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. 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. Several related terms are used differently in American and British English. 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.