Hervé Capart

Riemann wave description of erosional dam-break flows

This work examines the sudden erosional flow initiated by the release of a dam-break wave over a loose sediment bed. Extended shallow-water equations are formulated to describe the development of the surge. Accounting for bed material inertia, a transport layer of finite thickness is introduced, and a sharp interface view of the morphodynamic boundary is adopted. Approximations are sought for an intermediate range of wave evolution, in which equilibration of the sediment load can be assumed instantaneous but momentum loss due to bed friction has not yet been felt. The resulting homogeneous hyperbolic equations are mathematically tractable using the Riemann techniques of gas dynamics. Dam-break initial conditions give rise to self-similar flow profiles. The wave structure features piecewise constant states, two smoothly varied simple waves, and a special type of shock: an erosional bore forming at the forefront of the wave. Profiles are constructed through a semi-analytical procedure, yielding a geomorphic generalization of the Stoker solution for dam-break waves over rigid bed. For most flow properties, the predictions of the theoretical treatment compare favourably with experimental tests visualized using particle imaging techniques.

Formation of a jump by the dam-break wave over a granular bed

A previously unreported shock feature associated with the scouring of a horizontal granular bed by a dam-break wave is discussed. Near the wave centre, the present study shows, the free surface breaks backward and a hydraulic jump forms. This behaviour is described from the standpoint of shallow-water theory, suitably extended to deal with non-equilibrium sediment transport. The shock formation involves a particularly strong coupling between flow free-surface evolution and bed morphodynamics. Support for our conclusions is sought through experimental and numerical approaches. In order to magnify the observed phenomena, measurements were performed for the case of light bed particles moving in sheet and debris flow modes. A detailed picture of the transient two-phase flow is presented, based on whole field acquisition of the grain motions by particle tracking techniques. Corresponding shallow-water solutions are constructed numerically using a shock capturing scheme. Finally, an interpretation of the jump formation is proposed based on the theory of characteristics.

Rheological stratification in experimental free-surface flows of granular-liquid mixtures

Laboratory experiments are conducted to study the rheological behaviour of high-concentration granular-liquid mixtures. Steady uniform free-surface flows are obtained using a recirculating flume. Cases in which a loose deposit forms underneath the flow are contrasted with runs for which basal shear occurs along the flume bottom. The granular motions are observed through the channel sidewall, and analysed with recently developed Voronoi imaging methods. Depth profiles of mean velocity, solid concentration, and granular temperature are obtained, and complemented by stress estimates based on force balance considerations. These measurements are used to probe variations in rheological behaviour over depth, and to clarify the role of the granular temperature. The flows are found to evolve a stratified structure. Distinct sublayers are characterized by either frictional or collisional behaviour, and transitions between one and the other occur at values of the Stokes number which suggest that viscous effects intervene. The observed frictional behaviour is consistent with shear cell tests conducted at very low shear rates. On the other hand, the collisional data corroborate both the Bagnold description and the more recent kinetic theories of granular flows, provided that one accounts for the inertia of the interstitial liquid.

Numerical and experimental water transients in sewer pipes

The paper deals with transient transcritical flow in closed sewer pipe. Among the various shock-capturing schemes used for solving hyperbolic systems of conservation laws, an upwind scheme is adopted in such a way that automatic description of hydraulic jumps and bores becomes possible by a steep variation of hydraulic variables over a few grid points. The Pavia Flux Predictor scheme (P.F.P.) was selected, because of its simplicity, robustness and physical consistency. To validate the numerical model, experiments were carried out for a steep slope circular pipe. An application to flush flows at sewer heads is also presented in a context of parsimonious water consumption.

Set of measurement data from flume experiments on steady uniform debris flows

The paper documents a detailed set of measurements from laboratory experiments involving open–channel flows of granular–liquid mixtures. Polyvinyl Chloride pellets mixed with clear water are used, and represent simplified analogues of the mixtures of rocks and muddy water encountered in stony inertial debris flows. The flows are examined in steady uniform conditions, achieved by setting up a closed loop with an inclined channel and a high–speed conveyor belt. Such idealized conditions make it possible to estimate stresses within the flowing mixture, and to accumulate statistics of local granular configurations and motions. These are extracted from video sequences imaged through the sidewall, using algorithms based on the Voronoï diagram. Estimates of granular concentration and granular temperature are derived from local grain patterns and from a detailed analysis of Lagrangian and Eulerian velocity correlations. Depth profiles of kinematic and dynamic quantities are then obtained for various ratios of solid to liquid discharges. These data were earlier used to probe depth variations of rheological behaviour in granular–liquid flows. To make them available for other purposes, they are assembled here into a comprehensive dataset, and provided in digital form in the electronic supplement to this special issue.

