls manage the combined stormwater runoff from several developments before the runoff is released to the receiving water body. Typical SuDS options include: constructed wetlands, detention ponds and retention ponds.
Detention ponds or detention basins are temporary storage facilities that are ordinarily dry but are designed in such a manner that they are able to store stormwater runoff for relatively short periods of time. They are generally designed to detain runoff for a period of 24-72 hours and ensure the attenuation of the flood peak. Detention ponds have been commonly used in South Africa along side conventional systems to prevent flooding
- They are able to temporarily store large volumes of stormwater thus attenuating downstream flood peaks;
- Detention ponds are relatively inexpensive to construct and easy to maintain;
- Detention ponds may serve multiple purposes during drier seasons, particularly recreational purposes such as sports fields, play parks or commons; and
- If managed regularly, detention ponds can add aesthetic value to adjoining residential properties as well as presenting fewer safety hazards than wet ponds due to the absence of a permanent pool of water.
- Detention ponds are not very good at removing dissolved pollutants and fine material;
- They are most effective at or near their design flow; their efficacy drops off quite rapidly with very low or high flows;
- Siltation can be a problem;
- The floors of detention ponds can become swampy for some time after major rainfall;
- For best results, detention ponds have a large plan area. This takes valuable land; and
- Detention ponds are not very suitable in areas with a relatively high water table, or where the soil is very coarse, and there is a risk of groundwater contamination (Hobart City Council, 2006, Taylor, 2003).
Retention ponds have a permanent pool of water , much like a wetland. The difference is the size of the permanent pool and the vegetation cover. In wetlands these are generally both greater than in retention ponds. Retention Ponds are designed to detain and treat runoff for a period of 24-72 hrs.
- The incorporation of retention ponds into the natural landscape provides amenity and biodiversity benefits and can also be used for recreational purposes where adequate supervision is available;
- Retention ponds generally have the capacity to remove a wide range of common stormwater runoff pollutants;
- Retention ponds are one of the most cost-effective SuDS options;
- Retention ponds often have high community acceptability (Endicott & Walker, 2003, Woods-Ballard, et al., 2007); and
- Stormwater runoff that is captured in retention ponds can be reused for secondary domestic purposes and for various irrigation requirements.
- The permanent open pool of water creates health and safety concerns and therefore requires social impact considerations at the design stage;
- The permanent open pool of water could display floating debris and scum, in addition to the attraction of nuisances such as foul odours and mosquito breeding if maintained infrequently or irregularly;
- Retention ponds are normally restricted to sites with shallow slopes;
- Retention ponds require a baseflow or the addition of supplementary water to maintain a specified permanent water line; and
- Retention ponds normally require relatively large footprints and may be impractical in dense urban areas
Wetlands natural systems that are generally comprised of marshy areas of open water and aquatic vegetation. They may be categorised into: natural, modified natural or constructed. They provide a habitat for fish, birds and other wildlife – including rare and endangered species. Their aesthetic appeal encourages use for recreation as well for research and education as is the case at Century City in Cape Town. Stormwater wetlands are man-made systems designed to mimic the natural wetlands. They are most commonly used as regional controls although may be used on a local scale in the form of pocket wetlands
- Constructed wetlands perform significantly better in the removal of pollutants from stormwater runoff than other regional controls of equal volume;
- Constructed wetlands that are effectively incorporated into the urban landscape of neighbouring residences have the potential to add great aesthetic value to those properties provided there is an appropriate level of maintenance;
- Small aquaculture wetlands have the ability to produce various kinds of food (Hobart City Council, 2006); and
- Constructed wetlands can be retrofitted into existing ‘flood retarding basins’ (Environment Protection Authority – Melbourne Water Corporation, 1999).
- Constructed wetlands could potentially attract mosquitoes;
- Constructed wetlands are limited to application on relatively flat land, as they become costly to incorporate on steep and potentially unstable slopes;
- Constructed wetlands may require supplementary water during long dry periods; and
- Wind action can cause the re-suspension of organic solids where the water is shallow, potentially resulting in adverse changes in the soil chemistry; and