Location: Watershed Physical Processes Research2012 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.
3. Progress Report:
At its mouth the Missisquoi River has a history of exceedance of phosphorus concentration target levels endorsed by the governments of Vermont, Québec, and New York. Observations along the Missisquoi River have indicated that the river’s streambanks could be a significant source of the suspended sediment and hence phosphorus to Missisquoi Bay. Over the three years of this project we have investigated the extent of streambank erosion along portions of the Missisquoi River and several of its tributaries located in the United States, quantified the loadings of sediment and associated phosphorus emanating from streambanks, and evaluated mitigation scenarios to stabilize the streambanks. We found that significant portions of the studied reaches have greater than 50% of their banks failing. The average percent of reach failing increases nearly linearly with river kilometer from about 15% to about 70% at 140 km upstream of Missisquoi Bay. We carried out bank stability and toe erosion analyses using the model BSTEM at 27 study sites along the study reach using a 30-year, historic flow record. Predicted volumes of sediment eroded from the streambanks at each site ranged from 0 to 53.5 m3 of sediment per one meter reach under existing conditions with a median value of 11.5 m3/m and an interquartile range (IQR) of 30.3 m3/m. Contributions of sediment from streambank erosion along the U.S. study reaches of the Missisquoi River were found to be about 36% (31,600 t/yr) of the total suspended-sediment load entering Missisquoi Bay. Maximum associated phosphorus loadings of up to 1,540 kg/yr/km were calculated in the lower portion of Tyler Branch and the confluence of the Missisquoi and Trout Rivers. The median calculated phosphorus loading was 41.7 kg/yr/km and the IQR was 312 kg/yr/km. The calculated streambank erosion volumes contributed about 36% (52.0 t/yr/km) to the total phosphorus load entering Missisquoi Bay. We analyzed three mitigation scenarios to determine the percent reduction in loadings that can be obtained by stabilizing streambanks: mitigation scenarios 1) 25 year old mature trees on the bank top; 2) banks graded to a 2:1 (horizontal:vertical) slope; and 3) banks graded to a 2:1 slope in combination with 5-year old vegetative treatment on bank top and face. Mitigation scenarios 1 and 2 provided similar percent reductions in sediment loadings. Median erosion volumes were reduced by 93% for scenarios 1 and 2. The calculated reductions in eroded bank material volumes were greatest for scenario 3. The median value was reduced by 100%. Contributions of sediment from streambank erosion along the U.S. study reaches of the Missisquoi River to the total suspended-sediment load entering Missisquoi Bay were reduced by scenarios: 1) 21%, 2) 14%, and 3) 86%. The mitigation scenarios have a similar impact on phosphorus loadings as they have on sediment loadings, since phosphorus loadings are directly related to bank material loadings. Reductions in phosphorus loadings provided by mitigation scenarios 1 and 2 were moderate, whereas scenario 3 provided a significant reduction. The median value of calculated phosphorus loadings was reduced by 54% for scenario 1, 34% for scenario 2, and 97% for scenario 3. The calculated contribution to the total phosphorus load into Missisquoi Bay is reduced by 30% for scenario 1, 10% for scenario 2, and 82% for scenario 3. The overall impact of the accomplishments is that the governments of the states of Vermont and New York and the government of the province of Québec have new information on which to make decisions concerning streambank protection to minimize delivery of fine-grained sediments and phosphorus to Missisquoi Bay and Lake Champlain in order to improve water quality.