Pretty Rocks Landslide

Learn and Explore

Update: December, 2020
The Pretty Rocks Landslide, at Mile 45.5 of the of the Denali Park Road, traverses a precipitous slope high on the side of Polychrome Mountain and is one of several known landslides in the area and one of the more than 140 mapped unstable slopes along the entire Park Road. National Park Service (NPS) monitoring data indicate that the rate of movement in this area has increased dramatically in recent years.  

Pretty Rocks Landslide has the potential to disrupt transportation on the Park Road, and affect public safety. Park managers are weighing potential engineering solutions that will allow for continued access west of Polychrome Mountain for the foreseeable future. The NPS and Federal Highway Administration (FHWA) are collaborating to keep the road operational, resilient, and safe for travel under dynamic and changing conditions. 

On This Page:

• Comparison Photos
• Background Details About the Landslide
• Why the Park Road Traverses Polychrome

 

 

Pretty Rocks Landslide Comparison (November 2019 vs January 2020)

Late November, 2019. The area had fallen around 10' from grade since September. NPS Photo

Early January, 2020. The area had fallen significantly further since late November. NPS Photo

The vertical offset between the two points moved from zero in September 2019 (left) to 15 feet in January 2020 (right). The center of the road is near the left side of each photo. 
 
The rate of movement in 2020 was similar to the observed rate for 2019, and a total offset (slump) of 20 or more feet is expected by March, 2021, when the NPS road crew plans to reach the site and fill the slump for summer access. 

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Pretty Rocks Landslide Comparison (September 2018 vs March 2019)

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 19. NPS Photo

The eastern landslide scarp on March 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. NPS Photo

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Background

Landslide impacts have been experienced at Pretty Rocks since at least the 1960s and, until recently, only required maintenance every 2-3 years. During the 1990s, landslides in this section only caused small cracks in the road surface. Between 2016 and 2017, the full width of a 300-foot section of road slumped up to six inches per month, creating a slump that steepened the road gradient and limited sight lines for drivers approaching the sharp turn.

The landslide has evolved from a manageable maintenance concern into a much more extensive maintenance challenge. Landslide slumping increased to almost two inches per day in 2018 and 2019, then increased to three and a half inches per day by August, 2020. NPS road crews maintained a safe drivable surface during these years by importing up to 6,000 cubic yards of gravel to fill the slump. The NPS is exploring immediate, short and long term solutions to the increasing road movement at Pretty Rocks.

 

Next Steps

In May, 2020, NPS and FHWA partnered to conduct an Expert-Based Risk Assessment (EBRA) that evaluated the geotechnical attributes of the alternatives. Informed by the EBRA, a value analysis was conducted in July 2020 to determine which alternative provided the greatest value.

Informed by these evaluations, the NPS will address maintaining access west of Polychrome Mountain in three stages:

• Immediate Plan: The NPS and FHWA will maintain temporary access at Pretty Rocks by filling the Pretty Rocks slump during the spring.
• Short Term Plan: NPS will address sustained access at Pretty Rocks Corner while the long term solution is planned and implemented. Possible alternatives include continuing to fill the slump, building a bridge to span the landslide area, and removing material from above the road. The environmental compliance process which includes public participation, will be conducted in 2021.
• Long Term Plan: Based on the outcomes of the EBRA and Value Analyses, the NPS is considering a variety of options for a long term solution, including:
  • Option 1: Mainline (Existing Alignment)- Build a bridge at Pretty Rocks landslide and address additional geohazards along the existing route over Polychrome Pass
  • Option 2: North Route: realign the road to the north of Polychrome Pass
  • Option 3 (A and B): South Route: realign the road to the Plains of Murie below the current alignment

 

The September 2020 recommendation from FHWA is the Mainline Option. This will likely be the NPS proposed action for a long term solution. Implementation  would follow a period of planning, environmental analysis, and design, and is not expected to begin until 2025 or later.

NPS Graphic

Long Term Options

Based on the outcomes of the EBRA and Value Analyses, the NPS has explored three options for a long term solution:

Mainline Option

The mainline option would maintain the current road alignment over Polychrome Pass. It includes construction of engineered solutions at Pretty Rocks and five other geologic hazards within the area, including the slump at Bear Cave (mile 44.8).

This option was the recommended solution by Federal Highway Administration. It will be the NPS preferred alternative in required future compliance.

South Reroute Option

A south reroute would depart the existing alignment near the East Fork River (Mile 43), broadly paralleling the existing road on the valley floor to the south, and rejoin the road near Mile 48. This area has much lower topography, which decreases the chance of landslides and eases road constructability.

A gravel road would be constructed on upland areas and would bridge the numerous and wide floodplains. While the upland areas have relatively low topography, it likely contains a high amount of thaw-unstable permafrost. Therefore, the upland road sections may be easy to construct, but will involve greater maintenance needs due to the underlying permafrost.

North Reroute Option

Similar to the proposed South Reroute, a North Reroute would depart the existing alignment near the East Fork River (Mile 43), broadly parallel it on the northern side of Polychrome Mountain, and rejoin the road near Mile 48 (see Fig. 3). Similar geotechnical challenges are expected along this reroute compared to the existing route because it has similar geology, several known landslides, and likely contains abundant thaw-unstable permafrost. Several hundred-thousand to a few-million cubic yards of gravel would be needed to construct this reroute.

 

History of the "High Line"

The Alaska Road Commission (ARC) began to build the trans-park road into Mount McKinley National Park in 1922. Over the course of 16 years, crews carved a road out of wilderness. During this time, the National Park Service decided to place part of the road along the side of Polychrome Mountain—the "High Line."

Learn more about how and why the High Line became the park road route.