Effect of flood defence trees on control of overflow and its threshold breaking condition.

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The research was conducted to elucidate the effect of shear stress reduction due to plant and its breaking condition. Firstly, flume experiments were conducted. Porosities of vegetation model were selected 80% and 95%. In these experiments, water depth, velocity and drag force acting on vegetation model were measured. By using obtained experimental data, shear stress acting on slope of embankment model was calculated. Shear stress on slope of embankment model decreased with decreasing porosity. However, when porosity of vegetation model is small (85% porosity case), shear stress at rear side of vegetation model increased drastically. This indicates that to maintain the porosity of vegetation is very important for preventing the levee erosion.On the other hand, for elucidating breaking condition for real tree, tree pulling experiments that simulated flood action were conducted using Salix babylonica and Juglans ailanthifolia, exotic and invasive trees in Japanese rivers. The resulting damage was examined in order to assess the effects of physical tree characteristics and root architecture on the maximum resistive bending moment (M_<max>) for overturning. In situ soil shear strength tests were conducted in order to measure soil strength parameters. Significantly correlated (p<0.05) non-linear relationships were found between M_<max> and tree characteristics, and H*D_<bh>^2 provided the best predictor of the M_<max> within them. Non-linear models were fitted between M_<max> and D_<bh> for each species. The more efficient root architecture to withstand overturning is the heart-root system of S. babylonica, and the plate-root system of J. ailanthifolia is less efficient not with the same breast height diameter but with the same root volume. The average M_<max> of S. babylonica for overturning each species decreased linearly with increasing soil cohesion within the experimental range because root anchoring depth is restricted with increasing the soil cohesion.