Stormwater Management: Swales
Stormwater runoff is excess precipitation that flows into water bodies and local storm sewer systems largely due to the prevalance 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.
Swales, also referred to as “bioswales,” are shallow channels lined with vegetation that slow the conveyance of stormwater runoff into water bodies while filtering the water. Usually broad and linear, swales reduce stormwater runoff, filter pollutants from runoff and recharge groundwater. Swales also may contain check dams that facilitate pond formation, further increasing infiltration and decreasing stormwater flows. Swales can be used in place of gutter or sewer systems in low population areas with low rainfall.
RATIONALE
The U.S. Environmental Protection Agency (EPA) estimates that stormwater runoff is responsible for 70% of all water pollution in lakes, rivers and creeks. Swales reduce stormwater runoff, improving the quality of nearby water bodies, recharging groundwater and reducing erosion. Swales are one of the easiest stormwater management strategies to implement and apply to several different types of land uses.
Swales 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
Implementation is fairly easy if land is available. Construction of appropriate topography (i.e., slope) may be necessary. Appropriate plant selection is important, as plants should be erosion and flood tolerant plants – consultation with a plant nursery or horticulturist is advisable. Minimal maintenance is required. Periodic mowing, weeding and debris and sediment removal is recommended. Swales may require a permit.
BENEFITS
- Improved water quality in surrounding water bodies. Swales reduce stormwater runoff rates and volumes while improving the quality of the water by lessening the transfer of pollutants and chemicals into nearby water bodies.
- Flood prevention. Because swales reduce the velocity of stormwater runoff and can collect stormwater runoff in small ponds flooding is less likely to occur in surrounding areas.
- Groundwater recharge. As water seeps into the soil through infiltration it is stored underground and available for human use through well construction.
- Aesthetic appeal. Swales, especially when replacing other impervious stormwater management systems, provide greenspace and visual appeal.
- Less interference with aquatic ecosystems. Stormwater runoff is often warmer than the water in streams, lakes and rivers. As runoff enters water bodies, the increase in temperature negatively impacts aquatic ecosystems. For example, studies show that temperature changes among sensitive fish species can cause increased feeding, disorientation, increased metabolism, reduced reproduction and possible mortality.
- Wildlife habitat. Vegetated areas such as swales provide a habitat for animals, organisms and insects.
- Minimization of erosion. Stormwater quantity reductions minimize erosion in the nearby area and along the banks of water bodies.
RISKS
There is potential for pollutants to seep into the soil or surrounding area. Safety issues may be a consideration depending on size and slope. There are potential pest management issues.
ACTION AGENTS
Landscape architect, civil engineer, property owner, local government (permitting), horticulturist, plant nursery
COSTS
Costs vary according to location and design. In general, swales are a lower cost stormwater management Best Management Practice (BMP).
A basic unit of nature that includes a community of biological organisms and their nonliving environment linked by biological, chemical, and physical processes.Water from precipitation that flows over nonporous surfaces into sewer systems or receiving water bodies.
