Viscoplastic Bingham fluid — yield stress, surge dynamics, and deposition
Debris flows behave as Bingham plastics: they have a yield stress (τ_y) below which they move as a plug, and above which they flow with viscosity μ. This creates a characteristic snout morphology — a blunt, lobate front — and means they deposit abruptly when gravitational driving stress drops below τ_y. The ratio of inertial to viscous forces (Bingham number) controls whether the flow surges rapidly or creeps. Debris flows carry boulders in suspension and travel faster than their water content alone would suggest, making them extremely hazardous.