Christophe Ancey

Swiss Federal Institute of Technology in Lausanne | EPFL · Civil Engineering Institute

Measuring grain sizes in gravel‐bed rivers is crucial when studying river dynamics and sediment transport. Automated methodologies have been developed in recent years for detecting individual grains and measuring their size on digital imagery. These object‐based methodologies have mainly been applied to handheld imagery. Low‐cost and high‐resolutio...

For the prediction of landslide-generated waves, previous studies have developed numerous empirical equations to express the maximums of wave characteristics as functions of slide parameters upon impact. In this study, we built the temporal relationship between the wave characteristics and slide features. We gave specific insights into impulse wave...

The calculation of the impact pressure on obstacles in granular flows is a fundamental issue of practical relevance, e.g. for snow avalanches impacting obstacles. Previous research shows that the load on the obstacle builds up, due to the formation of force chains originating from the obstacle and extending into the granular material. This leads to...

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-a...

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 algorith...

We studied the segregation of single large intruder particles in monodisperse granular materials. Experiments were carried out in a two-dimensional shear cell using different intruder and media diameters, whose quotient defined a size ratio R that ranged from 1.2 to 3.333. When sheared, the intruders segregated and rotated at different rates, which...

Particles of differing sizes are notoriously prone to segregation in shear driven flows under the action of gravity. This has important implications in many industrial processes, where particle-size segregation can lead to flow problems and reduced product quality, as well as longer product development and start-up times. Particle-size segregation...

Steep streams on rough beds are generally characterised by supercritical flow conditions under which antidunes can develop and migrate over time. In this paper, we present flume experiments that we conducted to investigate the variability of antidune geometry and migration celerity, a variability observed even under steady‐state conditions. Quantif...

This paper presents image velocimetry measurements on turbulent flows adjacent to a permeable bed made of randomly packed glass particles. For measuring flow velocities inside the bed, the refractive index of the glass particles was matched with that of the fluid. By continuously scanning in the transverse direction, we measured the streamwise and...

Understanding the physical processes involved in snow avalanche‐obstacle interaction is essential to be able to estimate the pressure exerted on structures. Although avalanche impact pressure has been measured in field experiments for decades, the underlying physical principles are still elusive. Previous studies suggest that pressure is increased...

When colossal gravity-driven mass flows enter a body of water, they may generate waves which can have destructive consequences on coastal areas. A number of empirical equations in the form of power functions of several dimensionless groups have been developed to predict wave characteristics. However, in some complex cases (for instance, when the ma...

This paper outlines the various approaches used to calculate bedload transport. As bedload transport exhibits considerable spatial and temporal variations, computing the bedload transport rates and morphological changes experienced by streambeds is difficult. A large body of experimental work has revealed scaling laws relating the mean transport ra...

By the late nineteenth century, the scientific study of bedload transport had emerged as an offshoot of hydraulics and geomorphology. Since then, computing bedload transport rates has attracted considerable attention, but whereas other environmental sciences have seen their predictive capacities grow over time, particularly thanks to increased comp...

In the spring of 1818, ice avalanches from the Giétro Glacier created an ice dam, which in turn formed a glacial lake in the Drance Valley (Canton of Valais, Switzerland). Today, its maximum volume is estimated to have been 25×10⁶ m³. Cantonal authorities commissioned an engineer named Ignaz Venetz to mitigate the risk of the ice dam's failure. He...

Experimental studies of impulse-wave formation have mostly used rigid blocks or granular materials to mimic landslides at the laboratory scale. These studies have deduced that material deformability plays a key part in wave formation: the more rigid the sliding mass, the higher the impulse wave. It is, however, still unclear whether higher wave amp...

Mountain rivers exhibit sediment transport rate fluctuations that often cover more than two orders of magnitude. Bedform migration is often cited as the key process that causes giant fluctuations in the sediment transport rate. To quantify the effect of bedform migration on transport rate, we ran laboratory experiments in a 19-m long 60-cm wide flu...

Through experiments and discrete particle method (DPM) simulations we present evidence for the existence of a recirculating structure, that exists near the front of dense granular avalanches, and is known as a breaking size-segregation (BSS) wave. This is achieved through the study of three-dimensional bidisperse granular flows in a moving-bed chan...

Snow avalanches are a major hazard in mountainous areas and have a significant impact on infrastructures, economy and tourism of such regions. Obtaining a thorough understanding on the pressure exerted by avalanches on infrastructures is crucial for the development of design criteria so that they can withstand avalanche impact. Avalanches are chara...

