Experimental Studies on Debris Flow with Logs Focusing on Specific Weight Difference of Log Species

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.

Development and installation of bedload monitoring systems with submerged load cells

Bedload governs riverbed channel variations and morphology,it is necessary to determine bedload discharge through an arbitrary cross section in a mountain river.A new system with submerged load cells has been developed to directly measure bedload discharge.The system consists of:(1)an iron box which is 1 m long,0.5 m wide and 0.1 m in depth,(2) two submerged load cells 0.7 m apart,(3) a pressure sensor and,(4) an electromagnetic velocity meter.This system has been designed to exclude the effect of the hydraulic pressure of water on direct measurements of bedload particle weight.Initial tests in a laboratory were conducted to examine the accuracy of measurements with the system under aerial conditions.The system has been installed in the supercritical flume in Ashi-arai-dani River of the Hodaka Sedimentation Observatory of the Disaster Prevention Research Institute(DPRI) of KyotoUniversity to obtain bedload discharge under natural conditions.Flume tests were conducted in this channel by artificial supply of uniform sediment particles of several grain sizes.The average velocity of the sediment particles near the bed was estimated using cross-correlation functions for weight waves obtained by the two load cells.Bedload discharge calculations were based on time integration of the product of sediment velocity and sediment weight obtained by the two load cells.This study clarifies the reasons why bedload measurements are difficult,and provides some solutions using the monitoring systems with submerged load cells through the field measurements.Additionally,the applicability of bedload measurement with the submerged load cells is explained based on experimental artificial sediment supply data.