Quantifying and Comparing the Relative Effects of Riparian Root Networks on the Geotechnical, Hydrologic and Hydraulic Processes Acting on a Streambank

Riparian vegetation can both positively and negatively affect streambank stability. Previous research has shown that the effect of mechanical root-reinforcement on soil stability can be considerable, and can be successfully quantified and included in streambank stability models. However, root networks contained within a soil-matrix also have effects on the hydrologic and hydraulic processes acting on a streambank. Although these effects are often discussed they have generally been difficult to quantify. The work presented here summarizes the results of fieldwork, laboratory testing and computer simulations carried out to better quantify the effects of riparian vegetation on hydrologic and hydraulic processes occurring along streambanks. First, the evapotranspiration potentials of different riparian species were isolated by setting up an experiment to grow young riparian trees and switch grass in separate soil columns instrumented with tensiometers. The hydrological reinforcement provided to the soil from increased apparent cohesion as a result of enhanced matric suction was estimated to range from 1.0 to 3.1 kPa in spring when bank stability was most critical and up to a maximum of 5.0 kPa in the summer. Second, a vertical jet-test device was used to measure rates and volumes of scour in soils permeated by switch grass roots. Calculation of relative soil detachment rates (RSD) showed that with the highest rooting densities measured in the field jet-tests, eroded soil volume was 10 % of that in the tests with no roots. Third, the effects of enhanced matric suction due to evapotranspiration, and decreased soil erodibility because of the presence of plant roots were modeled using BSTEM 5.1 to quantify their effects on streambank factor of safety (Fs), and to compare with the effects of mechanical root-reinforcement. A sensitivity analysis showed that the change in soil matric suction due to evapotranspiration provided the greatest potential benefit to Fs but only during the summer months. During the winter and spring months, root-reinforcement remained the most important contributor to Fs. The sensitivity analysis conducted here also showed that whilst roots are capable of reducing the volume of hydraulic scour, the resulting effect on streambank geometry did not increase Fs as much as changes in soil matric suction and/or mechanical root-reinforcement.