Polychrome Area Plan

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Landslide movement at Pretty Rocks (mile 45.4 on the Denali Park Road) has accelerated in recent years and as of September 2021, it was determined that road maintenance efforts are no longer sustainable in the face of such movement. Construction will begin in Summer 2023 to address several geologic hazards, including Pretty Rocks Landslide, that threaten public safety and infrastructure between miles 44 and 46. Buses will travel no farther than mile 43 of the 92-mile road until the long-term solution is implemented to restore reliable road access west of the area.

Check the current conditions of the Park Road and visitor services. Read more about the science and monitoring of the Pretty Rocks Landslide and other geologic hazards.

 

NPS road maintenance staff made an extraordinary effort in Spring 2021 to repair the road at Pretty Rocks

NPS Photo

Planning Process

The collaboration between the National Park Service (NPS) and Federal Highways Administration (FHWA) to develop a long-term solution for the Park Road in the Polychrome Area began in the winter of 2017–2018. FHWA brainstormed several conceptual ideas, ruled them out one-by-one through preliminary evaluation, and narrowed it down to three alternatives for further consideration: bridging the landslide, removing the upper landslide, and detouring around the landslide. NPS identified four criteria to use for evaluating the options: feasibility of construction, continuity of access during and after construction, resiliency / life cycle, and how it supports the Denali National Park mission statement.

In 2019, FHWA conducted geotechnical investigations at Pretty Rocks and confirmed that the rock mass strength of the material at the east and west abutments was sufficient to support a bridge so bridging the landslide was retained as a viable alternative. Further analysis determined that removal of the upper landslide would cause the slope to become unstable again in 10–15 years due to the continuous exposure of the weak ash layer, so it was eliminated as a long-term solution.

In 2020, NPS and FHWA partnered to conduct an Expert-Based Risk Assessment (EBRA) and a Value Analysis (VA) that evaluated the remaining alternatives, including north and south re-routes and a mainline option that would retain the existing road alignment. Based on the outcomes of these expert panels, FHWA developed the Polychrome Project Delivery Plan and recommended the mainline option, which included building a bridge across the Pretty Rocks Landslide (mile 45.4) and constructing a retaining wall along the road above the Bear Cave Landslide (mile 44.8). NPS agreed that the mainline option offered several advantages over the re-routes such as retaining more of the historical character and visitor experience, minimizing the impacts to natural resources and wilderness, and restoring reliable access to the western portions of the park in the shortest amount of time. A summary of factors that went into the decision process to dismiss the reroute alternatives is provided in the table below. FHWA moved forward with the design for the bridge and the additional engineering solutions for the mainline option in Fall 2020.

 

Potential alignments considered by FHWA and NPS. The orange segments represent bridges.

Source: DG&A 2020

Comparison of Alignments

Factor	South Alignments	North Alignment	Mainline
Construction duration	11 to 13 years	12 to 14 years	1 to 2 years
Number of new bridges	5 to 8	8	1
Length of new road	5 to 6 miles	6 miles	600 feet
Project area in designated wilderness	780 to 830 acres	748 acres	11 acres
Construction Cost	$255–$275 million	$186 million	$91 million
Maintenance gravel need over 50-year design life	1 million cubic yards	1 million cubic yards	25,000 cubic yards

In 2021, additional geotechnical investigation was done to inform the design of the bridge as well as the preliminary design of the retaining wall at Bear Cave Landslide. FHWA provided NPS with preliminary (30%) design plans for a Pretty Rocks bridge. The next step in the planning process was to complete the required compliance, which assessed the potential environmental impacts of the proposed project.

 

FHWA engineers visited the site of the proposed Pretty Rocks bridge in August 2021

AECOM / Jon Isaacs

Project Overview

Although Pretty Rocks Landslide is the highest priority and will be addressed first, the NPS plans to take a comprehensive approach with several engineered solutions along approximately two miles of the Denali Park Road (Mile 44–46) to ensure that once access is achieved through Pretty Rocks, it won't be subsequently jeopardized by another nearby hazard. The project is referred to as the “Polychrome Area Improvements” and has been separated into two phases to expedite the start date of the first phase and allow time to secure funding and finalize designs for secondary phases:

 

Phase I

• Construction of an approximately 400-foot steel bridge to span the Pretty Rocks Landslide. A temporary platform will be installed on the east side to extend the staging area during bridge construction.
• Excavation to the east and west of the bridge to accommodate construction activities and traffic. The majority of the excavated material will be placed on the slope below the road.
• Slight realignment of the road on the west side of the bridge to allow for a proper turning radius for buses and other vehicles getting on and off the bridge.
• Installation of retaining walls and drainage improvements to the east of the bridge, on the uphill slope.
• Rock scaling, rock reinforcement, and/or creation of rockfall ditches to reduce rockfall risk.

