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Fresh debris flow deposit associated with 2020 Museum Fire of northern Arizona

Arizona geoscientists anchor ‘Advances in Wildfire-Related Earth Surface Processes’ at GSA Annual Conference

Wildfires are a catalyst for change in hydrologic, geomorphic and biogeomorphic processes. Recent research suggests that past fire management strategies, climate change, and shifts in land use are already contributing to increased fire frequency and intensity across the globe.  In steep, forested catchments, wildfire generates flooding, increased hillslope and channel erosion due to vegetation loss, water/air contamination from burned particulates, and impacts to fluvial/riparian ecosystems. In the Western US, these catchments are often primary water supplies for agricultural and municipal purposes. In this session, researchers will present a broad range of topics related to wildfires and earth surface processes, including field studies, hazard analyses, emerging trends, and ecosystem impacts.

Ann Youberg (Arizona Geological Survey at UArizona), Luke McGuire (Dept. of Geosciences, UArizona) and Francis Rengers (U.S. Geological Survey) are hosting two presentation sessions and a poster session at the Geological Society of America Connects 2021 Annual Conference in Portland, Oregon in October 2021 to address recent advances in wild-fire related Earth-surface processes.   (GSA Conference Website).

  •    Advances in Wildfire-Related Earth Surface Processes I   Tues. 12 Oct. 8am-12pm
  •    Advances in Wildfire-Related Earth Surface Processes (Poster) Tues. 12 Oct. 9am-1pm
  •    Advances in Wildfire-Related Earth Surface Processes II  Tues. 12 Oct. 1:30pm-5:30pm

Presentations by AZGS and UArizona Geosciences Researchers

Beers, R. and 7 others, Geomorphic Monitoring and Response in a Post-Fire Ephemeral Channel, A Case Study from High Elevation, Steep Slopes in Arizona

ABSTRACT.  The 2019 Museum Fire burned nearly 2000 acres in steep, forested terrain abutting Flagstaff city limits in northern Arizona. In addition to the immediate danger posed by the fire, multiple neighborhoods and businesses are currently threatened by post-fire-flooding hazards. Post-fire flooding presents a serious hazard to the city because the burn scar is predominantly confined to the Spruce Wash watershed, which drains into eastern Flagstaff neighborhoods. Furthermore, within the burn scar, Elden Lookout Road provides the only route to critical communication towers and is at high risk for post-fire-flooding and debris-flow damage. The risk to the public and critical infrastructure has prompted a multi-agency cooperation to evaluate watershed response to rainfall and snowmelt, document watershed recovery, and evaluate efficacy of applied mulch treatments and road stabilization projects in reducing erosion and peak runoff. From fall 2019 to present, we employed geomorphic surveying techniques such as channel head, rill, ground cover, and soil infiltration surveys, as well as seismic monitoring and rainfall gauges to evaluate these parameters. During this period, Flagstaff experienced its driest (2020) and second driest (2019) monsoons on record, followed by three extreme rainfall events in July 2021, which provided the opportunity to document watershed response under these unique circumstances. The recent, extreme rainfall events initiated multiple debris flows and hyperconcentrated flows in the watershed on July 13th, 14th, and 16th. Each of these events caused severe, but repairable, damage to the road and stabilization structures. Additionally, these events induced widespread flooding in downstream communities, prompting the City of Flagstaff and Coconino County to declare a state-of-emergency, resulting in an emergency declaration of $200,000 from Governor Ducey. Our preliminary results show extensive hillslope erosion, and severe scouring in multiple drainages, and distinct seismic responses during the events, constraining timing for debris flow initiation. As the monsoon continues through the summer, it is possible that more debris flows and flooding events will occur, further eroding channels and hillslopes, and threatening local communities and critical infrastructure.

