MECHANISM OF DEBRIS-FLOW MOMENTUM GROWTH DURING ENTRAINMENT OF WET BED SEDIMENT

Debris flows can grow dramatically in size and speed as they entrain material from their beds and banks, amplifying hazards and sediment delivery downstream, but momentum conservation seemingly implies that entrainment of static material should retard flow motion rather than enhance it. How, then, can flows that entrain bed sediment travel faster and further than those that don't? To answer this question, we conducted eight experiments in which nearly identical debris flows, each consisting of 6 m³ of water-saturated sand, gravel and mud (SGM), flowed across nearly uniform, ~12 cm-thick beds of the same SGM blanketing a 47-m reach of a steep (31º), 2-m wide concrete flume. We also conducted two control experiments with 6-m³ SGM debris flows but no bed sediment.

The key independent variable in our experiments was the bed-sediment volumetric water content, θ. We manipulated and measured θ through low-intensity sprinkling and real-time monitoring with tensiometers and electrical capacitance probes. At the time of debris-flow release, θ ranged from 0.15 to 0.28. We avoided creating positive pore-water pressure in the bed sediment, because significant positive pressures caused spontaneous bed failure (unsurprising, given the 40º internal and basal friction angles of the SGM).

Our data show that rapid loading by overriding debris flows generated high pore-water pressures in wet bed sediment, even though θ was always less than the full-saturation value (θ ≈ 0.4). The high pore pressures facilitated rapid downward scour (at rates ~10 cm/s) and reduced basal friction almost to zero behind flow fronts, while steepening and deepening the fronts themselves. These changes caused flow momentum to grow relative to that observed in control experiments, fostering further entrainment and resulting in positive feedback. The threshold θ necessary for development of high pore pressures and positive feedback was about 0.23. With θ > 0.23, pore water was apparently continuous enough to transmit pressure efficiently, whereas with θ < 0.23, compression must have occurred mostly in pore air. In such cases momentum transfer to the bed resulted in negative feedback and loss of debris-flow momentum, and minimal bed sediment was entrained.