Enhanced Application of Root-Reinforcement Algorithms for Bank-Stability Modeling

Sediment is one of the principal pollutants of surface waters of the United States and sediment eroded from streambank failures has been found to be the single largest contributor of suspended-sediment to streams draining unstable systems in the mid-continent. Riparian vegetation exerts mechanical and hydrologic controls on bank stability, with plant roots providing mechanical reinforcement to the soil matrix. Root reinforcement is largely a function of the strength of the roots crossing potential shear planes, and the number and diameter of such roots. Root densities vary in time and space and with species, and root tensile strength values have also been shown to vary by species. However, previous bank stability models have been constrained by limited field data pertaining to the architecture and extent of root networks within streambanks. In this paper, a method is developed to use previously published root-architecture data, to derive parameters required for modeling. Results showed that changes in root numbers over time can be estimated using sigmoidal regressions, which commonly represent the growth rates of organisms. The Bank Stability and Toe Erosion Model (BSTEM) was used to simulate the effect of different root distributions, all approximating the same average root-reinforcement over the top 1 m of the bank profile (5 kPa), but with differing vertical distributions (concentrated near surface, non-linear decline with depth, uniform over top meter). The results of these runs showed that the assumed vertical distribution of roots in the top meter of soil was most important in those banks with heights less than or equal to 1 m.