A
catch basin is a part of a storm drain or sewer system which is designed to trap
debris so that it cannot enter the drainage pipes. Catch basin are a large scale
version of the traps used in home drains to accomplish a similar function. Most
municipal sewer and storm drainage systems use catch basin, and the design is
basically the same all over the world, with a few small variations. Typically,
public works agencies maintain storm drains and catch basin, although individual
citizens would be well-advised to keep an eye on local storm drains so that they
can report problems promptly.
Catch basins have a wide sloping inlet which collects runoff, assuring that even
when high volumes of water are being dumped into the system, there is minimal
overflow. The inlet opens to a pipe which is covered with a grating. The grating
traps large debris, preventing it from entering the piping. As water floods the
catch basin, small particles which slip through the grate settle to the bottom.
Drainage pipes are located above the bottom of this vertical pipe, ensuring that
the water which flows into the drains is clear of sediment.
Maintenance of a catch basin includes regularly clearing the grating of debris
so that the drainage is not inhibited, and periodically cleaning out the trap at
the bottom. If particulates are allowed to rise high enough, they can start
spilling over into the drains, which can become an issue. Many cities clean
storm drains regularly, and pull out sediment after the rainy season.
In some cases, the catch basin may be designed in such a way that it prevents
backflow of gases from the pipes. This is common with catch basin in sewer
systems, so that the distinctive odor of the gases which build up in sewers does
not rise to the surface, where it could irritate passerby. The catch basin may
also include features which are meant to deter rats, as rats can be a common
problem in drainage and sewer systems.
It is also possible to use a catch basin in a home drainage system, although the
scale is usually smaller than that of a storm drain. Many plumbers are qualified
to put in catch basin, and they can also provide a quick overview of maintenance
techniques to ensure that the catch basin remains effective.
What they Are, What they Do
A storm drain, storm sewer (U.S.), stormwater drain (Australia and New Zealand)
or surface water system (UK) is designed to drain excess rain and ground water
from paved streets, parking lots, sidewalks, 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 and coastal towns which
experience regular storms.
Function
Inlet
Full view of a storm drain.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.
Piping
Pipes can come in many different shapes (rectangular, square, bread loaf shaped,
oval and, more commonly, circular) 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, concrete and even plastic in some cases.
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. 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.
Reducing stormwater flows
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. To reduce stormwater from rooftops, flows from eaves troughs
(rain gutters and downspouts) may be infiltrated into adjacent soil, rather than
discharged into 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.
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.
Combined sewers
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 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.
Building codes
Building codes 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 and bioswales may be
required in sensitive ecological areas to protect the watershed.
Exploration
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.
Ancient history
Archaeological 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 person to crawl
through. Other examples of early civilizations with elements of stormwater drain
systems include early people of Mainland Orkney such as Gurness and the Brough
of Birsay in Scotland.
