土石流先端への大岩塊の集中機構

Abstract

Observations of debris flows at the Kamikamihori valley on the eastern slope of Mt. Yake-dake in the Japan Alps show quantitatively that many large boulders would concentrate at thefrontal part of debris flows, and that the size of the largest boulder is nearly equal to the max-imum flow depth.According to a well-circulated hypothesis, this focusing phenomena is due to a combinationof an inverse grading process and higher velocity at the upper layers of a flow. Two types of ex-periments showed that the inverse grading process may occur due to the effects of kinetic sievingand dispersive pressure depending on experimental conditions. However these effects were unex-pectedly small. Furthermore, the effect of upward motion of the largest boulder whose diameteris nearly equal to the flow depth is rather minor.Results of model experiments of debris flow using particle mixtures of three sizes with wateron a sloping channel showed not only a clear focusing of larger particles to a flow front, but alsoa lack of incorporation of large particles into the rear part of the flow. A test comparing thevelocity of particles of three different sizes in a bore water flow showed that larger particleswould soon attain a higher velocity and approached the bore front faster than did smaller parti-cles.A theoretical motion analysis of a boulder in a bore flow on a steep valley slope of upperreach showed that the larger a boulder is, the higher velocity exceeding the bore-front velocitythe boulder attains, so that a larger boulder can catch up with the bore front faster.