Christophe Ancey | EPFL Graph Search

Christophe Ancey has both a PhD and an engineering degree granted by the Ecole Centrale de Paris and the Grenoble National Polytechnic Institute. Trained as a hydraulics engineer, he did his doctoral work under the supervision of Pierre Evesque from 1994 to 1997 on rheology of granular flows in simple shearing. He was recruited in 1998 as a researcher in rheology at the Cemagref as part of the Erosion Protection team directed by Jean-Pierre Feuvrier, which has since become the laboratoire "Storm Erosion, Snow and Avalanche Laboratory". Parallel to this research activity, with Claude Charlier He set up a consulting firm for engineering contracting called Toraval (www.toraval.fr), which has become the major player in the avalanche field in France. Since 2004, He is a fluid-mechanics professor at EPFL and he is the director of the Environmental Hydraulics Laboratory.

He is associate editor of Water Resources Research, one of the leading journal in the field.

Do Antidune Trains Undergo Morphological Changes When These Bedforms Migrate Upstream?

Christophe Ancey, Ivan Pascal

Upstream migrating antidunes develop in streams under supercritical flow conditions. These bedforms are often organised in trains (i.e. sequences of well-developed antidunes). In shallow flows over coarse sediments, the dynamics of a single antidune can be significantly influenced by the surrounding bedforms. The drivers and behaviour of these interactions remain unclear. We conducted an experimental study to investigate the morphodynamics of antidune trains in narrow streams with a particular focus on mechanisms that could systematically influence bedform geometry as antidunes migrate upstream. The incipient antidunes near the flume outlet (which is a bed discontinuity) often exhibited migration periods notably longer than the typical value observed further upstream. Far from the boundaries, antidune sequences were mostly affected by transient downstream travelling disturbances and we did not observe any systematic change of antidune shape and dynamics along their migration path. Such insights into potential sources of non-uniformity are helpful for interpreting the differences between the outcomes of experimental and numerical simulations.

IAHR International Association for Hydro-Environment Engineering and Research2022

Physics-based estimates of drag coefficients for the impact pressure calculation of dense snow avalanches.

Christophe Ancey, Johan Alexandre Philippe Gaume, Michael Lukas Kyburz

In avalanche engineering and hazard mapping, computing impact pressures exerted by avalanches on rigid structures has long been a difficult task that requires combining empirical equations, rules of thumb, engineering judgment and experience. Until the 1990s, well-documented avalanches were seldom, and the main source of information included back-analysis of damage to structures and scarce field measurements. By the 1990s, several field sites were equipped across Europe, and since then they have provided new insights into the physics of impact. The main problem has been the difficulty in interpreting and generalizing the results to propose sound methods for estimating impact pressure. Testing a wide range of flow conditions has also been difficult in the field. To go a step forward in the elaboration of new guidelines for computing avalanche forces, we developed a numerical code based on the Discrete Element Method (DEM), which made it possible to simulate how an avalanche interacts with a rigid obstacle and to study how impact pressure depends on obstacle shape and size, as well as the avalanche flow regime. We extracted pressure and velocity data from the Vallée de la Sionne database to validate the DEM code, calibrate the model parameters, and elaborate avalanche scenarios. We studied four avalanches scenarios related to distinct flow regimes of the avalanche’s dense core. In these scenarios, snow cohesion and velocity were imposed at the upstream boundary of the computational domain. Building on earlier work, we generalized an empirical equation for computing impact pressure as a function of snow cohesion, velocity, flow regime, and structure shape and size. Various coefficients were defined and calibrated from our DEM data. Within the range of tested values, we found good agreement between estimated pressure and field data.

2022

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

Christophe Ancey, Tomás Alfredo Trewhela Palacios

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields' interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche's leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt's results showed the potential for further theoretical developments in rheology and segregation-coupled models.

SPRINGER2021

Physics-based estimates of drag coefficients for the impact pressure calculation of dense snow avalanches.

Christophe Ancey, Johan Alexandre Philippe Gaume, Michael Lukas Kyburz

2022