Cabot Institute seminar: Hydrologic and geomorphological risk analysis after the 1980 Mt. St Helens eruption
Professor Dunne is one of the world’s leading fluvial geomorphologists with unparalleled expertise in field and theoretical studies of large river systems.
The eruption of Mt. St. Helens on May 18, 1980 caused extensive flooding and sedimentation, which have been documented by others (U. S. Geological Survey, 1981). In the following winters there was a possibility of new flood and sedimentation hazards in the Toutle and Cowlitz River valleys arising from landscape alterations caused by the eruption. There was considerable uncertainty about the nature and magnitude of these hazards, and large discrepancies existed between the predictions made by various agencies and individuals. Yet it was necessary to assess the hazards so that urgent decisions could be made about preventative and remedial actions or evacuation. It was necessary for scientists to make estimates before they had what would usually be regarded as sufficient data.
The following hazards were the major subjects of debate:
- Amplification of rain and snowmelt floods;
- Debris flows generated by pyroclastic flows over a thick snowpack;
- Catastrophic breaching of debris dams emplaced across valleys during the main eruption;
- High rates of erosion and sediment deposition that reduced the flood capacity of river channels.
These hydrologic and geomorphic hazards need to be incorporated into volcanic risk analyses on fragmental volcanoes through a combination of model development and field studies.