A storm drain, storm sewer (United Kingdom, U.S. and Canada), highway drain, 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. 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 rain gutters from houses and buildings can connect to the storm drain. Since many storm drainage systems are gravity sewers that drain untreated storm water into rivers or streams, any hazardous substances poured into the drains will contaminate the destination bodies of water. 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.
Function
Inlet
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.
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.
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.
Current practices Simple infrastructure such as open drains, pipes, and berms are still common. In modern times, more complex structures involving substantial earthworks and new technologies are common as well.
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