Describing bedload transport as a stochastic process is an idea that emerged in the 1930s with the pioneering work of Einstein. For a long time, the stochastic approach attracted marginal attention, but the situation has radically changed over the last decade with the recent advances in the theory of bedload transport. In parallel, the implementati...

We study the behaviour of a low-density granular material entering a water basin by means of a simplified two-dimensional model, with the aim to understand the dynamics of a snow avalanche impacting a water basin like an alpine lake or a fjord. The low density of the impacting mass induces an uplift buoyancy force and, consequently, a complicated i...

We report that the lift force on a single large particle segregating in a monodisperse dense granular flow is correlated with a downstream velocity lag. This correlation suggests a viscous-inertial origin for the lift force, similar to the Saffman lift force in (micro) fluids. This insight is relevant for modelling of particle-size segregation and...

When a snow avalanche enters a body of water, it creates an impulse wave whose effects may be catastrophic. Assessing the risk posed by such events requires estimates of the wave's features. Empirical equations have been developed for this purpose in the context of landslides and rock avalanches. Despite the density difference between snow and rock...

In gravel-bed rivers, bedload transport exhibits considerable variability in time and space. Recently, stochastic bedload transport theories have been developed to address the mechanisms and effects of bedload transport fluctuations. Stochastic models involve parameters such as particle diffusivity, entrainment and deposition rates. The lack of har...

The number of anthropized reservoirs threatened by snow avalanches is steadily increasing and impulse waves caused by avalanche impact are becoming a considerable risk for such basins. The dynamics of the impact of a snow avalanche into a water body is studied through laboratory experiments, where a granular material, with solid density slightly lo...

Debris and pyroclastic flows often have bouldery flow fronts, which act as a natural dam resisting further advance. Counter intuitively, these resistive fronts can lead to enhanced run-out, because they can be shouldered aside to form static levees that self-channelise the flow. At the heart of this behaviour is the inherent process of size segrega...

Gravity-driven flows can erode the bed along which they descend and increase their mass by a factor of 10 or more. This process is called “basal entrainment.” Although documented by field observations and laboratory experiments, it remains poorly understood. This paper examines what happens when a viscous gravity-driven flow generated by releasing...

The fair understanding of bed load dynamics in established transport conditions contrasts with the relatively poor knowledge and the rich variety of phenomena occurring close to the initiation of transport. Steep streams are also known to resist most of the existing predictive theories. In order to gain knowledge of the principal mechanisms involve...

This paper examines the importance of particle diffusion relative to advection in bedload transport. Particle diffusion is not included in existing approaches to bedload transport. Based on recent advances in the probabilistic theory of sediment transport, this paper emphasizes the part played by particle diffusion in bedload transport. The propose...

The advection-diffusion equation is one of the most widespread equations in physics. It arises quite often in the context of sediment transport, e.g., for describing time and space variations in the particle activity (the solid volume of particles in motion per unit streambed area). Phenomenological laws are usually sufficient to derive this equati...

When a mixture of particles, which differ in both their size and their density, avalanches downslope, the grains can either segregate into layers or remain mixed, dependent on the balance between particle-size and particle-density segregation. In this paper, binary mixture theory is used to generalize models for particle-size segregation to include...

In recent years, due to warmer snow cover, there has been a significant increase in the number of cases of damage caused by gliding snowpacks and glide avalanches. On most occasions, these have been full-depth, wet-snow avalanches, and this led some people to express their surprise: how could low-speed masses of wet snow exert sufficiently high lev...

We experimentally study particle scale dynamics during segregation of a bidisperse mixture under oscillatory shear. Large and small particles display an underlying asymmetry that is dependent on the local relative volume fraction, with small particles segregating faster in regions of many large particles and large particles segregating slower in re...

In this article, we present 10 experiments carried out in a steep, shallow water ume made of an erodible bed of natural, uniform gravel. We simultaneously recorded bed load particle motion, bed and water elevation using two high-speed cameras. Particle trajectories were reconstructed with the help of a robust, original tracking algorithm, available...

In recent years, the number of water reservoirs in high-altitude areas has increased. They are often used to provide water for various activities related with recreational (i.e. skiing) and production activities. Many of such reservoirs are threatened by snow avalanches. To investigate the phenomenon, an experimental study was carried out at the Ec...

This paper describes the relationship between the statistics of bedload transport flux and the time scale over which it is sampled. A stochastic formulation is developed for the probability distribution function of bedload transport flux, based on the Ancey et al. [2008] theory. An analytical solution for the variance of bedload transport flux over...

