Stormwater, also spelled storm water, is water that originates from precipitation events, including snow and ice melt. Stormwater can soak into the soil (infiltrate), be stored on the land surface in ponds and puddles, evaporate, or runoff. Most runoff is conveyed directly to nearby streams, rivers, or other water bodies (surface water) without treatment.
In meteorology, precipitation is any product of the condensation of atmospheric water vapour that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates". Thus, fog and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers."
Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be produced during volcanic eruptions, in a similar way in which the more dangerous lahars form.
Surface runoff is the flow of water that occurs when excess stormwater, meltwater, or other sources flow over the Earth's surface. This can occur when the soil is saturated to full capacity, and rain arrives more quickly than soil can absorb it. Surface runoff often occurs because impervious areas do not allow water to soak into the ground. Surface runoff is a major component of the water cycle. It is the primary agent of soil erosion by water. The land area producing runoff that drains to a common point is called a drainage basin.
In natural landscapes, such as forests, soil absorbs much of the stormwater. Plants also reduce stormwater by improving infiltration, intercepting precipitation as it falls, and by taking up water through their roots. In developed environments, unmanaged stormwater can create two major issues: one related to the volume and timing of runoff (flooding) and the other related to potential contaminants the water is carrying (water pollution).
Water pollution is the contamination of water bodies, usually as a result of human activities. Water bodies include for example lakes, rivers, oceans, aquifers and groundwater. Water pollution results when contaminants are introduced into the natural environment. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems. In turn, this can lead to public health problems for people living downstream. They may use the same polluted river water for drinking or bathing or irrigation. Water pollution is the leading worldwide cause of death and disease, e.g. due to water-borne diseases.
Stormwater is also an important resource as human population and demand for water grow, particularly in arid and drought-prone climates. Stormwater harvesting techniques and purification could potentially make some urban environments self-sustaining in terms of water.
Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.
With less vegetation and more impervious surfaces (parking lots, roads, buildings, compacted soil), developed areas allow less rain to infiltrate into the ground, and more runoff is generated than in the undeveloped condition. Additionally, conveyances such as ditches and storm sewers quickly transport runoff away from commercial and residential areas into nearby water bodies. This greatly increases the volume of water in waterways and the discharge of those waterways, leading to erosion and flooding. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather.
A parking lot or car park, also known as a car lot, is a cleared area that is intended for parking vehicles. Usually, the term refers to a dedicated area that has been provided with a durable or semi-durable surface. In most countries where cars are the dominant mode of transportation, parking lots are a feature of every city and suburban area. Shopping malls, sports stadiums, megachurches and similar venues often feature parking lots of immense area. See also multistorey car park.
A road is a thoroughfare, route, or way on land between two places that has been paved or otherwise improved to allow travel by foot or some form of conveyance, including a motor vehicle, cart, bicycle, or horse.
A building, or edifice, is a structure with a roof and walls standing more or less permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shapes, and functions, and have been adapted throughout history for a wide number of factors, from building materials available, to weather conditions, land prices, ground conditions, specific uses, and aesthetic reasons. To better understand the term building compare the list of nonbuilding structures.
A first flush is the initial runoff of a rainstorm. During this phase, polluted water entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters. 216:
First flush is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration. A concentration can be any kind of chemical mixture, but most frequently solutes and solvents in solutions. The molar (amount) concentration has variants such as normal concentration and osmotic concentration.
Urban runoff is surface runoff of precipitation created by urbanization. This runoff is a major source of flooding and water pollution in urban communities worldwide.
Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, and other land surfaces. Such pollutants include sediment, nutrients, bacteria, pesticides, metals, and petroleum byproducts.When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. While there is some attenuation of these pollutants before entering receiving waters, polluted runoff results in large enough quantities of pollutants to impair receiving waters.
A lawn is an area of soil-covered land planted with grasses and other durable plants such as clover which are maintained at a short height with a lawnmower and used for aesthetic and recreational purposes. Lawns are usually composed only of grass species, subject to weed and pest control, maintained in a green color, and are regularly mowed to ensure an acceptable length. Lawns are used around houses, apartments, commercial buildings and offices. Many city parks also have large lawn areas. In recreational contexts, the specialised names turf, pitch, field or green may be used, depending on the sport and the continent.
