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

Authors: Bankhead, Natasha; Simon, Andrew

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2008
Publication Date: 7/15/2008
Citation: Pollen-Bankhead, N., Simon, A. 2008. Enhanced Application of Root-Reinforcement Algorithms for Bank-Stability Modeling. Earth Surface Processes and Landforms, DOI: 10.1002/esp.

Interpretive Summary: Sediment is the main pollutant of surface waters of the Unites States, most of which comes from the banks of streams and channels. Streambank vegetation affects the mechanical strength of the soil and also the moisture of the soil. Plant roots increase the strength of the soil by reinforcing the bare soil in the same way that rebar is used to reinforce concrete. The amount of strength added to a soil by roots depends on the number of roots in the soil and their diameters. Root density can change over time and can vary between different locations in a streambank. Different species also have different root densities and the strength of their roots can vary. Previous attempts to model the effect of plant roots on the stability of streambanks have been limited because it is difficult to obtain data showing the number of roots in an area of soil. In this paper field data that has already been collected in other studies was used to model how roots affect the stability of streambanks over a number of years, as roots grow and develop in the soil. A curve was fitted to the data collected to be able to predict the number of roots within a bank after a certain number of years. The bank stability and toe erosion model (BSTEM) was also used to look at how a decline in the number of roots with increasing soil depth affects the stability of a streambank (more roots usually occur near the surface). An average value of 5kPa of root-reinforcement was added to a streambank, but the vertical distribution of roots was varied; one set of model runs used uniform root-reinforcement, one set included more roots near the surface and less roots deeper in the soil, and a third set of runs had roots concentrated only in the top layer of the soil with none in the lower layers of soil. The results of the model runs showed that the differences between the three sets of runs was greatest for banks that were less than or equal to 1m in height.

Technical Abstract: 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. Plant roots provide mechanical reinforcement to a soil matrix due to the different responses of soils and roots to stress. 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 root-architecture data obtained previously published, 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.