Bed load transport rate fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions even under constant sediment feeding and water discharge. The observed bed load transport rate pulses are periodic and a consequence of grain sorting. Moreover, the presence of large, relatively immobile boulders, such as erratic stones, which are often present in mountain streams, has an impact on flow conditions. The detailed analysis of a 13 h laboratory experiment is presented in this paper. Boulders were randomly placed in a flume with a steep slope (6.7%), and water and sediment were constantly supplied to the flume. Along with the sediment transport and bulk mean flow velocity, the boulder protrusion, boulder surface, and number of hydraulic jumps, which are indicators of the channel morphology, were measured regularly during the experiment. Periodic bed load transport rate pulses are clearly visible in the data collected during this long-duration experiment, along with correlated fluctuations in the flow velocity and bed morphology. The links among the bulk velocity, the time evolution of the morphology variables, and the bed load transport rate are analyzed via correlational analysis, showing that the fluctuations are strongly related. A phase analysis of all observed variables is performed, and the average shapes of the time cycles of the fluctuations are shown. Observations indicate that the detected periodic fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, which varies in time and space, clearly influences these bed load transport rate pulses. Finally, known bed load transport rate formulae are tested, showing that only the application of a drag shear stress allows a correct estimation of the time fluctuations.

No abstract

Most sediment transport equations overestimate the bedload transport rate by several times when applied to mountain rivers. This is due to the fact that the presence of large relatively immobile boulders, which disrupt the flow, is generally not taken into account. Sediment transport in steep channels with boulders was herein investigated using 41 laboratory experiments carried out on a tilting flume for three slopes (S 6.7%, 9.9%, and 13%). Sediment transport, mean flow velocities and morphology-describing variables were measured regularly during the experiments. The sediment transport capacity is clearly decreasing with dimensionless boulder distance and is better estimated in terms of critical discharge for incipient motion of mobile sediments than in terms of bed shear stress. A sediment transport formula based on excess discharge relative to a critical value, which depends not only on the channel slope but also on the boulder spatial density, is herein developed.bed area occupied by them, the distance between boulders and a drag coefficient (Yager et al., 2007, Bathurst, 1978, Canovaro et al., 2007.Most sediment transport formulae have the following general form:where q b is the bedload transport, which may be expressed in different units depending on the equation

Steep mountain rivers have hydraulic and morphodynamic characteristics that hinder velocity measurements. The high spatial variability of hydraulic parameters, such as water depth (WD), river width and flow velocity, makes the choice of a representative cross-section to measure the velocity in detail challenging. Additionally, sediment transport and rapidly changing bed morphology exclude the utilization of standard and often intrusive velocity measurement techniques. The limited technical choices are further reduced in the presence of macro-roughness elements, such as large, relatively immobile boulders. Tracer tracking techniques are among the few reliable methods that can be used under these conditions to evaluate the mean flow velocity. However, most tracer tracking techniques calculate bulk flow velocities between two or more fixed cross-sections. In the presence of intense sediment transport resulting in an important temporal variability of the bed morphology, dead water zones may appear in the few selected measurement sections. Thus a technique based on the analysis of an entire channel reach is needed in this study. A dye tracer measurement technique in which a single camcorder visualizes a long flume reach is described and developed. This allows us to overcome the problem of the presence of dead water zones. To validate this video analysis technique, velocity measurements were carried out on a laboratory flume simulating a torrent, with a relatively gentle slope of 1.97% and without sediment transport, using several commonly used velocity measurement instruments. In the absence of boulders, salt injections, WD and ultrasonic velocity profiler measurements were carried out, along with dye injection technique. When boulders were present, dye tracer technique was validated only by comparison with salt tracer. Several video analysis techniques used to infer velocities were developed and compared, showing that dye tracking is a valid technique for bulk velocity measurements. RGB Euclidean distance was identified as being the best measure of the average flow velocity.

RÉSUMÉ. -Suite à un incident survenu en 2000 sur la Ligne à Grande Vitesse (LGV) Paris-Lyon sur la commune de Sarry dû à un emportement de ballast par débordement des eaux du drainage longitudinal de la plateforme ferroviaire, la SNCF a engagé en tant que gestionnaire délégué de l'infrastructure un ensemble de démarches afin de mieux caractériser le « risque Sarry ». Une campagne d'expertise a été ainsi initiée sur les LGV les plus anciennes afin de repérer les sites à risque de débordement de drainage pouvant impacter les circulations ferroviaires. Par la suite, des travaux de régénéra-tion ont été réalisés sur les sites priorisés les plus à risque afin d'éliminer le risque hydraulique. En parallèle, la LGV Paris-Marseille est surveillée en permanence par des radars météorologiques pour permettre le déclenchement d'alertes en cas de dépassement de seuils pluviométriques. Ce déclenchement permet l'avertissement du centre de régulation des trains en contact avec les conducteurs de TGV et entraîne une tournée intempéries par les agents de maintenance. Des recherches ont été menées en collaboration avec l'Ecole Polytechnique Fédérale de Lausanne (EPFL) avec la réalisation d'un modèle physique à l'échelle 1/3. Les résultats ont permis de mieux appréhender le phénomène de transport de ballast et d'identifier les limites d'emportement de ballast sur piste pour les deux critères de détermination du risque : la hauteur d'eau et la vitesse de l'écoulement. Ces démarches établies à différentes échelles permettent de réduire la vulnérabilité des circulations ferroviaires vis-à-vis du risque de débordement de drainage, appelé « risque Sarry ». Mots-clés : risque, débordement, modèle physique, ferroviaire Analysis of the risks for rail traffic in the event of overflowing of the drainage system on high speed linesFollowing an incident at the village of Sarry in 2000 involving the overflowing of the longitudinal drains of the Paris-Lyon High-Speed Line (HSL), the SNCF, responsible for the maintenance of the infrastructure, outlined a wide range of initiatives to define the "Sarry risk". A campaign of visits by experts has been initiated on the oldest HSLs in order to detect the sites at risk of overflowing and which could impact the railway traffic. In areas classified as "high risk areas", regeneration works are undertaken to eliminate the risk. At the same time, the HSL between Paris and Marseille is monitored permanently by meteorological radars with a view to issuing alerts in case of exceedance of pluvio metrical thresholds. This alert allows the railway regulation centre, in contact with the HSL drivers to be kept informed and results in a surveillance visit by the maintenance staff. Research has been undertaken in collaboration with EPFL (Ecole Polytechnique Fédérale de Lausanne) to realise a physical model at 1/3 scale. Results are used to better understand the phenomena of ballast loss and identify limits of ballast removal for two criteria : water level and flow velocity which are the criteria used in determining the...