storm drain storm drain a storm drain, storm sewer (united kingdom, u.s. and canada), surface water drain/sewer (united kingdom), or stormwater drain (australia and new zealand) 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. this article is about tunnels that carry runoff from urban streets. for the early sewers designed to carry both waste and storm water, see combined sewer. for the modern sewer system that carries waste, see sanitary sewer. storm drain grate on a street in warsaw, poland storm drain with its pipe visible beneath it due to construction work drains receive water from street gutters on most motorways, freeways and other busy roads, as well as towns in areas with heavy rainfall that leads to flooding, and coastal towns with regular storms. even gutters from houses and buildings can connect to the storm drain. many storm drainage systems are gravity sewers that drain untreated storm water into rivers or streams—so it is unacceptable to pour hazardous substances into the drains. storm drains sometimes cannot manage the quantity of rain that falls in heavy rains or storms. inundated drains can cause basement and street flooding. many areas require detention tanks inside a property that temporarily hold runoff in heavy rains and restrict outlet flow to the public sewer. this reduces the risk of overwhelming the public sewer. some storm drains mix stormwater (rainwater) with sewage, either intentionally in the case of combined sewers, or unintentionally. nomenclature several related terms are used differently in american and british english. termamericanbritishcomments combined sewera sewer designed and intended to serve as a sanitary sewer and a storm sewer, or as an industrial sewer and a storm sewer: 121 same as american englishstormwater mixed with sewage storm sewer, surface water sewer, or surface sewera sewer designed and intended to carry only stormwater, surface runoff, street wash waters, and drainage: 668 a sewer designed and intended to carry only rainwater runoffonly stormwater stormwater bypasssame as british englisha combined sewer discharge pipeline intended to bypass wastewater treatment plants during peak runoff eventsstormwater mixed with sewage road channelsee roadside ditcha roadside channel to prevent uncontrolled runoff along roadway surfacesonly stormwater road gullysee roadside ditchconsists of a gully grating on a chamber that connects to a surface water sewer / drain, ditch, or watercourseonly stormwater roadside ditcha roadside channel to prevent uncontrolled runoff along roadway surfacessee road gullyonly stormwater function american-style curbside storm drain receiving urban runoff inlet full view of a storm drain (ontario, canada) there are two main types of stormwater drain (highway drain or road gully in the uk) inlets: side inlets and grated inlets. side inlets are located adjacent to the curb and rely on the ability of the opening under the back stone or lintel to capture flow. they are usually depressed at the invert of the channel to improve capture capacity. many inlets have gratings or grids to prevent people, vehicles, large objects or debris from falling into the storm drain. grate bars are spaced so that the flow of water is not impeded, but sediment and many small objects can also fall through. however, if grate bars are too far apart, the openings may present a risk to pedestrians, bicyclists, and others in the vicinity. grates with long narrow slots parallel to traffic flow are of particular concern to cyclists, as the front tire of a bicycle may become stuck, causing the cyclist to go over the handlebars or lose control and fall. storm drains in streets and parking areas must be strong enough to support the weight of vehicles, and are often made of cast iron or reinforced concrete. storm drain in küstrin (now kostrzyn nad odrą in poland) some of the heavier sediment and small objects may settle in a catch basin, or sump, which lies immediately below the outlet, where water from the top of the catch basin reservoir overflows into the sewer proper. the catchbasin serves much the same function as the "trap" in household wastewater plumbing in trapping objects. in the united states, unlike the plumbing trap, the catch basin does not necessarily prevent sewer gases such as hydrogen sulfide and methane from escaping. however, in the united kingdom, where they are called gully pots, they are designed as true water-filled traps and do block the egress of gases and rodents. most catchbasins contain stagnant water during drier parts of the year and can, in warm countries, become mosquito breeding grounds. larvicides or disruptive larval hormones, sometimes released from "mosquito biscuits", have been used to control mosquito breeding in catch basins. mosquitoes may be physically prevented from reaching the standing water or migrating into the sewer proper by the use of an "inverted cone filter". another method of mosquito control is to spread a thin layer of oil on the surface of stagnant water, interfering with the breathing tubes of mosquito larvae. the performance of catch basins at removing sediment and other pollutants depends on the design of the catchbasin (for example, the size of the sump), and on routine maintenance to