Eric Lajeunesse

• Professor
• Senior at Paris Institute of Earth Physics

During a rainfall event, water infiltrates into the ground where it accumulates in porous rocks. This accumulation pushes the underlying groundwater towards neighbouring streams, where it runs to the sea. After the rain has stopped, the aquifer gradually releases its excess water, as the water table relaxes, until the next rain. In the absence of r...

The distribution of stress generated by a turbulent flow matters for many natural phenomena, of which rivers are a prime example. Here, we use dimensional analysis to derive a linear, second-order ordinary differential equation for the distribution of stress across a straight, open channel, with an arbitrary cross-sectional shape. We show that this...

We develop a phenomenological model of suspended sediment transport on the basis of data acquired in the Capesterre river, which drains a small tropical catchment in Guadeloupe. The model correctly represents the concentration of suspended sediment during floods, provided that the relation between concentration and water level forms a counterclockw...

Bedload transport, entrainment of coarse sediment by a river, is inherently a stochastic and intermittent process whose monitoring remains challenging. Here, we propose a new method to characterize bedload transport in the field. Using an uncrewed aerial vehicle (UAV) equipped with a high-resolution camera, we recorded yearly images of a bar of the...

We develop a phenomenological model of suspended-sediment transport on the basis of data acquired in the Capesterre river, which drains a small tropical catchment in Guadeloupe. The model correctly represents the transport of suspended sediment during floods, provided that the relation between concentration and water-level forms a counterclockwise...

The geometry of alluvial river channels both controls and adjusts to the flow of water and sediment within them. This feedback between flow and form modulates flood risk, and the impacts of climate and land-use change. Considering widely varying hydro-climates, sediment supply, geology and vegetation, it is surprising that rivers follow remarkably...

Titan's rich and dense atmosphere, composed mainly of methane and nitrogen, maintains a methane cycle that shapes its surface, like the water cycle on Earth. Methane precipitations erodes Titan's surface and forms complex river networks observed at all latitudes by the Cassini-Huygens mission. However, precipitation rates are poorly constrained and...

Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the...

In a shallow channel, the flow transfers most of its momentum vertically. Based on this observation, one often neglects the momentum that is transferred across the stream – the core assumption of the shallow-water theory. In the context of viscous flows, this approximation is referred to as the ‘lubrication theory’, in which one assumes that the sh...

Significance Rivers carry and deposit sediment, thereby shaping most landscapes around us. In doing so, their malleable channels change shape to accommodate the sediment load. Here, we show how fluid stress, gravity, and the erratic trajectories of traveling grains combine together to determine the shape of a river. We find that the stress on the b...

Rivers transports coarse sediment (gravel, cobbles, or boulder) as bedload. During a flood, when the discharge is high enough, the sediment grains move by rolling and bouncing on the river bed. Measuring bedload transport in the field is notoriously difficult. Here, we propose a new method to characterize bedload transport by floods. Using a drone...

Abstract. Many cities and settlements are organized around alluvial rivers, which are self-formed channels composed of gravel, sand and mud. Much of the time alluvial river channels are oversized, in that they could accommodate greater water flow; yet during extreme storms they are woefully undersized, and potentially catastrophic flooding can occu...

Couder et al. [] discovered that the behavior of droplets bouncing on a vibrated bath mimics a variety of quantum phenomena, through their coupling with a Faraday wave. Here, we introduce a continuous model inspired from this experiment, which encapsulates its essential features into a system of three simple differential equations. We show, numeric...

An alluvial river builds its own bed with the sediment it transports; its shape thus depends not only on its water discharge but also on the sediment supply. Here we investigate the influence of the latter in laboratory experiments. We find that, as their natural counterpart, laboratory rivers widen to accommodate an increase of sediment supply. By...

The French Critical Zone research infrastructure, OZCAR-RI, gathers 20 observatories sampling various compartments of the critical zone, each having developed their own data management and distribution systems. A common information system (Theia/OZCAR IS) was built to make their in situ observation FAIR (findable, accessible, interoperable, reusabl...

Plain Language Summary Rainwater infiltrates into the ground, accumulates in porous rocks, and eventually flows toward a neighboring stream. Although this underground travel often takes millennia, groundwater can contribute quickly to floods. To understand how an underground flow can be so responsive, we have recorded the motion of the groundwater...