The 1996 Lake Ha! Ha! breakout flood, Quebec: Test data for geomorphic flood routing methods

This paper describes a set of field data suitable for the testing and comparative assessment of geomorphic flood routing methods. The data pertain to a particularly severe and unusually well–documented flood event: the Lake Ha! Ha! breakout flood of July 1996 in the Saguenay Region of Québec. In this event, heavy rains combined with the incision of a new lake outlet caused a major flood, which significantly reworked the downstream valley. Published and unpublished data from multiple sources are assembled and co–registered in a common frame of reference. These data include vertical and oblique air photos, hydrological records, surface geology information, and digital terrain models of the pre– and post–flood valley topography. The spatial coverage encompasses the drained lake as well as the full length of the downstream valley. To meet the respective needs of two– and one–dimensional approaches, the topography is sampled on a Cartesian mesh as well as interpolated along evenly–spaced cross–sections. The data set described in the paper is provided in digital form in the electronic supplement to this special issue.

Two-layer shallow water computation of mud flow intrusions into quiescent water

The present computational study addresses two-layer shallow flows in which the superposed layers differ in velocity, density and rheology. The geomorphological phenomena motivating this model are confluence problems in which mud and debris surges slump into upland lakes and rivers. Specifically, the flows of interest are assumed to be sharply stratified, with a clear water layer flowing over a moving layer of mud modelled as a Herschel–Bulkley fluid.A finite volume computational scheme suitable for the simulation of such flows is presented and applied to various validation cases. The scheme extends to two-layer flows the robust method of Harten, Lax and Van Leer. Special care is devoted to the following numerical issues: the treatment of pressure along interfaces, the ”contact points“ at which the thickness of either layer can vanish, and the finite yield stress characterizing the mud rheology. Results are presented for the intrusion of mud surges into shallow quiescent water.

Underwater sand bed erosion and internal jump formation by travelling plane jets

Theory and experiments are used to investigate the water and sediment motion induced along a sea bed by travelling plane jets. Steadily moving jets are considered, and represent an idealization of the tools mounted on ships and remotely operated vehicles (ROVs) for injection dredging and trenching. The jet-induced turbulent currents simultaneously suspend sand from the bed and entrain water from the ambient. To describe these processes, a shallow-flow theory is proposed in which the turbulent current is assumed stratified into sediment-laden and sediment-free sublayers. The equations are written in curvilinear coordinates attached to the co-evolving bed profile. A sharp interface description is then adopted to account rigorously for mass and momentum exchanges between the bed, current and ambient, including their effects on the balance of mechanical energy. Travelling-wave solutions are obtained, in which the jet-induced current scours a trench of permanent form in a frame of reference moving with the jetting tool. Depending on the operating parameters, it is found that the sediment-laden current may remain supercritical throughout the trench, or be forced to undergo an internal hydraulic jump. These predictions are confirmed by laboratory experiments. For flows with or without jump in which the current remains attached to the bed, bottom profiles computed by the theory compare favourably with imaging measurements.

Digital imaging characterization of the kinematics of water-sediment interaction

Digital imaging techniques especially geared towards the laboratory characterization of the kinematics of water-sediment interaction are presented. More specifically, the methods proposed apply to the motion of cohesionless spherical particles in transient water flow, with the aim of obtaining both particle velocity and concentration fields from sequences of digital images. A special particle identification algorithm is devised in order to deal with densely packed particles (in contrast to the sparse seedings of tracers used in studies of pure fluid kinematics) and to allow application of digital particle tracking velocimetry (DPTV). A procedure for extracting the concentration field from the knowledge of the discrete particle positions is then detailed. Finally, the various tools are illustrated for the laboratory case of a dambreak wave over a movable bed.

Influence of aspect ratio on the distribution of porosity and velocity in columns of spheres

Abstract Internal imaging experiments are used to study assemblies of mono‐size spheres in a cylindrical flow cell. An optical measurement technique is adopted, whereby we scan a laser light sheet across the column composed of refractive index matched solid and liquid materials. To probe the influence of boundary confinement, we examine two different aspect ratios (ratios of cylinder diameter to sphere diameter), for which we compare radial distributions of bed porosity and liquid velocity. The measurements highlight the competing roles of near‐wall ordering and axial frustration in columns of low aspect ratios.