Igloo Debris Slide
On September 30th and October 18th 2018, the Igloo Debris Slide partially blocked the Denali Park Road. In each event, the Denali road crew cleared approximately 500 cubic yards (450 m3) from the roadway within a few hours. Like the 2013 event, 12’ (4 m) blocks of ice-rich, unconsolidated debris slid on the same or similar unfrozen clay layer. 2013 and 2018 both experienced unusually mild fall weather. Such weather further thaws permafrost, which decreases cohesion and increases pore water pressure. Therefore, this combination of processes can cause and trigger landslides. As more permafrost has thawed and the slide has continued to move, the area of the slide has increased. Several smaller events have filled the road ditch since September 30th.
Geology and engineering experts from the NPS and Federal Highways Administration expect activity at this site to continue in subsequent years. Therefore, we are currently developing designs to reduce risk at this site and others as part of the Unstable Slope Management Plan for Federal Land Management Agencies.
In late October 2013, road maintenance staff discovered that a 600’ (180 m)-long, 110’ (35 m)-wide debris slide had blocked the park road near Mile 38.
Blocks of ice-rich, unconsolidated debris as thick as 15’ (5 m) and the size of a small cabin had slid on a slippery, unfrozen clay that acted as the failure plane. With winter snows held off by unseasonably warm weather, the Denali road crew managed to clear the road of debris after considerable effort.
The trigger for the slide remains unknown. Ground, aerial, and satellite imagery of the site in the years and months before to the event indicate that a small slide had previously occurred here, groundwater seeped from the area, and the ground was beginning to move slightly. In the days preceding discovery of the slide, the area was experiencing temperatures that fluctuated near the freezing point. Therefore, the forces associated with the expansion of ice during the repeated freezing and thawing of water near the surface may have triggered the slide. Alternatively, we also know that a thick layer of permafrost slid on an unfrozen layer of clay.
Regionally, permafrost is thawing; while the trend at the site is presently unknown, thaw in the area would be consistent with regional trends. Therefore, it is possible that the permafrost thinned through the clay layer, which triggered the slide. Many other triggers are also possible and are being examined.