Urban runoff issurface runoff of rainwater, landscape irrigation, and car washing[1] created byurbanization.Impervious surfaces (roads,parking lots andsidewalks) are constructed duringland development. Duringrain, storms, and otherprecipitation events, these surfaces (built from materials such asasphalt andconcrete), along withrooftops, carry pollutedstormwater tostorm drains, instead of allowing the water topercolate throughsoil.[2]
This causes lowering of thewater table (becausegroundwater recharge is lessened) andflooding since the amount of water that remains on the surface is greater.[3][4] Most municipal storm sewer systems discharge untreated stormwater tostreams,rivers, andbays. 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 ofurban flooding andwater pollution in urban communities worldwide.
Water running offimpervious surfaces in urban areas tends to pick upgasoline,motor oil,heavy metals,trash, and other pollutants from roadways and parking lots, as well asfertilizers andpesticides from lawns. Roads and parking lots are major sources ofpolycyclic aromatic hydrocarbons (PAHs), which are created as the byproducts of thecombustion of gasoline and otherfossil fuels, as well as of the heavy metalsnickel,copper,zinc,cadmium, andlead. Roof runoff contributes high levels ofsyntheticorganic compounds andzinc (fromgalvanized gutters). Fertilizer use on residential lawns, parks and golf courses is a measurable source ofnitrates andphosphorus in urban runoff when fertilizer is improperly applied or when turf is over-fertilized.[3][5]
Eroding soils or poorly maintained construction sites can often lead to increasedsedimentation 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 disruptaquatic ecosystems. Excessive levels of sediment andsuspended 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 behindreservoir. This reduction of reservoir capacities can lead to increased expenses for public land agencies while also impacting the quality of water recreational areas.[6]
Runoff can also inducebioaccumulation andbiomagnification of toxins in ocean life. Small amounts of heavy metals are carried by runoff into the oceans, which can accumulate within aquatic animals to causemetal poisoning. This heavy metal poisoning can also affect humans, since ingesting a poisoned animal increases the risk of heavy metal poisoning.[7][8]
As stormwater is channeled into storm drains and surface waters, the naturalsediment 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.[9][clarification needed]
Overwatering throughirrigation by sprinkler may produce runoff reaching receiving waters duringlow flow conditions.[10] Runoff carries accumulated pollutants to streams with unusually low dilution ratios causing higher pollutant concentrations than would be found during regional precipitation events.[11]
Urban runoff is a major cause ofurban flooding, the inundation of land or property in a built-up environment caused by rainfall overwhelming the capacity ofdrainage systems, such asstorm sewers.[12] Triggered by events such asflash flooding,storm surges, overbank flooding, orsnow melts, urban flooding is characterized by its repetitive, costly, and systemic impacts on communities, even when not withinfloodplains or near any body of water.[13]
There are several ways in which stormwater entersproperties: 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.[citation needed]
Urban runoff contributes towater quality problems. In 2009 the USNational Research Council published a comprehensive report on the effects of urban stormwater and stated that it continues to be a major contamination source in many watersheds throughout the United States.[14]: vii The report explained that "...further declines in water quality remain likely if the land-use changes that typify more diffuse sources of pollution are not addressed... These include land-disturbing agricultural, silvicultural, urban, industrial, and construction activities from which hard-to-monitor pollutants emerge during wet-weather events. Pollution from these landscapes has been almost universally acknowledged as the most pressing challenge to the restoration of waterbodies and aquatic ecosystems nationwide."[14]: 24
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The runoff also increases temperatures in streams, harmingfish 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 groundwateraquifers.[15]
One of the most pronounced effects of urban runoff is on watercourses that historically contained little or no water during dry weather periods (often calledephemeral streams). When an area around such a stream isurbanized, the resultant runoff creates an unnatural year-roundstreamflow that hurts the vegetation,wildlife andstream 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 asmeanders andsandbars, and creates severe erosion—increasing sediment loads at the mouth while severely carving thestream bed upstream. As an example, on manySouthern 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 ofalgae blooms that soon consume most of theoxygen. 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 asstream pools and the pools behinddams,weirs, and somedrop structures. 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 hardenedembankments and similar control structures using concrete and masonry materials. Use of these hard materials destroys habitat for fish and other animals.[16] 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.[17]SeeRiver 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.[citation needed] 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.[citation needed]
Effective control of urban runoff involves reducing the velocity and flow of stormwater, as well as reducing pollutant discharges. Local governments use a variety of stormwater management techniques to reduce the effects of urban runoff. These techniques, calledbest management practices for water pollution (BMPs) in some countries, may focus on water quantity control, while others focus on improving water quality, and some perform both functions.[18]
Pollution prevention practices includelow impact development (LID) orgreen infrastructure techniques - known asSustainable Drainage Systems (SuDS) in the UK, andWater-Sensitive Urban Design (WSUD) in Australia and the Middle East - such as the installation ofgreen roofs and improved chemical handling (e.g. management of motor fuels & oil, fertilizers, pesticides androadway deicers).[9][19] Runoff mitigation systems includeinfiltration basins,bioretention systems,constructed wetlands,retention basins, and similar devices.[20][21]
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 sweeping vacuums collect particles of dust and suspended solids often found in public parking lots and roads that often end up in runoff.[22]
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.[23]
Thermal pollution from runoff can be controlled by stormwater management facilities that absorb the runoff or direct it intogroundwater, 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.[18]: p. 5–58
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 astoilet flushing andirrigation.[24]
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