A roof is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights; it provides protection against rain, snow, sunlight, extremes of temperature, and wind. A roof is part of the building envelope.
A farm is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is used for specialised units such as arable farms, vegetable farms, fruit farms, dairy, pig and poultry farms, and land used for the production of natural fibres, biofuel and other commodities. It includes ranches, feedlots, orchards, plantations and estates, smallholdings and hobby farms, and includes the farmhouse and agricultural buildings as well as the land. In modern times the term has been extended so as to include such industrial operations as wind farms and fish farms, both of which can operate on land or sea.
This section contains too many or overly lengthy quotations for an encyclopedic entry. (August 2019)
In addition to the pollutants carried in stormwater runoff, urban runoff is being recognized as a cause of pollution in its own right. In natural catchments (watersheds) surface runoff entering waterways is a relatively rare event, occurring only a few times each year and generally after larger storm events. Before development occurred most rainfall soaked into the ground and contributed to groundwater recharge or was recycled into the atmosphere by vegetation through evapotranspiration.
Modern drainage systems, which collect runoff from impervious surfaces (e.g., roofs and roads), ensure that water is efficiently conveyed to waterways through pipe networks, meaning that even small storm events result in increased waterway flows.
In addition to delivering higher pollutants from the urban catchment, increased stormwater flow can lead to stream erosion, encourage weed invasion, and alter natural flow regimes. Native species often rely on such flow regimes for spawning, juvenile development, and migration.
In some areas, especially along the U.S. coast, polluted runoff from roads and highways may be the largest source of water pollution. For example, about 75 percent of the toxic chemicals getting to Seattle, Washington's Puget Sound are carried by stormwater that runs off paved roads and driveways, rooftops, yards, and other developed land.
For Class V stormwater injection wells 3 In addition to the problem of chemical contaminants in stormwater, this USGS report also identifies problems of physical habitat disturbance that Best Management Practices (BMPs) do not eliminate, “Some of the most substantial biological changes caused by development are directly or indirectly related to altered hydrology. Despite efforts to use BMPs to attenuate the hydrologic effects of development, increased peak flows and more flashy runoff will cause physical modifications to the channel shape, bed substrate, and banks of receiving waters, with corresponding effects on aquatic habitat and biota. Loss of forest canopy, increases in paved area, and shallow and(or) muddy detention areas also may cause thermal pollution problems, which can exacerbate chemical stressors on aquatic organisms in receiving waters.” :6 U.S. Congress prohibits Class V stormwater wells to be authorized by permit or by rule where they endanger drinking water sources.the U.S. Environmental Protection Agency reports “the contaminants that have been observed above drinking water standards or health advisory limits in storm water drainage well injectate are aluminum, antimony, arsenic, beryllium, cadmium, chloride, chromium, color, copper, cyanide, iron, lead, manganese, mercury, nickel, nitrate, pH, selenium, TDS, turbidity, zinc, benzene, benzo(a)pyrene, bis(2-ethylhexyl) phtlalate, chlordane, dichloromethane, fecal coliforms, methyl-tertbutyl- ether, pentachlorophenol, tetrachloroethylene, and trichloroethylene.” The U.S. Geological Survey (USGS) reports “Many of the contaminants normally associated with runoff from the Nation's highways have the potential for biological effects. ... Highway-runoff contaminants of particular interest throughout the United States include deicers, nutrients, metals, industrial/urban-organic chemicals, sediment, and agricultural chemicals from industrial, commercial, residential, agricultural, and highway sources.” :
Stormwater is a major cause of urban flooding. Urban flooding is the inundation of land or property in a built-up environment caused by stormwater overwhelming the capacity of drainage systems, such as storm sewers. Although triggered by single events such as flash flooding or snow melt, urban flooding is a condition, characterized by its repetitive, costly and systemic impacts on communities. In areas susceptible to urban flooding, backwater valves and other infrastructure may be installed to mitigate losses.
