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Urban runoff flowing into a storm drain
Urban runoff is surface runoff of rainwater created by urbanization. This runoff is a major source of water pollution in many parts of the United Statesmarker and other urban communities worldwide.

Overview

Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain storms and other precipitation events, these surfaces (built from materials such as asphalt, cement, and concrete), along with rooftops, carry polluted stormwater to storm drains, instead of allowing the water to percolate through soil. This causes lowering of the water table (because groundwater recharge is lessened) and flooding since the amount of water that remains on the surface is greater. Most municipal storm sewer systems discharge stormwater, untreated, to streams, rivers and bays.

Pollutants in urban runoff

A creek filled with urban runoff after a storm
Water running off these impervious surfaces tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants from roadways and parking lots, as well as fertilizers and pesticides from lawns. Roads and parking lots are major sources of polycyclic aromatic hydrocarbons (PAHs), which are created as combustion byproducts of gasoline and other fossil fuels, as well as of the heavy metals nickel, copper, zinc, cadmium, and lead. Roof runoff contributes high levels of synthetic organic compounds and zinc (from galvanized gutters). Fertilizer use on residental lawns, parks and golf courses is a significant source of nitrates and phosphorus.

As stormwater is channeled into storm drains and surface waters, the natural sediment load discharged to receiving waters decreases, but the water flow and velocity increases. In fact, the impervious cover in a typical city creates five times the runoff of a typical woodland of the same size.

One of the most pronounced effects of urban runoff is on watercourses that historically contained little or no water. When the area is urbanized, urban runoff then creates an unnatural year-round flow that hurts the vegetation, wildlife and riverbed of the waterway. Containing little or no sediment, urban runoff rushes down stream channels and creates severe erosion - increasing sediment loads at the mouth while severely incising the streambed upstream. As an example, on many Southern California beaches at the mouth of a waterway, urban runoff carries trash, pollutants, excessive silt, and other wastes, and can pose moderate to severe health hazards.

Effects of urban runoff

A 2008 report by the United States National Research Council identified urban runoff as a leading source of water quality problems:

The runoff also increases temperatures in streams, harming fish and other organisms. (A sudden burst of runoff from a rainstorm can cause a fish-killing shock of hot water.) Also, road salt used to melt snow on sidewalks and roadways can contaminate streams and groundwater aquifers.

Prevention and mitigation of urban runoff



Effective control of urban runoff involves reducing the velocity and flow of stormwater, as well as reducing pollutant discharges. A variety of stormwater management practices and systems may be used to reduce the effects of urban runoff. Some of these techniques, called best management practices in the U.S., focus on water quantity control, while others focus on improving water quality, and some perform both functions.

Pollution prevention practices include low impact development techniques, installation of green roofs and improved chemical handling (e.g. management of motor fuels & oil, fertilizers and pesticides). Runoff mitigation systems include infiltration basins, bioretention systems, constructed wetlands, retention basins and similar devices.

See also



References

  1. Water Environment Federation, Alexandria, VA; and American Society of Civil Engineers, Reston, VA. "Urban Runoff Quality Management." WEF Manual of Practice No. 23; ASCE Manual and Report on Engineering Practice No. 87. 1998. ISBN 1-57278-039-8. Chapter 1.
  2. Schueler, Thomas R. "The Importance of Imperviousness." Reprinted in The Practice of Watershed Protection. 2000. Center for Watershed Protection. Ellicott City, MD.
  3. Chapter 2.
  4. U.S. Environmental Protection Agency (EPA). Washington, DC. "Protecting Water Quality from Urban Runoff." Document No. EPA 841-F-03-003. February 2003.
  5. United States. National Research Council. Washington, DC. "Urban Stormwater Management in the United States." October 15, 2008. pp. 18-20.
  6. United States Geological Survey. Atlanta, GA. "The effects of urbanization on water quality: Urban runoff."
  7. EPA. "Preliminary Data Summary of Urban Storm Water Best Management Practices." Chapter 5. August 1999. Document No. EPA-821-R-99-012.
  8. EPA. "Fact Sheet: Low Impact Development and Other Green Design Strategies." October 9, 2008.
  9. California Stormwater Quality Association. Menlo Park, CA. "Stormwater Best Management Practice (BMP) Handbooks." 2003.
  10. New Jersey Department of Environmental Protection. Trenton, NJ. "New Jersey Stormwater Best Management Practices Manual." April 2004.


Further reading




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