IN SITU MEASUREMENTS OF SEDIMENT ENTRAINMENT FROM THE HEADWATERS OF A NATURAL DEBRIS-FLOW BASIN

When mixtures of soil, rock and water flow down slope as a debris flow they can entrain large amounts of bed sediment, significantly increasing the mass and volume of the flow. Empirical evidence and scaling considerations demonstrate that increases in flow volume result in increases in inundation area, inundation height, runout length and flow velocity, all of which represent a serious escalation of debris-flow hazard. A major shortcoming of many continuum debris-flow models is the neglect of mass exchange with the subjacent bed. Although adding entrainment terms is mathematically simple, the measurements needed to confirm the physical reality of such terms are few, especially in natural settings.

We present data from the headwaters of a debris-flow basin at Chalk Cliffs, Colorado, where we measured entrainment of channel sediment during three different debris-flow events. We combine in-situ measurements of bed and flow properties (pore-fluid pressure, basal normal stress, flow stage, and soil moisture) with direct measurements of entrainment from sensors buried in the sediment bed. These data characterize the style, rates, and mechanism of sediment entrainment during a natural debris-flow event. During the three separate debris-flow events approximately 1.1 m, 0.5 m and 0.4 m of unconsolidated bed sediment were entrained. Entrainment stopped when the flow reached bedrock. In all events, erosion began with the arrival of a steep granular front, but in one event erosion rates peaked during passage of the watery tail behind the fronts.

When the bed sediment was unsaturated, entrainment manifested as relatively slow, progressive erosion of bed sediment, generally taking several minutes to entrain all the sediment (mean rates of 4 mm/s to 6 mm/s). In one event where the bed sediment was saturated, progressive entrainment occurred via episodic, rapid failures of thicker packages of bed sediment, as well as by slower progressive erosion. During this event, 20 cm and 10 cm packages of bed sediment were eroded in less than 0.2 seconds (rates up to 1 m/s). These data support a progressive-entrainment model, as opposed to bulk failure, for flows over unsaturated bed material, and highlight the important role soil moisture plays in controlling the rates of entrainment.