Experimental Study of the Grain‐Flow, Fluid‐Mud Transition in Debris Flows

We have performed a series of laboratory experiments that clarify the nature of the transition between fluid‐mud and grain‐flow behavior. The surface velocity structure and the speed of the nose of debris flows in channels with semicircular cross sections were measured with several cameras and visual tracers, while the mass flow rate was recorded using a load cell at the exit chamber. Other rheological tests were used to calculate independently the yield strength and matrix viscosity of the debris‐flow mixture. Shear rates were varied by nearly an order of magnitude for each mixture by changing the channel radius and slope. Shear rates were significantly higher than expected (6–55 s−1), given the modest slopes examined (10.7°–15.2°). The large values were primarily a result of the concentration of shear into narrow bands between a central nondeforming plug and the sidewall. As a result, the shear rate of interest was calculated by using the width of the shear band and the plug velocity, as opposed to the flow depth and front velocity. The slurries exhibited predominantly fluid‐mud behavior with finite yield strength and shear‐thinning rheologies in the debris‐flow body, while frictional behavior was often observed at the front, or snout. The addition of sand or small amounts of clay tended to make the body of the flows behave in a more Bingham‐like fashion (i.e., closer to a linear viscous flow for shear stresses exceeding the yield stress). The addition of sand also tended to accentuate the frictional behavior at the snout. Transition to frictional grain‐flow behavior occurred first at the front, for body friction numbers on the order of 100. Similar behavior has been observed in an allied field site in the Italian Alps. In the experiments, it was hypothesized that the snout‐grain‐flow transition was a result of concentration of the coarsest material at the flow front, reduced shear near the snout, and loss of matrix from the snout to the bed. Regardless of the frictional effects at the snout, flow resistance in the body was nearly always regulated by yield‐stress and shear‐thinning properties, with no discernible boundary slip, despite volumetric sand contents in excess of 50%.