The Role of Riparian Roots in Resisting the Hydraulic Scour of Streambanks

Riparian vegetation interacts in a number of ways with the geotechnical and hydraulic processes operating in streambanks. Previous research has shown that roots provide mechanical reinforcement to soil through the production of root-soil matrix, in which the roots resist tensile forces and strengthen banks against mass failure. In addition, plants have been shown to reduce pore-water pressures, and thus increase the stability of streambanks through removal of water by evapotranspiration. However, to date, very little has been documented about the effect of roots on hydraulic scour at bank toes, which when eroded, greatly increase bank instability. Quantification of the effects of riparian vegetation on streambank stability is important if we are to be able to design channel restoration projects that use biological strategies based on geomorphic effects as well as ecological and aesthetic benefits. In this study a vertical jet-test device was used to measure rates and volumes of scour in soils permeated by switchgrass roots. Twenty tests were carried out over a range of root densities. Root volume per unit volume of soil for the jet tests ranged from 0.0132 to 0.172 cm³<inf arrange="stagger">roots/cm³<inf arrange="stagger">soil. Erosion rates calculated for each jet-test showed stepped profiles for tests with roots (compared to smooth profiles for non root-permeated soils) because the roots protected the soil from erosion, initially at the surface and then at other points in the scour hole where roots were concentrated. Results show 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. The r² for these relationships is fairly high despite the fact that root distributions relative to the shear stress field induced by the jet varied between tests, even under similar root volumes. The shape of the non-linearly declining functions between scoured soil volume and root biomass/root length density/root volume, suggest that a threshold exists at a relatively low root volume or biomass, above which, scour-hole volume was very small.