|Thomas, Robert -|
Submitted to: American Geophysical Union
Publication Type: Book / Chapter
Publication Acceptance Date: August 6, 2010
Publication Date: July 1, 2011
Citation: Simon, A., Bankhead, N.L., Thomas, R.E. 2011. Development and application of a deterministic bank-stability and toe-erosion model (BSTEM) for stream restoration. In: Stream Restoration in Dynamnics Fluvial Systems: Scientific Approaches, Analyses and Tools. American Geophysical Union. Geophysical Monograph Series. 194:500. Interpretive Summary: The Bank-Stability and Toe Erosion Model (BSTEM) is a simple tool that can be used to estimate the erosion of streambanks. The model has been used for a number of rivers, and has been used to investigate what conditions would cause a bank to fail, and what can be done to that streambank to stop it failing in the future. The model has also been used to see what would happen to a streambank after a sequence of storms. The BSTEM model can also include vegetation on the streambanks to see if trees, grasses and shrubs would have a reinforcing effect on the bank. The results obtained from BSTEM can be used to estimate the number of times a streambank might fail in a certain period of time. It can also provide an estimate of the amount of bank material that is likely to be eroded and end up in the channel. If the results are used together with stream surveys that provide an indication of recent stability along the length of the channel, the amount of sediment entering the channel from the streambanks over a period of time can be calculated. Results of studies carried out on different rivers in the USA have shown that the amount of sediment coming from the banks can vary greatly. In the agricultural watershed of the Big Sioux River, SD, only 10 % of the sediment in the channel is likely to be coming from the streambanks. But in the steep forested watershed of the Lake Tahoe Basin, CA, up to 50% of the sediment in the channels likely comes from the streambanks. For the rivers studied the model results have shown that if the base of the bank can be protected in some way, for example using riprap, the erosion of the streambank can be reduced greatly, by 85 – 100 %.
Technical Abstract: The Bank-Stability and Toe Erosion Model (BSTEM) is a simple spreadsheet tool to simulate streambank erosion in a completely mechanistic framework. It has been successfully used in a range of alluvial environments in both static mode to simulate bank-stability conditions and design of streambank stabilization measures, and iteratively over a series of hydrographs to evaluate surficial, hydraulic erosion, bank failure frequency and thus, the volume of sediment eroded from a bank over a given period of time. In combination with the sub-model RipRoot, the reinforcing effects of riparian vegetation can be quantified and included in analysis of mitigation strategies. In addition, the model has been shown to be very useful in testing the effect of potential mitigation measures that might be used to reduce the frequency of bank instability and decrease sediment loadings emanating from streambanks. Finally, the results of iterative BSTEM analysis can be used to spatially extrapolate bank-derived volumes of sediment from individual sites to entire reaches when used in conjunction with Rapid Geomorphic Assessments (RGA’s) conducted at regular intervals along the study reach. Results of these case studies have shown that the relative contribution of suspended sediment from streambanks can vary considerably, ranging from as low as 10% in the predominantly low gradient, agricultural watershed of the Big Sioux River, SD to more than 50% in two steep, forested watershed of the Lake Tahoe Basin, CA. Modeling of streambank mitigation strategies has also shown that the addition of toe protection to eroding streambanks can reduce overall volumes of eroded sediment up to 85-100%, notwithstanding that hydraulic erosion of the toe in this particular case makes up only 15-20% of total bank erosion.