Youberg, A. and 4 others, Hydro-Geomorphic Impacts of Successive Wildfires in Areas with different Fire Exposures: Lessons Learned from the Santa Catalina Mountains, Tucson, AZ  

ABSTRACT. The Santa Catalina Mountains (SCM), located just north of Tucson, Arizona, have a rich history of wildfires. Prior to 1995, the largest historical wildfires documented in the SCM were less than 7,000 ac. Since then, however, the size and characteristics of wildfires have dramatically changed as drought conditions have intensified. For example, in 2002 the Bullock Fire burned ~ 32,000 ac. east of the Catalina Highway, which divides the mountain, and in 2003 the Aspen Fire burned ~ 85,000 ac. west of the highway. In 2017, the Burro Fire burned ~ 27,000 ac. on the far east side of the SCM, reburning a portion of the Bullock burn scar. Most recently, in June 2020, the Bighorn Fire burned ~ 120,000 ac. across most of the SCM, reburning through the Aspen, Bullock and Burro burn scars. In this study, we assess the hydro-geomorphic impacts of areas with different fire exposures by stratifying sites based on number of fires in the last 25 years (0, 1, 2+) and soil burn severity of each fire (unburned, low, moderate-high), a key driver of post-fire hydro-geomorphic impacts. We used minidisk infiltrometers to evaluate soil hydrologic properties (saturated hydraulic conductivity, sorptivity, wetting front potential), as well as rainfall data and field observations of flows (debris flow, flood, no response) to document geomorphic responses. Immediately after the Bighorn Fire, we established 6 study sites to assess soil hydrologic properties, and monitored 20+ basins, 6 of which were instrumented with pressure sensors or nodal seismometers and rain gauges. The 2020 monsoon was abnormally dry, with only 8 storm days recorded at the highest elevations and 3 days at lower elevations. Debris flows in high elevation, forested basins were triggered by a rainstorm with a peak 15-minute rainfall intensity (I15) of 55 mm/h, the highest I15 recorded at that gauge since the Bighorn Fire. At other rain gauges, however, higher I15 intensities were recorded on other days, including an I15 of 93 mm/h in a low elevation, desert basin, yet debris flows were not generated. During spring 2021, we added 20 additional study sites to assess soil hydrologic properties from areas with different fire exposures, and we continue to monitor geomorphic responses in study basins.

Catalina Fire History
Modern fire history of the Santa Catalina Mountains, southeastern Arizona.

Presentations/Posters that include AZGS staff as co-authors

McGuire, L, Youberg, A. and 3 others including R. Beers,  Geomorphic and Hydrologic Impacts of Successive Wildfires: Insights from the 2021 Telegraph Fire In Central Arizona (Poster)

ABSTRACT. The geomorphic and hydrologic impacts of wildfire are often considered to be independent of prior wildfires, with the magnitude of the impact often being a function of soil burn severity in the most recent fire. In many conceptual models for post-fire recovery, geomorphic and hydrologic systems return to a state that is similar to nearby areas that have not recently burned. As fire and climate regimes change, however, landscapes may begin experiencing fire on more frequent intervals that allow for minimal recovery between fires. For example, the 2021 Telegraph Fire burned a large portion of the Pinal Mountains in central Arizona, including much of an area that had recently burned in the 2017 Pinal Fire. In this study, we examine soil hydraulic properties and debris flow activity in the first several months following the 2021 Telegraph Fire and compare them with observations and measurements made after the 2017 Pinal Fire. Following the 2017 Pinal Fire, we established five monitored watersheds (< 1 km2) to assess changes in soil hydraulic properties and debris flow activity as the landscape recovered. Several of these watersheds burned again in the 2021 Telegraph Fire, providing an opportunity to explore how successive fires influence hydrologic and geomorphic response. Field measurements of soil hydraulic properties made with mini disk tension infiltrometers indicate reductions in sorptivity and wetting front potential following the Telegraph Fire that are roughly equivalent to those observed immediately after the Pinal Fire, despite the fact that the area experienced moderate-high soil burn severity during the Pinal Fire and low soil burn severity during the Telegraph Fire. Preliminary observations of increased debris-flow activity within areas burned during both the Pinal and Telegraph Fires, relative to observations following the Pinal Fire, suggest that areas burned by both fires may be more susceptible to extreme geomorphic and hydrologic responses.

geomorphic study
UArizona & AZGS researchers investigating conditions on the slopes of Green Mountain.

Cover Photo (B. Beers) - A 3000 m3 debris flow deposit in the Museum Fire burn scar outside Flagstaff, Arizona resulting from intense rainfall on August 17th, 2021.


The Geological Society of America Connects 2021, Annual Conference in Portland, Oregon, USA  10-13 Oct. 2021