Definition: A catchment defined by storm drain infrastructure
emptying into a common outlet.
Examples: Defined as an analogy to the concept of natural
watershed, which refers to an area draining to a single point in a stream
network, sewersheds are determined by curbs, storm drains, settling basins,
pipes, and outfalls to streams. Often storm sewers or
the collecting curbs and drains cross the boundaries of the watersheds that
existed before urbanization.
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Sewershed Figure 1.
Sewersheds as defined by storm sewer drain networks in west Baltimore City.
Dark blue indicates remaining surface streams in the Gwynns Falls
watershed, whereas lighter blue indicates the storm drains, with thickness of
the lines proportional to the diameter of the pipes. The yellow patch near the center of the map
is Watershed 263, a 932 acre storm drain catchment comprising part or all of 11
neighborhoods. Green indicates
parklands. Baltimore Harbor is the large
body of water to the lower right. The
boundary of the Gwynns Falls watershed is indicated by the dotted line. Graphic provided by Brian McGrath from his
Urban Design Studio at Columbia University.
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Sewershed Figure 2. Removal of some of the unneeded
pavement in Watershed 263 aims to reduce the stormwater flow in this engineered
watershed, and to provide other environmental benefits desired by the community. This improvement in the schoolyard Franklin
Park Elementary School of was carried out by the City Department of Public
Works with design input from the students at the school. Photo courtesy of Guy Hager, Parks &
People Foundation.
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Why important: Storm sewersheds are intended to rapidly carry
off rainwater from city streets, roof gutters, and other large areas of
impervious surface. The intent is to
prevent flooding of basements and streets, and to avoid standing water that
might serve as breeding sites for disease carrying organisms such as
mosquitoes. Storm drainage networks,
although they are referred to as sewers, are in many cases, separate from the
network of pipes designed to carry the septic effluent in sanitary sewers. Leaks or purposeful connections between storm
and sanitary sewers contribute to pollution of streams, lakes, and the coastal
waters into which storm sewers drain.
Storm sewersheds are increasingly being seen as targets for improved
design to reduce the amount of storm water that is generated within them. Greening, reducing impervious surface, and
on-site retention and management of rain water using green roofs, rain gardens,
and the like, are strategies to reduce the negative outputs from storm
sewersheds.
More information:
Cadenasso,
M.L., S.T.A. Pickett, P.M. Groffman, G.S. Brush, M.F. Galvin, J.M. Grove, G.
Hagar, V. Marshall, B.P. McGrath, J.P.M. O'Neil-Dunne, W.P. Stack, and A.R.
Troy. 2008. Exchanges across land-water-scape boundaries in urban systems:
strategies for reducing nitrate pollution. Annals of the New York Academy of
Sciences 1134:213-232.
