Urban runoff
Urban runoff refers to the water that flows into bodies of water, such as streams and rivers, from precipitation events like rainfall and melting snow in developed areas. Unlike natural landscapes, where vegetation absorbs much of the rain, urban areas are characterized by impervious surfaces—like roofs and roads—that prevent water from soaking into the ground. This results in stormwater runoff that can lead to environmental issues such as flooding and water pollution.
Urban runoff is a significant source of nonpoint source pollution, meaning it is difficult to trace back to a single origin. Common pollutants include automotive fluids, heavy metals, and chemicals from lawns. The increase in urbanization exacerbates flooding, with small floods becoming significantly more frequent as urban land coverage rises. Traditional stormwater management methods have evolved from simple conveyance systems to more sophisticated techniques, like using swales, ponds, and wetlands to manage and filter runoff. Recent regulations have also emphasized the need for municipalities to control pollution from stormwater, encouraging residents to adopt practices such as disconnecting roof drains and utilizing rain barrels. Through these initiatives, communities can work towards reducing the negative impacts of urban runoff on local waterways.
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Urban runoff
DEFINITION: Water that flows into streams, rivers, and other bodies of water from lawn irrigation, rainfall, and snowmelt in cities and other developed areas
In urban areas, runoff can contribute to environmental problems such as flooding and water pollution.
When rain falls on natural landscapes, much of it is caught by vegetation or soaks into the ground. A coniferous forest, for example, can intercept as much as 50 percent of the rain that falls on it annually. The rain that reaches the ground percolates into the soil and makes its way into the or travels slowly through the soil to reach the nearest stream hours, days, or even months later.
In developed areas, rainwater falls on impervious surfaces—roofs, roads, and other nonporous materials—where it is prevented from soaking into the ground. The runoff, often referred to as stormwater, then flows across those surfaces in large quantities, collecting and transporting pollutants, until it reaches a storm sewer, stream, or natural area. This large amount of water flowing into streams almost immediately after a storm can cause local flooding. Research has shown that the number of small floods increases up to ten times when a reaches 20 percent urbanization. Conversely, since less water is allowed to enter the soil and groundwater, stream flows are greatly reduced in the dry season. This rapid fluctuation of water levels causes stream and siltation and destroys for fish and other aquatic life.
The carried by urban is a form of nonpoint source pollution—pollution that is not easily traced to a particular point, such as the end of a pipe, but instead comes from all over the environment. Automobiles, combined with impervious surfaces, are responsible for a large part of the pollution in urban runoff. In addition to motor oil that is improperly dumped down storm drains, oil and other automotive fluids leak onto parking lots and roads and are picked up by runoff. Heavy metals, such as zinc and copper, accumulate from the dust of tire and brake wear and can be major pollutants in urban streams. Lawns can be relatively impervious as well, and heavy storms or excessive watering can wash pesticides, herbicides, and fertilizers into streams, disrupting the already damaged aquatic environment. Airborne pollutants that fall on impervious surfaces, substances dumped on roads or in storm drains, bird and pet wastes, and street litter also contribute to the toxic mix of stormwater runoff.
An additional runoff problem is caused by the erosion of disturbed soil in areas of active building and development. Sediment that is washed into streams, either through storm drains or directly, can cover gravel that is critical to aquatic insects and fish and can stream channels, contributing to downstream flooding.
The first attempts at urban stormwater management consisted of the construction of gutters and the digging of ditches to convey excess water quickly to the nearest natural waterway. In 1869 famed landscape architectFrederick Law Olmsted was the first to design a system of underground pipes to convey muddy, horse-manure-laden runoff from the streets of the new of Riverside, Illinois, to the nearby Des Plains River. Many cities soon thereafter developed systems that combined storm sewers with sanitary sewers, but as communities grew and the amount of land covered by impervious surfaces increased, these systems often became overloaded and then overflowed, sending stormwater and raw into rivers and streams. These cities later had to work to separate the storm and sanitary systems, with engineering emphasis placed on storing or detaining runoff during storms and then releasing it to the streams later to reduce flooding.
Large-scale stormwater management relies on swales, ponds, and wetlands. Runoff from a town or development can be routed through a swale, a depression in the landscape that directs the runoff to another place or holds it long enough for it to evaporate or soak into the soil. Vegetation in the swale slows the water and also filters out some of the pollutants. A pond or wetland also detains the water and allows some of the pollutants to settle or be filtered by the plants. Some wetland organisms can actually break down oils and other pollutants into harmless elements.
In 1988 the reauthorization of the Clean Water Act required municipalities in the United States to do more to control nonpoint source pollution, most of which reaches waterways through stormwater. The listing of various species of fish as endangered or threatened under the Endangered Species Act also forced some cities to improve fish habitats by limiting pollution and in local streams.
Because nonpoint source pollution, and itself, accumulates drop by drop from many small sources, the small actions of many people throughout a watershed can help to solve the problem. In many cities, residents are encouraged to disconnect their roof drains, where appropriate, and divert stormwater to their own landscapes. In addition, rain barrels and cisterns can detain stormwater to be used for landscape watering and other purposes. Naturescaping, or gardening with native plants that are naturally adapted to the local climate and resistant to native pests, requires fewer chemical applications and less supplementary watering. The careful use and disposal of landscape and household chemicals and automotive fluids can also reduce pollutants in stormwater. In these ways, residents of a watershed can become part of the solution to the problem of urban flooding and local stream pollution.
Bibliography
American Society of Civil Engineers. Urban Runoff Quality Management. Reston, Va.: Author, 1998.
Debo, Thomas N., and Andrew J. Reese. Municipal Stormwater Management. 2d ed. Boca Raton, Fla.: Lewis, 2003.
"EPA Facility Stormwater Management." Environmental Protection Agency, 2 Jan. 2024, www.epa.gov/greeningepa/epa-facility-stormwater-management. Accessed 22 July 2024.
Hill, Marquita K. “Water Pollution.” In Understanding Environmental Pollution. 4th ed. New York: Cambridge University Press, 2020, pp. 206-241.
Hunt, Constance Elizabeth. Thirsty Planet: Strategies for Sustainable Water Management. New York: Zed Books, 2004.
Sullivan, Patrick J., Franklin J. Agardy, and James J. J. Clark. “Water Pollution.” In The Environmental Science of Drinking Water. Burlington, Mass.: Elsevier Butterworth-Heinemann, 2005.