REFINING THRESHOLDS FOR RUNOFF-GENERATED DEBRIS FLOWS: INSIGHTS FROM A RECENTLY BURNED AREA IN THE GILA NATIONAL FOREST, NEW MEXICO, USA

Debris flows are often triggered by runoff in steep, recently burned terrain. Rainfall intensity-duration (ID) thresholds are frequently used for hazard assessment purposes to estimate the likelihood of post-fire debris flows. It is well established that the magnitude of rainfall ID thresholds varies among regions and among watersheds within individual burned areas due to differences in drainage basin morphology, burn severity, and soil properties. In general, however, we lack a physically-based explanation for the spatial variability of rainfall ID thresholds within burned areas. In this study, we monitored debris flow and flood activity within several small (0.01-1 km²) drainage basins following the 2018 Buzzard Fire in the Gila National Forest, New Mexico. By using pressure transducers to constrain the timing of debris flows within rainstorms, we determined that debris flows tended to initiate once a critical rainfall ID threshold had been exceeded. Although there were differences in debris flow activity across our study area, water-dominated floods generally occurred when 15-minute rainfall intensities were less than approximately 40 mm/h while debris flows often initiated at higher rainfall intensities. Using a hydrologic model constrained by field measurements of soil hydraulic properties, we determined that the observed rainfall ID threshold is generally consistent with a recently derived hydrodynamic threshold for the initiation of runoff-generated debris flows. Further numerical experiments of runoff in response to observed and idealized rainstorms then revealed several aspects of basin morphology that appear to play important roles in determining debris flow potential in our study area, including hillslope length. Results provide additional constraints on the rainfall intensities needed to produce post-fire debris flows in the Southwestern U.S. and offer process-based insights into why particular basin morphologies may promote debris flows rather than floods.