A Professional Monitoring System Helped the National Park Service Make Critical Infrastructure Decisions
Alaska’s Denali National Park—also known as the “Crown Jewel of the North”—comprises six million acres of celebrated wildlife, outdoor adventures and breathtaking natural beauty, including Pretty Rocks Landslide. While active for at least 60 years, Pretty Rocks Landslide has only recently grown from a minor maintenance nuisance to a significant safety issue causing road restrictions.
Prior to 2014, geologists and road maintenance crews noticed small cracks in the road surface, but the cracks were easily repairable. Each year since 2014, staff have observed substantial increases in landslide-caused road movement. Denali National Park and Preserve (Denali)—a division of the U.S. Department of Interior’s National Park Service—implemented a monitoring program in 2016 and by 2021, the rate of road movement evolved from dropping 6-12” per year to dropping up to .65” per hour. At this point, standard maintenance activities were no longer deemed safe or effective, so major road closures began while Denali determined how to re-establish the road through this area with a more resilient and long-term solution.
Crucial to this monitoring project was that Denali needed to determine the rate of movement to evaluate safety and to understand the underlying cause for the movement. They had previously noted a correlation between rain and unusually warm years, but wanted to know if there were other factors leading to the quick road decline.
Composite photo of the Pretty Rocks Landslide area from 2015. Approximate landslide outline indicated by red dashed line. (NPS photo and caption from article)
Pretty Rocks Landslide location (NPS photo)
Beyond maintenance–overcoming challenges to understand the root cause
The National Park Service studied landslide monitoring options and identified a very low cost, yet unproven, method they could implement alone. However, the team felt the safer and more effective option was to go with an “out of the box” proven monitoring system. The team turned to Trimble® to strategize and determine next steps.
Trimble has provided manual monitoring systems for Pretty Rocks Landslide since 2016 and has extensive knowledge of, and experience with, challenging monitoring projects in remote locations. The existing manual monitoring campaign involved 30 installed monuments, rebar with aluminum caps installed in the landslide surface, and Trimble RTK GNSS receiver to measure the monuments on a periodic basis. This system accurately measured movement but didn’t capture finer temporal scales. The RTK allowed just as many data points; however, there was a much lower temporal resolution and longer time between data collection and availability, due to post time. Therefore, the Trimble team of monitoring experts recommended an automated monitoring total station (AMTS) system for accuracy, precision, near-real-time capabilities and cost effectiveness.
“The Trimble monitoring team implemented a system that met our needs for making safety and maintenance decisions for the Pretty Rocks Landslide project,” said Denny Capps, the park geologist. “Before implementing the automated monitoring system, we had a poor understanding of what meteorological conditions were the primary drivers of increasing displacement. Heavy rain is often a predictor, but warming conditions are also important because the site contains abundant ice-rich permafrost.”
September 16, 2021 photo of eastern side of the Pretty Rocks Landslide. The displacement of approximately 14 vertical feet seen below the person standing on the stable road surface (for scale) occurred over two weeks following the cessation of maintenance and road use on September 2, 2021. (NPS photo and caption from article)
(NPS photo and caption from article)
Pretty Rocks Landslide’s rugged and remote location was the primary monitoring challenge. The landslide traverses steep topography and loose volcanic rock; therefore, the ability to monitor the movement from a safe distance was a distinct advantage. Even then, staff required fall protection equipment to access the AMTS. Because of the distance from other infrastructure, the monitoring team utilized a solar installation for power. Considering the lack of cellular and local networks, the monitoring team set up a satellite system with the right blend of data transmission capacity and cost to maintain 24/7 communication. This option enabled the National Park Service team to receive data from the system in near-real-time at any location with an internet connection and the proper login credentials.
A system in place to identify and measure displacement
The monitoring team installed the following system for the Pretty Rocks Landslide project:
- Trimble S5 AMTS, to accurately measure distances and angles from about 35 small prisms installed within and around the landslide. The team chose prisms as a cost-effective option over GNSS sensors because they expected damage to the monitoring equipment by rockfall and debris flows. The team took initial measurements to establish a baseline and the total station repeated the measurement at hourly intervals, 24 hours per day.
- Trimble Settop M1, a total station controller that’s a combination of a field computer, device server, router, cellular modem and remote switch, which controls and powers the total station enabling data collection and transferring the data in real-time to the office software.
- Trimble T4D Control™ (T4D) software, to manage data and post to a web interface that allows staff to view the data remotely at any time. T4D was crucial to creating graphs of the prism locations. The team configured analysis, reporting and alarms, based on knowledge of the project site. Prism locations adjacent to the road were the most valuable because they present clear data for monitoring interpretation. T4D is equipped with weather and movement threshold alarms that automatically alert staff when the preset thresholds are exceeded. Thresholds were set to monitor for excessive movement, heavy rain, etc., and proved extremely helpful to the monitoring team.
- Weather station, which includes a shielded thermometer, anemometer (wind speed and direction) and tipping-bucket rain gauge providing environmental information which can be correlated to movements on site.
- Time-lapse camera, that provided context for, and confirmation of, instrumental readings. Time-lapse videos are also powerful visuals to help nontechnical audiences understand complex 4D processes.
- Solar-power system, that utilizes solar panels to charge numerous batteries that power the system during low-light times.
- Satellite internet system, to relay all the data from the above system to a computer at park headquarters.
Working through learning curves and identifying the root cause
Through drilling and additional investigation, a broader team of experts determined Pretty Rocks is actually a rock glacier that is likely accelerating due to warming ground temperatures and heavier precipitation.
Monitoring data from the AMTS was key to showing the rate of increase and movement levels of the landslide and helping the Denali team make informed decisions throughout the project, including the difficult decision to put road restrictions in place for short periods in 2019 and 2021, an early closure in 2021, and a complete closure in 2022.
“During all phases of the project, the Trimble monitoring team delivered a fully functional monitoring campaign that met our needs,” recalled Capps.
Sightseers and adventure seekers should soon be back on the road to experiencing the park’s natural beauty, including the Pretty Rocks area.
Scientific illustration of the surface and subsurface of the Pretty Rocks landslide by Laurel Mundy (NPS illustration taken from article)