Effects of local scouring and saturation of soil due to flooding on maximum resistive bending moment for overturning Robinia pseudoacacia

摘要

The effects on Robinia pseudoacacia (an exotic and invasive plant in Japanese rivers) of local scouring and saturation of the soil in the root-anchoring zone due to flooding were investigated. Scouring has been defined as the removal of substrate in the root-anchoring zone, exposing the tree roots. Tree-pulling experiments were conducted, simulating flood action, and the resulting damage was examined in order to assess the effect of local scouring on the maximum resistive bending moment (M_(max)) for over- turning. Scouring was artificially created to three different depths, 0, 25, and 50 cm. A nonlinear model was developed that included soil strength characteristics to calculate the critical overturning moment (M_(cri)) under dry and saturated soil conditions. Significant correlations (p < 0.05) of M_(max) with different tree and root-soil plate characteristics, such as diameter at breast height (D_(bh)), tree weight, root depth, and root-soil plate radius, were developed in order to elucidate the effects of scouring on M_(max). M_(max) was slightly reduced with scouring depth for trees with D_(bh) <10 cm (small) trunks, and it was significantly and negatively (p < 0.05) correlated with scouring depth for trees with 10 < D_(bh) < 20 cm (medium) trunks. However, M_(max) did not change significantly with scouring depth for trees with a D_(bh) >20 cm (big) trunks. The nonlinear model was useful for determining the M_(cri) of R. pseudoacacia under dry and saturated soil conditions. The overturning moments of all (small, medium, and big) trees were considerably reduced under the saturated soil condition. It could be concluded that medium-sized trees were greatly affected by scouring, and that small and big trees were mainly affected by saturation of the soil under severe flooding conditions.