The distribution and strength of riparian tree roots in relation to riverbank reinforcement

The main influences of plants on the mass stability of riverbanks are those that affect the strength of bank sediments. Plants enhance bank strength by reducing pore-water pressures and by directly reinforcing bank material with their roots. In this paper we do not consider bank hydrology but focus on quantifying increases in sediment strength due to root reinforcement. Root reinforcement is a function of root strength, interface friction between the roots and the soil, and the distribution of roots within the soil. Field and laboratory studies of Australian riparian trees, river red gum (Eucalyptus camaldulensis) and swamp paperbark (Melaleuca ericifolia), indicate that bank reinforcement, due to the roots of these species, decreases exponentially with depth below the soil surface and distance away from the trees. Differences in the spatial distribution of root reinforcement are illustrated by a comparison of the apparent cohesion due to roots (c(r)) with the effective cohesion of the saturated bank material (15 kPa). Directly below the river red gum, root reinforcement is equal to effective cohesion at 1.7 m depth. At the dripline (17 m from the trunk), root reinforcement is equal to material strength at 0.4 m depth. For the swamp paperbark, c(r) = 15 kPa at 1.1 m depth beneath surface, at the trunks, and at 0.4 m depth at the dripline (2.5 m from the trunk). A description of the spatial distribution of root reinforcement is important in the riverbank context. We find that interspecies differences in the strength of living roots have less significance for bank reinforcement than interspecies differences in root distribution. Copyright (C) 2001 John Wiley & Sons, Ltd.