The coupling of sediment transport with the flow that drives it allows rivers to shape their own bed. Cross-stream fluxes of sediment play a crucial, yet poorly understood, role in this process. Here, we track particles in a laboratory flume to relate their statistical behavior to the self organization of the granular bed they make up. As they trav...

The coupling of sediment transport with the flow that drives it allows rivers to shape their own bed. Cross-stream fluxes of sediment play a crucial, yet poorly understood, role in this process. Here, we track particles in a laboratory flume to relate their statistical behavior to the self-organization of the granular bed they make up. As they trav...

A fluid flowing over a granular bed can move its superficial grains, and eventually deform it by erosion and deposition. This coupling generates a beautiful variety of patterns, such as ripples, bars and streamwise streaks. Here, we investigate the latter, sometimes called ‘sand ridges’ or ‘sand ribbons’. We perturb a sediment bed with sinusoidal s...

Core Ideas OZCAR is a network of sites studying the critical zone. OZCAR covers various disciplines. OZCAR will help disciplines to work together for a better representation and modeling of the critical zone. The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique–Application et Recherche or Critical Zone Observatories...

The grain-size distribution of ancient alluvial systems is commonly determined from surface samples of vertically exposed sections of gravel deposits. This method relies on the hypothesis that the grain-size distribution obtained from a vertical cross section is equivalent to that of the riverbed. Such an hypothesis implies first that the sediments...

The grain-size distribution of ancient alluvial systems is commonly determined from surface samples of vertically exposed sections of gravel deposits. This method relies on the hypothesis that the grain-size distribution obtained from a vertical cross-section is equivalent to that of the river bed. We report a field test of this hypothesis on sampl...

We use the erosion–deposition model introduced by to numerically simulate the evolution of a plume of bed load tracers entrained by a steady flow. In this model, the propagation of the plume results from the stochastic exchange of particles between the bed and the bed load layer. We find a transition between two asymptotic regimes. The tracers, ini...

We use the erosion-deposition model introduced by Charru et al. (2004) to simulate numerically the evolution of a plume of bedload tracers entrained by a steady flow. In this model, the propagation of the plume results from the stochastic exchange of particles between the bed and the bedload layer. We find a transition between two asymptotic regime...

Using laboratory experiments, we investigate the growth of an alluvial fan fed with two distinct granular materials. Throughout the growth of the fan, its surface maintains a radial segregation, with the less mobile sediment concentrated near the apex. Scanning the fan surface with a laser, we find that the transition between the proximal and dista...

More than a century of experiments have demonstrated that many features of natural rivers can be reproduced in the laboratory. Here, we revisit some of these experiments to cast their results into the framework of the threshold-channel theory developed by . In all the experiments we analyze, the typical size of the channel conforms to this theory,...

Using laboratory experiments, we investigate the growth of an alluvial fan fed with two distinct granular materials. Throughout the growth of the fan, its surface maintains a radial segregation, with the less mobile sediment concentrated near the apex. Scanning the fan surface with a laser, we find that the transition between the proximal and dista...

More than a century of experiments have demonstrated that many features of natural rivers can be reproduced in the laboratory. Here, we revisit some of these experiments to cast their results into the framework of the threshold-channel theory developed by Glover and Florey (1951). In all the experiments we analyze, the typical size of the channel c...

The Bayanbulak Grassland, Tianshan, P. R. China, is located in an intramontane sedimentary basin where meandering and braided gravel-bed rivers coexist under the same climatic and geological settings. We report and compare measurements of the discharge, width, depth, slope and grain size of individual threads from these braided and meandering river...

At the outlet of mountain ranges, rivers flow onto flatter lowlands. The associated change of slope causes sediment deposition. As the river is free to move laterally, it builds conical sedimentary structures called alluvial fans. Their location at the interface between erosional and depositional areas makes them valuable sedimentary archives. To d...

The Bayanbulak Grassland, Tianshan, China is located in an intramountane sedimentary basin where meandering and braided gravel-bed streams coexist under the same climatic and geological settings. We report on measurements of their discharge, width, depth, slope and grain size. Based on this data set, we compare the morphology of individual threads...

Guadeloupe Island is a natural laboratory, ideally suited to the study of biogeochemical processes in tropical and mountainous volcanic environments. The island’s east–west rainfall gradient (1200–8000 mm/yr) is superimposed on a north–south age gradient (2.7 Ma to present), providing a unique opportunity to investigate the influence of rainfall an...

