Stormwater is a term used to describe water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt or runoff water from overwatering that enters the stormwater system. Stormwater that does not soak into the ground becomes surface runoff, which either flows directly into surface waterways or is channeled into storm sewers, which eventually discharge to surface waters.
Stormwater is of concern for two main issues: one related to the volume and timing of runoff water (flood control and water supplies) and the other related to potential contaminants that the water is carrying, i.e. water pollution.
Since the era that humans began living in concentrated village or urban settings, stormwater runoff has presented itself as an issue. Such dwelling styles can be generally related to the Bronze Age when considerable amounts of impervious surface emerged as a factor in the design of early human settlements. Some of the early incorporation of stormwater engineering is evidenced in ancient Greece.
Because impervious surfaces (parking lots, roads, buildings, compacted soil) do not allow rain to infiltrate into the ground, more runoff is generated than in the undeveloped condition. This additional runoff can erode watercourses (streams and rivers) as well as cause flooding when the stormwater collection system is overwhelmed by the additional flow. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather.
Pollutants entering surface waters during precipitation events is termed polluted runoff. Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, etc. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. While there is some attenuation of these pollutants before entering the receiving waters, the quantity of human activity results in large enough quantities of pollutants to impair these receiving waters.
In addition to the pollutants carried in stormwater runoff research by Australian researchers is identifying urban runoff as a cause of pollution in its own right.
In natural catchments (watersheds) surface runoff entering waterways is a relatively rare event, occurring only a few times each year and generally after larger storm events. Before development occurred most rainfall soaked into the ground and contributed to groundwater recharge or was recycled into the atmosphere by trees as evapotranspiration.
Modern drainage systems which collect runoff from impervious surfaces (e.g., roofs and roads) ensure that water is efficiently conveyed to waterways through pipe networks, meaning that even small storm events result in increased flows in waterways.
In addition to delivering higher pollutants from the urban catchment increased stormwater flow can lead to stream erosion, encourage weed invasion and can alter natural flow regimes which native species rely on for a range for activities including spawning, juvenile development and migration.
Many researchers believe that polluted runoff from roads and highways is the largest source of water pollution in coastal areas today. For example, about 75 percent of the toxic chemicals getting to Seattle, Washington's Puget Sound are carried by stormwater that runs off paved roads and driveways, rooftops, yards and other developed land.
Managing the quantity and quality of stormwater is termed,
"Stormwater Management." The
Management Practice (BMP) is often used to refer to both
structural or engineered control devices and systems (e.g. retention ponds) to treat polluted
stormwater, as well as operational or procedural practices. There
are many forms of stormwater management and BMPs, including:
• manage stormwater to control flooding and erosion;
• manage and control hazardous materials to prevent release of pollutants into the environment (source control);
• plan and construct stormwater systems so contaminants are removed before they pollute surface waters or groundwater resources;
• acquire and protect natural waterways where they still exist or can be rehabilitated;
• look for opportunities to build "soft" structures such as ponds, swales or wetlands to work with existing or "hard" structures, such as pipes and concrete channels;
• revise current stormwater regulations to address comprehensive stormwater needs;
• enhance and enforce existing ordinances to make sure property owners consider the effects of stormwater before, during and after development of their land;
• educate ourselves about how our actions affect the quality of our water, and about what we can do to improve water quality; and
• plan carefully to create solutions before problems become too great.
Integrated water management (IWM) of stormwater has the potential to address many of the issues affecting the health of waterways and water supply challenges facing the modern urban city.
Also known as low impact development in the United States, IWM has the potential to improve runoff quality, reduce the risk and impact of flooding and deliver an additional water resource to augment potable supply.
The development of the modern city often results in increased demands for water supply due to population growth, while at the same time altered runoff predicted by climate change has the potential to increase the volume of stormwater that can contribute to drainage and flooding problems. IWM offers several techniques including stormwater harvest (to reduce the amount of water that can cause flooding), infiltration (to restore the natural recharge of groundwater), biofiltration or bioretention (e.g., rain gardens) to store and treat runoff and release it at a controlled rate to reduce impact on streams and wetland treatments (to store and control runoff rates and provide habitat in urban areas).
There are many ways of achieving low impact development (LID). The most popular is to incorporate land-based solutions to handle stormwater runoff through the use of retention ponds, bioswales, infiltration trenches, sustainable pavements (such as Pervious concrete), and others noted above. LID can also be achieved by utilizing engineered, manufactured products to achieve similar, or potentially better, results as land-based systems (underground storage tanks, stormwater treatment systems, biofilters, etc.). The proper LID solution is one that balances the desired results (controlling runoff and pollution) with the associated costs (loss of useable land for land-based systems versus capital cost of manufactured solution).
IWM as a movement can be regarded as being in its infancy and brings together elements of drainage science, ecology and a realization that traditional drainage solutions transfer problems further downstream to the detriment of our environment and precious water resources.
In the United States, the Environmental Protection Agency (EPA) is charged with regulating stormwater pursuant to the Clean Water Act (CWA). The goal of the CWA is to restore all "Waters of the United States" to their "fishable" and "swimmable" conditions. Point source discharges, which originate mostly from municipal wastewater (sewage) and industrial wastewater discharges, have been regulated since enactment of the CWA in 1972. Pollutant loadings from these sources are tightly controlled and limited. However, despite these controls, thousands of water bodies in the U.S. remain classified as "impaired," meaning that they contain pollutants at levels higher than is considered safe by EPA for the intended beneficial use of the water. Much of this impairment is due to polluted runoff.
Under the CWA, point source discharges to "Waters of the United States" require National Pollution Discharge Elimination System (NPDES) permits. To address the nationwide problem of stormwater pollution, in 1987 Congress broadened the CWA definition of "point source" to include industrial stormwater discharges and municipal separate storm sewer systems ("MS4"). These facilities were required to obtain NPDES permits. This 1987 expansion was promulgated in two phases: Phase I and Phase II. Phase I required that all municipalities of 100,000 persons or more, industrial dischargers, and construction sites of 5 acres (20,000 m2) or more have NPDES permits for their stormwater discharges. Phase I permits were issued in much of the U.S. in 1991. Phase II required that all municipalities, industrial dischargers, construction sites of 1-acre (4,000 m2) or more, and other large property owners (such as school districts) have NPDES permits for their stormwater discharges. Phase II rules came into effect in 2003.
On May 16, 2008 the United States Environmental Protection Agency announced the issuance of a new Construction General Permit (CGP) to replace the permit that expires on July 1, 2008. This new permit has no substantive changes to the current Construction General Permit save the extension of the current conditions through July 1, 2010.
The EPA has authorized 45 states to issue NPDES permits. In addition to implementing the NPDES requirements, many states and local governments have enacted their own stormwater management laws and ordinances, and some have published stormwater treatment design manuals. Some of these state and local requirements have expanded coverage beyond the federal requirements. For example, the State of Maryland requires erosion controls and sediment controls on construction sites of 5,000 sq ft (460 m2) or more.
Agricultural runoff (except for concentrated animal feeding operations, or "CAFO") is considered by the CWA to be nonpoint source pollution. It is not included in the CWA definition of "point source" and therefore not subject to NPDES permit requirements. The 1987 CWA amendments established a non-regulatory program at EPA for nonpoint source pollution management consisting of research and demonstration projects. Related programs are conducted by the Natural Resources Conservation Service (NRCS) in the U.S. Department of Agriculture.