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 debris flows, which can be an extremely hazardous hydrologic process commonly associated with volcanic eruptions, particularly at snow-clad stratovolcanoes. To provide a realistic basis for modelling the behavior of large (> 1000 m(3)/s) debris flows, this paper summarizes kinematic, volumetric and hydraulic characteristics of ten large historic volcanic debris flows from four different volcanoes. Although debris flows larger than these are known to have occurred in the past, the ten summarized here define a practical upper range in magnitude of more typical flows to be considered for future hazard prediction. Peak flow velocities 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 locally 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 up 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 flows. Flows generally achieved supercritical flow and deposited minimal volumes of sediment on gradients steeper than 0.02 m/m. Subcritical flow and active deposition predominated on gradients less than 0.01 m/m, although flows travelled tens of kilometers on such low gradients while laying down deposits. Total distances travelled (as debris flows) were as far as 120 km.