GEOMORPHIC MONITORING AND RESPONSE IN A POST-FIRE EPHEMERAL CHANNEL, A CASE STUDY FROM HIGH ELEVATION, STEEP SLOPES IN ARIZONA

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 13^th, 14^th, and 16^th. 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.