Sediment entrainment by debris flows: In situ measurements from the headwaters of a steep catchment

Abstract

[1] Debris flows can dramatically increase their volume, and hence their destructive potential, by entraining sediment. Yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited. Using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris-flow events. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time-averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry. Bed sediment was entrained from the sediment-surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water-rich, inter-surge flow. En masse failure of bed sediment along the sediment-bedrock interface was never observed. Large-magnitude, high-frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment. Within this near surface layer, concomitant fluctuations in Coulomb frictional resistance are expected, irrespective of the influence of pore fluid pressure or fluctuations in shear stress. If the near-surface sediment was wet as it was overridden by a flow, additional large-magnitude, highfrequency pore pressure fluctuations were measured in the near-surface bed sediment. These pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner. The depth to which large excess pore pressures propagated was typically less than 10 cm, but scaled as (D/f i ) 0.5 , in which D is the hydraulic diffusivity and f i is the frequency of a particular pore pressure fluctuation. Shallow penetration depths of granular-normal-stress fluctuations and excess pore pressures demonstrate that only near-surface bed sediment experiences the full dynamic range of effective-stress fluctuations, and as a result, can be more easily entrained than deeper sediment. These data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment.

The importance of entrainment and bulking on debris flow runout modeling: examples from the Swiss Alps

“…From large-scale debris-flow erosion experiments at the US Geological Survey debris flow flume, Reid et al (2011) reported erosion rates ranging from 0.05 to 0.10 ms −1 . McCoy et al (2012) reported a maximum erosion rate of about 0.14 ms −1 within the headwaters of a natural debris flow catchment at Chalk Cliffs, Colorado, USA. The maximum erosion rate measured in the Illgraben (0.25 ms −1 ) is somewhat larger than reported in the other studies; however, the flows at the Illgraben are also larger in size than in the other studies.…”

“…Physically based numerical models were developed to investigate runout distance and inundation patterns as well as flow heights and flow velocities (Crosta et al, 2003;D'Ambrosio et al, 2003;Medina et al, 2008;Hungr and McDougall, 2009;Christen et al, 2012). Only recently have researchers focused on better understanding the process by which debris flows entrain sediment from the bed of a torrent channel as part of the erosion process (e.g., Hungr et al, 2005;Mangeney et al, 2007;Berger et al, 2011;McCoy et al, 2010McCoy et al, , 2012McCoy et al, , 2013.…”

Visualization of the internal flow properties and the material exchange interface in an entraining viscous Newtonian gravity current

“…Bearing in mind that the gravity current studied was a simple viscous Newtonian fluid, this transition layer is reminiscent of the mixing layer in Le and Pitman [19]; and the stepwise entrainment of debris in field studies [6,23] allows a qualitative comparison with our findings.…”

“…Berger et al [6] used a scour sensor to measure erosion rates during debris flows and they found that entrainment of sediment occurred progressively, and was correlated with high pressure and pressure fluctuations near the flow front, caused by particle impacts. McCoy et al [23] also found progressive entrainment at the flow front, as well as during more dilute flows, but they found no correlation with flow depth and other bulk flow properties. Instead they found high entrainment rates to be caused by high-frequency pore-pressure fluctuations near the surface when the bed sediment was saturated.…”

Methods for detecting channel bed surface changes in a mountain torrent – experiences from the Dorfbach torrent

“…Erosion sensors, described by Berger et al (2010), are an exclusive version that can be constructed with more affordable materials (cf. McCoy et al, 2012).…”