LESSONS FROM ONTAKE-SAN - A COMPARATIVE-ANALYSIS OF DEBRIS AVALANCHE DYNAMICS

Dynamic aspects of the long runout Ontake-san debris avalanche are eva luated by a comparison of several models. An unsteady numerical model assumes two-dimensional flow of an incompressible biviscous or Newtoni an fluid, represented as a continuum with a free surface. Internal def ormation of the flowing mass is considered, as well as boundary resist ances. Thus flow thinning and deposit shape as well as flow kinematics may be modeled. Parameters are adjusted to match observed runout, wit h additional constraints on velocity and emplacement time. With abunda nt constraints for Ontake-san, from careful field investigations by Ja panese research teams, our analysis indicates that a substantial decre ase in flow resistance occurred as a function of displacement. Constan t-property models that match runout tend to overestimate the peak velo cities and to underestimate the emplacement times. A staged increase i n mobility in both constant volume and variable volume models leads to results consistent with field data. Runout in a channel overflow area was also modeled. Qualitatively similar results have been obtained by other researchers using simple sliding block models with empirical pa rameters, a slide block model with rational parameter selection, a mod ified flood simulation, and a multi-element frictional slide model. Th e relative merits of these models are compared. The field mechanisms a ssociated with this mobility increase with displacement are poorly und erstood, but the question is now identified as a target for future res earch at debris avalanche sites, and some plausible mechanisms are con sidered. The main reason probably involves the entrainment of river wa ter and saturated sediment, leading to enhanced efficiency of fluid pr essure mechanisms with undrained shear; in addition, progressive shear ing reduced the mean particle size and angularity, and the cohesion an d friction (and apparent viscosity) of avalanche debris near the wette d perimeter. Hydroplaning - the shearing of water films and slurries - may have occurred locally.