1a. Objectives (from AD-416):
Determine the rates of potential stream channel widening and down cutting in watersheds that are impacted by increasing urbanization. U.S. EPA Stormwater Policy requires the nation's streams be assessed based on impacts of urbanization on water quality and channel degradation.
1b. Approach (from AD-416):
Numerous studies exist across the U.S. that relate river and watershed relationship characteristics such as drainage area, two-year flow, slope, and D50 to channel width and depth. Regional relationships have been developed, but there are currently no national relationships or a comprehensive national set of relationships. In this study, we will assemble all existing channel studies into a database, examine existing regional relationships, and develop a national set of equations relating channel width and depth to watershed characteristics. USGS regression equations are currently available by State that relate bank-full flow to watershed land use, including urbanization. U.S. EPA estimates of increasing urbanization will be used in the USGS equation for flow impact, which will then be used in the equations that relate flow to channel width and depth. Thus, urbanization impacts on channel widening and down cutting will be estimated, and miles of streams in the U.S. requiring rehabilitation will be determined.
3. Progress Report:
United States Environmental Protection Agency (EPA) is required to develop a national Stormwater Protection Plan to determine the impacts of urbanization and potential mitigation strategies. One of the major consequences of urbanization is stream widening and downcutting that can degrade roads, bridges, pipelines, parks, and private property. In this study, we are developing a modified version of the Soil and Water Assessment Tool (SWAT), called SWAT-DEG (DEGradation), that simulates the impact of urbanization and impoundment structures on channel widening and downcutting. In the first phase of the project, a database of existing channel morphology has been developed with over 1,300 channel cross sections from the conterminous U.S. Regional relationships of channel width and depth have been developed based on drainage area and 2-year frequency stream flows. Current development is focused on estimating channel erodibility and vegetation characteristics, with the ultimate goal of applying SWAT-DEG to small (1-10mi**2) urban watersheds across the conterminous U.S.