砂礫州上に繁茂する植生の洪水時流失限界評価

抄録

Trees in rivers sometimes become driftwoods at flood events, and it might cause a disaster. It is important to understand the uprooting condition of the trees by floods. 2D unsteady depth-averaged flow model was applied to the Arakawa River between 76 km to 82 km for evaluating September 2007 flood and October 2006 flood. This model expressed well the peak water levels of the floods. Threshold drag moment and shear stresses for up-rooting the representative vegetation, Salix subfragilis and Robinia pseudoacacia, which grew on the gravel-bed bar in the upstream of the Kumagaya-Ohashi was evaluated according to the result of numerical analysis and the washed-out situation of the plants. Threshold drag moment for tree-uprooting on the gravel-bed was larger compared with the previous experimental results by pulling test. Flood discharge for which all plants on this gravel-bed bar were washed away was evaluated by comparing the calculated shear stress in each discharge condition and the threshold shear stress for uprooting obtained by the analysis.