8 September 2021
The Pretty Rocks landslide in Denali National Park in Alaska
Last year I posted about landslides in Denali National Park in Alaska, including a very interesting case study, the Pretty Rocks Landslide. This failure, located at 63.537, -149.815 if you wish to take a look, is heavily disrupting the single road through the national park. Time magazine has an online article about this landslide that is worth a look. Sitting behind that is considerable more detail on the Denali National Park website, including a set of images of the landslide:
The Denali National Park article notes that technically this is a rock glacier, and that it is moving quite rapidly. The rate of movement of the landslide has increased in recent years in response to the warming climate, with highttps://www.nps.gov/dena/learn/nature/pretty-rocks.htmh rates being noted in particular in periods of summer rainfall. The website describes the change in pattern of movement of the landslide:-
The landslide at Pretty Rocks has been active since at least the 1960s, but has evolved from a manageable maintenance concern into a much more extensive maintenance challenge. Before 2014, the landslide only caused small cracks in the road surface and required maintenance every 2-3 years.
Between 2016 and 2017, the full width of a 100-yard (90-m) section of road slumped up to 0.2” (0.5 cm) per day, steepening the road gradient and limiting sight lines for drivers approaching the sharp turn
. but not warranting long-term closures.
However, by 2018 the slumping increased to almost half an inch per day, and then to three and a half inches per day by August 2020. Early August rains in 2021 appear to have triggered the rate to increase significantly, with much of the landslide currently moving downhill at over ten inches per day.
NPS road crews maintained a safe drivable surface for most of the summer of 2021 by importing as much as 100 dump-truck loads of gravel per week to fill the slump. On August 24, park managers recognized that this solution was no longer tenable or safe and enacted a road closure to the east of Pretty Rocks. As the area becomes more unstable, traditional road maintenance methods have become inadequate. The park’s ability to adapt to future conditions will require transitioning to more expensive and novel construction methods to maintain road-based access.
Denali National Park has produced a wonderful time lapse video of the landslide movement that is incredibly instructive:
Note the continuous movement of the slide, wbich extends right to the toe of the slope. The dumping of gravel to maintain the road is clearly evident, as is the pitfall of this approach. The gravel is steadily loading the head of the landslide, increasing the driving force. Resistance is not increasing at the same rate, so the tipping is likely to be making the problems with the slope slightly worse.
The National Park are right to cease this approach. Unfortunately, as the National Park notes, alternative ways to maintain the road are likely to be costly and difficult.