Experimental studies on debris flow with logs focusing on specific weight difference of log species

Abstract

There are many experimental approaches, field investigations and numerical calculations for movements of woods in a clear water and debris flow. However, kinematic conditions for accumulated logs and the interactions between a main flow and logs have not been fully evaluated. Mitigations for woods need taking into account the characteristics of tree species such as conifer and broad-leaf trees and of shapes such as root swells and crown. In the present study, we focus on the differences in specific weight of conifer and broad-leaf trees with some moisture in a sediment-water mixture flow with narrow flow width, and consider that conifer and broad-leaf tree are floating and submerged solid phase, respectively. Flume tests are conducted in steady flow of clear and debris flow over a rigid bed in order to evaluate conifer and broad-leaf tree movement in clear water and debris flow. Experimental data indicates that dimensionless transverse diffusion coefficient can be 0.1 to 0.4 and 0.3 to 0.9 in flow direction. Those diffusive characteristics seem to be independent of Reynolds number and Froude number, but dependent of bed slope, i.e., gravity, though detailed considerations are needed to discuss about flow characteristics such as spatial eddy structures, momentum transfer induced by interactions of logs and so on.