Where properties are built with basements, urban flooding is the primary cause of basement and sewer backups. Although the number of casualties from urban flooding is usually limited, the economic, social and environmental consequences can be considerable: in addition to direct damage to property and infrastructure (highways, utilities and services), chronically wet houses are linked to an increase in respiratory problems and other illnesses.Sewer backups are often from the sanitary sewer system, which takes on some storm water as a result of Infiltration/Inflow.
Urban flooding has significant economic implications. In the U.S., industry experts estimate that wet basements can lower property values by 10 to 25 percent and are cited among the top reasons for not purchasing a home.According to the Federal Emergency Management Agency almost 40 percent of small businesses never reopen their doors following a flooding disaster. In the UK, urban flooding is estimated to cost £270 million a year (as of 2007) in England and Wales; 80,000 homes are at risk.
A study of Cook County, Illinois, identified 177,000 property damage insurance claims made across 96 percent of the county’s ZIP codes over a five-year period from 2007 to 2011. This is the equivalent of one in six properties in the County making a claim. Average payouts per claim were $3,733 across all types of claims, with total claims amounting to $660 million over the five years examined.
An example of an urban flooding control project is the Brays Bayou Greenway Framework in Houston, Texas. Brays Bayou and its tributaries drain a watershed of approximately 88,000 acres south of downtown Houston. The federally-funded improvement project created a short-term solution by improving the bayou's drainage capacity, and identified a broad set of potential recreation and open space opportunities along the 35 miles of the bayou and tributaries.
An example of urban stormwater creating a sinkhole collapse is the February 25, 2002 Dishman Lane collapse in Bowling Green, Kentucky where a sinkhole suddenly dropped the road under four traveling vehicles. The nine-month repair of the Dishman Lane collapse cost a million dollars but there remains the potential for future problems.
In undisturbed areas with natural subsurface (karst) drainage, soil and rock fragments choke karst openings thereby being a self-limitation to the growth of openings. 189–190, 196 The undisturbed karst drainage system becomes balanced with the climate so it can drain the water produced by most storms. However, problems occur when the landscape is altered by urban development. :28 In urban areas with natural subsurface (karst) drainage there are no surface streams for the increased stormwater from impervious surfaces such as roofs, parking lots, and streets to runoff into. Instead, the stormwater enters the subsurface drainage system by moving down through the ground. When the subsurface water flow becomes great enough to transport soil and rock fragments, the karst openings grow rapidly. :190 Where karst openings are roofed by supportive (competent) limestone, there frequently is no surface warning that an opening has grown so large it will suddenly collapse catastrophically. :198 Therefore, land use planning for new development needs to avoid karst areas. :37–38 Ultimately taxpayers end up paying the costs for poor land use decisions.:
Managing the quantity and quality of stormwater is termed, "Stormwater Management."The term Best Management Practice (BMP) or stormwater control measure (SCM) is often used to refer to both structural or engineered control devices and systems (e.g. retention ponds) to treat or store polluted stormwater, as well as operational or procedural practices (e.g. street sweeping). Stormwater management includes both technical and institutional aspects.
Integrated water management (IWM) of stormwater has the potential to address many of the issues affecting the health of waterways and water supply challenges facing the modern urban city. IWM is often associated with green infrastructure when considered in the design process. Professionals in their respective fields, such as urban planners, architects, landscape architects, interior designers, and engineers, often consider integrated water management as a foundation of the design process.
Also known as low impact development (LID)in the United States, or Water Sensitive Urban Design (WSUD) in Australia, IWM has the potential to improve runoff quality, reduce the risk and impact of flooding and deliver an additional water resource to augment potable supply.
The development of the modern city often results in increased demands for water supply due to population growth, while at the same time altered runoff predicted by climate change has the potential to increase the volume of stormwater that can contribute to drainage and flooding problems. IWM offers several techniques, including stormwater harvest (to reduce the amount of water that can cause flooding), infiltration (to restore the natural recharge of groundwater), biofiltration or bioretention (e.g., rain gardens), to store and treat runoff and release it at a controlled rate to reduce impact on streams and wetland treatments (to store and control runoff rates and provide habitat in urban areas).
