Storm drain
A storm drain, storm sewer, highway drain, 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.
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 with sewage, either intentionally in the case of combined sewers, or unintentionally.
Nomenclature
Several related terms are used differently in American and British English.| Term | American | British | Comments |
| Combined sewer | A sewer designed and intended to serve as a sanitary sewer and a storm sewer, or as an industrial sewer and a storm sewer | Same as American English | Stormwater mixed with sewage |
| Storm sewer, Surface water sewer, or surface sewer | A sewer designed and intended to carry only stormwater, surface runoff, street wash waters, and drainage | A sewer designed and intended to carry only rainwater runoff | Only stormwater |
| Stormwater bypass | Same as British English | A combined sewer discharge pipeline intended to bypass wastewater treatment plants during peak runoff events | Stormwater mixed with sewage |
| Road channel | See roadside ditch | A roadside channel to prevent uncontrolled runoff along roadway surfaces | Only stormwater |
| Road gully | See roadside ditch | Consists of a gully grating on a chamber that connects to a surface water sewer / drain, ditch, or watercourse | Only stormwater |
| Roadside ditch | A roadside channel to prevent uncontrolled runoff along roadway surfaces | See road gully | Only stormwater |
Function
Inlet
There are two main types of stormwater drain 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, 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. Drainage systems may have many different features including waterfalls, stairways, balconies and pits for catching rubbish, sometimes called Gross Pollutant Traps. 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 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 have been common as well.Geotextiles
New storm water drainage systems incorporate geotextile filters that retain and prevent fine grains of soil from passing into and clogging the drain. Geotextiles are synthetic textile fabrics specially manufactured for civil and environmental engineering applications. Geotextiles are designed to retain fine soil particles while allowing water to pass through. In a typical drainage system, they would be laid along a trench which would then be filled with coarse granular material: gravel, sea shells, stone or rock. The geotextile is then folded over the top of the stone and the trench is then covered by soil. Groundwater seeps through the geotextile and flows through the stone to an outfell. In high groundwater conditions a perforated plastic pipe is laid along the base of the drain to increase the volume of water transported in the drain.Alternatively, a prefabricated plastic drainage system made of HDPE, often incorporating geotextile, coco fiber or rag filters can be considered. The use of these materials has become increasingly more common due to their ease of use, since they eliminate the need for transporting and laying stone drainage aggregate, which is invariably more expensive than a synthetic drain and concrete liners.
Over the past 30 years, geotextile, PVC filters, and HDPE filters have become the most commonly used soil filter media. They are cheap to produce and easy to lay, with factory controlled properties that ensure long term filtration performance even in fine silty soil conditions.
21st century alternatives
Public Utilities created a pilot program called Street Edge Alternatives Project. The project focuses on designing a system "to provide drainage that more closely mimics the natural landscape prior to development than traditional piped systems".The streets are characterized by ditches along the side of the roadway, with plantings designed throughout the area.
An emphasis on non-curbed sidewalks allows water to flow more freely into the areas of permeable surface on the side of the streets. Because of the plantings, the run off water from the urban area does not all directly go into the ground, but can also be absorbed into the surrounding environment.
Monitoring conducted by Seattle Public Utilities reports a 99 percent reduction of storm water leaving the drainage project.
Drainage has undergone a large-scale environmental review in the recent past in the United Kingdom. Sustainable urban drainage systems are designed to encourage contractors to install drainage system that more closely mimic the natural flow of water in nature. Since 2010 local and neighbourhood planning in the UK is required by law to factor SUDS into any development projects that they are responsible for.
Slot drainage is a channel drainage system designed to eliminate the need for further pipework systems to be installed in parallel to the drainage, reducing the environmental impact of production as well as improving water collection. Stainless steel, concrete channel, PVC and HDPE are all materials available for slot drainage which have become industry standards on construction projects.
In the construction industry
The civil engineer is responsible for drainage in construction projects. During the construction process, they set out all the necessary levels for roads, street gutters, drainage, culverts and sewers involved in construction operations.Civil engineers and construction managers work alongside architects and supervisors, planners, quantity surveyors, and the general workforce, as well as subcontractors. Typically, most jurisdictions have some body of drainage law to govern to what degree a landowner can alter the drainage from their parcel.
Drainage options for the construction industry include:
- Point drainage, which intercepts water at gullies. Gullies connect to drainage pipes beneath the ground surface, so deep excavation is required to facilitate this system. Support for deep trenches is required in the shape of planking, strutting or shoring.
- Channel drainage, which intercepts water along the entire run of the channel. Channel drainage is typically manufactured from concrete, steel, polymer or composites. The interception rate of channel drainage is greater than point drainage and the excavation required is usually much less deep.