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Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (, and ) are constructed during land development. During rain, storms, and other precipitation events, these surfaces (built from materials such as Asphalt concrete and concrete), along with , 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 since the amount of water that remains on the surface is greater. 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. . Chapter 1. Most municipal storm sewer systems discharge untreated stormwater to , , and . This excess water can also make its way into people's properties through basement backups and seepage through building wall and floors. Urban runoff can be a major source of urban flooding and water pollution in urban communities worldwide.
Eroding soils or poorly maintained construction sites can often lead to increased sedimentation in runoff. Sedimentation often settles to the bottom of water bodies and can directly affect water quality. Excessive levels of sediment in water bodies can increase the risk of infection and disease through high levels of nutrients present in the soil. These high levels of nutrients can reduce oxygen and boost algae growth while limiting native vegetation growth, which can disrupt aquatic ecosystems. Excessive levels of sediment and suspended solids have the potential to damage existing infrastructure as well. Sedimentation can increase surface runoff by plugging underground injection systems. Increased sedimentation levels can also reduce storage behind reservoirs. This reduction of reservoir capacities can lead to increased expenses for public land agencies while also impacting the quality of water recreational areas.
Runoff can also induce bioaccumulation and biomagnification of toxins in ocean life. Small amounts of heavy metals are carried by runoff into the oceans, which can accumulate within aquatic animals to cause adverse effects. This heavy metal poisoning can also affect humans, since ingesting a poisoned animal increases the risk of heavy metal poisoning.
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.
There are several ways in which stormwater enters real property: backup through sewer pipes, toilets and sinks into buildings; seepage through building walls and floors; the accumulation of water on the property and in public rights-of-way; and the overflow of water from water bodies such as rivers and lakes. Where properties are built with basements, urban flooding is the primary cause of basement flooding.
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 .
One of the most pronounced effects of urban runoff is on watercourses that historically contained little or no water during dry weather periods (often called ephemeral stream). When an area around such a stream is urbanization, the resultant runoff creates an unnatural year-round streamflow that hurts the vegetation, wildlife and stream bed of the waterway. Containing little or no sediment relative to the historic ratio of sediment to water, urban runoff rushes down the stream channel, ruining natural features such as and , and creates severe erosion—increasing sediment loads at the mouth while severely carving the stream bed 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.
Because of fertilizer and organic waste that urban runoff often carries, eutrophication often occurs in waterways affected by this type of runoff. After heavy rains, organic matter in the waterway is relatively high compared with natural levels, spurring growth of algae blooms that soon consume most of the oxygen. Once the naturally occurring oxygen in the water is depleted, the algae blooms die, and their decomposition causes further eutrophication. These algae blooms mostly occur in areas with still water, such as and the pools behind , , and some . Eutrophication usually comes with deadly consequences for fish and other aquatic organisms.
Excessive stream bank erosion may cause flooding and property damage. For many years governments have often responded to urban stream erosion problems by modifying the streams through construction of hardened levee and similar control structures using concrete and masonry materials. Use of these hard materials destroys habitat for fish and other animals. Such a project may stabilize the immediate area where flood damage occurred, but often it simply shifts the problem to an upstream or downstream segment of the stream. See River engineering.
There are many different ways that polluted urban runoff could harm humans, such as by contaminating drinking water, disrupting food sources and even causing parts of beaches to be closed off due to a risk of illness. After heavy rainfall events that cause stormwater overflows, contaminated water can impact waterways in which people recreate or fish, causing the beaches or water-based activities to be closed. This is because the runoff has likely caused a spike in harmful bacterial growth or inorganic chemical pollution in the water. The contaminants that we often think of as the most damaging are gasoline and oil spillage, but we often overlook the impact that fertilizers and insecticides have. When plants are watered and fields irrigated, the chemicals that lawns and crops have been treated with can be washed into the water table. The new environments that these chemicals are introduced to suffer due to their presence as they kill native vegetation, invertebrates, and vertebrates.
Pollution prevention practices include low impact development (LID) or green infrastructure techniques - known as Sustainable Drainage Systems (SuDS) in the UK, and Water-Sensitive Urban Design (WSUD) in Australia and the Middle East - such as the installation of and improved chemical handling (e.g. management of motor fuels & oil, fertilizers, pesticides and roadway deicers). Runoff mitigation systems include infiltration basins, bioretention systems, constructed wetlands, , and similar devices.California Stormwater Quality Association. Menlo Park, CA. "Stormwater Best Management Practice (BMP) Handbooks." 2003.New Jersey Department of Environmental Protection. Trenton, NJ. "New Jersey Stormwater Best Management Practices Manual." April 2004.
Providing effective urban runoff solutions often requires proper city programs that take into account the needs and differences of the community. Factors such as a city's mean temperature, precipitation levels, geographical location, and airborne pollutant levels can all affect rates of pollution in urban runoff and present unique challenges for management. Human factors such as urbanization rates, land use trends, and chosen building materials for impervious surfaces often exacerbate these issues.
The implementation of citywide maintenance strategies such as street sweeping programs can also be an effective method in improving the quality of urban runoff. Street sweeper collect particles of dust and suspended solids often found in public parking lots and roads that often end up in runoff.
Educational programs can also be an effective tool for managing urban runoff. Local businesses and individuals can have an integral role in reducing pollution in urban runoff simply through their practices, but often are unaware of regulations. Creating a productive discussion on urban runoff and the importance of effective disposal of household items can help to encourage environmentally friendly practices at a reduced cost to the city and local economy.
Thermal pollution from runoff can be controlled by stormwater management facilities that absorb the runoff or direct it into groundwater, such as bioretention systems and infiltration basins. Bioretention basins tend to be less effective at reducing temperature, as the water may be heated by the sun before being discharged to a receiving stream.
Stormwater harvesting deals with the collection of runoff from creeks, gullies, ephemeral streams, and other ground conveyances. Stormwater harvesting projects often have multiple objectives, such as reducing contaminated runoff to sensitive waters, promoting groundwater recharge, and non-potable applications such as Flush toilet and irrigation.
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