Pretty Rocks Landslide

Learn and Explore

A section of the Denali Park Road, called Pretty Rocks, traverses a precipitous slope high on the side of Polychrome Mountain. Built in the 1920s and 1930s as a scenic high route, this section of the road is at roughly the mid-way point on the 92-mile long road. The Pretty Rocks Landslide at Mile 45.5 of the park is one of several known landslides in that general area and one of the more than 140 mapped unstable slopes along the entire park road. The park routinely monitors this area with aerial and ground surveys. Recent data indicate the rate of movement in this area increased dramatically during the summer of 2019. We have monitored the site monthly through the winter and the speed of the slide has remained high.

Because the Pretty Rocks Landslide has the potential to disrupt transportation, negatively impact visitor experience, curtail commercial services and affect public safety, park managers are evaluating geologic data and weighing potential engineering solutions that will address this geologic issue. We are committed to keeping the road operational, resilient, and safe for travel under these dynamic and changing conditions. In spite of our best efforts, travel restrictions may sometimes be necessary. Our work is being supported by the Federal Highways Administration (FHWA). 
 

 
 
Pretty Rocks Landslide Comparison (November 2019 vs January 2020)
a man standing on a dirt road, on a portion lower than the main surface of the road a man standing on a dirt road, significantly lower than in the previous photo
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
These two locations were directly connected on September 25, 2019; the vertical offset between the two points in January was 15 feet. The center of the road is near the left side of each photo.

If this rate of movement continues, a total offset of 20 or more feet is expected by mid-February when the snow plow crew is likely to reach the site. 



 
Pretty Rocks Landslide Comparison (September 2018 vs March 2019)
woman and child sitting on a dirt road with a large crack in the middle of it a woman standing on a dirt road, on a section that has cracked off and slumped below the main roadbed
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



 
a map of denali illustrating its location in central alaska. a red mark indicates the location of pretty rocks, in roughly the middle of the park's sole road
Shaded-relief map of Denali National Park and Preserve outlined in green. Black line is the Denali Park Road, red asterisk indicates location of the Pretty Rocks Landslide. Red line is the George Parks Highway. Inset map shows location of the park (green line) in Alaska.

Detailed 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 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 swale that steepened the road gradient and limited sight lines.

The landslide has evolved from anticipated, mangeable maintenance concerns into a much more extensive maintenance challenge. Landslide slumping increased in 2018 and 2019, to almost two inches per day since August, 2019.

The National Park Service is committing the resources necessary to ensure that the Denali Park Road will be open for the 2020 season by maintaining the current alignment until a permanent solution is complete. NPS and the Federal Highway Administration (FHWA) are working proactively, with partners, to plan for a long term solution to the access problems caused by changes in road dynamics at Polychrome Pass. NPS and FHWA are also working diligently to complete contingency plans if a natural disaster temporarily displaces park visitors and other road users.

 
aerial image of a dirt road on a precipitous mountainside with a line circling an area indicating a landslide section
Oblique view of the Pretty Rocks Landslide. Background is a composite image from June 15, 2015 with the approximate landslide outline indicated by red dots.

NPS Image

Options to Improve Resiliency or Relocate the Road

Existing Road Alignment Across Polychrome Pass

Several options exist to mitigate the risks at Pretty Rocks within the existing road corridor. While there are challenging locations, most of the problematic areas are known. Consideration of risks (safety, construction, maintenance, etc.) posed by all landslides in this section of road is critical when choosing the most appropriate option as even small realignments may be unfeasible.

Popular visitor destinations including Polychrome Overlook are in areas adjacent to the Pretty Rocks Landslide. Farther west along the road, visitors can access Stony Overlook and the Eielson Visitor Center. The State of Alaska (through the State Historic Preservation Office) and the National Park Service recognize the current road alignment as a historic resource worth protecting and have nominated the Mount McKinley National Park Road Historic District for listing in the National Register of Historic Places.

There were four basic in-place design options identified for further analysis. Each had potential to sufficiently improve safety and resiliency at the Pretty Rocks Landslide without significantly rerouting the road. Two were determined to no longer be feasible:

  • Resist: build engineered structures to arrest landslide movement

  • Tunnel: build a tunnel in bedrock behind landslide

The two other options remain viable:

  • Earthwork: remove rock, debris, and ice until risk adequately reduced

  • Bridge: build a bridge over the landslide

As described below, two options to move this section of road away from the hazard are being developed.

South Reroute Option

A south reroute would depart the existing alignment near the East Fork River (Mile 43), broadly paralleling the existing road to the south, and rejoin the road near Mile 48. There is relatively high confidence in understanding the risk of constructing a roadway here. Most importantly 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 bridge numerous and wide floodplains. While the upland areas have relatively low topography, it likely contains a high amount of thaw-unstable permafrost. Permafrost is thawing regionally. Through monitoring, it has been demonstrated that disturbing thaw-unstable permafrost for the construction of infrastructure causes negative subsequent impacts to infrastructure and natural and cultural resources. Therefore, the upland road sections may be easy to construct, but will involve greater maintenance needs due to the underlying permafrost.

NPS Management Policies (2006, Subsection 4.6.4 – Floodplains) states that we must:

  • Protect, preserve, and restore the natural resources and functions of floodplains

  • Avoid the . . . environmental effects associated with the occupancy and modification of floodplains

  • Avoid direct and indirect support of floodplain development that could adversely affect the natural resources

  • When it is not practicable to locate or relocate development or inappropriate human activities to a site outside and not affecting the floodplain, the Service will . . . use non-structural measures as much as practicable to reduce hazards to human life and property, while minimizing the impact to the natural resources of floodplains

As a result, floodplains would likely need to be bridged. While this presents a lesser engineering challenge, bridges are expensive, particularly in this remote environment.

Upland Reroute Option

Similar to the proposed South Reroute, an upland reroute would depart the existing alignment near the East Fork River (Mile 43), broadly parallel it to the north, and rejoin the road near Mile 48 (see Fig. 3). There is relatively low confidence in understanding the risk of constructing a roadway here. This option must either be substantially longer or cross even more topography than the existing road corridor. 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. Additionally, several hundred-thousand to a few-million cubic yards of gravel would be needed to construct this reroute; reserves of that magnitude are not identified. Gravel sources outside the park would be prohibitively expensive and hauling would create adverse effects on the road. As a result, this option will continue to be investigated, but it has limited feasibility.

Cost Estimates

We have only begun to calculate reliable cost estimates for the above-proposed alternatives. The Federal Highways Administration expects to generate preliminary cost estimates by the first quarter of 2020 once permafrost and engineering data become available.

 

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.
 

Last updated: February 24, 2020

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Denali Park, AK 99755

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