FIRES, FLOODS, AND DEBRIS FLOWS: UNDERSTANDING SEDIMENT FLUX, GEOMORPHIC IMPACTS, AND FLUVIAL SYSTEM RECOVERY FOLLOWING DISTURBANCES

Extreme events are predicted to increase in step with climate change, altering Earth surface processes and landscapes. Of particular concern are disturbances that increase sediment delivery to channels because of the effects of increased sedimentation on aquatic ecosystems, channel and floodplain restructuring, and water resource management including water quality and quantity. Within the last seven years, extreme flooding, wildfire, and debris flows have occurred within the Colorado Front Range, resulting in widespread hillslope erosion, morphologic channel change and avulsion, sediment pulse evolution and transport, and extensive deposition. Because damage to communities and infrastructure commonly accompanies extreme events, mitigating the effects of sediment disturbance is of broad societal interest. We draw upon the results of several studies that assess the effects of extreme events on sedimentation and recovery. Analysis of flood effects from a substantial storm in September 2013 in two basins, one burned by wildfire in 2012 (260 km²), and one unburned (100 km²), indicates that 10s to 100s of years of sediment were eroded during the storm and transported to downstream receiving waters. Field data collection indicates that sediment fluxes returned to pre-disturbance values by year three after the fire. We also characterize 11 debris flows that initiated and transported sediment to valley bottoms during this same storm. Measured debris fan surface areas exceeded 100,000 m² and runout distances were >4 km. Gullying within debris flow scars, and scarp and levee diffusion has been the dominant post-disturbance response. Projected increased frequency of intense rainstorms, and increased fire intensity, frequency and extent in the coming decades requires that multiple, compound disturbances be addressed in post-disturbance response studies.