Stormwater Control Measures (SCM)
SCMs are permanent, engineered structures built and maintained to remove pollutants from urban stormwater runoff. In civic planning some secondary goals may be listed for SCMs, such as improving water quality and controlling the quantity of runoff that reaches streams and drinking water supplies.
SCMs are a critical part of overall stormwater management, since pollutants in urban runoff can impair downstream water quality and aquatic life if they’re allowed to enter streams and lakes unfiltered. When streams and lakes are degraded, drinking water reservoirs also suffer in quality.
SCM engineers have access to a wide variety of structures to meet their goals, including dry detention basins and wet detention ponds, stormwater wetlands, bioretention cells, and infiltration trenches. Each of these have advantages and disadvantages and are most suitable in specific situations. We’ll look at each of them.
Infiltration Trenches and Vegetated Strips
Infiltration trenches and vegetated strips are often installed parallel to roads, highways, sidewalks, or other impervious surfaces that shed water in sheets rather than concentrating or absorbing it.
These trenches are typically filled with gravel, allowing water to soak into the ground, and bordered by vegetated strips which reduce flow rates, prevent erosion, remove debris and large sediment, and add aesthetic appeal. Infiltration trenches are not expected to treat water from large storms, and in fact may be lined with geotextile fabric to prevent the formation of small rivulets along the edge of the trench.
Vegetated strips originated as an agricultural practice but have more recently been adopted in urban stormwater management. They can be relatively effective at pollutant removal when sheet flow is maintained, but they do require relatively large amounts of space for small drainage areas. Vegetated filter strips are designed to temporarily hold runoff water for a period of no more than 48-72 hours in order to provide adequate filtration while preventing mosquitoes from breeding. Typical vegetation may feature turf or meadow grasses, shrubs, other native vegetation and even trees. Overall, infiltration by soil and plants in filter strips can potentially reduce stormwater runoff by up to 85% in most storm events, but they’re less effective during very large storms.
Dry Detention Basins and Ponds
Dry detention basins (sometimes called dry ponds) are designed to capture, temporarily hold, and gradually release stormwater runoff over a brief period, typically about 24 hours. Often one of the first stages of an overall stormwater management plan, their purpose is to delay and minimize stormwater peaks. These structures help to minimize the erosive effects caused by large volumes of extremely fast-moving water by significantly slowing the stormwater movement, enabling it to drop most large sediment before it exits the basin, all in a relatively short period of time. After exiting the basin, it can be directed for further decontamination and eventual infiltration into the soil.
Dry detention basins can accept flow from stormwater hot spots (areas where surface runoff is likely to be highly contaminated, such as gas stations, vehicle repair areas and waste storage areas) if they are lined with an impermeable geotextile liner to prevent groundwater infiltration. Because of the nature of hot spot contamination, liners should be selected based on durability, resistance to physical damage such as tears and punctures, and resistance to degradation from chemical exposure. Liners built to contain hazardous products like diesel, pesticides, and wastewater are ideal for these applications.
Wet Retention Basins and Ponds
Wet retention ponds (sometimes called wet basins) are intended to capture and hold contaminated stormwater runoff for several days. They’re easily distinguished because retention ponds are designed to hold water at all times and are generally larger than detention basins. These ponds help prevent flooding by capturing and holding stormwater, while erosion is minimized by surrounding the ponds with plentiful vegetation including grasses, reeds, shrubs and trees.
Sediment settling is one of the most effective processes for reducing pollutants and improving water quality. While a detention basin may permit large sediment particles to settle, many stormwater pollutants are extremely small and require much longer to settle out. Retention ponds hold water long enough for that to happen, meaning that water exiting a retention pond after a few days is higher quality than when it entered.
Retention ponds are available in a variety of designs intended to address specific site limitations or improve a pond’s effectiveness. A basic retention pond design includes a forebay, where incoming water is slowed and has an opportunity to drop larger sediment particles. The forebay is separated from the main pond by a berm set at the same level as the outlet in the main pond. The main pond features a wide vegetated shelf where aquatic plants help filter the water and absorb pollutants. Some ponds may include baffles to direct water movement in a path that further reduces flow speed and extends retention time.
Water typically arrives in a retention pond through an underground pipe, allowing the pond to accept large amounts of water without eroding the surrounding areas. A small outlet is placed at the other end to allow water to eventually flow out while maintaining minimum water levels. For severe storms, an emergency overflow allows water to pass directly out of the pond.
Retention ponds are attractive stormwater control measures because they’re simple to build, can substantially improve water quality without additional equipment, support important wildlife habitats, improve property values, and may even be used for recreation if they’re large enough. However, they’re not always suitable because they require relatively large parcels of land and must be carefully designed to avoid making the problem worse rather than better.
Constructed Wetlands
Wetlands represent the most effective natural flood-control measures available, and many of the world’s most dire problems with flooding can be traced back to the elimination of wetlands in favor of expanding urban development. In this time of changing weather patterns, communities everywhere are choosing to rehabilitate or reconstruct wetlands in order to manage worsening problems with flooding, polluted stormwater, and environmental damage.
Wetlands, at their most basic level, help to keep water circulation in balance within a specific area by taking up floodwater and gradually releasing it during dry periods. They also help maintain water quality by filtering out pollutants, helping maintain the health of the local ecology
Artificial wetlands function in a similar fashion as wet ponds, in that they use shallow zones and vegetation to filter and remove pollutants from stormwater, as well as deeper zones for slowing the water flow and extending retention time. Well-designed wetlands are especially effective at reducing BOD, removing nutrients, hydrocarbons and even toxic metals.
Like wet ponds, wetlands provide valuable wildlife habitats, but on a much larger scale. In fact, wetlands are among the most productive ecosystems in the world, not unlike rain forests and coral reefs. Wetlands can also be attractive features for local residents, offering a chance to get close to nature, enjoy an abundance of wildlife and learn about the ecologies that surround them.
