Stormwater runoff is excess precipitation that flows into water bodies and local storm sewer systems largely due to the prevalence of hard surfaces, such as impervious concrete. Large quantities of water that would ordinarily be absorbed into the ground in the natural environment instead enter streams and lakes. Stormwater runoff collects pollutants, chemicals and debris as it flows over paved surfaces and into water bodies. It also causes erosion, decreases groundwater recharge and alters aquatic environments.
Retention ponds or “wet ponds” are ponds constructed to treat and store stormwater runoff. Retention ponds are permanent pools of standing water and eventually empty into a receiving water body. Forebays can be included in the design of a retention pond to "pretreat" the stomrwater before it spills over into the major water feature. Water is treated through sedimentation and nutrient uptake.
RATIONALE
Retention ponds are one of the most common forms of stormwater management. In contrast to detention or “dry” ponds, retention ponds hold water from storm events for extended periods of time, effectively treating the stormwater. According to the Metropolitan Council of Minnesota, more than 90% of the pollutant removal in retention ponds occurs between rainfall events. The storage and treatment of water reduces the overall quantity and quality of stormwater runoff, reducing the impact of runoff on surrounding water bodies.
Retention ponds can also play a role in policy. Many cities have developed stormwater policies limiting the amount of impervious cover or requiring a reduction in stormwater runoff from each site.
EFFORT REQUIRED
Significant effort is required for initial pond contsruction. As with any stormwater management strategy, some maintenance is required. Regular inspections for pests and erosion are recommended. Buffer areas must be mowed regularly. Sedimentation, debris and excess algae should be removed periodically. Retention Ponds may require a permit.
BENEFITS
- Improved water quality in surrounding water bodies. Retention ponds manage stormwater quantity and quality, lessening the transfer of pollutants and chemicals into nearby water bodies.
- Aesthetic appeal. A retention pond may enhance aesthetics via plant selection, plant placement, topography and pedestrain access options.
- Increased biodiversity. Biodiversity is defined as the variation of life forms in a given ecosystem. Landscaping with ponds and different varieties of plants can add to the the natural environment.
- Wildlife habitat. Retention ponds provide habitats for animals, organisms and insects.
- Water conservation. Water that is stored in retention ponds is available for non-potable human uses such as irrigation.
- Flood prevention. Stormwater quantity reductions minimize the risk of flooding in nearby areas.
- Minimization of erosion. Stormwater quantity reductions minimize erosion in the nearby area and along the banks of water bodies.
RISKS
There are some risks associated with retention ponds. Improper installation and maintance can lead to problems with nutrient and metal release. Depending on the size, there is the risk of drownings if the pond is not monitored regularly or located remotely. Structural problems may arise if the pond is not designed or constructed properly. Temperature sensitive fish may be affected by pond disharges, which are usually warmer than receiving waters. Also, underground utiliy placement should be considered during design and construction.
ACTION AGENTS
Civil engineer, engineer, property owner, horticulturist, plant nursery, local government (permitting), wetlands scientist
COSTS
Construction of retention ponds requires a moderate financial investment and a large land area.
