THE GENERATION OF WAVES BY A LANDSLIDE: SKAGWAY, ALASKA - A CASE STUDY

Frederic Raichlen, Jiin Jen Lee, Catherine Petroff, Philip Watts

Abstract


On November 3, 1994, at about 7:10 p.m. Alaska time, a combined aerial and sub-aerial landslide, which was not seismically induced, occurred at a railway dock on the eastern side of the harbor at Skagway, Alaska. A small portion of the landslide, consisting in part of rip-rap, was initially above water. The submerged landslide material was composed of loose alluvium accumulated along the eastern side of the harbor on an underwater slope with an average angle between 0 deg. and 30 deg. Witnesses reported waves 30 ft to 36 ft high at the eastern side of the harbor. The slide and the generated water waves claimed the life of one construction worker and caused an estimated $21 million damage. A map of the Skagway Harbor is presented in Fig. 1 showing the location of the landslide and the surrounding area including the Taiya Inlet which is an arm of the much longer Lynn Canal. A floating dock used as an Alaska state ferry terminal is located in the middle of the harbor at the end of a 660 ft long jetty, and a small boat harbor is located near the east side of the harbor. The chains which moored the floating dock were broken during the event. At the time of the landslide the tide elevation was at about -4 ft MLLW; the tide range at Skagway varies from about 9.8 ft to about 24.6 ft over a month. The record from a tide gage located at a dock on the west side of the harbor is presented in Fig. 2 showing the waves generated by the landslide. The tide gage trace is truncated for extremely low water surface elevations because, for those readings, the travel of the gage recording pen was stopped by the chart paper guide. The purpose of the study reported in this paper was to more fully understand the wave events which could have taken place in Skagway Harbor after the landslide. The interest is not in precisely modeling the landslide, but in defining the response of the harbor and of the moored floating ferry dock to waves generated by a moving boundary which is used as an analogy to the landslide for purposes of wave generation.

Keywords


case study; wave generation; landslide

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