FLOW CHARACTERISTICS OF LARGE ERUPTION-TRIGGERED DEBRIS FLOWS AT SNOW-CLAD VOLCANOS - CONSTRAINTS FOR DEBRIS-FLOW MODELS

Theoretical modelling is not yet adequate to predict the behavior of d ebris flows, which can be an extremely hazardous hydrologic process co mmonly associated with volcanic eruptions, particularly at snow-clad s tratovolcanoes. To provide a realistic basis for modelling the behavio r of large (> 1000 m(3)/s) debris flows, this paper summarizes kinemat ic, volumetric and hydraulic characteristics of ten large historic vol canic debris flows from four different volcanoes. Although debris flow s larger than these are known to have occurred in the past, the ten su mmarized here define a practical upper range in magnitude of more typi cal flows to be considered for future hazard prediction. Peak flow vel ocities of the ten debris flows studied were indirectly measured to be between about 5 and 20 m/s on gradients of 0.005 to 0.25 m/m, but loc ally they were as great as 40 m/s. Hydraulic (average) depths were as great as 25 m, but were more commonly between 5 and 15 m in channels u p to 400 m wide. Computed peak discharges (volumetric flow rates) were as high as 10(5) m(3)/s, and total volumes were as much as 10(8) m(3) . Total flow volumes increased by as much as four times in relatively steep channels as eroded sediment was incorporated into the debris flo ws. Flows generally achieved supercritical flow and deposited minimal volumes of sediment on gradients steeper than 0.02 m/m. Subcritical fl ow and active deposition predominated on gradients less than 0.01 m/m, although flows travelled tens of kilometers on such low gradients whi le laying down deposits. Total distances travelled (as debris flows) w ere as far as 120 km.