Geophones were used as a proxy for determining the behavior of sediment transport in boulder mountain river. Field observations aggregated by a statistical analysis (PCA) allowed to identifying periods in which the behaviour between the water discharge (Qw), the sediment flux (Qs) and the meteorologic conditions was correlated. Within each period a...

The paper reviews our recent attempts at modelling bed load transport in mountain rivers. This is a longstanding issue that has attracted considerable attention over the last century. While a number of field and laboratory studies have been instrumental in getting the big picture, there is a clear lack of efficient methods for predicting bed evolut...

In this paper, we present an experimental study about the motion of bed load particles in water. Recent developments in the stochastic theory of bed load transport call for precise experimental data to validate the models. We set up the experiment in a tilted narrow flume, where we could control water discharge. The erodible bed was made of natural...

This article examines the spatial dynamics of bed load particles in water. We focus particularly on the fluctuations of particle activity, which is defined as the number of moving particles per unit bed length. Based on a stochastic model recently proposed by Ancey and Heyman [2014], we derive the second moment of particle activity analytically; th...

This paper concerns a model of bed load transport, which describes the advection and dispersion of coarse particles carried by a turbulent water stream. The challenge is to develop a microstructural approach that, on the one hand, yields a parsimonious description of particle transport at the microscopic scale and, on the other hand, leads to avera...

L'expertise d'avalanche désigne non seulement l'analyse de la menace que font peser les avalanches sur des infrastructures et des zones habitées, mais également la détermination des meilleurs moyens de s'en prémunir. Si cette double nécessité a mobilisé les hommes depuis longtemps, les moyens d'y répondre ont évolué.

Geophones were used as a proxy for determining the behavior of sediment transport in boulder mountain river. Field observations aggregated by a statistical analysis (PCA) allowed to identifying periods in which the behaviour between the water discharge $(Q_{w})$, the sediment flux $(Q_{s})$ and the meteorologic conditions was correlated. Within eac...

The phenomena of bed forms exist widely in the natural rivers and are still not fully understood. The detailed sediment dynamics near the bed is essential for this problem. However, the fluid dynamics near the bed, which drives the sediment motion, is not clear. In this talk, we focus on the fluid dynamics of supercritical flow over a sinusoidal wa...

In order to simulate a simple entraining geophysical flow, a viscous Newtonian gravity current is released from a reservoir by a dam-break and flows along a rigid horizontal bed until it meets a layer of entrainable material of finite depth, identical to the current. The goal is to examine the entrainment mechanisms by observing the interaction bet...

This paper addresses the dam-break problem for particle suspensions, that is, the flow of a finite volume of suspension released suddenly down an inclined flume. We were concerned with concentrated suspensions made up of neutrally buoyant non-colloidal particles within a Newtonian fluid. Experiments were conducted over wide ranges of slope, concent...

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 avail...

We have carried out laboratory experiments to determine the internal structure of segregating dense granular avalanches and test the recent theoretical predictions of the existence of breaking size-segregation waves [Thornton & Gray, 2008]. Measurements were performed on a quasi-stationary avalanche that flows down an inclined upward-moving conveyo...

The use of geo-sensors in mountain rivers becomes more and more frequent. It provides valuable continuous time series of bedload transport discharge at high temporal resolution that are essential to evaluate, compare and fit theoretical model. This increasing amount of in-situ data has to be interpreted and analyzed carefully at least for two reaso...

Introduction Modeling avalanches Bibliography

Introduction Typology of torrential flows Initiation, motion and effects of debris flows Modeling debris flows Bibliography

We experimentally studied the flow behavior of a fixed volume of granular suspension, initially contained in a reservoir and released down an inclined flume. Here “granular suspension” refers to a suspension of non-Brownian particles in a viscous fluid. Depending on the solids fraction, density mismatch, and particle size distribution, a wealth of...

We present flume experiments showing plastic behavior for perfectly density-matched suspensions of non-Brownian particles within a Newtonian fluid. In contrast with most earlier experimental investigations (carried out using coaxial cylinder rheometers), we obtained our rheological information by studying thin films of suspension flowing down an in...

Bed-load transport process over steep slope is of large fluctuations. This kind of fluctuation widely exists over different time scales. Due to this random behavior, the stochastic model can make a better performance on the description of bed-load transport process than the deterministic one. Based on the result of the early study, an alternative m...

