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Title: Enhancements of a Bank-Stability and Toe-Erosion Model and the Addition of Improved Mechanical Root-Reinforcement Algorithms

item Bankhead, Natasha
item Simon, Andrew
item Langendoen, Eddy

Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2007
Publication Date: 5/15/2007
Citation: Pollen, N.L., Simon, A., Langendoen, E.J. 2007. Enhancements of a bank-stability and toe-erosion model and the addition of improved mechanical root-reinforcement algorithms. World Environmental and Water Resources Congress, May 15-19, 2007, Tampa, FL. 11 pages.

Interpretive Summary: Sediment eroded from streambanks is the largest source of suspended-sediment in unstable streams of the mid-continent of the USA. With many stream restoration projects underway to remedy this, a method was needed to be able to predict conditions under which streambanks are unstable and stable, and what effect vegetation has on bank stability. A bank stability model was developed in the late 1990s at the USDA, ARS, National Sedimentation Laboratory, and this has undergone many changes since then. The original model allowed the user to enter up to 5 different soil layers within the streambank, accounted for the saturated and unsaturated parts of the bank, and took into account the flow of the stream in the channel. The most recent version (BSTEM 5.0) also includes a calculation of erosion at the base of the bank during flow events of different sizes, and allows for failure by undercutting (Cantilever) or planar failure. The vegetation growing on the banks is also taken into account in the model using a root-reinforcement model called RipRoot, which requires the user to enter the species, their age and an indication of root density (Sparse, medium or dense). Common species found on streambanks of the USA are included in the model and different combinations of species can also be taken into account. The linking of the BSTEM and RipRoot models allows improved predictions of bank stability under different conditions.

Technical Abstract: Sediment eroded from streambank failures has been found to be the single largest contributor to suspended-sediment loads to streams draining unstable systems in the mid-continent. With the recent focus on stream restoration, a quantitative means was needed to predict critical conditions for stability and the effects of riparian vegetation on attaining stable bank geometries. A deterministic bank-stability model was developed in the late 1990s at the USDA-ARS National Sedimentation Laboratory and has undergone substantial enhancements since that time. The original model allowed for 5 unique layers, accounted for pore-water pressures on both the saturated and unsaturated parts of the failure plane, and the confining pressure from streamflow. The enhanced Bank Stability and Toe Erosion Model (BSTEM Version 5.0) includes a sub-model to predict bank-toe erosion and undercutting by hydraulic shear, and allows the user to select between cantilever or planar failure modes. Riparian vegetation is now accounted for by adding cohesion due to individual layers due to the mechanical reinforcement of the soil by plant roots. The BSTEM (Version 5.0) includes a direct link to an improved root-reinforcement model (RipRoot), where users enter an approximate root density. Individual species or assemblages of common North American riparian species, each with differing root densities can be accounted for. Estimates of root-reinforcement are then input directly into BSTEM 5.0 for use in the stability algorithms. The integration of these two models represents a significant advance in the way that riparian vegetation is accounted for in bank-stability analyses.