MONITORING THE EFFICACY OF NOVEL FLOOD-MITIGATION STRUCTURES BELOW THE 2022 PIPELINE FIRE BURN SCAR

Wildfires pose a significant threat to communities living in the wildland-urban-interface (WUI). While the immediate wildfire danger persists for weeks to months, subsequent post-fire flooding can persist for years in communities downstream of a burned area. The post-fire flooding risk to WUI and downstream communities creates a need to increase community resiliency to flood events through cost-effective, quickly implemented post-fire flood control mitigation. Near Flagstaff, Arizona, multiple WUI communities have experienced significant flooding following recent fires, including the 2010 Schultz Fire, 2019 Museum Fire, and the 2022 Tunnel and Pipeline Fires, which reburned the lower and upper slopes of the Schultz Fire.

In response to post-fire flooding following the 2022 fires, the Coconino County Flood Control District (CCFCD) constructed eight mitigation structures in watersheds upstream of neighborhoods on the eastern side of the burned area and the City of Flagstaff constructed three detention basins on the western side. Instead of constructing traditional detention basins, CCFCD is employing a novel mitigation strategy. They are reshaping existing alluvial fans and floodplains to create broad, low-relief surfaces in an effort to slow flood-flow velocities, promote infiltration and sediment deposition, and reduce impacts to downstream communities. A potential benefit of this strategy is that these surfaces are expected to behave similarly to natural alluvial surfaces and thus may not require repeated maintenance and associated costs linked to detention basins. This technique has been used in Coconino County before, but those structures were constructed three years after the 2010 Schultz Fire, and three years after the 2019 Museum Fire, after substantial hydrologic recovery of the affected areas. Because the Pipeline mitigation structures were constructed between the first and second post-fire monsoon, we can assess the efficacy of this strategy before the watersheds recover. Here, we present monitoring results from the 2023 monsoon and assess performance of this novel strategy against traditional detention basins and an untreated, control fan.