Experimental study on the entrainment of bed material into debris flow

Abstract

The present study describes entraining characteristics of bed material into debris flow, based on flume tests, numerical and dimensional analyses. Flume tests are conducted to investigate influences of bed sediment size on erosion rate by supplying debris flows having unsaturated sediment concentration onto erodible beds. Experimental results show that the relative erosion rate, E/E₀, decreases monotonically with increase of relative sediment size, d/d₀, although E/E₀ changes slightly with sediment concentration of debris flow. Herein, E is the erosion rate of bed sediment of size, d, E₀ is the erosion rate when solid particle size, d₀, of debris flow, are the same of the erodible bed material. According to the relation between E/E₀ and d/d₀, erosion rate, E, can be estimated by using Egashira's formula for E₀. Therefore, the validity of erosion rate formula for E₀ is tested by solving numerically for debris flow characteristics in terms of governing equations. In addition, critical size of bed sediment entrainment is discussed by introducing non-dimensional effective bed shear stress which is formulated by using fluid shear stress (total shear stress minus yield stress), bed sediment size, d, specific weight of sediment particle in water and acceleration due to gravity, and it is found that the critical non-dimensional effective shear stress takes a value similar to critical Shields parameter for bed load movement.