Phase II

• Construction of a partially buried retaining wall and drainage improvements along 1,000 feet on the south side of the road above Bear Cave Landslide.
• Temporary widening of the road above Bear Cave Landslide to accommodate traffic during construction.
• Rock scaling, rock reinforcement, and/or creation of rockfall ditches to reduce rockfall risk.

The total estimated cost for Phase I is $58 million and $25 million in funding has been made available through the Bipartisan Infrastructure Law. Phase I will require approximately five months of excavation plus three months of bridge construction and will take approximately two years to complete. Road access through Polychrome is not anticipated during Phase I. Visitor transportation will be allowed to travel as far as mile 43 of the Park Road and buses will turn around at the East Fork Toklat bridge or the East Fork Cabin site. Access to Kantishna inholdings will be primarily by air until the bridge is completed.

An additional $65 million is needed to complete Phase II and it will be implemented at a later date. Road access through Polychrome is anticipated during Phase II, likely with some delays or temporary restrictions expected at the construction sites. Visitor transportation to destination points west of Pretty Rocks and regular traffic to Kantishna inholdings will be allowed during Phase II.

 

Duration:

1 minute, 58 seconds

Take a brief tour of the bridge designed to span the landslide at Pretty Rocks. No audio.

 

NEPA and NHPA Compliance

The National Environmental Policy Act (NEPA) mandates an environmental assessment of any proposed federal action that has the potential to “significantly affect the quality of the human environment." Section 106 of the National Historic Preservation Act (NHPA) requires that federal agencies identify and assess the effects its actions may have on historic properties and consider public views and concerns about historic preservation issues when making final project decisions. Compliance with the NHPA is conducted concurrent with, but as a separate process to, the NEPA compliance process.

Environmental and cultural compliance for the Polychrome Area Improvements project was formally initiated in October 2021 and was completed in March 2022:

• Step 1. Conducted consultations and civic engagement (July–Oct ‘21)
• Step 2. Analyzed impacts and assessed the effect to historic properties (Oct–Nov ‘21)
• Step 3. Identified mitigations and prepared environmental document (Nov–Dec ‘21)
• Step 4. Posted environmental document for public review (Jan–Feb ‘22)
• Step 5. Reviewed public comments and prepared decision document (Feb ‘22)
• Step 6. Released final decision document to the public (March ‘22)

The NPS contracted an environmental consulting firm (AECOM) to prepare the Environmental Assessment (EA), which considered the environmental consequences of a “no action alternative” (no new bridge and the road remains closed) and an “action alternative” (construct Pretty Rocks Bridge and Polychrome Road Improvements).

Prior to the development of the EA, the NPS consulted with federally recognized Tribes, Alaska Native Corporations, State and local governments, and the State Historic Preservation Office. The NPS also hosted two virtual public meetings in October 2021 to identify and document specific concerns, issues, and potential impacts of both alternatives. Issue topics analyzed in the EA included geology, socioeconomics, visual resources, visitor use and experience, noise / soundscape, wildlife, wetlands and vegetation, cultural resources, and wilderness. Following the publication of the EA in January 2022, two additional virtual public meetings were held to discuss the analysis and provide information to aid the public in reviewing and commenting on the compliance document.

After careful consideration of resource impacts, consultation with stakeholders, and review of public comments, the NPS concluded that the Polychrome Area Improvements project would not have a significant effect on the environment. The Finding of No Significant Impact (FONSI) was approved by the NPS Alaska Regional Director and released to the public in March 2022. With the completion of the compliance process, design of Phase I is underway.

 

How Can I Get Involved?

Learn about other plans and become familiar with the Denali Planning Portfolio.

Check the NPS Planning, Environment, and Public Comment (PEPC) website for documents available for review and comment.

If you have questions, please email us.