On Numerical Simulation of the Landslide-Generated Tsunami of November 3, 1994 in Skagway Harbor, Alaska

A three-dimensional, shallow-water numerical model for a viscous landslide with full slide-wave interaction (Kulikov et al.; 1996; Fine et al., 1998) has been modified to include the subaerial component of the landslide. The model is used to simulate the November 3, 1994 tsunami in Skagway, Alaska generated by collapse of the PARN Dock. Results show that the dock slide moved down the steep (30–35°) slope of Taiya Inlet and was guided along the trough at the base of the slope, consistent with geomorphological findings. The leading tsunami wave, propagating in front of the advancing slide, impacted the Alaska State Ferry Terminal and the NOAA tide gauge site as a positive wave (crest), consistent with the tide gauge record and with the results of laboratory modelling by Raichlen et al. (1996). Computed wave heights for the PARN Dock failure (13 m at the Ferry Terminal, 7.7 m at the tide gauge site, and 1.3 in the Small Boat Harbor) agree closely with the tide gauge record and eyewitness accounts. The computed 3.0 min period for the fundamental long-wave mode for Skagway Harbor is nearly identical to the observed period. Estimates of the Q-factor (Q≈24) are comparable to observed values (Q≈21), suggesting significant tsunami energy retention in the harbour. Energy loss appears to be through radiation damping rather than from frictional effects. A detailed examination of the slide motion and associated tsunami waves in the vicinity of the PARN Dock reveals that, in the first few seconds, a “wall of water” would have formed opposite the dock and that the floating Ferry Terminal would have been impacted 15 to 20 s after onset of the event, consistent with eyewitness accounts. The floating debris observed at the still-standing northern portion of the dock was apparently carried alongshore by a secondary wave crest originating near the collapsed southern part of the dock.As with similar phenomena in other coastal regions of the world ocean, and with an earlier landslide and tsunami generated in Skagway Harbor in October 1966, the November 1994 Skagway event is linked to critical overloading of the slope materials at the time of extreme low tide. An examination of the physical mechanism linking coastal landslides and low tides indicates that the 1994 Skagway tsunami was largely the result of a subaerial component of the landslide. This supports our contention that the slide was caused by failure of the PARN Dock. We conclude that our numerical model of the landslide associated with the 1994 PARN Dock failure in Skagway Harbor accounts for all aspects of the observed wave field without additional assumptions concerning simultaneous, hypothetical, geophysical, geological or hydrometeorological events in the adjoining region.