A lahar is a natural hazard involving:

Concept: Natural hazards are classified by the material that moves and the mechanism that drives it. A volcanic mass flow is a gravity-driven mixture of water with loose volcanic ash, rock fragments, and other debris; it often travels rapidly along valleys and stream channels.

Application: The term “lahar” denotes this water-saturated volcanic-debris flow. Therefore, the description “a fast-moving flow of water mixed with volcanic ash and rock debris” identifies the phenomenon precisely.

• Destructive high-velocity winds describe a windstorm or cyclone mechanism, in which moving air is the principal hazard.

• Large ocean waves caused by sudden water displacement describe a tsunami mechanism, in which a displaced water column propagates through the sea.

• A simultaneous episode of high winds and large water waves can occur during a severe coastal storm, but it is not a volcanic debris-flow mechanism.

Cross-check: Replace the hazard name with its defining moving material. Only the water–volcanic-debris description preserves both the volcanic source and the gravity-driven flow, so the result is a fast-moving flow of water mixed with volcanic ash and rock debris.