26 D9. Using Geological Archives to Understand and Document Earth Surface Processes and Past Climate

Wednesday, 13 May 2020
1:30 PM - 5:30 PM
Madera Room

POST-WILDFIRE SEDIMENT TRANSFERS FROM DRAINAGE DIVIDES TO ALLUVIAL FANS DOCUMENTED WITH NESTED SCALES OF TOPOGRAPHIC MONITORING

Paper number: 26-5
Program: Technical Sessions
Time: 2:50 PM – 3:10 PM

Authors

Abstract

In mountainous semiarid regions, wildfire is an increasingly important disturbance that leads to drastic increases in runoff and erosion that can pose hazards to downstream aquatic biota and human infrastructure in wildland-urban interfaces. Most hazardous are post-fire debris flows, which are generated from the bulking of runoff through direct entrainment of sediments derived from large areas of the burned watershed rather than discrete landslides. In soil-mantled watersheds this broad source area has hampered our understanding of the dominant processes involved, which are conceptualized as a competition between extensive shallow hillslope erosion and confined deep channel erosion. We seek to answer the following questions: (1) Does channel erosion become the dominant process at critical thresholds over larger drainage areas? (2) What longitudinally-varying topographic characteristics of the trunk channels control the degree of sediment export from burn areas?

To answer these questions, we used a nested scale approach of analyzing change in a first-order catchment (~0.07 km2) with ground-based lidar and unmanned aerial-based photogrammetry, and airborne lidar at the full upland catchment scale (1-10 km2). Sediment budgets derived from the hillslope-to-first order channel transition reveal that the dominant erosional mechanism fueling debris flows rapidly toggles over to deeper channelized incision of post-fire dry ravel and pre-fire channel fill. However, monitoring at canyon mouth debris basins revealed muddy streamflow as opposed to debris-laden flows documented in the uplands. Preliminary results from airborne lidar change detection shows that debris flows became progressively attenuated once channels became unconfined and lower gradient (<10%). These results show that channel network characteristics and basin morphology are important considerations for debris flow export from burn areas and should be incorporated into post-fire hazard models.

Geological Society of America Abstracts with Programs. Vol. 52, No. 4

doi: 10.1130/abs/2020CD-346997

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