How Government Agency Planning Can Preserve Life Safety from Postfire Debris Flows

In 2020 the Grizzly Creek wildfire burned both sides of the narrow and deep Glenwood Canyon in Colorado, USA. Within the canyon there is a major Interstate Highway (I-70, the only east-west interstate highway across the state of Colorado), a major railroad (the Union Pacific), and a critical waterway (the Colorado River that supplies water to millions of downstream users). Within this canyon, there is a history of life-threatening postfire debris flows from two previous fires (the 1994 South Canyon Fire and the 2002 Coal Seam Fire) that both produced debris flows a few months following the wildfires. Based on this historical knowledge, several government agencies used their combined expertise to coordinate on life-safety decision-making following the Grizzly Creek Fire. After the Grizzly Creek Fire, nine large debris flows were triggered by rainstorms in the summer of 2021, followed by three small debris flows in the summer of 2023. Despite the disruptive postfire debris flow activity, there were no fatalities during these storms, which was largely due to a tiered strategy of hazard assessment/forecasting, monitoring, and adaptation. Many different government agencies worked together to share knowledge and inform decision-making to preserve life safety during these events, including: the U.S. Forest Service, U.S. Geological Survey, Colorado Department of Transportation (CDOT), and the National Weather Service (NWS). Weather forecasts and estimates of debris-flow likelihood, volume, and triggering rainfall thresholds were used to anticipate the location, triggering rainfall, and debris flow volume. These forecasts were compared with rainfall thresholds to determine when to deliver warnings to the public and advise canyon closures. After debris flow triggering rainstorms, the rainfall thresholds were re-evaluated. If a forecast was above the debris-flow rainfall threshold then the NWS would issue a watch or a warning. If the NWS issued a watch, CDOT staff would be positioned at either end of the canyon, and then if the NWS upgraded the watch to a warning CDOT staff would close the highway. This helped to make sure that the public was out of the canyon when there was a potential for debris flows. As the burn area recovered the warnings were adapted based on observations from monitoring. This collaborative model may be helpful in future wildfire situations in areas with critical infrastructure where the mandate for life safety falls across multiple jurisdictions.