What Are Debris Avalanches and Debris Flows?
Debris avalanches and debris flows (both popularly called “mudslides”) are shallow landslides, saturated with water, that travel rapidly downslope as muddy slurries. The flowing mud carries rocks, bushes, and other debris as it pours down the slopes. A debris avalanche (Figure 1) is a fast-moving debris flow that travels faster than about 10 mph or approximately 25 yards in about 5 seconds. Speeds in excess of 20 mph are not uncommon, and speeds in excess of 100 mph, although rare, do occur locally. Figure 1. Sketch of a typical debris avalanche scar and track. Although this figure shows the “zone of deposition” as quite near the source, debris avalanches can travel thousands of feet or, in exceptional cases, miles from the point of origin. Original drawing by Janet K. Smith. What Dangers Are Posed by Debris Avalanches? Debris avalanches pose hazards that are often overlooked. Houses in the path of debris avalanches can be severely damaged or demolished. Persons in these structures can