We investigate the response of a laboratory aquifer submitted to artificial rainfall, with an emphasis on the early stage of a rain event. In this almost two-dimensional experiment, the infiltrating rainwater forms a groundwater reservoir which exits the aquifer through one side. The resulting outflow resembles a typical stream hydrograph: the wate...

When they reach a flat plain, rivers often deposit their sediment load into a cone-shaped structure called alluvial fan. We present a simplified experimental setup that reproduces, in one dimension, basic features of alluvial fans. A mixture of water and glycerol transports and deposits glass beads between two transparent panels separated by a narr...

Submarine canyons are the main conduits for sediment transfer from continental shelves to deep abyssal plains. A large number of bathymetric and seismic surveys provide detailed information on their morphology and structure, shedding light on the mechanisms involved in their formation. However, because of the difficulty in deploying instruments in...

Water flowing over a loose granular bed organizes into a braided river, a network of ephemeral and interacting channels. The temporal and spatial evolution of this network of braided channels is not yet quantitatively understood. In ∼1 m-scale experiments, we found that individual channels exhibit a self-similar geometry and near-threshold transpor...

Erosive effects of the tropical storm Helena that hit the volcanic island of Basse Terre (Guadeloupe — Lesser Antilles Arc) on 24 October 1963 has been measured using 80 aerial images acquired by the French geographic institute (IGN — Institut Géographique National) at the approximate scale of 1/8000 less than three months after the storm Helena. O...

Secondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the e...

We investigate experimentally the statistical properties of bedload transport induced by a steady, uniform, and laminar flow. We focus chiefly on lateral transport. The analysis is restricted to experiments where the flow-induced shear stress is just above the threshold for sediment transport. We find that, in this regime, the concentration of movi...

Bedload particles entrained by rivers tends to disperse as they move downstream. In this paper, we use the erosion-deposition model of Charru et al. (2004) to describe the velocity and the spreading of a plume of tracer particles. We restrict our analysis to steady-state transport above a flat bed of uniform sediment. The transport of tracer partic...

A viscous fluid flowing over plastic grains spontaneously generates single-thread channels. With time, these laminar analogues of alluvial rivers reach a reproducible steady state, showing a well-defined width and cross section. In the absence of sediment transport, their shape conforms with the threshold hypothesis which states that, at equilibriu...

Particles entrained by rivers tend to disperse as they move downstream. Understanding the mechanisms responsible for particle dispersion is therefore important to describe the transport of solid-phase contaminants in streams or the accumulation of cosmogenic radionuclides in sediment grains during transport. While the prediction of the sediment tra...

Despite several decades of investigations, accounting for the effect of the wide range of grain sizes composing the bed of rivers on bedload transport remains a challenging problem. We investigate this problem by studying experimentally the influence of grain size distribution on bedload transport in the simple configuration of a bimodal sediment b...

In the absence of sediment transport, the equilibrium width of a river results from the balance of gravity and fluid friction at the scale of a sediment particle. However, this theory breaks down when the river transports sediment. We present a laboratory investigation of bedload transport in laminar flows. The analysis of individual grain trajecto...

Secondary flow cells are commonly observed in straight laboratory channels. However, their existence in rivers is still unclear, despite the recent development of field measurement techniques such as Acoustic Doppler Current Profilers (ADCP). A possible explanation is the limited spatial resolution of these instruments, which precludes ADCPs from d...

Alluvial sediment transport is often modelled using either linear or non linear diffusion equations. These equations are then used to study the evolution through time of alluvial systems and their dependance on various parameters (such as discharge, subsidence or uplift), initial and boundary conditions . Much of these equations though are valid on...

Hillslopes bring rainfall water to the drainage network by surface runoff and groundwater flow. To identify the contribution of groundwater to river discharge, we need to characterize flow regimes in unconfined aquifers. Here we investigate the dynamics of free-surface Darcy flows in a two-dimensional laboratory model. Although very simple, this ex...

River beds usually exhibit granulometric patches. Previous studies focused on the patches characterization (size, spatial organization, grain sizes distribution) and the links between these characteristics and the sediment transport rate in rivers. However, few studies have attempted to quantify the time evolution of patches as a results of flow an...

