土石流における岩屑の粒度偏析過程 その2 -1985年7月焼岳上々堀沢の土石流における粒度偏析と, 剪断による粒度偏析の実験-

Abstract

Debris flow brings about a larger damage due to its distinctive accumulation of largeboulders in the flow front rather than a muddy debris-flow motion without such an accumu-lation. In 1985, we got a set of data of a typical stony debris flow at our observation siteon Mt. Yakedake, Japan. The examination of the relationship between the segregation ofdebris in that flow and the hydraulic factors of the flow showed us the following facts. Thefrontal part of the main flow, with a swollen bore shape owing to the remarkable accumula-tion of large boulders, took rather low surface velocity in comparison with the rear part.After the rear part of the main flow with lower concentration of boulders and with a littlesmaller flow depth took a maximum value of surface velocity, the hydraulic values in the rearpart of the flow decreased gradually. The evaluation of the change in size segregation ofdebris in the flow clarified that the falling down of a flow from a high dam did not destroythe accumulation structure in the frontal part, and after that the degree of the accumulationstate decreased slightly in the lower gentle channel.Accumulation of large boulders in the flow front can be ascribed to the size segregationwithin the flow plus the addition of large boulders immiscible with the main flow. The formeris known to occur with inverse grading and higher flow velocity at the upper layers. Theeffect of dispersive pressure, kinetic sieving and collisions of large boulders with the bottomsurface are considered to promote an inverse grading process. The second effect, the kineticsieving was studied experimentally by shearing a particle mixture. This test showed us thathigh concentration of large particle produced inverse grading, and low concentration of thatdid normal grading as a result of the active valance between the density segregation effectand the kinetic sieving effect.