26 July 2022
The risk of large rockslides at the port of Skagway in Alaska
Skagway is a small borough in Alaska with a population in the 2020 census of 1,240 people. In the summer months it becomes a key tourist destination – according to Wikipedia it attracts over a million visitors per year, with the vast majority arriving on cruise ships. The port also processes iron ore mined from within the hinterland. Thus, the port, shown in the Google Earth image below, is of critical importance to the region.
As the image above shows, one section of the port abuts a steep rockslope, which is about 220 m high. This section provides mooring for large cruise ships. In 2017 two rockslides occurred on this slope – indeed, the scar of one of these events can be seen in the image above. In June 2022 a further rockslide occurred, causing a cruise ship to divert.
The port authority is the White Pass and Yukon Railroad. The local authority, the Municipality of Skagway, has commissioned an initial inspection of the slope by Shannon and Wilson, geotechnical consultants. Admirably, their initial assessment has been published online and is discussed in a recent article by KTOO. It does not make easy reading.
Included in the report is the image below, showing the slope and the recent rockslides, and providing a decent context for the hazards posed by failures from the slope:-
There are some really interesting observations in the report:
- The slope shows evidence of past failures, including scars and debris piles;
- The larger of the two 2017 failures (the North Slide in the image above) came from an area high on the slope that underwent toppling failure. There is a large mass on the slope that is at risk of further collapse;
- An extensometer installed in this mass after 2017 has shows an average movement rate of about 40 mm per year. Recently this has increased to about 65 mm per year.
The consultants conclude that this part of the port at Skagway is at risk from a largescale rockslope failure form the mass at the top of the North Slide:
Given our onsite observations, reported historic movement of the slide mass, and the apparent increase in movement rates measured by the instrumentation, we believe that there is a significant risk associated with complete failure of this feature. It is our opinion that the slide mass will eventually fail and the consequences of such failure will be catastrophic in nature with significant risks to life and property. The timing of such a failure is difficult to predict but accelerating movement rates suggest that the failure event is approaching.
The consultants recommend major engineering works to mitigate the risk in the long term. In the short term they recommend the installation of a warning system and evacuation plan.
The long term measures to protect the cruise port at Skagway are likely to be a significant financial burden for small community. There is an interesting interview on the Alaska Public Radio website with Reba Hylton from the Skagway Assembly, which highlights that there are ongoing discussions regarding responsibility for the works (the rockslope might be on municipal land). Meanwhile, additional measures have been put in place to protect port users.