QUANTIFYING SEDIMENT LOADINGS FROM STREAMBANK EROSION IN SELECTED AGRICULTURAL WATERSHEDS DRAINING TO LAKE CHAMPLAIN
Watershed Physical Processes Research Unit
2010 Annual Report
1a.Objectives (from AD-416)
The primary objectives of the proposed work are to determine rates and loadings of sediment from streambank erosion along main stem and selected tributaries of a large agricultural watershed draining to Lake Champlain.
1b.Approach (from AD-416)
The scope of the project is the main stem and four tributaries (Hereford Brook, Trout River, Tyler Branch and Black Creek) of the Missisquoi River, VT. Tests of the geotechnical and hydraulic erosion resistance of bank materials will be conducted at each site that is selected for detailed investigation. Surveys of bank geometry and vegetation characteristics (species, age and density) will also be conducted at the sites. These data will be used to determine appropriate input parameters for each bank layer at the selected sites for bank-stability modeling. The major controlling processes responsible for bank erosion will be modeled iteratively using the Bank-Stability and Toe-Erosion Model (BSTEM) developed by the ARS, National Sedimentation Laboratory that has been successfully used previously for these purposes in the Mid South and other regions. Simulations for existing bank conditions along the main stem and tributaries will be conducted for each site over a range of annual hydrographs representing the 99th, 95th, 90th, 75th, 50th, and 25th percentile flow year. Discharge data from U.S. Geological Survey (USGS) gauging stations will be converted to flow depths for each significant storm event for input into the toe-erosion sub-model. Combined toe-erosion and mass failure will then be simulated for each event at each site in an iterative fashion. Results will then be extrapolated to the remainder of the channel lengths based on field and aerial reconnaissance of the extent of streambank failures.
The research approach centers on quantifying the hydraulic and geotechnical resistance of streambank sediments for the purpose of simulating streambank erosion rates using the Bank-Stability and Toe-Erosion Model (BSTEM). Field-data collection has been completed at 20 sites in the Mississquoi River Basin. An additional 10 sites will be tested as part of a phase II. The additional sites will be located in the upper part of the watershed that was not included in Phase 1. Geotechnical tests will be conducted in situ within boreholes with a borehole shear-test device (BST). Two BST tests will be conducted within each layer of the studied banks. Measurements of the resistance of bank-surface materials to hydraulic erosion will be made with jet-test devices. A minimum of two jet tests will be conducted for each bank surface, at each site. Whereas bank-stability modeling is being conducted at 20 sites for existing conditions only, BSTEM modeling in Phase 2 will test various mitigation strategies to reduce sediment loadings from the banks. This will be done for all 30 sites. Analysis of a range of alternatives including bank-slope reduction, the addition of vegetation and bank-toe protection will be expressed in terms of potential sediment-load reductions. A cost-basis analysis of these potential measures will also be included based on BSTEM results and cost information obtained from local contractors. All data from the phase I sites (20) have been input into BSTEM and model results have been completed for 25% of those sites. Preliminary modeling results were presented to the cooperator at a symposium in Burlington, VT. Project monitoring is conducted by conference calls and by meetings in the field area.