The Influence of Frost Weathering on Debris Flow Sediment Supply in an Alpine Basin

F. K. Rengers,Luke A. McGuire,Jason W. Kean,Joel B. Smith,Nadine G. Reitman,Jeffrey A. Coe

doi:10.1029/2019jf005369

Abstract

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 sediment accumulation between debris flow seasons and identifying the control on sediment production. To address these knowledge gaps, we measured changes in sediment production 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 contributed to channel refilling between active debris flow periods. This work shows that 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 during 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 balance of debris flow watersheds in alpine areas.

Highlights

Debris flows are mixtures of water and unconsolidated sediment that flow quickly downhill and pose a natural hazard for infrastructure and human life (Major & Iverson, 1999)
Understanding the debris flow hazard in this, and similar alpine settings, requires determining the magnitude of sediment accumulation between debris flow seasons and identifying the control on sediment production. To address these knowledge gaps, we measured changes in sediment production 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 contributed to channel refilling between active debris flow periods
Our observations show a strong link between sedimentation and temperature using several years of data collection (Figure 8), which implicates frost weathering as a likely driver of sediment production, supporting prior suggestions of this linkage (Bennett et al, 2013)

Summary

Introduction

Debris flows are mixtures of water and unconsolidated sediment that flow quickly downhill and pose a natural hazard for infrastructure and human life (Major & Iverson, 1999). Unlike floods, which are extreme water-dominated flow events that breach the natural channel boundaries, debris flows incorporate more than 40% sediment by volume and move with greater velocity and depth than flood-dominated flows (e.g., Kean et al, 2016). The likelihood that a debris flow will be triggered depends on the availability of (1) sufficient precipitation and (2) sediment (Bennett et al, 2014; Church & Miles, 1987). Fewer studies have investigated how debris flow channels refill with sediment between events (e.g., Bennett et al, 2014). The volume of a new debris flow depends on the amount of available material stored in channels

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Discussion

Conclusion