Pretty Rocks Landslide

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The Pretty Rocks Landslide at Mile 45.4 of the park road is one of several known landslides in that general area and one of more than 140 mapped “unstable slopes” along the entire park road. The Pretty Rocks Landslide has the potential to greatly disrupt transportation, negatively impact visitor experience, cause resource damage, curtail commercial services, and affect public safety.
 
Pretty Rocks Landslide Comparison (September 2018 vs April 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 19th. The photo below was taken six months later in the same location. NPS Photo
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. 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
Figure 1. 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.

Background

Over the last two decades, the Pretty Rocks Landslide (Figure 2) has evolved from a minor maintenance concern to an existential threat to the current model of visitation. During the 1990s, maintenance staff easily repaired small cracks in the road surface caused by the landslide. By 2016, the Pretty Rocks Landslide was moving the road down the slope approximately 2.6 feet/year. In 2017, the rate had approximately doubled. By the fall of 2018, the rate had doubled again and was moving the full width of a 100-yard section of road approximately 12 feet/year vertically and horizontally. This movement creates a swale that steepens the road gradient and limits sight lines, which hinders travel.

If the Pretty Rocks Landslide velocity continues to increase, the park will soon reach a threshold where the National Park Service (NPS) and our partners, including the Federal Highways Administration (FHWA), can no longer maintain the current road alignment. Steep slopes above and below the road preclude simple reroutes. More complex reroutes are feasible but will be long (approximately five miles), expensive, and enter federally designated wilderness, thus requiring congressional action.
 
aerial image of a dirt road on a precipitous mountainside with a line circling an area indicating a landslide section
Figure 2. 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. The road has existed here for almost 90 years. 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 Stoney 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 are four basic in-place design options identified for further analysis. Each has potential to sufficiently improve safety and resiliency at the Pretty Rocks Landslide without significantly rerouting the road:
  • Resist - build engineered structures to arrest landslide movement
  • Rebalance - remove material from top of landslide until risk adequately reduced
  • Bridge - build a bridge over landslide
  • Tunnel - build a tunnel in bedrock behind landslide

As described below, in addition to considering combinations of the above items to reduce, arrest, or avoid movement of the Pretty Rocks Landslide, two options to move this section of road away from the hazard are being developed. A road terminus option is also being developed.

South Reroute Option


A south reroute (see Fig. 3) 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.

Road Terminus at Mile 43


Ending the Road near the East Fork River (Mile 43; see Fig. 3) is another alternative. This option is not preferable because it would have substantial impacts to tourism (and therefore the regional economy), including loss of overland access to the Toklat River Rest Stop, Eielson Visitor Center, the Wonder Lake Campground, in-holder properties (private property and commercial businesses) in Kantishna, other important infrastructure and cultural and natural resources. The park would need to construct an appropriate turnaround and facilities near the East Fork River. As a result the park and stakeholders would need to significantly modify visitation patterns. Such a step would likely lead to higher impacts to resources within the shortened road corridor.

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.
 
relief map of a mountain. color overlays indicate possible new routes for a road, on the north side of a mountain and on the south side. third area indicates a possible turn-around location for the park road.
Figure 3. Image depicts two possible reroute corridors and one possible road terminus.

Completed Work and Next Steps


Following is a list of past, ongoing and future work addressing this important issue:
  • In 2018, completed emergency evacuation plans/route evaluations and conversations with inholders, partners, and tourism industry stakeholders;
  • A May, 2019, multi-agency workshop was held to further refine the road terminus option since that scenario would be likely as a temporary state in the event of road failure and for any major reconstruction/reroute options;
  • In fall 2019, host a multi-agency evacuation drill/exercise with State of Alaska Department of Homeland Security;
  • Install a real-time monitoring system within the Pretty Rocks Landslide that will allow rapid detection and response to changes in landslide velocity;
  • Collaborate with academic partners to monitor permafrost and its impacts on the location, mechanism, magnitude, and timing of landslide events along all potential routes;
 

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: December 12, 2019

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Denali Park, AK 99755

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