retain the storage available in the sump to capture sediment. municipalities typically have large vacuum trucks that perform this task. catch basins act as the first-line pretreatment for other treatment practices, such as retention basins, by capturing large sediments and street litter from urban runoff before it enters the storm drainage pipes. a storm drain culvert under the main road empties into a bigger open channel piping pipes can come in many different cross-sectional shapes (rectangular, square, bread-loaf-shaped, oval, inverted pear-shaped, egg shaped, and most commonly, circular). drainage systems may have many different features including waterfalls, stairways, balconies and pits for catching rubbish, sometimes called gross pollutant traps (gpts). pipes made of different materials can also be used, such as brick, concrete, high-density polyethylene or galvanized steel. fibre reinforced plastic is being used more commonly for drain pipes and fittings. outlet most drains have a single large exit at their point of discharge (often covered by a grating) into a canal, river, lake, reservoir, sea or ocean. other than catchbasins, typically there are no treatment facilities in the piping system. small storm drains may discharge into individual dry wells. storm drains may be interconnected using slotted pipe, to make a larger dry well system. storm drains may discharge into human-made excavations known as recharge basins or retention ponds. environmental impacts water quantity a truck for cleaning storm drains storm drains are often unable to manage the quantity of rain that falls during heavy rains and/or storms. when storm drains are inundated, basement and street flooding can occur. unlike catastrophic flooding events, this type of urban flooding occurs in built-up areas where human-made drainage systems are prevalent. urban flooding is the primary cause of sewer backups and basement flooding, which can affect properties repeatedly. clogged drains also contribute to flooding by the obstruction of storm drains. communities or cities can help reduce this by cleaning leaves from the storm drains to stop ponding or flooding into yards. snow in the winter can also clog drains when there is an unusual amount of rain in the winter and snow is plowed atop storm drains. runoff into storm sewers can be minimized by including sustainable urban drainage systems (uk term) or low impact development or green infrastructure practices (us terms) into municipal plans. to reduce stormwater from rooftops, flows from eaves troughs (rain gutters and downspouts) may be infiltrated into adjacent soil, rather than discharged into the storm sewer system. storm water runoff from paved surfaces can be directed to unlined ditches (sometimes called swales or bioswales) before flowing into the storm sewers, again to allow the runoff to soak into the ground. permeable paving materials can be used in building sidewalks, driveways and in some cases, parking lots, to infiltrate a portion of the stormwater volume. many areas require that properties have detention tanks that temporarily hold rainwater runoff, and restrict the outlet flow to the public sewer. this lessens the risk of overburdening the public sewer during heavy rain. an overflow outlet may also connect higher on the outlet side of the detention tank. this overflow prevents the detention tank from completely filling. restricting water flow and temporarily holding the water in a detention tank public this way makes it far less likely for rain to overwhelm the sewers. water quality main article: urban runoff the drain cover, located in tallinn, estonia, with a mention of sewers proximity to the sea the first flush from urban runoff can be extremely dirty. storm water may become contaminated while running down the road or other impervious surface, or from lawn chemical run-off, before entering the drain. lack of proper storm drains and sewer systems in kalibari community in mymensingh, bangladesh – a common situation in urban slums in developing countries 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 nickel, copper, zinc, cadmium, lead and polycyclic aromatic hydrocarbons (pahs), which are created as combustion byproducts of gasoline and other fossil fuels. roof runoff contributes high levels of synthetic organic compounds and zinc (from galvanized gutters). fertilizer use on residential lawns, parks and golf courses is a significant source of nitrates and phosphorus. separation of undesired runoff can be achieved by installing devices within the storm sewer system. these devices are relatively new and can only be installed with new development or during major upgrades. they are referred to as oil-grit separators (ogs) or oil-sediment separators (oss). they consist of a specialized manhole chamber, and use the water flow and/or gravity to separate oil and grit. mosquito breeding volunteers clearing gutters in ilorin, nigeria, during a volunteer sanitation day. even when there is adequate infrastructure for sanitation, plastic pollution can interfere with stormwater runoff creating space for mosquitos to breed in water, and causing flooding. some sewage systems in the global south are frequently overwhelmed by the waste, such as in bangkok, thailand.