Uncertainty and sensitivity in a bank stability model: Implications for estimating phosphorus loading

Authors: LAMMERS, RODERICK - Colorado State University; BLEDSOE, BRIAN - Colorado State University; Langendoen, Eddy

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2016
Publication Date: 3/30/2017
Publication URL: https://handle.nal.usda.gov/10113/5763095
Citation: Lammers, R.W., Bledsoe, B.P., Langendoen, E.J. 2017. Uncertainty and sensitivity in a bank stability model: Implications for estimating phosphorus loading. Earth Surface Processes and Landforms. 42(4):612-623. doi:10.1002/esp.4004.

Interpretive Summary: Bank erosion can be a significant source of nutrient loading. Substantial effort has been made to develop mechanistic models to predict bank erosion and instability in stream systems; however, these models do not account for inherent natural variability in input values. In a collaborative study, scientists from the Watershed Physical Processes Research Unit, Oxford, MS and Colorado State University conducted uncertainty and sensitivity analyses using the USDA, ARS Bank Stability and Toe Erosion Model (BSTEM). Generally, bank height, soil cohesion, and plant species were found to be most influential in determining stability of cohesive banks. In addition to these three inputs, groundwater elevation, stream stage, and bank angle were also identified as important in non-cohesive banks. Slope and bank height are the dominant variables in fluvial erosion modeling. In addition, a probabilistic Monte Carlo modeling approach was applied to data from a watershed-scale sediment and phosphorus loading study on the Missisquoi River, Vermont to quantify uncertainty associated with these published results. While our estimates indicated that bank erosion was likely a significant source of sediment and phosphorus to the watershed, the uncertainty associated with these predictions suggests that they should be considered order of magnitude estimates only. These results can help, in general, to develop guidelines for data collection efforts as they identified the important variables, while also indicating which less influential variables may be set to assumed values.

Technical Abstract: Eutrophication of aquatic ecosystems is one of the most pressing water quality concerns in the U.S. and around the world. Bank erosion has been largely overlooked as a source of nutrient loading, despite field studies demonstrating that this source can account for the majority of the total phosphorus load in a watershed. Substantial effort has been made to develop mechanistic models to predict bank erosion and instability in stream systems; however, these models do not account for inherent natural variability in input values. To quantify the impacts of this omission, uncertainty and sensitivity analyses were performed on the Bank Stability and Toe Erosion Model (BSTEM), a mechanistic model developed by the USDA-ARS that simulates both mass wasting and fluvial erosion of streambanks. Generally, bank height, soil cohesion, and plant species were found to be most influential in determining stability of clay (cohesive) banks. In addition to these three inputs, groundwater elevation, stream stage, and bank angle were also identified as important in sand (non-cohesive) banks. Slope and bank height are the dominant variables in fluvial erosion modeling, while erodibility and critical shear stress had low sensitivity indices; however, these indices do not reflect the importance of critical shear stress in determining whether erosion will occur. These results identify those important variables upon which data collection efforts should be focused while also indicating which less influential variables may be set to assumed values. In addition, a probabilistic Monte Carlo modeling approach was applied to data from a watershed-scale sediment and phosphorus loading study on the Missisquoi River, Vermont to quantify uncertainty associated with these published results. While our estimates indicated that bank erosion was likely a significant source of sediment and phosphorus to the watershed, the uncertainty associated with these predictions suggests that they should be considered order of magnitude estimates only.