Abstract
AbstractRainfall intensity‐duration (ID) thresholds are commonly used to assess the potential for runoff‐generated debris flows, but the sensitivity of these thresholds to sediment supply, which can change rapidly with time, is relatively unexplored. Furthermore, debris flows often self‐organize into distinct surges, but the factors controlling the magnitude and frequency of these surges, including sediment supply and grain size, are poorly constrained. We use a combination of numerical modeling and debris flow monitoring data from Chalk Cliffs, Colorado, USA, to explore how sediment supply influences rainfall ID thresholds for debris flows and surge properties. Results suggest that rainfall ID thresholds only become sensitive to sediment supply below a sediment thickness threshold. Surge magnitude is a nonmonotonic function of sediment supply (i.e., channel bed sediment thickness and grain size) with the largest surges tending to form at intermediate values of sediment availability with intermediate grain sizes.
Highlights
Runoff-generated debris flows, which are common in recently burned areas (Cannon, 2001; Gabet & Bookter, 2008; Kean et al, 2011; Nyman et al, 2011) and unburned alpine settings (Berti et al, 1999; Coe et al, 2008), represent a significant and widespread hazard
Surge magnitude is a nonmonotonic function of sediment supply with the largest surges tending to form at intermediate values of sediment availability with intermediate grain sizes
Low-gradient channel reaches, debris flow surge formation may result from progressive instabilities that share many similarities with the roll waves that commonly form in water-dominated flows and viscous fluids (Zanuttigh & Lamberti, 2007)
Summary
Runoff-generated debris flows, which are common in recently burned areas (Cannon, 2001; Gabet & Bookter, 2008; Kean et al, 2011; Nyman et al, 2011) and unburned alpine settings (Berti et al, 1999; Coe et al, 2008), represent a significant and widespread hazard. Rainfall intensity-duration (ID) thresholds are commonly used to forecast the initiation of runoff-generated debris flows (Staley et al, 2013, 2017). Low-gradient channel reaches, debris flow surge formation may result from progressive instabilities that share many similarities with the roll waves that commonly form in water-dominated flows and viscous fluids (Zanuttigh & Lamberti, 2007). Kean et al (2013) demonstrated that debris flow surges at Chalk Cliffs, Colorado, USA, often (though not exclusively) form through this type of regressive instability where bedload sediment preferentially deposits in low-slope portions of the channel and continues to build until the resulting sediment dam fails en masse. Despite the potential hazard caused by debris flows, comparatively little is known about the factors, including sediment supply and grain size, that control the frequency and magnitude of debris flow surges that form through regressive instabilities
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