Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical root-reinforcement the whole story?

Abstract

Riparian vegetation has a number of effects on the mechanisms by which streambanks fail, some positive and some negative. 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. Root networks contained within a soil-matrix, however, also have effects on the hydrologic and hydraulic processes acting on a streambank, and although these effects are often discussed they have generally been difficult to quantify. This paper summarizes the results of field data collection, 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, each instrumented with tensiometers at 30 cm and 70 cm depths, and compared against bare control columns. 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. Results showed that the volume of soil scoured during a test declined non-linearly with increasing root volume, per unit volume of soil, and with increasing root length density (RLD) and increasing root biomass. 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 from evapotranspiration, and decreased soil erodibility because of the presence of plant roots were modeled using BSTEM 5.1 to quantify the effects on streambank factor of safety ( F s), and to compare with the effects of mechanical root-reinforcement. The sensitivity analysis showed that the change in soil matric suction from evapotranspiration provided the greatest potential benefit to F s but only during the summer months. During the winter and spring months, root-reinforcement remained the most important contributor to F s. 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 F s as much as changes in soil matric suction and/or mechanical root-reinforcement.