Sewershed

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.

  

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.

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.

 

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.