Steep slope streams show large fluctuations of sediment discharge across several time scales. These fluctuations may be inherent to the internal dynamics of the sediment transport process. A probabilistic framework thus seems appropriate to analyze such a process. In this paper, we present an experimental study of bedload transport over a steep slo...

Geophysical gravity flows such as avalanches and debris flows belong to a special class of hazardous environmental events, 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...

We investigated the dam-break problem for Herschel–Bulkley fluids: a fixed volume of a viscoplastic material (a polymeric gel called Carbopol ultrez 10) was released and flowed down an inclined flume. Using Particle Image Velocimetry techniques, we measured the velocity profiles far from the sidewalls, the front position as a function of time, and...

We experimentally investigated the spreading of fluid avalanches (i.e., fixed volumes of fluid) down an inclined flume. Emphasis was given to the velocity field within the head. Using specific imaging techniques, we were able to measure velocity profiles within the flowing fluid far from the sidewalls. We studied the behavior of Newtonian and visco...

Predicting the occurrence and spatial extent of extreme avalanches is a longstanding issue. Using field data pooled from various sites within the same mountain range, authors showed that the avalanche size distribution can be described using either an extreme value distribution or a thick-tailed distribution, which implies that although they are mu...

Large fluctuations in the sediment transport rate are observed in rivers, particularly in mountain streams at intermediate flow rates. These fluctuations seem to be, to some degree, correlated to the formation and migration of bedforms. Today the central question is still how to understand and account for the strong bedload variability. Recent expe...

Even though it oversimplifies reality, the dam-break problem for frictionless fluids provides predictions relevant to various manmade and natural dam-break floods. The exact solution can also be used to test numerical schemes. However, as far as we are aware, exact solutions derived from the nonlinear shallow-water equations hold only for infinitel...

Introduction Particle-laden, gravity-driven flows occur in a large variety of natural and industrial situations. Typical examples include turbidity currents, volcanic eruptions, and sand-storms (see Simpson 1997 for a review). On mountain slopes, debris flows and snow avalanches provide particular instances of vigorous dense flows, which have speci...

A general continuum theory for particle-size segregation and diffusive remixing in polydisperse granular avalanches is formulated using mixture theory. Comparisons are drawn to existing segregation theories for bi-disperse mixtures and the case of a ternary mixture of large, medium and small particles is investigated. In this case, the general theo...

Dam break floods on steep slopes occur in diverse settings and can encompass natural events such as mudflows and avalanches as well as the failure of manmade dams. Laboratory experiments were undertaken to investigate the dam break flows of pure water down a variably sloping, rectangular fixed bed channel. A high-speed digital camera was used to tr...

Optical measurement techniques such as particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are now routinely used in experimental fluid mechanics to investigate pure fluids or dilute suspensions. For highly concentrated particle suspensions, material turbidity has long been a substantial impediment to these techniques, which explai...

We experimentally investigated how a binary granular mixture made up of spherical glass beads size ratio of 2 behaved when flowing down a chute. Initially, the mixture was normally graded, with all the small particles on top of the coarse grains. Segregation led to a grading inversion, in which the smallest particles percolated to the bottom of the...

Dam-break flows and related phenomena: • Laboratory insight • Focus: highly concentrated flows ◦ The Newtonian paradigm ◦ Origin of plasticity in particle suspensions ◦ Evidence of two-phase effect • Flow structure ◦ particle segregation ◦ deposition and levee formation

The aim of this study is to understand the interactions between the hydrodynamic characteristics (flow field, water depth), the morphological structures, and the sediment transport rate. A better knowledge of these processes, i. e. the coupling between the water flow and sediment transport, is needed to improve modeling of river bed dynamics. We ob...

Bedload sediment transport of two-size coarse spherical particle mixtures in a turbulent supercritical flow was analyzed with image and particle tracking velocimetry algorithms in a two-dimensional flume. The image processing procedure is entirely presented. Experimental results, including the size, the position, the trajectory, the state of moveme...

The idea of stochastic sediment transport models emerged in the 1930s, notably with the doctoral work of Hans A. Einstein (1936). Einstein's seminal work gave impulse to several stochastic models, which usually led to thin-tailed or bounded distributions for the particle-transport rate. Experimental observations together with field surveys suggest...

The aim of spatial analysis is to quantitatively describe the behavior of environmental phenomena such as precipitation levels, wind speed or daily temperatures. A number of generic approaches to spatial modeling have been developed[1], but these are not necessarily ideal for handling extremal aspects given their focus on mean process levels. The a...