Inundation and flow properties of a runoff‐generated debris flow following successive high‐severity wildfires in northern Arizona, USA

Abstract

Burned slopes are susceptible to runoff‐generated debris flows in the years following wildfire due to reductions in vegetation cover and soil infiltration capacity. Debris flows can pose serious threats to downstream communities, so quantifying variations in flow properties along debris‐flow runout paths is needed to improve both conceptual and quantitative models of debris‐flow behaviour to help anticipate and mitigate the risk associated with these events. Changes in flow properties along the runout paths of the runoff‐generated debris flows that follow fire may be particularly dramatic, since they initiate when a water‐dominated flow rapidly entrains sediment and later transition back to a water‐dominated flow once they reach greater drainage areas and lower slopes. Here, we study the properties of a debris flow that initiated 1 month following the 2022 Pipeline Fire in northern Arizona, USA. We categorized flow type into two classes, granular debris flow and muddy debris flow, along the 7‐km runout path and examined how flow properties varied between the phases. Changes in channel gradient and confinement likely facilitated the transition between the flow phases, which were characterized by significant differences in maximum clast size, but similar clay content and fine fractions. We also found that the volume and runout distance of the debris flow were 28 and six times greater, respectively, than that of a debris flow that initiated in the same watershed following a fire 12 years earlier. We attribute these differences to the combined effects of two high‐severity fires, suggesting that consideration of recent fire history could improve post‐fire debris‐flow hazard assessments. Results of this study provide quantitative constraints on changes in post‐fire debris‐flow properties along a runout path. Data collected in this study add to a small number of debris‐flow inundation datasets that can be used to test runout models in post‐fire settings.

A Prefire Approach for Probabilistic Assessments of Postfire Debris‐Flow Inundation

“…Liu et al (2016), O'Brien and Julien (1988), and O'Brien et al (1985, 1993. The VC range was established based on the uncertainty in the volume of debris flow deposits observed by Gorr et al (2024) and the hydrograph simulations (Figure 2). Manning's n values were selected to cover a broad spectrum from 0.03 to 0.1 m s 1/3 , which align with the conventional values attributed to barren terrains and mixed forests, and are similar to those calibrated in recently burned watersheds in the Earth's Future 10.1029/2023EF004318…”

“…Postfire debris flow volume models have been developed in the western USA and Australia (Gartner et al, 2014;Nyman et al, 2015;Santi & Morandi, 2013;Wall et al, 2023) while PFDFs have been reported much more broadly (e.g., Conedera et al, 2003;Esposito et al, 2023;Wang et al, 2022). Employing PFDF volume models outside regions where they were developed has been met with mixed results (Gorr et al, 2024;Nyman et al, 2015) and is likely to increase uncertainty associated with flow volume, a critical input for inundation models (Barnhart et al, 2021). An alternative approach to estimating inflow hydrographs for debris-flow runout modeling is the use of a bulking factor (e.g., Gusman et al, 2009).…”

“…The Copeland headwater drainages were burned by the 2010 Schultz Fire and again by the 2022 Pipeline Fire. PFDFs were documented following both fires (Gorr et al, 2024;Youberg & McGuire, 2019). Data from the PFDF that occurred on 15 July 2022, shortly following the Pipeline Fire is used in this study.…”

“…We refer to this particular flow as the Copeland debris flow. The Copeland debris flow traveled ∼7 km from the headwaters to the downstream neighborhood built on the unincised Holocene alluvial fan (Gorr et al, 2024). The Fort Valley site, which is located on the southwest slope of the San Francisco Peaks (Figure 1d), also consists of two watersheds (1.61 and 0.97 km 2 ) and an alluvial fan with elevations ranging from 2,355 to 3,766 m above sea level.…”

“…Detailed hydrogeomorphic field data, including the measurements of flow depth, extent of inundation, debris-flow timing and rainfall hyetographs for the triggering rainstorm, and soil-hydrologic properties may be required for model set up, calibration, and evaluation. Here, we take advantage of recently acquired field observations and hydrogeomorphic data from a runoff-generated debris flow following the 2022 Pipeline Fire in northern Arizona (Gorr et al, 2024). We propose a modeling framework for a probabilistic PFDF inundation assessment in a prefire context.…”