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

研究了植物及其断裂条件对剪应力降低的影响。首先进行水槽试验。植被模型孔隙度分别为80%和95%。在这些实验中，测量了作用在植被模型上的水深、流速和阻力。利用得到的试验数据，计算了作用在路堤模型边坡上的剪应力。路堤模型边坡剪应力随孔隙率的减小而减小。而当植被模型孔隙度较小时（85%孔隙度），植被模型后侧剪应力急剧增大。这说明保持植被孔隙度对防止堤防侵蚀具有重要意义。另一方面，为了阐明真实树木的断裂条件，以日本河流中的外来和入侵树种——古巴柳（Salix babylonica）和绿枝菊（Juglans ailanthifolia）为研究对象，进行了模拟洪水作用的拔树实验。为了评估树木物理特性和根系构型对最大抗弯弯矩（M_<max>）的影响，对造成的损伤进行了检测。为了测量土的强度参数，进行了土的原位抗剪强度试验。M_<max>与树的性状呈显著相关（p<0.05）的非线性关系，其中H*D_<bh>^2是M_<max>的最佳预测因子。在M_<max>和D_<bh>之间拟合非线性模型。在不同胸径和根体积的情况下，叶仙花的心根体系抵御倾覆的效率较高，而叶仙花的板根体系抵御倾覆的效率较低。在试验范围内，由于根系锚固深度受土壤黏聚力的限制，各树种的平均M_<max>随土壤黏聚力的增加而线性减小。

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.