The Pretty Rocks Landslide at Mile 45.4 of the Denali Park Road has the potential to disrupt transportation, impact visitor experience, cause resource damage and affect public safety.
Over the last two decades, the landslide has evolved from a minor maintenance concern to a threat. During the 1990s, the landslide caused small cracks in the road surface.
In the spring of 2019, the landslide was displacing the full width of a 100-yard (90-m) section of road up to .45 inches (1.1 cm) per day vertically and horizontally, creating a swale that steepens the road gradient and limits sight lines.
If the landslide continues to increase in velocity, we will soon reach a threshold where the National Park Service can no longer maintain the current road alignment.
The management of Denali National Park and Preserve has made substantial gain in its knowledge of the Pretty Rocks Landslide and associated risk reduction techniques; however, ongoing and proposed work will provide higher-value decision-making tools. Long-term solutions are likely very costly, so park managers must make well-informed decisions early in the process.
Changing environmental conditions such as permafrost thaw and rainfall that is increasing in amount and severity will likely aggravate the issue. When the efforts listed below are completed, managers will have the tools to prioritize risk-reduction actions. This process also offers a transition from reactive to proactive management of this and other geohazards along the Denali Park Road.
Federal Highway Administration and NPS staff installed monitoring equipment in two bore holes below the road surface in 2003. The bore holes confirmed the presence of permafrost and substrate type; however, landslide motion sheared the monitoring equipment before staff obtained useful measurements.
Landslide velocity increases immediately after material is added to the swale created by the slide. This was noticeable in 2014 and 2018, and is consistent with landslide science—more material increases the weight and translates into additional movement.
NPS maintenance staff adds minimal material to the swale as needed to provide a safe road surface. However, road gradient is steepening once again and sight distance is becoming limited. We will need to add more material in 2019 for safe passage and this may further increase movement.
The road hazard was quantified in 2016 through the Unstable Slope Management Program for Federal Land Management Agencies (USMP) and ranked highest of 141 unstable slopes analyzed along the road.
Also in 2016, park staff created a probability-based landslide susceptibility model and associated map.
Federal Highway Administration and park staff collaborated with the Army Corps of Engineers in 2016 to determine subsurface characteristics of bedrock and permafrost affecting the site. Mapping showed permafrost containing massive ice below the road. Permafrost is known to be thawing regionally.
Beginning in 2015, we have obtained aerial photogrammetry of the site at least once, but often several times, per year. The photos and associated digital elevation models validate past observations of landslide velocity and show displacement patterns.
Denali hosted a multidisciplinary risk assessment workshop in 2017 that identified unstable slopes as the greatest risk to the road with permafrost degradation also rating high as a causative agent.
Three highly rated unstable slopes along the road were prioritized in 2017 for risk reduction work: Pretty Rocks (Mile 45); a recurrent landslide at Bugstuffer Creek (Mile 52); and recurrent rockfall at Toklat Bluffs (Mile 53).
We update the USMP rating and susceptibility model as necessary. USMP ratings change as site conditions evolve. While the first susceptibility model is now complete, new information or techniques will improve the model and subsequent outputs.
A multidisciplinary work group is monitoring and responding to geohazard threats. The team is developing short- and long-term response plans. We completed a draft report summarizing existing knowledge of the landslide in 2018. We expect to complete a peer-reviewed document by mid 2019.
We will systematically monitor the landslide with aerial photogrammetry surveys and analyses.
USGS and park staff are mapping the surficial geology of the entire road corridor. The data will help management of unstable slopes identified in the USMP and assist with planning for potential reroutes.
Federal Highway Administration and park staffs are investigating the feasibility of rerouting the road if stabilization efforts are impractical or fail. The reroutes are complex because of their length (five miles), terrain and location in designated wilderness.
In the summer of 2018, FHWA supervised the drilling of five new boreholes, instrumentation installation, and monitoring. The drilling and instrumentation provide data on substrate composition, ground temperature (permafrost), water level, and ground displacement location and magnitude. Additionally, we installed a rain gauge and thermometer at the surface to help determine if temperature and rainfall substantially affects landslide displacement. We will continue to monitor these boreholes as long as they continue to provide useful information.
In the summer of 2018, park staff installed and began monitoring a network of 30 monuments to track surface displacement with survey-grade GPS. We will repeat this monitoring at least twice a year for the foreseeable future.
Researchers will create a permafrost model to predict ice-rich permafrost thaw through time. FHWA will use the results of this model, borehole data, and other information to create a displacement model of the landslide to inform risk reduction techniques.
In coordination with U.S. Geological Survey, park staff will conduct a geohazard vulnerability assessment of the road, with emphasis on the landslide. Managers must understand the geohazards and associated risks, as well the ways in which visitors, staff and assets are vulnerable to the hazard.
Park staff, along with Federal Highway Administration, will conduct a benefit/cost analysis for the most significant unstable slopes in the USMP. We will then use the benefit/cost analysis to develop a prioritized list for programming based on the greatest benefit to stakeholders.
Researchers will combine observations of spatial and temporal patterns of landslides in Denali with permafrost monitoring to predict the location, mechanism, magnitude, and timing of landslide events in the road corridor. The researchers will build a predictive model that the NPS can use to identify both high-risk and stable sections of the road.
Igloo Debris Slide
On September 30th and October 18th 2018, the Igloo Debris Slide partially blocked the Denali Park Road. In each event, the Denali road crew cleared approximately 500 cubic yards (450 m3) from the roadway within a few hours. Like the 2013 event, 12’ (4 m) blocks of ice-rich, unconsolidated debris slid on the same or similar unfrozen clay layer. 2013 and 2018 both experienced unusually mild fall weather. Such weather further thaws permafrost, which decreases cohesion and increases pore water pressure. Therefore, this combination of processes can cause and trigger landslides. As more permafrost has thawed and the slide has continued to move, the area of the slide has increased. Several smaller events have filled the road ditch since September 30th.
Geology and engineering experts from the NPS and Federal Highways Administration expect activity at this site to continue in subsequent years. Therefore, we are currently developing designs to reduce risk at this site and others as part of the Unstable Slope Management Plan for Federal Land Management Agencies.
In late October 2013, road maintenance staff discovered that a 600’ (180 m)-long, 110’ (35 m)-wide debris slide had blocked the park road near Mile 38.
Blocks of ice-rich, unconsolidated debris as thick as 15’ (5 m) and the size of a small cabin had slid on a slippery, unfrozen clay that acted as the failure plane. With winter snows held off by unseasonably warm weather, the Denali road crew managed to clear the road of debris after considerable effort.
The trigger for the slide remains unknown. Ground, aerial, and satellite imagery of the site in the years and months before to the event indicate that a small slide had previously occurred here, groundwater seeped from the area, and the ground was beginning to move slightly. In the days preceding discovery of the slide, the area was experiencing temperatures that fluctuated near the freezing point. Therefore, the forces associated with the expansion of ice during the repeated freezing and thawing of water near the surface may have triggered the slide. Alternatively, we also know that a thick layer of permafrost slid on an unfrozen layer of clay.
Regionally, permafrost is thawing; while the trend at the site is presently unknown, thaw in the area would be consistent with regional trends. Therefore, it is possible that the permafrost thinned through the clay layer, which triggered the slide. Many other triggers are also possible and are being examined.
Rockfall Audio Clip
Rocks plummet from great height and strike a glacial moraine, echoing across the steep-walled valley. Recorded by the Yentna-Lacuna Confluence Sound Station.
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