Stormwater management is required to protect and preserve the streams and the environment. As we con- tinue to develop our urban areas, there is potential for an increase in the pollutants that are generated as well as the volume of stormwater runoff due to the increase in impervious areas. The effects from development can be severely detrimental if mitigation measures are not incorporated successfully. Stormwater runoff can cause erosion, flooding, pollution, and other problems for local infrastructure, streams, and the environment.
Most jurisdictions in the United States are required to comply with National Pollutant Discharge Elimination System (NPDES) permits and required by the U.S. Environmental Protection Agency (EPA) to provide treatment measures for pollutants. In addition to minimizing pollutants, a good stormwater management strategy also involves controlling the amount of stormwater that enters storm drain sys- tems and local Municipal Separate Storm Sewer Systems (MS4s). Some regions of the United States still have combined sewers systems and combined sewer overflows (CSO). Retaining stormwa-ter onsite prevents overloading these CSOs discharging to the streams with- out treatment.
Retention basins are a common best management practice (BMP) that are used by engineers to hold the water onsite by storing and infiltrating into native soils because it mimics natural hydrology. Retention basins collect runoff from impervious surfaces and other sources, such as rooftops, parking lots, and drive- ways. Stormwater can also be retained and stored onsite for future use.
Stormwater Retention BMPs
Stormwater retention can be addressed through the use of structural and/or non- structural techniques. Where possible, structural BMPs should be designed, con- structed and operated to trap pollutants in the stormwater and promote infiltration into native soils. Selection of appropriate BMPs should be based on a site-specific investigation, which includes evalua- tions of terrain, soil conditions, ground- water depth and existing and proposed development.
The implementation of several individual retention BMPs within a watershed has been shown to have a cumulative positive impact on the reducing the stormwater runoff downstream of the watershed. The most common non-structural retention BMPs include site design best management measures, which incorporate a combination of the following strategies:
- Reducing impervious areas and preserving natural areas when designing the site plan or layout.
- Preventing excessive compaction of newly graded landscaped areas and pervious areas to effectively retain stormwater runoff.
- Diverting impervious roof drains, parking, and streets towards existing and landscaped pervious areas onsite.
Some typical structural retention BMPs are described in more detail below.
Stormwater retention/infiltration basins/ponds: This is typically an exca- vated pond design to collect stormwater and infiltrate into the ground. This BMP will rely on the infiltration rates of the native soils onsite. These areas can be vegetated with grass. A retention pond can also be lined with an impermeable liner to store a certain volume of water onsite without infiltration and excess inflow can be bypassed. They can be blended into landscape design by adding aesthetic ele- ments like fountains, waterfalls, etc.
Rain garden: Retention basins can also be converted to a rain garden by planting native vegetation. Rain gardens can also be blended into the landscaping design for a more aesthetic look within the project site.
Stormwater wetlands: Constructed wet- lands temporarily store runoff in shallow pools that support conditions suitable for the growth of wetland plants. Constructed stormwater wetlands must be used with other BMPs, such as settling basins or sedi- ment forebays to provide pre-treatment
Bioretention/biofiltration basins: These BMPs are like retention basins, but they are covered by layers of gravel and soil media above the native soils. These are vegetated to help provide pretreatment and remove pollutants before infiltration into the ground. This also helps when the infiltration rates are slower or for subsur- face storage of the stormwater.
Infiltration trench: A linear gravel-filled trench allows stormwater to soak into the ground. Infiltration trenches can be designed with an overflow, an underdrain or vegetative cover. Optional pre-treatment elements can also be provided to prevent clogging, which helps with maintenance and extends the operational life of the installation.
Dry wells: A structural retention BMP that is like an infiltration trench, but it has a smaller footprint and is excavated deeper into the ground. Dry wells need pre-treat- ment to remove pollutants and they have higher potential to contaminate groundwa- ter. Many jurisdictions have restrictions on dry wells so it may not be a preferred option based on location and other site conditions.
Trees or tree wells: Trees can help retain stormwater by interception and holding rainwater in its leaves and branches. A larger tree canopy can hold a larger volume of water. Runoff draining to its root areas is retained by root uptake and evapotranspira- tion. Tree roots also help with infiltration of stormwater. The area around the tree root can be excavated and filled with previous structure soil to provide extra storage and soil stability. Tree wells are ideal for areas adjacent to walkways, driveways and park- ing areas to capture and treat stormwater runoff flowing from these surfaces.
Green roofs: A green roof, which can also be described as a “living roof,” is cre- ated by covering the roof of a building with vegetation. A rooftop garden is created in a soil media planted over an impermeable membrane for waterproofing and root barrier. In addition, water retention green roofs can help mitigate the heat island effect in urban areas during the day.
Permeable pavement: Pavements which are traditionally impervious and increase runoff can be made permeable to help reduce runoff by infiltrating stormwater and remove pollutants. These are con- structed using alternative materials like pervious asphalt, pervious concrete, inter- locking pavers and plastic grid pavers to help allow rain and snowmelt to infiltrate into underlying layers of gravel and soil.
Rain barrels/cisterns: Cisterns are simple and cost-effective solutions to collect and retain rainwater draining from the roof by installing a container or a barrel under the roof drain. Cisterns are like rain barrels but larger and can be installed at ground level or subsurface. Many jurisdictions are providing free rain barrels and training workshops to promote the use of rain barrels in residential areas to store stormwater for future use.
Advantages of Retention BMPS
- BMPs help collect and divert stormwater runoff from streets, parking lots and driveways into the ground instead of draining directly into streams via storm drains/sewers.
- Infiltration increases groundwater recharge, which helps to reduce water pollution, a significant problem in some areas due to high rates of development and industrialization.
- Infiltration helps mitigate the effects of drought because more precipitation moving into an aquifer means there is more water available for use during dry spells.
- Help reduce inflow into MS4 collection systems and will reduce wet weather flow volumes and associated detention or treatment costs.
- Less water flowing through the collection system means less resources required for maintenance and reduced power consumption if the systems include pumps or other equipment saving significant operating costs over time.
- Infiltration also has benefits by improving the capacity of ecosystems, such as wetlands and streams, by providing cleaner water with less sediment and pollutants. Infiltrating groundwater allows for greater levels of dissolved oxygen when com- pared with non-infiltrated waters, helping healthier habitat for fish and other aquatic organisms living in these water bodies.
Disadvantages of Retention BMPS
The suitability of BMPs depends on the site conditions, which may prevent selection of retention BMPs, especially those relying on infiltration.
- Infiltration may adversely impact the stability of the soils and structures near the infiltration BMP locations.
- Infiltration may pollute and adversely impact the ground water if there is not enough separation to achieve removal of contaminant before the stormwater comingles with the groundwater.
- If the soil onsite is contaminated, then infiltrated stormwater will leach these contaminants and pollute the groundwater.
- Soil properties may not be suitable for infiltration or infiltration may be impacted overtime due to excessive sedimentation and poor maintenance.
The preferred BMP for stormwater treatment and reduction of runoff are infiltration BMPs that can mimic the natural hydrologic regime. A combination of both structural and non- structural retention BMPs implemented at multiple sites throughout the watershed will have a cumulative positive impact on managing the stormwater runoff within the watershed and urban areas. This overall effect will improve the downstream quality and health of our receiving waters.
Kiran Pallachulla is a senior water resources engineer at NV5. Pallachulla can be reached at [email protected].