THE LEGACY OF A CHRONIC HAZARD-- SEDIMENT REDISTRIBUTION FOLLOWING EXPLOSIVE ERUPTIONS

Transient hazards associated directly with explosive eruptions are extremely serious and appropriately receive focused attention. However, chronic hazards related to sediment redistribution after an eruption can be equally devastating, yet have received relatively scant attention. Posteruption sediment redistribution commonly is the most enduring problem associated with explosive volcanic activity, and it can cause environmental, social, and economic harm that equals or exceeds that caused directly by an eruption. Following an eruption, rivers adjust to changed hydrologic and hydraulic conditions and redistribute sediment from sources to sinks. Sources may include older sediment stored in valleys as well as that newly deposited. Sediment redistribution typically triggers geomorphic instability, reduces channel navigability, and reduces channel conveyance capacity which increases flood risk. Geomorphic responses to recent eruptions worldwide suggest that sediment redistribution is initially dominated by lahars triggered by rainfall, snowmelt, or breaching of impounded waters. In addition to the effects noted above, secondary lahars can overwhelm mitigation structures designed to control sediment migration, and they can inundate areas initially unaffected by an eruption. Fluvial redistribution of sediment can be equally detrimental as enhanced sediment yields can linger for years to decades after an eruption. At Mount St. Helens, for example, suspended-sediment yields remain 10 to 100 times above pre-1980 background levels in some basins.

Costs to mitigate posteruption hydrologic hazards and associated sedimentation can tax resources in developed countries, and generally are overwhelming for developing countries. In the aftermath of the 1980 Mount St. Helens eruption, mitigation costs exceeded $300 million from 1980-1983 alone; sediment redistribution ultimately commanded construction of an ~$250 million retention dam to inhibit downstream sediment migration. Land managers and emergency planners should anticipate persistent geomorphic problems following explosive eruptions.