Experimental assessment of riverbed sediment reinforcement by vegetation roots

Vegetation roots are known to increase soil stability on hillslopes, crops and river banks, thus reducing slide hazards, surface erosion and general processes affecting lateral channel migration. Many studies have tackled this classical engineering problem from either experimental or modelling viewpoints, in general describing the role of roots entanglement of soil particles as an additional cohesion, though non-physical. On river bedforms, the alluvial material on which pioneering plants grow is often non-cohesive. Moreover, both local and non-local erosion processes drive the mechanisms of vegetation uprooting. While the first are influenced by the ratio between non-porous obstacles diameter vs sediment grain size, the second have rather a morphodynamic origin, and are responsible for the onset and migration of alluvial bedforms. In both cases, ablation of particles does not occur as the failure of a volume along preferential sliding planes, but rather gradually when local hydrodynamic bed shear stress conditions exceed some critical value. Hence, classic soil stability models are inadequate to assess the role of roots on stabilizing riverbed sediment, and experimental methods are also not well established. This study proposes an analysis of the data available from the experimental setup of (Pasquale et al. 2011, Pasquale et al. in press), in order to inquire the effect of root reinforcement of river bedforms alluvial material. We compared the amount of sediment scouring recorded inside square plots (4 m(2)) with growing Salix cuttings with that occurring around them and caused by pure morphodynamic erosion, i.e. under the absence of obstacles inducing either deposition or erosion. We show that for those cases where the aforesaid assumptions hold, then bedload transport models can be used in an inverse way in order to explain the reduced erosion in the presence of roots as an increased critical shear stress condition, which might be more useful to morphodynamic modelling purposes. We discuss the accuracy of the proposed methodology in relation to data availability, and propose improvements to design more effective laboratory experiments.