The amount and distribution of rainfall, type of soil, geography, size of a site, and density of development are factors that can help determine the best stormwater mitigation strategy. There are several general categories that describe some of the best stormwater management practices.
: When possible, this is usually the most preferred stormwater management technique. Rainwater is channeled to vegetative, ponding areas and permitted to evaporate and infiltrate on-site over the course of a couple of days. The most commonly know example of this strategy is a .
: Biofiltration is the channeling of stormwater through vegetative canals, often called .
and : There are some areas with larger stormwater management needs, such as those of a large-scale development with large impervious areas. Creating stormwater wetlands or sediment ponds allows ponds and to store and treat stormwater runoff.
Rainwater Harvesting: It is possible to store rainwater inor for later use. This water is usually used for irrigation, but can be cleaned and used for drinking water.
Permeable paving: There is an increasing availability of paving options that allow water to drain, either directly through the paving material or through joints between modular pavers.
Peak flow rate occurs after a rain event when the current in the receiving body (typically a stream) dramatically increases. Because of the speed at which the curb and pipe system conveys water, even after a small rain event there is a dramatic increase in both the volume and the speed of water that flows into streams. The streambed erosion is related to this volume and speed of water. Fast-moving water picks up sediment and conveys it downstream, stripping the sides and the bottom of the streambed. With fast-moving water the natural process of sedimentation, or redeposition of sediment, does not occur. Over time, this process results in eroded and continually eroding streams that are unable to sustain the plant life that typically stabilizes stream banks. Bodies of water further downstream have a resulting increase in particulate matter.
Runoff tends to pick up pollution. With the typical stormwater system, there is nothing to prevent this pollution from ending up in lakes, rivers, and streams. Biological processes can remove or mitigate many of these pollutants. In the traditional method of stormwater management, there is no biological buffer between pollutants from road surfaces, roof runoff, or other impervious areas and the receiving body. Without a buffer or filter, pollutants are washed to receiving bodies with every rainfall. These bodies, which are often wildlife habitat, hunting and fishing venues, and recreational areas, are sensitive to pollution levels. The adoption of strategies upstream to improve water quality at the source has a significant impact on the quality farther downstream.
Wildlife benefits from well-designed stormwater areas. Birds and other small animals can use raingardens and other stormwater management areas for habitat. Linear stormwater features can act as wildlife corridors, providing connective areas for wildlife populations. Proper design can ensure that these features do not create nuisances, such as mosquito breeding areas.
Notes on Use
The stormwater strategy that is best suited for a particular project depends on the size of the project, soil type, geography, amount of impervious area, and available space. For small projects with available areas of well draining soil, raingardens ( ) are often the best strategy, as they do not necessitate a stormwater system to carry the water off-site. Larger projects and those with building footprints that take up most of their sites might not have the open area needed for infiltration and would need to use or another mechanism where the majority of the water is not retained on-site, but water quality is improved and the speed of the water is slowed before it flows off-site. Large-scale projects or those that require specific water treatment (i.e. industrial wastewater) may benefit from or other large-scale water treatment methods. Projects that have a large amount of impervious surface area and ample space might use a , which acts in some ways like a bioretention area. The larger the impervious area and the amount of rainfall or the higher the level of pollution, the greater the necessity for design assistance. Landscape architects and civil engineers can provide assistance with the design of stormwater management systems.
Another important consideration to choosing a stormwater strategy is the scale of implementation. Creating a raingarden might be the best option on a single parcel of land, but in a planned unit development or neighborhood development, significant savings and improvement in water quality can be achieved through grouping infiltration areas into a neighborhood-scale network of water management sites.
Options and Analysis
Stormwater Management Alternatives
Some systems require an overflow mechanism where water may also be diverted into the storm sewer or other drainage mechanism during heavy rain. Raingardens are a common application of this strategy. Stormwater is channeled to a well-draining vegetated depression where the water infiltrates over the course of a couple of days.
Constructed wetlands can be a viable solution for stormwater management, even where wetlands were not originally present. Typically, these require larger amounts of land than a rain garden or swale, but can handle larger volumes of stormwater. Constructed wetlands are a viable option for treating stormwater when there is a large tract of land, an abundance of impermeable paving with the chance of mild contamination, or a large amount of water that needs primary treatment.