There are many ways of achieving LID. The most popular is to incorporate land-based solutions to reduce stormwater runoff through the use of retention ponds, bioswales, infiltration trenches, sustainable pavements (such as permeable paving), and others noted above. LID can also be achieved by utilizing engineered, manufactured products to achieve similar, or potentially better, results as land-based systems (underground storage tanks, stormwater treatment systems, biofilters, etc.). The proper LID solution is one that balances the desired results (controlling runoff and pollution) with the associated costs (loss of usable land for land-based systems versus capital cost of manufactured solution). Green (vegetated) roofs are also another low cost solution.
IWM as a movement can be regarded as being in its infancy and brings together elements of drainage science, ecology and a realization that traditional drainage solutions transfer problems further downstream to the detriment of the environment and water resources.
In the United States, the Environmental Protection Agency (EPA) is charged with regulating stormwater pursuant to the Clean Water Act (CWA). 15The goal of the CWA is to restore all "Waters of the United States" to their "fishable" and "swimmable" conditions. Point source discharges, which originate mostly from municipal wastewater (sewage) and industrial wastewater discharges, have been regulated since enactment of the CWA in 1972. Pollutant loadings from these sources are tightly controlled through the issuance of National Pollution Discharge Elimination System (NPDES) permits. However, despite these controls, thousands of water bodies in the U.S. remain classified as "impaired," meaning that they contain pollutants at levels higher than is considered safe by EPA for the intended beneficial uses of the water. Much of this impairment is due to polluted runoff, generally in urbanized watersheds (in other US watersheds, agricultural pollution is a major source). :
To address the nationwide problem of stormwater pollution, Congress broadened the CWA definition of "point source" in 1987 to include industrial stormwater discharges and municipal separate storm sewer systems ("MS4"). These facilities are required to obtain NPDES permits.In 2017, about 855 large municipal systems (serving populations of 100,000 or more), and 6,695 small systems are regulated by the permit system.
EPA has authorized 47 states to issue NPDES permits.In addition to implementing the NPDES requirements, many states and local governments have enacted their own stormwater management laws and ordinances, and some have published stormwater treatment design manuals. Some of these state and local requirements have expanded coverage beyond the federal requirements. For example, the State of Maryland requires erosion and sediment controls on construction sites of 5,000 sq ft (460 m2) or more. It is not uncommon for state agencies to revise their requirements and impose them upon counties and cities; daily fines ranging as high as $25,000 can be imposed for failure to modify their local stormwater permitting for construction sites, for instance.
Agricultural runoff (except for concentrated animal feeding operations, or "CAFO") is classified as nonpoint source pollution under the CWA. It is not included in the CWA definition of "point source" and therefore not subject to NPDES permit requirements. The 1987 CWA amendments established a non-regulatory program at EPA for nonpoint source pollution management consisting of research and demonstration projects.Related programs are conducted by the Natural Resources Conservation Service (NRCS) in the U.S. Department of Agriculture.
Education is a key component of stormwater management. A number of agencies and organizations have launched campaigns to teach the public about stormwater pollution, and how they can contribute to solving it. Thousands of local governments in the U.S. have developed education programs as required by their NPDES stormwater permits.
One example of a local educational program is that of the West Michigan Environmental Action Council (WMEAC), which has coined the term Hydrofilth to describe stormwater pollution,as part of its "15 to the River" campaign. (During a rain storm, it may take only 15 minutes for contaminated runoff in Grand Rapids, Michigan to reach the Grand River.) Its outreach activities include a rain barrel distribution program and materials for homeowners on installing rain gardens.
Other public education campaigns highlight the importance of green infrastructure in slowing down and treating stormwater runoff. DuPage County Stormwater Management launched the "Love Blue. Live Green." outreach campaign on social media sites to educate the public on green infrastructure and other best management practices for stormwater runoff.Articles, websites, pictures, videos and other media are disseminated to the public through this campaign.
