The Denali Park Road, especially a section on Polychrome Pass, is needing more monitoring, planning and more maintenance.
“Landslide activity has intensified along the road, especially at Pretty Rocks,” said Denny Capps, park geologist. “In 2018, we saw downhill movement of the Pretty Rocks landslide increase to 0.4 inches a day, which became our new norm, but starting this August, we began measuring 1.5-2 inches a day.”
The Pretty Rocks section, at Mile 45.4, required extensive rebuilding this spring before buses could safely travel the one road into Denali National Park. The section is right in the middle of Polychrome Pass.
The increased movement is due to an especially wet fall. August saw 19.01 inches of rainfall at Eielson Visitor Center, making it the wettest August since 17.62 inches of rain fell in August 2000. On Aug. 16, Eielson sustained a record rainfall of 4.15 inches in a 24-hour period (the previous record was 2.36 inches). That led to a road closure that kept 300 visitors and 17 shuttle buses at the Toklat Rest Area for three hours as maintenance crews cleared a debris flow near the Polychrome Overlook.
Of concern now is what will occur during upcoming winter months.
“The road’s stability is hugely dependent on the underlying permafrost,” said Capps. “Within the Pretty Rocks landslide, a layer of ice sits approximately 30 feet beneath the surface.”
Geotechnical experts and park resource teams are taking core samples in the area.
“The ice-rich permafrost within the landslide is likely thawing,” Capps said. “As this ice melts, it adds more water to the already saturated soils and reduces cohesion, which further increases the likelihood of downhill movement.”
Using data collected so far, the park is expecting a 13-foot or greater drop in the Pretty Rocks landslide portion of the road over the coming winter. This 2020 estimate is double the actual drop experienced along the road when it was cleared this past spring, which itself was a doubling of the drop experienced in 2017.
The stability of the road is dependent upon how cold it gets this winter, Capps said.
“Colder temperatures generally mean less snow and a more stable base,” he added “Warmer temperatures generally mean a less stable base and more snow, resulting in more soil saturation and more deformation.”
There are no current plans to delay the road opening in 2020 or to implement any road restrictions. Park officials encourage visitors to continue to plan their 2020 trips to the park.