Curb and Gutter
This is the traditional model for stormwater management and is meant to prevent flooding in developed areas. Curbs and gutters channel water from impervious areas and pipe it to a receiving body. There are several downsides to this type of system. The high peak flow rate immediately after a rain event contributes to erosion of receiving bodies. Litter, particulates, and other pollutants that are picked up by stormwater are often not filtered in this system, creating water polluted with trash.
There are some devices that have been integrated into curb and gutter systems that aid in their efficacy in pollutant removal, including traps and filters at drainage gates and in-line of the pipe system. One barrier to peak flow reduction using a curb and gutter system is the large amount of holding capacity needed to store water for a enough time to allow for lower peak flow discharge rates.
Rainwater catchments refer to the practice of channeling stormwater off of impervious areas to be stored for on-site use. The most common application of this method on a single family residential scale is rain barrels. These systems can be simple—a barrel at the end of a downspout—and provide short-term water storage on-site to be used for irrigation. More complex systems can provide water for toilet flushing and other non-potable uses. Advanced systems incorporating cleaning mechanisms provide water suitable for drinking and can range in size from a system suited to a single-family house to a large commercial building.
Cisterns are larger containers used for larger buildings and projects and can be as simple as a concrete box with a lid. Cisterns can be used for drinking water, but are more often used for watering plants and other non-potable uses.
Cisterns and rain barrels can also serve as a buffer for rainwater runoff, though this requires significant capacity, as a 1-inch rain event on a 700 square foot roof needs 436 gallons of storage. If these systems are used to lower the storm surge amount, they are typically set up to slowly drain the water that is collected, ensuring that they are empty for the next rainfall.
The sediment from erosion contributes to pollution downstream from the source and can result in negative on-site impacts, such as the loss of topsoil and the destabilization of ground. Mitigating erosion on a site is especially important during construction and can be achieved by creating a break or diversion for the water that is causing the erosion. The most common strategies during development are to limit the amount of earth that is exposed and to construct barriers to the flow of water across a site, such as silt fencing or mulch ground cover. Vegetative shoreline and other buffer areas can be used to catch runoff before they go farther downstream. Regardless of the erosion control method used, it must correspond to the site conditions and be properly installed and maintained.
Permeable Paving and Turf Options
Permeable paving allows water to drain either directly through the paving material or through joints between pavers. Because parking lots and other large impervious areas are a large source stormwater runoff, reducing the amount of impervious areas reduces the amount of runoff. There are some drawbacks to these systems and a careful look at the pattern of use and available drainage area for a project should dictate which strategy is best suited for the conditions on the site.
There is also a growing market for turf reinforcing products that can allow the occasional use of a grassed area for parking or pass through. These products reinforce the earth and limit the damage from tires and the loads of automobiles, even when wet. These strategies are increasingly being used for intermittent needs, such as locations that need occasional overflow or event parking.
Municipalities are increasingly channeling the cost of stormwater management to the users. This cost can be lowered through on-site water management, as the upfront price to developing a bioretention system can often be lower than a curb and gutter system, particularly for a new development. The Somerset project is an example of the cost-effectiveness of these strategies. On these 80-acres of single-family houses in Maryland, the developer decided to implement rain gardens in lieu of other management methods. The estimated cost of stormwater systems for the 199-homes was $400,000 for the traditional management technique (not including curbs and gutters) and around $100,000 for the rain gardens. That is a total cost savings of $300,000, or around $1,500 per unit.2 Also, many homeowners view their stormwater management areas, like raingardens and bioswales, as a positive part of their yard. Only a small amount of maintenance is required for many of these techniques and they can provide a beautiful component to the landscaping design.
Minnesota Sustainable Building Guidelines. 2008. Center for Sustainable Building Research. University of Minnesota. 20 Feb. 2008. www.msbg.umn.edu.
2 US Environmental Protection Agency. "'Rain Gardens' Control Residential Runoff." Nonpoint Source News-Notes. Aug/Sep 1995. pp 5-7. 7 July 2008. www.epa.gov/owow/info/NewsNotes/pdf/42issue.pdf.