In August 2020, the Grizzly Creek Fire ignited in the rugged Glenwood Canyon of central Colorado. Glenwood Canyon is considered one of the most scenic corridors on the U.S. Interstate Highway System and is a critical route for road and rail traffic across the state as well as providing recreation opportunities for hiking, biking, hunting, and river rafting. Over an approximate four-month period the fire altered forest lands along the steep canyon walls and forested connecting drainages above Interstate 70 (I-70) and the Colorado River.
The following winter provided a quiet recovery period for the canyon, but this was only temporary as the summer months in this region of Colorado generate intense thunderstorms with runoff that can overwhelm heathy drainages. Unfortunately following a forest fire, the storm runoff on burned and bare soils can be orders of magnitude more destructive. The summer monsoon season of 2021 was no exception, with several storms generating sediment laden post-wildfire debris flows that covered and damaged I-70 and the nearby Amtrak railway, deposited sediment in the Colorado River, and also stranded travelers in the canyon at times. The events resulted in weeks of highway closure for this critical corridor during the summer travel season, causing adverse economic impacts to nearby communities and measurable disruptions to interstate commerce.
To reduce the potential for future disruptions, the Colorado Department of Transportation (CDOT) engaged with BGC to understand how ground conditions are changing following the wildfire and 2021 post-fire debris flows. Through this additional understanding, CDOT can prioritize mitigation projects on the basis of greatest need and cost-benefit, while also advancing predictive models that consider the relationship between burned conditions, slope, changes in terrain, and precipitation thresholds that can lead to disruptive debris flows.
Debris flow deposition on I-70 bridge approach.
To measure continuous ground change over the entire burn area, BGC contracted with an aerial survey firm to collect and process airborne lidar for over 100 square miles (260 square kilometers) of the Glenwood Canyon and Grizzly Creek Fire area. This newly collected lidar data was processed against existing public lidar data collected in 2016. These two sets of lidar data were entered into Cambio, BGC’s software platform, to deliver an interactive lidar change detection layer across the entire burn area. This processing of change between two different lidar data sets uses a patent-pending change detection algorithm to calculate positive and negative change over this entire area. Using Cambio, this type of change detection processing can be turned around within 24 hours.
BGC continues to work with CDOT and other partner agencies, such as the United States Geological Survey, to understand how the Canyon slopes have changed after the fire and 2021 debris flow season, and to plan mitigation efforts that can be implemented in the summer of 2022 and beyond. A better understanding of the post-fire debris flow events in Glenwood Canyon may also help CDOT and other stakeholders understand their risk exposure to debris flow impacts from future burn scars.
The Geohazards paradigm is so different to what a lot of the other members of CDOT are used to being exposed to. It is often difficult to convey the severity of an event to people outside of the response, but Cambio is such a great tool to do this alongside the other utility it provides.
Mark Vessely, M.Sc., PE.
Principal Geotechnical Engineer
Mark Vessely has over 25 years of experience in geologic hazard and risk assessment, emergency response to slope and other ground movements, and design for bridge foundations, retaining walls, pavements, and slope stabilization projects.