Since humans began living in concentrated village or urban settings, stormwater runoff has been an issue. During the Bronze Age, housing took a more concentrated form, and impervious surfaces emerged as a factor in the design of early human settlements. Some of the early incorporation of stormwater engineering is evidenced in Ancient Greece.
A specific example of an early stormwater runoff system design is found in the archaeological recovery at Minoan Phaistos on Crete.
A storm drain, storm sewer, surface water drain/sewer, or stormwater drain is infrastructure designed to drain excess rain and ground water from impervious surfaces such as paved streets, car parks, parking lots, footpaths, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems.
The Clean Water Act (CWA) is the primary federal law in the United States governing water pollution. Its objective is to restore and maintain the chemical, physical, and biological integrity of the nation's waters; recognizing the responsibilities of the states in addressing pollution and providing assistance to states to do so, including funding for publicly owned treatment works for the improvement of wastewater treatment; and maintaining the integrity of wetlands. It is one of the United States' first and most influential modern environmental laws. As with many other major U.S. federal environmental statutes, it is administered by the U.S. Environmental Protection Agency (EPA), in coordination with state governments. Its implementing regulations are codified at 40 C.F.R. Subchapters D, N, and O.
The Nationwide Urban Runoff Program (NURP) is a research project conducted by the United States Environmental Protection Agency (EPA) between 1979 and 1983. It was the first comprehensive study of urban stormwater pollution across the United States.
A retention basin, sometimes called a wet pond,wet detention basin or stormwater management pond, is an artificial pond with vegetation around the perimeter, and includes a permanent pool of water in its design. It is used to manage stormwater runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay.
Bioswales are linear channels designed to concentrate and convey stormwater runoff while removing debris and pollution. Bioswales can also be beneficial in recharging groundwater.
A combined sewer is a sewage collection system of pipes and tunnels designed to simultaneously collect surface runoff and sewage water in a shared system. This type of gravity sewer design is no longer used in almost every instance worldwide when constructing new sewer systems. Modern-day sewer designs exclude surface runoff from sanitary sewers, but many older cities and towns continue to operate previously constructed combined sewer systems.
The United States Environmental Protection Agency (EPA) Storm Water Management Model is a dynamic rainfall–runoff–subsurface runoff simulation model used for single-event to long-term (continuous) simulation of the surface/subsurface hydrology quantity and quality from primarily urban/suburban areas. It can simulate the Rainfall- runoff, runoff, evaporation, infiltration and groundwater connection for roots, streets, grassed areas, rain gardens and ditches and pipes, for example. The hydrology component of SWMM operates on a collection of subcatchment areas divided into impervious and pervious areas with and without depression storage to predict runoff and pollutant loads from precipitation, evaporation and infiltration losses from each of the subcatchment. Besides, low impact development (LID) and best management practice areas on the subcatchment can be modeled to reduce the impervious and pervious runoff. The routing or hydraulics section of SWMM transports this water and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, ponds, storages, pumps, orifices, weirs, outlets, outfalls and other regulators. SWMM tracks the quantity and quality of the flow generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period composed of multiple fixed or variable time steps. The water quality constituents such as water quality constituents can be simulated from buildup on the subcatchments through washoff to a hydraulic network with optional first order decay and linked pollutant removal, best management practice and low-impact development removal and treatment can be simulated at selected storage nodes. SWMM is one of the hydrology transport models which the EPA and other agencies have applied widely throughout North America and through consultants and universities throughout the world. The latest update notes and new features can be found on the EPA website in the download section. Recently added in November 2015 were the EPA SWMM 5.1 Hydrology Manual and in 2016 the EPA SWMM 5.1 Hydraulic Manual and EPA SWMM 5.1 Water Quality Volume (III) + Errata”
Nonpoint source (NPS) pollution is pollution resulting from many diffuse sources, in direct contrast to point source pollution which results from a single source. Nonpoint source pollution generally results from land runoff, precipitation, atmospheric deposition, drainage, seepage, or hydrological modification where tracing pollution back to a single source is difficult.
