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

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, high‐frequency 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.