Braided rivers are complex systems in which a network of ephemeral, interacting channels continually migrate to create a rapidly changing landscape. We present results of a set of ˜ 1m-scale experiments of braided rivers forming over a bed of monodisperse glass beads. The experiments evolve from an initial flat bed, allowing us to study the approac...

Despite several decades of investigations, accounting for the effect of the wide range of grain sizes composing the bed of rivers on bedload transport remains a challenging problem. We investigate this problem by studying experimentally the influence of grain size distribution on bedload transport in the simple configuration of a bimodal sediment b...

Introduction Bedload transport Channel morphology and flow dynamics Bed topography and flow depth Conclusions Acknowledgements References

In the tropic, the small watersheds are affected by intense meteorological events playing an important role on the erosion of soils and therefore on the associated organic carbon fluxes. We studied the geochemistry of three small watersheds around the Basse-Terre volcanic Island (FWI) during a four years period, by measuring DOC, POC and DIC concen...

Measurements were performed during two complete flow seasons on the Urumqi River, a proglacial mountain stream in the northeastern flank of the Tianshan, an active mountain range in Central Asia. This survey of flow dynamics and sediment transport (dissolved, suspended and bed loads), together with a 25-year record of daily discharge, enables the a...

In gravel-bed rivers, sediments are often sorted into patches of different grain-sizes, but in braided streams, the link between this sorting and the channel morpho-sedimentary elements is still unclear. In this study, the size of the bed sediment in the shallow braided gravel-bed Urumqi River is characterized by surface-count and volumetric sampli...

he volume of sediment exported from a tropical watershed is dramatically increased during extreme climatic events, such as storms and tropical cyclones (Dadson et al. 2004; Hilton et al. 2008). Indeed, the exceptionally high rainfall rates reached during these events generate runoff and trigger landslides which accumulate a significant amount of se...

Measuring the velocity field in large rivers remains a challenge, even with recent measurement techniques such as Acoustic Doppler Current Profiler (ADCP). Indeed, due to the diverging angle between its ultrasonic beams, an ADCP cannot detect small-scale flow structures. However, when the measurements are limited to a single location for a sufficie...

In this paper we present chemical composition data for major elements in rivers from three islands of the Lesser Antilles. The Lesser Antilles are a tropical volcanic subduction arc and are characterized by steep gradients of relief, bedrock age and precipitation. They constitute a natural laboratory where the response of the weathering engine to l...

base de l'érosion et du transport flu-vial, parce qu'elles offrent la possibili-té de contracter le temps, sont un outil précieux dans notre quête d'une com-préhension globale de l'évolution des paysages de notre planète. Là où terre et océan se rencontrent, la mer laisse de nombreuses traces, concentrées près de sa surface (appe-lée par fois nivea...

Organic matter is an important factor that cannot be neglected when considering global carbon cycle. New data including organic matter geochemistry at the small watershed scale are needed to elaborate more constrained carbon dynamic and climatic models. The objectives are to estimate precisely the DOC, POC and DIC fluxes in Guadeloupaen Rivers that...

The concentration of solutes in rivers typically decreases as water discharge increases, although more slowly than simple dilution would account for. Most theories describing this phenomenon are based either on multiple reservoirs dynamics, or on the distribution of travel times associated with distinct flow paths (such as runoff and groundwater fl...

Bedload transport, which results from the motion of particles rolling, sliding or saltating along the bed of a stream, is of fundamental importance for river morphodynamics as (1) it may represent an important fraction of the total sediment flux transported in a river; (2) it is involved in many aspects of morphologic changes in rivers including ba...

The regime equations of alluvial rivers at low transport rates are well established: the mechanical equilibrium of a sediment grain imposes, for a constant water discharge, both the width and the longitudinal slope of the channel. However, the effect of bedload on regime equations is still a matter of debate, partly because the notion of equilibriu...

The study of landscape evolution often meets with the difficulty of characterizing systems with data of limited temporal or spatial resolution, where boundary conditions may also be poorly constrained. Experiments can provide a useful testing ground for the development of reliable methods of quantification, and experimental delineation of the scale...

We report measurements performed during two complete flow seasons on the Urumqi River, a proglacial mountain stream in the northeastern flank of the Tianshan, an active mountain range in Central Asia. This survey of flow dynamics and sediment transport (dissolved, suspended and bed loads), together with a 25-year record of daily discharge, enables...

