Retention ponds

Component: Retention ponds

SuDS component retention pond

Description

Retention ponds can provide both stormwater attenuation and treatment. They are designed to support emergent and submerged aquatic vegetation along their shoreline.

Runoff from each rain event is detained and treated in the pool. The retention time promotes pollutant removal through sedimentation and the opportunity for biological uptake mechanisms to reduce nutrient concentrations.

Advantages & disadvantages

Advantages	Disadvantages
Can cater for all storms Good removal capability of urban pollutants Can be used where groundwater is vulnerable, if lined Good community acceptability High potential ecological, aesthetic and amenity benefits May add value to local properties.	No reduction in runoff volume Anaerobic conditions can occur without regular inflow Land take may limit use in high density sites May not be suitable for steep sites, due to requirement for high embankments Colonisation by invasive species could increase maintenance Perceived health & safety risks may result in fencing and isolation of the pond.

Where component can be used

Residential: Yes
Commercial/industrial: Yes
High density: Unlikely
Retrofit: Unlikely
Contaminated sites: Yes (with liner)
Sites above vulnerable groundwater: Yes (with liner)

Performance
Peak flow reduction: Good
Volume reduction: Poor
Water quality treatment: Good
Amenity potential: Good
Ecology potential: Good

Quantity
Ponds can be designed to control flow rates by storing floodwater and releasing it slowly once the risk of flooding has passed (also known as a balancing pond). The stored water will change the water level, and ponds should be designed to function in both dry and wet weather. Quantity can also be influenced by the amount of water that can be allowed to infiltrate into the ground if there is no risk to groundwater quality.

Quality
Ponds treat runoff in a variety of ways:
•   settlement of solids in still water. Having plants in the water enhances calm conditions and promotes settlement
•   adsorption by aquatic vegetation
•   biological activity

Amenity
Ponds offer many opportunities for the landscape designer. Permanently wet ponds can be used to store water for reuse, and offer excellent opportunities for the provision of wildlife habitats. Ponds can be part of public open space.

Maintenance

•   Litter/debris removal
•   Inlet/outlet cleaning
•   Vegetation management
•   Sediment monitoring and removal when required.