Effect of erosion inhibit by root system of trees and criterion for falling of trees by river bed erosion in riparian zones

Abstract

The trees in the riparian zone would fall at the time of floods and those become the driftwoods and cause disasters. The cause of falling of the trees in the riparian zones could be regarded as erosion of the base materials of the trees. In this study, a field survey was performed to clarify the falling limit of the trees by erosion of the base materials. The tractive force at the time of the recent flood was analyzed and the grain size distribution of the materials under the riparian trees was measured. The maximum diameter of the base materials of the tree (d₉₅) was smaller than diameter of stones (d_τmax) that correspond to tractive force at the time of the flood, and it was thought that the root system of the tree inhibited the erosion. As a result of the analysis, it was revealed that, at the base of survive tree, as smaller the root system interval become, the ratio of d_τmax to d₉₅ becomes larger.