We report the results of an experimental investigation of bimodal bedload transport. The experiments are carried out in a tilted rectangular flume, partially filled with an erodible bed composed of a mixture of quartz grains of two different sizes (0.7 and 2.2 mm) sheared by a steady and spatially uniform turbulent flow. A high-speed video imaging...

We report a new methodology to estimate the impact of extreme events on sediment transport in rivers. Our approach relies on two complementary instruments developed and tested during a 1.5 years field survey performed from june 2007 to january 2009 on the Capesterre river located on Basse-Terre island (Guadeloupe archipelago, Lesser Antilles Arc)....

We describe an optical method known as moiré for acquiring quasi-simultaneous measurements of bed topography and flow depth in laboratory experiments. The moiré method is based on projecting a fringe pattern (grating) on the bed and analyzing the deformation of the pattern caused by the topography with respect to a reference plane. The height of th...

We report measurements performed during two complete flow seasons on the Urumqi River, a proglacial mountain stream in the northeastern ank of the Tianshan, an active mountain range in Central Asia. This survey of flow dynamics and sediment transport (dissolved, suspended and bed loads), together with a 25-year record of daily discharge, enables th...

Here the focus is on the geochemistry of rivers draining volcanic arc regions. Although data are rather sparse, first order estimates show that volcanic arc settings are amongst the faster eroding regions of the Earth because they combine high precipitation regimes with high hydrothermal contributions in which increased temperature and increased ac...

We describe an optical method known as moiré for acquiring quasi-simultaneous measurements of bed topography and flow depth in laboratory experiments. The moiré method is based on projecting a fringe pattern (grating) on the bed and analyzing the deformation of the pattern caused by the topography with respect to a reference plane. The height of th...

Rivers continuously discharge dissolved material to the oceans. Dissolved compounds partially result from water–rock interactions, which produce a large range of water chemical and isotopic compositions. These waters are collected by rivers, that are commonly assumed to be well-mixed with regard to their different tributaries, as a result of turbul...

A series of recent works have underlined that the flux of material exported outside of a watershed is dramatically increased during extreme climatic events, such as storms, tropical cyclones and hurricanes [Dadson et al., 2003 and 2004; Hilton et al., 2008]. Indeed the exceptionally high rainfall rates reached during these events trigger runoff and...

In laboratory experiments, most transport relationships are express ed with respect to the bed shear stress and incorporate a power law regime with a 3/2 exponent above a critical value for motion inception. Nevertheless, in gravel bed streams, available studies seemed to show that conditions for this asymptotic regime to be observed are only rarel...

Despite several decades of investigations, accounting for the effect of the wide range of grain sizes composing the bed of rivers on bedload transport remains a challenging problem. Field and experimental data show indeed that the granulometric distribution of the sediment bed exerts a strong influence which is not correctly modeled in the transpor...

Sediment transport by rivers and erosion dynamics evolve continuously with respect to climatic conditions. In addition, the morphological makers of erosion are often scarce, fragmented and they correspond to a very discontinuous record. At the outlet of drainage basins submitted to erosion, sedimentary series often constitute more frequent, perenni...

We report an experimental investigation of the motion of bed load particles under steady and spatially uniform turbulent flow above a flat sediment bed of uniform grain size. Using a high-speed video imaging system, we recorded the trajectories of the moving particles and measured their velocity and the length and duration of their flights, as well...

The formation of beach rhomboid pattern by swash is investigated experimentally. This centimeter-scale structure is classically interpreted as the mark of stationary gravity waves generated by obstacles in supercritical flows. However, thanks to the use of water-based fluids of various viscosity, our experiments show that a rhomboid pattern can dev...

Over the last twenty years, sediment transport by rivers and erosion dynamics have been the focus of a huge research effort. However, the morphological makers of erosion are often scarce, fragmented and they correspond to a very discontinuous record. At the outlet of drainage basins submitted to erosion, sedimentary series often constitute more fre...

The prediction of the runout length L of large dry debris flows has long been the subject of a considerable research effort, primarily due to the obvious concern caused by their destructive power. One seemingly well established feature is the increase of the mobility M of a rock avalanche, defined as the ratio of the runout distance to the fall hei...

We report the results of a grain-scale investigation of bedload transport in a small experimental river. The trajectories of the grains were visualized using a high-speed video imaging system. From these measurements, the velocity and surface density of the moving grains were derived, together with the lengths and durations of the saltation flights...