Coarse-grained debris-flows: Hysteresis and time-dependent rheology

Rheometer tests of coarse-grained debris-flow material show hysteresis of yield stress, the degree of which depends on maximum grain size, solids concentration, and normal stress. Coarse-grained debris-flow material did not exhibit well-defined flow curves based on instantaneous data; shear stresses at a given shear rate varied between +/-50-100% of the mean value. Over periods of seconds to minutes this variability is accompanied by more gradual variation of shear stress at constant shear rate. The rheometry of coarse-grained debris-flows resembles that of dense suspensions in its poor precision and reproducibility of viscometric data.