LANDSLIDE-GENERATED TSUNAMIS IN COASTAL EMBAYMENTS AND FJORDS

Slope failures in high-relief coastal areas account for many regionally devastating tsunamis. Numerous examples from Norway, Alaska, British Columbia, Greenland, and Chile show that considerable loss of life and property have resulted when coastal communities have been inundated by waves up to tens of meters in height generated by subaqueous or subaerial failures. We have carried out three-dimensional modeling of both types of tsunami in British Columbia and Alaska with sources ranging from block slides through visco-plastic (Bingham) debris flows to viscous flows. Modeling results are generally in good agreement with observation despite the many uncertainties, especially with respect to the triggering failure. These uncertainties include the pre-failure seafloor morphology and initial volume (vs. total failure volume), and rate and style of failure. While the physics of wave generation and propagation are well known, there is often uncertainty with respect to the conversion of gravitational energy to wave energy related to the source of the tsunami (subaerial or subaqueous), location of the failure mass on the slope (for subaqueous slides), slide volume, and rate of failure.

While most assessments of tsunami impact assume that the largest waves are the ones arriving directly from the failure, numerical modeling in a fjord in northern British Columbia has shown that, at certain coastal sites, the largest waves are associated with edge waves that have propagated along the coastline at a much slower velocity than the direct waves. This points to the need to conduct numerical simulations over an appropriate distance from the source region to identify and distinguish between direct, reflected, and edge wave effects.

Assessment of landslide-generated tsunami hazards during coastal infrastructure planning is rarely undertaken. It is recommended that a progressive approach -- similar to that being developed for seismically generated tsunamis in the open ocean -- be undertaken involving searching for historical or anecdotal accounts of past events, physical evidence of slides along coastlines or subaqueous slopes, bathymetric and geophysical investigations of seabed morphology and sediment thicknesses, geotechnical assessment, and numerical modeling of tsunamis for a suite of potential failure scenarios.