火砕流の発生と流動の機構

Abstract

A model is presented for the physical processes that occur in the formation of pyroclasticflows generated by the fluidization of the bed which is composed of essential, degassing, fragmental material. Irn the bed of fine material, fluidization takes place rather easily andthe whole bed including the non-fluidized part which exists beneath the fluidized part willbegin to move downward along the slope. In case the slope angle is steep, it is able to erodethe ground and continue to run down even if degassing is ceased. In the bed of coarsematerial, the rate of degassing may not be plenty enough to form a fluidized bed, and onlyan upper part of the nonfluidized bed will begin to flow. In this case the flow will stop assoon as the degassing ceases.A formula which describes the velocity of the pyroclastic flow is presented. This formulatakes the momentum transfer into account, which is due to both inter-particle collisions andturbulent mixing. The laboratory flume experiments confirmed the validity of the presentedformula.