Landslides & Debris Flows

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NPS Photo / Riley Tingue

 

2019 Geohazard Analysis

The data below illustrates the most severe weather-related geohazards that developed on the Denali Park Road during August 2019.

The data is evaluated using a "SPE" model—Severity, Probability, Exposure. The model assigns a numeric value to each geohazard event based on its severity, its probability to recur, and its exposure. The values are multiplied, and an overall score from 1 to 100 is the result. A high score indicates a bigger risk or problem. 

A brief description of the event, the mitigation of it, and notes for the future are also available in the dataset.

details of geohazards that occurred along the denali park road in 2019

APPROX MILE	EVENT NAME	FUTURE SEVERITY	FUTURE PROBABLILITY	FUTURE EXPOSURE	FUTURE VALUE & RISK LEVEL	DESCRIPTION	ESTIMATED CUBIC YARDS DISTURBED	MAT ACROSS ROAD?	MITIGATION TO DATE (SEPTEMBER 2019)	PROPOSED

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Pretty Rocks Landslide

^{Section last updated: April 2, 2019}

The Pretty Rocks Landslide at Mile 45.4 of the Denali Park Road has the potential to disrupt transportation, impact visitor experience, cause resource damage and affect public safety.

Over the last two decades, the landslide has evolved from a minor maintenance concern to a threat. During the 1990s, the landslide caused small cracks in the road surface. In the spring of 2019, the landslide was displacing the full width of a 100-yard (90-m) section of road up to .45 inches (1.1 cm) per day vertically and horizontally, creating a swale that steepens the road gradient and limits sight lines. If the landslide continues to increase in velocity, we will soon reach a threshold where the National Park Service can no longer maintain the current road alignment.

Read more about the Pretty Rocks Landslide

 

Pretty Rocks Landslide Comparison (September 2018 vs April 2019)

Left image
The eastern landslide scarp through the road on September 29, 2018. The road was last graded on September 14th and closed to most vehicle traffic on September 19th. The photo below was taken six months later in the same location.
Credit: NPS Photo

Right image
The eastern landslide scarp through the road on 3/22/2019. Survey rod held by park employee is 6.5 feet (2.0 m) tall and is placed near center-line of the road. Note employees had recently shoveled dirty snow in foreground to aid in measuring the scarp.
Credit: NPS Photo

 

Igloo Debris Slide

2018

On September 30^th and October 18^th 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 30^th.

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.

2013

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.

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• Rockfall Audio Clip

  
  

  Rocks plummet from great height and strike a glacial moraine, echoing across the steep-walled valley. Recorded by the Yentna-Lacuna Confluence Sound Station.

  Credit / Author:

  NPS Media / Davyd Betchkal

  Date created:

  2020-10-14 00:00:00.0

  Audio Transcript

  Sound of rocks falling.