Component: Retention ponds
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
- 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
High density: Unlikely
Contaminated sites: Yes (with liner)
Sites above vulnerable groundwater: Yes (with liner)
Peak flow reduction: Good
Volume reduction: Poor
Water quality treatment: Good
Amenity potential: Good
Ecology potential: Good
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.
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
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.
• Litter/debris removal
• Inlet/outlet cleaning
• Vegetation management
• Sediment monitoring and removal when required.