The influence of frost weathering on the debris flow sediment supply in an alpine basin

Rocky, alpine mountains are prone to mass wasting from debris flows. The Chalk Cliffs
study area (central Colorado, USA) produces debris flows annually. These debris flows
are triggered when overland flow driven by intense summer convective storms mobilizes
large volumes of sediment within the channel network. Understanding the debris flow

hazard in this, and similar alpine settings, requires determining the magnitude of sed-
iment accumulation between debris flow seasons, and identifying the control on sediment
production. To address these knowledge gaps, we measured changes in sediment produc-

tion using a sediment retention fence to quantify how sedimentation was influenced by
temperature at the plot scale. These measurements were extrapolated to a larger area,
where we extended the sediment fence results to explore how rockfall sedimentation con-
tributed to channel refilling between active debris flow periods. This work shows debris

flow channel refilling is correlated with low temperatures and time in the frost-cracking
window, implicating frost weathering mechanisms as a key driver of sedimentation. This
sediment production process resulted in a large amount of sediment accumulation dur-

ing a single winter season in our study reach (up to 0.4 m in some locations). Using these
observations, we develop a channel refilling model that generally describes the mass bal-
ance of debris flow watersheds in alpine areas.