Bank Stability and Toe Erosion (BSTEM) Modeling of Bank Retreat and Riparian Tree Root Protection in Composite Streambanks

Abstract

Abstract. Streambank retreat can be a significant contributor to total sediment and nutrient loading to streams. Process-based bank stability models, such as the Bank Stability and Toe Erosion Model (BSTEM), have been used to determine critical factors affecting streambank erosion and failure and to estimate retreat rates over time. BSTEM has been successfully applied on a number of cohesive streambanks, but less so on composite banks consisting of both cohesive and noncohesive soils in highly sinuous streams. Composite streambanks can exhibit rapid and episodic bank retreat. The objective of this research was to systematically apply BSTEM to a series of 10 composite streambanks distributed along the Barren Fork Creek in eastern Oklahoma in order to assess model performance. This research also aimed to document the influence of riparian conservation practices on bank retreat rates and evaluated simplistic methods for incorporating such practices into such process-based models. Therefore, sites modeled included historically unprotected sites with no riparian vegetation and historically protected sites with riparian vegetation present during all or part of the 2003 to 2010 study period. With calibration the model was able to match both the observed total amount of retreat as well as the timing of retreat at both the protected and unprotected sites as derived from aerial imagery. During calibration BSTEM was not sensitive to the specific value of the additional soil cohesion added due to roots for the cohesive topsoil layer, suggesting simplified techniques could be used to estimate root cohesion values. Protected sites had less bank retreat but with more variability in retreat rates per year. The BSTEM modeling also provided an advantageous assessment tool for evaluating retreat rates compared to in situ bank retreat measurements due to the magnitude and episodic nature of streambank erosion and failures. Process-based models, such as BSTEM, may be necessary to incrementally model bank retreat in order to quantify long-term bank retreat rates.