The effects of ice on debris flow mobility and initiation processes–results from the large-scale experimental USGS debris flow flume.

Ice-rock avalanches generate unusual debris flows known for their high mobility, long runout distances, and potential hazard. Ice is thought to reduce friction and, during larger time intervals, reduce shear resistance because of increased pore pressure associated with melting. In the context of climate warming, a degrading cryosphere redistributes stresses and destabilizes slopes in high alpine regions as well as at ice-clad volcanoes. This can lead to more frequent ice-rock avalanches threatening communities downstream. Consequently, ice-rock avalanches have recently received more attention. However, most studies are based on numerical models, rotating drums, or small-scale flume experiments, which exhibit problematic scaling effects (for example, disproportional effects of pore water pressure, viscous flow resistance, and grain inertia) and thus not represent physical processes well.

We present results from the large-scale experimental USGS debris flow flume (95 m long, 2 meters wide, 1.2 m deep inclined on a 31° slope that tapers off onto a 2° runout pad towards its end) showing how ice affects debris flow mobility and initiation processes. In a series of mobility experiments, sediment-ice mixtures were placed behind a gate that was suddenly opened. In a series of initiation experiments, the flume was modified by attaching a retaining wall inside the flume, placing the sediment-ice mixture behind the wall, and watering the mixture (emulating groundwater inflow) until failure occurred. We conducted six large-scale debris flow experiments (8 m³ mixtures) with ice volume ranging from 100% to 0 %, at 20% intervals, and three initiation experiments (6.2 m³ mixtures) with volumetric ice content of 65%, 30%, and 0% ice. To isolate the effects of ice, we used sediment containing no silt and clay, which are known to enhance mobility by maintaining elevated pore pressure within the flow. The sediment-ice mixture was fully saturated at the start of each mobility experiment. Increasing ice content created a nonlinear trend of decreased mobility, in terms of runout distance and velocity, until a critical ice content was reached. As ice content increased beyond a critical value, mobility and velocity of the mixture increased and surpassed that of debris flow with no ice.

During initiation experiments, sediment-ice mixtures and sediment only (control) experiments were saturated until slope failure. Mixtures containing ice caused pore water pressures to stay elevated longer than those without ice before the failure. However, peak pore-water pressure of the sediment-ice mixtures during slope failure was lowered than that of the control (no ice) experiment and exhibited a hampered or sluggish failure.