Empirical Study on the Concentration of Debris Flow Based on the Effects of Fine Particle Sediments

On July 21, 2009, Hofu City in Yamaguchi, Japan recorded a precipitation of 256 mm/day which was the all-time high in the observation record. This rain triggered a disaster of debris flow containing large amount of fine particle sediment at the Tsurugi River located in a granite zone which reached houses and Route 262 at the valley exit and killed two people. Unlike pebble-type debris, the debris flow occurred in this disaster contained fine particle sediment, and high concentration of the debris reached the valley exit. Debris flow containing large amount of fine particle sediment like this is said to result in flows with higher concentration than pebble-type debris flow. Thus, hydraulic model experiments were conducted in this study to observe equilibrium concentration of debris flow in various waterway gradients using the sediment from the collapsed areas at the Tsurugi River which contained large amount of fine particle sediment. The experiment used collapsed sediment collected in the Tsurugi River where large amount of sediment was actually discharged. Using the outcome of these experiments and Takahashi’s equilibrium debris concentration formula as a basis, the authors suggested a method for calculating the concentration of debris flow and the amount of discharged debris which takes account of the effect of fine particle sediment while incorporating the concept of boundary grain size which defines fine particle sediment. This study found that conventional calculations for the amount of discharged debris targeting pebble-type debris flow may underestimate the amount of discharged debris in basins where debris flow containing fine particle sediment might occur.