Effect of available entrainable material on a viscous gravity current including run-out characteristics and internal flow properties.

It has long been accepted that entrainment of loose material by geophysical gravity flows such as dense snow avalanches and debris flows may change their behaviour significantly. Run-out distances and bulk-flow velocities are notable examples of susceptible behaviours. It is still disputed how this has an effect but it has been noted that the availability of entrainable material is a principal parameter. Laboratory and numerical results are studied side-by-side to demonstrate the effects of a finite erodible bed of varying length and depth, which is placed in the path of a flowing gravity current. Both the current and the bed are composed of the same material. Natural geophysical flows are simulated as idealized viscous gravity currents at zero degrees inclination in order to study the link between the internal dynamics and the bulk features in the simplest case. In the laboratory, a PIV configuration using a laser sheet allows the visualization of a vertical stream-wise cross section of the flow in the transition region from rigid to erodible bed, far from the side-walls. This allows the study of the velocity field within the cross-section of the flow in the entrainable region. Run-out speeds and distances are measured after the current exits the erodible bed and flows over a rigid base once more. A relationship is sought between the released volume, the erodible bed dimensions (that is, length and depth) and the run-out characteristics of the flow. This bulk run-out behaviour is investigated with reference to the internal flow dynamics as measured by PIV. This work is supplemented by results obtained modelling the same system using the open source CFD software OpenFOAM. We were able to track the front of the current during the flow and found that even the presence of a shallow entrainable bed (3 mm deep) significantly advanced the run-out front compared to the no-bed case. A further increase in bed depth led to a slight increase in run-out. The length of the bed was also a strongly controlling parameter, highlighting the change in flow characteristics across this region, and underlining the necessity to investigate the internal dynamics when a gravity current flows over an entrainable bed.

Laboratory-scale dam-break study of gravity currents with basal entrainment: PIV measurements of a viscous newtonian fluid over a horizontal bed of the same ﬂuid.

Christophe Ancey, Belinda Margaret Bates

Geophysical gravity flows such as avalanches and debris flows belong to a special class of hazardous environmental event, in which a mixture of solids and fluids (e.g. debris and mud, snow and air) flow as a liquid and may run out much further than expected over a slope less steep than the critical angle of repose. Exchange of material between the overriding flow and a loose bed layer underneath is known to affect the characteristics of such a flow however it is not well understood how this plays a part, nor the mechanism of such an exchange, due to the difficulties of seeing inside a flowing mass in the field and experiments. Research has been carried out on an entraining viscous gravity current in an laboratory flume using Particle Image Velocimetry. The aim was to investigate the flow properties in a vertical slice of fluid in the downstream direction, far from the side-walls, when a flowing layer overrides a basal layer of the same material. The parameter varied here is the volume of fluid released from the reservoir, which affects the height and velocity of the encroaching fluid current. An interface was identified between the overriding fluid and the initially stationary bed fluid. PIV was used to show the evolution of the velocity and shear fields throughout the system compared to the location of this interface and how this changes with the volume of fluid released. The overriding fluid displaces the bed fluid, either by plunging or spilling into the erodible layer depending on the initial volume, thus inciting the stationary material downstream and below to move. Initially, a large amount of bed material is suddenly mobilized by the front of overriding fluid and little deposition occurs. Shortly after this we see the clear development of a depositional region (with low velocities, predominantly in the downwards direction) and an eroding region (with high, almost uniformly horizontal velocities) within the front of material that enters the bed, separated by a region of high shear. For larger released volumes, multiple shear bands develop, thus showing distinct regions with different rates of material deposition.

2012

Fluid dynamics of basal entrainment by geophysical gravity-driven flows

Belinda Margaret Bates

Geophysical gravity-driven flows -- including avalanches, debris flows, pyroclastic flows and submarine turbidity currents -- are multiphase natural hazards that flow under the influence of gravity. Despite their differences, they share much of the same physics, having the potential to pick up material from beneath, a process called basal entrainment during which the flow may increase in volume and velocity manyfold. Due to their complexity and unpredictability there are still many unanswered questions about their mechanics, so that many of the theoretical models in use are based on insufficient data sets and may not apply to a general case. Here, basal entrainment by geophysical gravity-driven flows is studied by isolating the process in idealised laboratory experiments. The avalanche is simplified and controlled in such a way that any changes can be confidently attributed to the entrainment process alone. Further, the methods available in the laboratory allow the continuous, passive study of entrainment, so that for the first time full data sets of internal measurements are obtained, from experiments ranging from simple to complex. The data obtained is easily exploited for confirmation of the mathematical models developed in this thesis. Experiments which simulated entraining avalanches as Newtonian dam-breaks along a horizontal flume and as viscoplastic dam-breaks along an inclined flume both showed an increase in front position, dependent on the amount of material available, amongst other changes. The experimental results allow the development of a theoretical thin-film model in both cases which is solved numerically and compares favourably with the data obtained. The Newtonian model reproduced the flow characteristics excellently and the viscoplastic model successfully simulated the effects of entrainable material. The possibility of performing similar experiments using a granular suspension is also investigated, with promising results. This work shows that the effect of entrainment on gravity-driven flows can be quantified and modelled mathematically as a non-local transport process. This has implications for hazard modelling: if the quantity of available loose material is known, and its characteristics are similar to those of the flowing avalanche, the avalanche and the entrainable bed can be modelled as a continuous flow over a rigid base. Thus it is suggested that the models developed be tested in more realistic cases, e.g. in the case of an avalanche entraining material with different characteristics, or in a more complex geometry, in order to better mimic what happens in nature.

EPFL2015

Fluid dynamics of basal entrainment by geophysical gravity-driven flows

Belinda Margaret Bates

EPFL2015

Laboratory-scale dam-break study of gravity currents with basal entrainment: PIV measurements of a viscous newtonian fluid over a horizontal bed of the same ﬂuid.

Christophe Ancey, Belinda Margaret Bates

2012