Debris flows: behaviour and hazard assessment

Debris flows are water‐laden masses of soil and fragmented rock that rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form lobate deposits when they spill onto valley floors. Because they have volumetric sediment concentrations that exceed 40 percent, maximum speeds that surpass 10 m/s, and sizes that can range up to ～10⁹ m³, debris flows can denude slopes, bury floodplains, and devastate people and property. Computational models can accurately represent the physics of debris‐flow initiation, motion and deposition by simulating evolution of flow mass and momentum while accounting for interactions of debris’ solid and fluid constituents. The use of physically based models for hazard forecasting can be limited by imprecise knowledge of initial and boundary conditions and material properties, however. Therefore, empirical methods continue to play an important role in debris‐flow hazard assessment.