Landslides, Permafrost and a 92-Mile Road: Managing Geohazards in Denali

Denali staff collecting GPS data at the Pretty Rocks landslide near mile 45 of the Park Road on March 22, 2019. The survey rod is 6.5 feet (2.0 m) tall, is placed near the centerline of the road and shows the displacement since September 14, 2018. NPS/Denny Capps.

Denali's 92-mile (148-km) park road is how visitors and staff reach the park, and landslides, rockfalls and debris flows have repeatedly blocked and damaged it. The 2016 Eagle's Nest landslide fell in July, in the busy season, and blocked the road completely for four days and limited traffic for six more. Three authors from the National Park Service and the Federal Highway Administration describe how the park tracks and reduces such geohazards.

What counts as a geohazard

The authors define geohazards as geologic processes that pose a risk to humans and infrastructure. They say geohazards can generally be forecast through scientific evaluation, can be analyzed for risk using probabilities, are linked to one another and to the physical environment (earthquakes and heavy rain can cause landslides), have growing impacts as infrastructure expands, and can be managed by minimizing their consequences.

Denali is a prime place to study them, with over 20,000 feet (6,000 m) of topographic relief, 1,400 square miles (3,700 km2) of glaciers, active tectonics and widespread permafrost. Its geohazards include earthquakes, landslides, rockfall, debris (mud) flows, glacier outburst floods, ice and snow avalanches and river erosion and deposition. Avalanches have killed at least 49 mountaineers since 1976, but landslides, rockfalls and debris flows cause the greatest concern because they have repeatedly blocked and damaged the road.

The 2013 Igloo landslide; two people are circled for scale and the road grade bench is marked by a dashed orange line. Image from NPS's page.

The Igloo landslide has blocked the road numerous times, once for four days, but those closures came in September and October, after the busy summer season; the Eagle's Nest landslide came in July.

A road that matters

In 2017 Denali had approximately 600,000 visitors, who spent $632 million and supported 8,150 jobs and $924 million in general economic output, so the park and its road are a major economic engine for the region. The Alaska Road Commission built the road between 1922 and 1938 to give visitors and staff access and as a haul road for mining. The State of Alaska and the Park Service have nominated its current alignment for the National Register of Historic Places. Traffic is restricted: most visitors ride buses driven by highly trained staff, and all other drivers must watch a Denali-specific training video on traveling the road safely.

Shaded-relief map of Denali National Park and Preserve (green outline), the Denali Park Road (black line) and the George Parks Highway (red line). Image from NPS's page.

What drives the hazards

Plate tectonics builds the high relief of the Central Alaska Range, and steep slopes invite landslides, debris flows and rockfall. Earthquakes happen daily in Denali, though most are too small to notice; several are felt each year, particularly in the Kantishna area. A rare large one, magnitude 7.9, struck 19 miles (30 km) east of the park on November 2, 2002, and the article, citing a personal communication, says it released more energy than all the earthquakes in the lower 48 states over the previous 30 years combined. Earthquakes matter because they can trigger landslides, debris flows and rockfall.

Climate matters too. Mean annual temperatures in 2014 and 2015 were considerably warmer than the 1981-2010 normal, and forecasts through the end of the century indicate that Denali is likely to see increasing warming (about 4.0 °C) and more variable seasonal precipitation. Warming thaws permafrost, which can destabilize slopes. Models indicate that about 75% of Denali was underlain by shallow permafrost in the 1950s and project that shallow permafrost will cover only 6% of the park by the 2050s and 1% by the end of the century. The authors say permafrost thaw is already influencing landslide activity in the park, and that rapid thaw raises landslide susceptibility by altering the physical and hydrologic properties of hillslope materials, including reducing the cohesion of soils, which can let landslides occur on slopes as shallow as a few degrees.

The 2016 Eagle's Nest landslide, its margins marked by a yellow dotted line; two front loaders with seven-cubic-yard (five-cubic-meter) buckets are on the road near the top center. Image from NPS's page.

Why the park acts

The park's first goal is to use research to improve safety. The authors read the 1916 Organic Act's mandate both to conserve park resources and to provide for their enjoyment as implying a responsibility for visitor safety. Denali is very large, over six million acres, and remote, so attention goes where visitors spend the most time, a matter of exposure: a hazard near a seldom-used trail matters less than one over a busy road or housing area. Maintenance staff also work for hours, days or weeks at sites with known geohazards. The authors add that hazard research can help protect the government from successful tort claims, citing a 2008 federal case and a 2015 Washington State case in which courts found no liability because the government had shown a proactive, strategic approach to managing hazards.

Resilience is not new in Denali. In 1927 the president of the Alaska Road Commission wrote that it was cheaper and better in the long run to spread road work over several seasons, and to apply final surfacing only after the sub-grade had thawed, drained and stabilized.

The program today

The 2013 Igloo landslide pushed managers toward a more proactive stance, at the time the Unstable Slope Management Program for Federal Land Management Agencies (USMP) was being developed, and Denali joined its development early. With the Federal Highway Administration it now tracks over 140 unstable slopes along the road and has carried out a quantitative risk assessment of the ten highest-ranking sites. Conceptual designs and cost estimates for all high-ranking sites will give managers a range of options, from education and signage to complex engineering, and funding will be sought where the benefits outweigh the costs. A separate landslide inventory database records all known landslides in the road corridor, whereas the USMP records only slopes that have affected the road or trails, and a susceptibility model estimates landslide probability along the corridor.

Two projects with the U.S. Geological Survey are under way: a digital map and database of the surficial geology along the road corridor, begun in 2017, and a second, planned to start in 2019, to determine vulnerability, meaning the exposure, sensitivity and adaptive capacity of people and facilities. Federal Highway Administration borings have been instrumented to monitor water level, temperature and land displacement, and government and university researchers use the data to refine thermal, deformation and rockfall models. The U.S. Army Corps of Engineers' Cold Regions Research and Engineering Laboratory used geophysical techniques to look for permafrost and the depth to bedrock along approximately 10 miles (16 km) of road with past geotechnical problems.

The authors hope to support more permafrost studies, monitor more sites, refine the susceptibility models, maintain the USMP, improve funding structures, continue risk reduction, expand partnerships and increase geohazard awareness. They stress that the risk can never be fully eliminated in such a dynamic environment.

Sources

• Denny Capps (Denali National Park and Preserve), Douglas A. Anderson (Federal Highway Administration) and Molly McKinley (Denali National Park and Preserve), Geohazard Risk Reduction along the Denali National Park Road, Alaska Park Science 18(1), 2019, pp. 44-51, National Park Service; rewritten in hubnx's own words.