storm drains, instead of allowing the water to percolate through soil.[2]
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.[3][4] Most municipal storm sewer systems discharge untreated stormwater to streams, rivers, and bays. 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.
Pollutants
Water running off
nitrates and phosphorus 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 increased
reservoir. 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 induce
metal 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 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.[9][clarification needed]
Effects
Dry weather discharges
Overwatering through irrigation by sprinkler may produce runoff reaching receiving waters during low 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]
There are several ways in which stormwater enters properties: 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]
National 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]
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.[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 called
sandbars, 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 stream pools and the pools behind dams, weirs, and some drop structures. Eutrophication usually comes with deadly consequences for fish and other aquatic organisms.
An oil-grit separator is designed to capture settleable solids, oil and grease, debris and floatables in runoff from roads and parking lots
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 embankments 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]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.[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, called best 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]
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 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.[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 as toilet flushing and irrigation.[24]
^Schueler, Thomas R. (2000) [initial publ. 1995]. "The Importance of Imperviousness". In Schueler; Holland, Heather K. (eds.). The Practice of Watershed Protection. Ellicott City, MD: Center for Watershed Protection. pp. 1–12. Archived from the original(pdf) on 2014-03-27. Retrieved 2014-12-24.
McGinn, Anne Platt (2004). Human Activities That Threaten the Worlds Oceans. Farmington Hills, MI: Glennhaven Press. pp. 144–157.
Berland, Adam; Shiflett, Sheri A.; Shuster, William D.; Garmestani, Ahjond S.; Goddard, Haynes C.; Herrmann, Dustin L.; Hopton, Matthew E. (1 June 2017). "The role of trees in urban stormwater management". Landscape and Urban Planning. 162: 167–177.
Tromp, Karin; Lima, Ana T.; Barendregt, Arjan; Verhoeven, Jos T. A. (15 February 2012). "Retention of heavy metals and poly-aromatic hydrocarbons from road water in a constructed wetland and the effect of de-icing". Journal of Hazardous Materials. 203–204: 290–298.
Peer, Wendy Ann; Baxter, Ivan R.; Richards, Elizabeth L.; Freeman, John L.; Murphy, Angus S. (2005). "Phytoremediation and hyperaccumulator plants". Molecular Biology of Metal Homeostasis and Detoxification. Topics in Current Genetics. Vol. 14. pp. 299–340.
Fletcher, T. D.; Andrieu, H.; Hamel, P. (1 January 2013). "Understanding, management and modelling of urban hydrology and its consequences for receiving waters: A state of the art". Advances in Water Resources. 51: 261–279.
Selbig, William R.; Loheide, Steven P.; Shuster, William; Scharenbroch, Bryant C.; Coville, Robert C.; Kruegler, James; Avery, William; Haefner, Ralph; Nowak, David (1 February 2022). "Quantifying the stormwater runoff volume reduction benefits of urban street tree canopy". Science of the Total Environment. 806 (Pt 3): 151296.
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