A storm drain
, storm sewer
), stormwater drain
and New Zealand
) or surface water
) is designed to drain
and ground water
from paved streets, parking lots
, and roofs. Storm drains vary in design from small residential dry wells
to large municipal systems. They are fed by street gutters
on most motorways
, freeways and other busy roads, as well as towns in areas which experience heavy rainfall, flooding
towns which experience regular storms
Most storm sewers are provided with gratings or grids to prevent large objects from falling into the sewer system. However, their bars are fairly widely spaced so that the flow of water is not impeded. Consequently, many small objects can fall through.
Many of these small objects are caught by the catchbasin, or sump, which lies immediately below the grating. Water from the top of the catchbasin drains 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 trap, the catchbasin does not necessarily prevent sewer gases such as hydrogen sulfide and methane from escaping. However in the United Kingdom, where they are called gulley-pots, they are designed as true water-filled traps and do block the egress of gases and rodents.
Most catchbasins will contain stagnant water during the drier parts of the year and can be used by mosquitoes for breeding. The performance of catchbasins at removing sediment and other pollutants depends on the design of the catchbasin (e.g., the size of the sump), and routine maintenance to retain the storage available in the sump to capture sediment. Municipalities typically have large vacuum trucks that perform this task.
Catchbasins act as pretreatment for other treatment practices, such as retention basins, by capturing large sediments.
Pipes can come in many different shapes (rectangular, square, bread loaf shaped, oval and, more commonly, circle) and have many different features (including waterfalls
, stairways, balconies and pits for catching rubbish or Gross Pollutant Traps (GPTs). Several different materials can also be used, such as brick
and even plastic
in some cases.
Most drains have a single large exit at their point of discharge (often covered by a grating
) into a canal
, sea or ocean
. 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 man-made excavations known as recharge basins.
Relationship to sanitary sewer systems
Storm drains are often operated independently from sanitary sewer
systems. The separation of storm sewers from sanitary sewers helps to prevent sewage treatment plants becoming overwhelmed by the huge influx of water during a rainstorm, which can result in untreated sewage being discharged into the environment.
Many storm drainage systems are designed to drain the storm water, untreated, into rivers or streams. Special care must be taken to ensure the citizenry is aware of this, lest waste be dumped into the storm drain system. In the city of Cleveland, Ohio, for example, all new catch basins installed have inscriptions on them not to dump any waste, and usually include a fish imprint as well.
Runoff into storm sewers can be minimized by including sustainable urban drainage systems (UK term) or low impact development practices (U.S. term) into municipal plans. Eaves troughs should not discharge directly into the storm sewer system but rather onto the ground where it has a chance to soak into the soil. Where possible, storm water runoff should be directed to unlined ditches before flowing into the storm sewers, again to allow the runoff to soak into the ground.
The first flush
from storm 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 sewer. For this reason, some jurisdictions subject the runoff from storm drains to sewage treatment
when there is sufficient capacity to spare. In the U.S., these systems are called combined sewers
. In these systems a sudden large rainfall that exceeds sewage treatment capacity will be allowed to overflow directly from the storm drains into receiving waters via structures called combined sewer overflows
Washington, D.C. and other cities with older combined systems have this problem due to a large influx of storm water after every heavy rain. Some cities have dealt with this by adding large storage tanks or ponds to hold the water until it can be treated. Chicago has a system of tunnels, collectively called the Deep Tunnel, underneath the city for storing its stormwater.
Separation of undesired runoff can be done within the storm sewer system, but such devices are new to the market 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.
vary greatly on the handling of storm drain runoff. New developments might be required to construct their own storm drain processing capacity for returning the runoff to the water table
may be required in sensitive ecological areas to protect the watershed
An international subculture
has grown up around the exploration of stormwater drains. Societies such as the Cave Clan
regularly explore the drains underneath cities. This is commonly known as 'urban exploration
', but is also known as 'draining' when in specific relation to storm drains.
studies have revealed use of rather sophisticated stormwater runoff
systems in ancient cultures. For example, in Minoan Crete
approximately 4000 years before present, cities such as Phaistos
were designed to have storm drains and channels to collect precipitation runoff. At Cretan Knossos
storm drains include stone lined structures large enough for a man to crawl through. Other examples of early civilizations with elements of stormwater drain systems include early people of Mainland Orkney
and the Brough of Birsay