THE INFLUENCE OF LARGE WOODY DEBRIS ON POST-WILDFIRE DEBRIS-FLOW SEDIMENT STORAGE

Debris flows transport water and granular material, such as sediment and wood, and this water-particle slurry can have devastating impacts on life and infrastructure. The proportion of large woody debris (LWD) incorporated into debris flows can be enhanced in forested areas recently burned by wildfire because wood recruitment into channels is typically higher in burned forests than in unburned forests. In this study, we explored how LWD transported in post-wildfire debris flows retained sediment within small headwater channels in the Tadpole fire, which burned 45 square kilometers of the Gila National Forest in New Mexico in June and July, 2020. Debris flows occurred in areas where the pre-fire vegetation was dominated by Ponderosa Pine (Pinus ponderosa) between July and September 2020. We found that LWD promoted sediment retention and that the largest amount of debris-flow sediment was stored in channel reaches with a ratio of LWD length to channel width between 0.25 and 1. Moreover, the LWD was effective at promoting deposition of debris-flow sediment in low-angle (<25 degrees) channel reaches. Here we show how sediment storage behind LWD results in punctuated sediment storage patterns that differ from those predicted by a commonly used post-fire debris-flow volume model. Finally, we demonstrate a method for estimating debris-flow velocity based on estimates of the critical velocity required to break wood with various diameters.