Dam-break induced geomorphic flows and the transition from solid- to fluid-like behaviour across evolving interfaces

Thèse de doctorat - UCL, 200

Three-dimensional Voronoi imaging methods for the measurement of near-wall particulate flows

A set of stereoscopic imaging techniques is proposed for the measurement of rapidly flowing dispersions of opaque particles observed near a transparent wall. The methods exploit projective geometry and the Voronoi diagram. They rely on purely geometrical principles to reconstruct 3D particle positions, concentrations, and velocities. The methods are able to handle position and motion ambiguities, as well as particle-occlusion effects, difficulties that are common in the case of dense dispersions of many identical particles. Fluidization cell experiments allow validation of the concentration estimates. A mature debris-flow experimental run is then chosen to test the particle-tracking algorithm. The Voronoi stereo methods are found to perform well in both cases, and to present significant advantages over monocular imaging measurements

Laser stripe measurements of near-wall solid fraction in channelflows of liquid-granular mixtures

A simple, robust, and accurate imaging technique is proposed to measure granular concentration profiles in channel flows of liquid-granular mixtures. We focus on moderate to high granular concentrations (5–50%), for which optical access is restricted to regions close to a transparent wall. To measure concentrations in this range, we illuminate solid grains moving near the wall using a transverse laser light sheet. The evolving shape of the laser stripe, deformed by passing grains, is then monitored using an oblique camera. Statistics of the granular distance to wall can thus be acquired and converted to volumetric solid fraction measurements. The method is verified using fluidization cell tests and applied to open-channel sheet flow experiments. Free of any parameter adjustment, the laser stripe method is found to yield good results, and allows joint measurements of granular velocity and solid fraction profiles

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 concentrations 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.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Intense bedload associated with unsteady dam-break surges

Sediment transport has traditionally been tackled with vastly phenomenological approaches, and the proposed empirical relations contain calibration coefficients that have been adjusted on the basis of a combination of flume and field data. Since recently, the study of bedload is benefiting from the progress made in granular physics, aimed at accounting for the collisional and frictional interactions between grains, especially for bedload layers several grains thick. The development of a specific rheology requires a deeper understanding of the internal structure of bedload layers, in particular in terms of velocity and concentration profiles. We investigate these profiles experimentally, by looking at intense bedload associated with unsteady dam-break surges in a laboratory flume. We resort to a novel technique to simultaneously measure such profiles by imaging the flows with a fast camera combined with a normal-to-wall laser light-sheet. Velocities are obtained by tracking particle positions and concentration is obtained by an original method measuring the penetration depth of the laser light into the granular assembly. Over the wide range of flow conditions covered by our dam-break experiments, from the sparse grain-by-grain movements upstream of the wave to the hyperconcentrated bedload invading the whole flow depth near the downstream wavefront, observations suggest a linear variation of both velocity and concentration throughout the transport layer, though with a concentration at its base that may be noticeably lower than the reference motionless bed value. We extend to unsteady flow a theory for intense bedload recently proposed by Capart and Fraccarollo (2011), and compare predictions with our experimental measurements. The theory lets the granular concentration at the base of the bed-load layer vary with flow conditions, regulating both the frequency of collisions with the bed and the density stratification across the layer. Predictions include longitudinal profiles of bed, water and transport layer boundaries, as well as vertical profiles of velocity and concentration. For the first time, we obtain such predictions using a theory that is free from adjustable coefficients, and provide detailed comparisons with local velocity and concentration measurements, with excellent agreement

SCOURING AT THE INTERFACE BETWEEN FIXED AND MOBILE BED IN STEEP SLOPED CHANNELS

The intense scouring occurring at the interface between fixed and mobile bed can have a strong impact on the stability of structures and on the global morphological changes in rivers. Often, this transition is studied in flat or mild-slope channels. However, in order to better understand this phenomenon, it is important also to understand what can happen in steeper channels. The research presented here focuses on an experimental study of scouring at the interface between fixed-mobile beds in a steep channel with a 5% bed slope. The experiments are conducted in an 8m long flume featuring a 2m long fixed bed followed by a mobile bed portion. Results are captured using two types of cameras: a classical camera capturing 12 pictures per second and a high-speed camera capturing 500 pictures per second. The first one is used to capture the bed- and water-level evolution during the scouring process. The second one allowed the study of the movement of the individual sand particles. The flow is controlled by a fixed discharge upstream and a 0.30m high weir downstream. The experiments are run using four different constant discharges. All tests are repeated 3 to 5 times and showed good repeatability. The results highlighted four different stages in the scouring process as well as the links between the velocity field and the evolution of the bed morphology. The experimental results are then compared to numerical simulations using a one-dimensional finite-volume model

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 issu

Measurement of the free-surface elevation in a steady flow in complex topography using photogrammetry

Experiments are widely used to obtain validation data for numerical models in well-controlled flow conditions. One difficulty when conducting experiments is the selection of appropriate measurement techniques. Most of the current techniques can only capture a small portion of the area of interest and some of them are also intrusive. This study uses photogrammetry to determine the water elevation in a wide area representing a scale model of the Dahan creek in Taiwan, without perturbing the flow. In this work, steady flows over a fixed bed (without sediment transport) are investigated. Based on the obtained results, the capabilities and limitations of the photogrammetry in capturing the flow features are discussed

Stereoscopic imaging measurements of solid concentrations in dense granular flows

The paper outlines imaging methods developed for the measurement of volumetric granular concentrations in the vicinity of a transparent interface. The aim is to construct an imaging indicator which remains robust and sensitive across the whole range of concentrations, and in particular in the dense limit. As monocular (i.e. single camera) techniques have been found in past attempts to present inherent limitations, the present work resorts to a stereoscopic principle. Efficient techniques based on the 2D Voronoï diagram and the epipolar line concept are first developed to reconstruct the three-dimensional positions of visible grains from pairs of digital images. Concentration is then estimated from these three-dimensional positions, using the 3D Voronoï diagram to homogenise the discrete data and obtain a near-wall estimate which is not biased by occlusion effects. The approach is validated using fluidisation cell tests. The comparison between bulk measurements and stereoscopic imaging estimators shows the latter to be endowed with both robustness and sensitivity across the whole examined range