Remobilization of Ash by Water

When volcanic ash mixes with sufficient water it can be eroded, transported and later deposited downslope. When moving, it can look like a mass of wet concrete. Such flows are known as secondary lahars, which can vary in size and speed. These flows can deposit ash and other debris over large areas requiring clean up.

Hundreds of lahars swept down from nearby Unzen Volcano in Japan buried, crushed, or carried away more than a thousand homes along the Mizunashi River. Between August 1992 and July 1993, lahars triggered by heavy rains damaged about 1,300 houses and required considerable clean up of the deposited sediment. Each period of heavy rain required the sudden evacuation of several thousand residents along two rivers heading on the volcano.