Urbanization and intensive agricultural activities significantly alter the hydrology of the natural watersheds. To accommodate the higher rates and volumes of stormwater runoff in urban development, storm sewer conveyance systems are installed to provide efficient drainage of the landscape. Additional protection is provided through detention and storage structures to control release rates to downstream systems. Traditional design considerations for stormwater systems have been the prevention of damage to the development site, streams, drainageways, streets, public and private property from excessive runoff quantities, and to the reduction of soil erosion. With the implementation of the stormwater NPDES Phase I and II regulations, stormwater runoff quality is now an additional management goal for some communities.
The management of stormwater quantity and quality has to be based on a comprehensive planning and design principles and widely-accepted management criteria (Minimum stormwater management criteria; and Unified Sizing Criteria). Assessment of the regional area, specific site conditions, site constraints, site hydrology, and project type are central to successful planning to minimize pollutants during development as well as during the life of the project. In an effort to protect the Nation's waters from pollution, federal and state agencies have developed a system of regulations and reporting to ensure that proper steps are taken to reduce damages from flood events and to reduce the impacts to receiving waters from the high concentrations of pollutants contained in the runoff.