Best management practices (BMPs) is a term used in the United States and Canada to describe a type of water pollution control. Historically the term has referred to auxiliary pollution controls in the fields of industrial wastewater control and municipal sewage control, while in stormwater management and wetland management, BMPs may refer to a principal control or treatment technique as well.
A Discharge Monitoring Report (DMR) is a United States regulatory term for a periodic water pollution report prepared by industries, municipalities and other facilities discharging to surface waters. The facilities collect wastewater samples, conduct chemical and/or biological tests of the samples, and submit reports to a state agency or the United States Environmental Protection Agency (EPA). All point source dischargers to ”Waters of the U.S.” must obtain a National Pollution Discharge Elimination System (NPDES) permit from the appropriate agency, and many permittees are required to file DMRs.
Nutrient pollution, a form of water pollution, refers to contamination by excessive inputs of nutrients. It is a primary cause of eutrophication of surface waters, in which excess nutrients, usually nitrogen or phosphorus, stimulate algal growth. Sources of nutrient pollution include surface runoff from farm fields and pastures, discharges from septic tanks and feedlots, and emissions from combustion. Excess nutrients have been summarized as potentially leading to:
Water quality laws govern the release of pollutants into water resources, including surface water, ground water, and stored drinking water. Some water quality laws, such as drinking water regulations, may be designed solely with reference to human health. Many others, including restrictions on the alteration of the chemical, physical, radiological, and biological characteristics of water resources, may also reflect efforts to protect aquatic ecosystems more broadly. Regulatory efforts may include identifying and categorizing water pollutants, dictating acceptable pollutant concentrations in water resources, and limiting pollutant discharges from effluent sources. Regulatory areas include sewage treatment and disposal, industrial and agricultural waste water management, and control of surface runoff from construction sites and urban environments.
Although pollution of surface waters began with human settlements in North America, predating the founding of the United States, it increased significantly with growth in the population, and the development of mechanised agriculture, mining and industry. The problems caused by water pollution became more apparent during the 19th century with the rapid growth of cities and extensive industrialization.
Point source water pollution comes from discrete conveyances and alters the chemical, biological, and physical characteristics of water. In the United States, it is largely regulated by the Clean Water Act (CWA). Among other things, the Act requires dischargers to obtain a National Pollutant Discharge Elimination System (NPDES) permit to legally discharge pollutants into a water body. However, point source pollution remains an issue in some water bodies, due to some limitations of the Act. Consequently, other regulatory approaches have emerged, such as water quality trading and voluntary community-level efforts.
Los Angeles County Flood Control District v. Natural Resources Defense Council, Inc., 568 U.S. 78 (2013), is a United States Supreme Court case in which the Natural Resources Defense Council and Santa Monica Baykeeper challenged the Los Angeles County Flood Control District (District) for violating the terms of its National Pollutant Discharge Elimination System (NPDES) permit as shown in water quality measurements from monitoring stations within the Los Angeles and San Gabriel Rivers. The Supreme Court, by a unanimous 9-0 vote, reversed and remanded the Ninth Circuit's ruling on the grounds that the flow of water from an improved portion of a navigable waterway into an unimproved portion of the same waterway does not qualify as a "discharge of a pollutant" under the Clean Water Act.
A runoff footprint is the total surface runoff that a site produces over the course of a year. According to the United States Environmental Protection Agency (EPA) stormwater is "rainwater and melted snow that runs off streets, lawns, and other sites". Urbanized areas with high concentrations of impervious surfaces like buildings, roads, and driveways produce large volumes of runoff which can lead to flooding, sewer overflows, and poor water quality. Since soil in urban areas can be compacted and have a low infiltration rate, the surface runoff estimated in a runoff footprint is not just from impervious surfaces, but also pervious areas including yards. The total runoff is a measure of the site’s contribution to stormwater issues in an area, especially in urban areas with sewer overflows. Completing a runoff footprint for a site allows a property owner to understand what areas on his or her site are producing the most runoff and what scenarios of stormwater green solutions like rain barrels and rain gardens are most effective in mitigating this runoff and its costs to the community.