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Viewing fresh debris flow deposits

Post-fire flooding and debris flow impacts in the Museum Fire burn scar

The Museum Fire ignited on July 21, 2019, one mile north of Flagstaff, Arizona, and burned approximately 2000 acres of steep, forested terrain. The fire prompted evacuations for Flagstaff residents near the flames, and although the immediate fire danger has passed, many residents remain in danger of post-fire flooding. To mitigate damage to homes and businesses downstream of the burn scar, Coconino County Flood Control (CCFC) and the City of Flagstaff (CoF) deployed sandbags and jersey barriers and made stream channel improvements to prevent damage to underground gas and electrical lines. Additionally, the Coconino National Forest (CNF) implemented structures such as cable barriers and articulating-concrete-block mats to protect Elden Lookout Road, the only roadway to critical communication towers, from irreversible damage.

From a geological perspective, we want to know how the burned landscape will respond to rainfall and snowmelt, how it will recover over time, and to analyze if road stabilization projects and aerial mulch treatments are effective in reducing roadway and landscape erosion. Multiple agencies are currently working together to answer these questions through geomorphic monitoring led by myself (AZ Geological Survery [AZGS], and formerly Northern Arizona University [NAU]) and Pete Robichaud (Rocky Mountain Research Station [RMRS]), seismic monitoring led by Ryan Porter (NAU), game cameras and rain gauge monitoring with data assistance provided by Joe Loverich (JE Fuller Hydrology and Geomorphology, Inc. [JEF]), Elden Lookout Road engineering reports provided by Sean Untalan (CNF), and additional support from Ed Schenk (CoF), Taylor Joyal (NAU), Jacob Dahlin (CNF), Ann Youberg (AZGS), and CCFC. From fall 2019 to present, I monitored stream channel erosion, hillslope rilling (slope erosion), soil infiltration rates, and changes to vegetation cover. Seismometers were also implemented to measure vibrations in the earth during debris flow events, which helped us narrow-down the exact timing of the events. Although minimal flood activity occurred in 2019 and 2020 (the second driest, and driest monsoons in Flagstaff recorded history, respectively), July 2021 has brought strong monsoons and subsequent flooding and debris flows.

On July 14th, 2021, I was joined by UArizona Ph.D. student Alex Gorr and NAU graduate student Chad Kwiatkowski to investigate fresh post-fire debris flow and flood deposits initiated by intense rainfall over the Museum Fire burn scar. The debris flows and floods were initiated on July 13th after 2.16 inches of rain fell on the scar, with 1.06 inches falling in less than 15 minutes (Source: JEF Museum Fire North rain gauge). Upon investigation, we found that the debris flows near the first Elden Lookout Road switchback had moved boulders onto the road, the largest boulder being about 2.5 feet across (cover figure). Footage of the flood event on Elden Lookout Road was captured by Enrique Rubio, a local Flagstaff runner who was momentarily stranded by the flood waters as they rushed over the road. Enrique graciously provided video of the event that can be viewed at '30-s flood video'. In addition to moderate road damage, the storm event caused extensive flooding along roadways in the downstream community.

Heavy rain fell again on July 14th, causing more damage to Elden Lookout Road and flooding in neighborhoods. Although this second storm produced less rainfall (1.22 inches total, with a maximum 15-minute intensity of 0.75 inches [Source: JEF Museum Fire South rain gauge]), damage to Elden Lookout Road was more substantial than the preceding storm (Figure 2). This is likely due to the soils being saturated from the previous days’ rain, causing a higher percentage of Wednesday’s rainfall to runoff, rather than infiltrate into the soil. Interestingly, this storm event also produced a microburst approximately 1.5 miles away from the Museum burn scar in the Fanning Wash watershed, resulting in at least one debris flow that impacted trails in the Elden foothills, and raging floodwaters that damaged homes and other property in the Greenlaw and Swiss Manor neighborhoods.


Fresh debris flow
Figure 2: Boulders, cobbles, and a tree lay across Elden Lookout Rd., just below the first road switchback following the July 14th downpour. Pictured: Alex Gorr.

Intense rain fell yet again on the burn scar on Friday, July 16th, producing a total of 1.42 inches with a peak 15-minute intensity of 0.63 inches (Source: JEF Museum Fire North rain gauge). This storm was the most destructive of the three storms, and left Elden Lookout Road severely damaged, but still repairable. The erosive power of the floodwaters was captured by JE Fuller, and left the road impassable in at least one location until road crews were able to access the area (Figure 3A). Seismometers in the scar also distinctly recorded the debris flow and flooding events, which show that there were likely two pulses to the events (Figure 3B).

Figure 3
Figure 3: (A) Timelapse photos at Elden Lookout Road crossing from JEF showing flood progression on Friday, July 16th. Notice the severe erosion of the road-bed and introduction of small boulders. (B) Seismograph from the Friday, July 16th debris flow events, provided by Ryan Porter at NAU. In the top graph, the large “squiggles” show seismic energy produced by two back-to-back debris flow events in a tributary channel of the Spruce Wash. The purple bars on the graph show rainfall at the Museum Fire North rain gauge, and the blue bars show rainfall at the Museum Fire East rain gauge. Both rain gauges are operated by JEF. The middle graph simply shows power to the seismometer instrument, and the bottom graph shows seismic energy in terms of frequency. 

Overall, our research demonstrates that the Museum Fire burn scar is very responsive to rainfall runoff after two years of extreme drought. As we continue our research, we will further evaluate how erosion from the mid-July events has impacted the landscape within the scar. However, as the summer monsoon progresses, the possibility for additional flood and debris flow events remains.

Acknowledgements: Funding for this research is provided through the U.S. Forest Service and I want to thank all volunteers and agencies who have made this work possible including Rocky Mountain Research Station, Northern Arizona University, JE Fuller Hydrology and Geomorphology Inc., Coconino County Flood Control, Arizona Geological Survey, City of Flagstaff, and Coconino National Forest.

Posted by Rebecca Beers, AZGS Research Scientist (7/28/2021)

Cover figure: Chad Kwiatkowski (left) and Alex Gorr (right) stand amongst boulders and cobbles deposited on Elden Lookout Rd., just below the first road switchback after the July 13th storm.