Sediment transport, sedimentary structures and bed stability in step-pool channels

Sediment transport, sedimentary structures and bed stability in step-pool channels Step-pool channels are characterized by moderate to steep slopes. The step is composed of coarse bed material clustering into structures aligned more or less perpendicularly to the flow direction. It is immediately followed downstream by a pool having a depth lower than the mean bed elevation. While step-pools have been studied more extensively during the last ten years, the processes that explain the interactions between the morphology, sediment transport and flow dynamics are still obscured. In natural environments, only a few studies have been conducted in step-pools because of several difficulties associated with step-pool systems such as site accessibility, abundant vegetation, steep slopes, coarse material, scales of roughness, and metric boulders. These difficulties limit the application of the methods and sampling strategies that are generally used in graver-bed rivers.; Considering the areas where knowledge is lacking as it is made evident from an exhaustive literature review, the objectives of this thesis are presented in two complementary facets: methodological and dynamic. The objectives are: (1) to test a new tracing technique of particles to quantify sediment transport, (2) to map and describe sedimentary structures (e.g. pebble clusters and ribs) in step-pool channels from detailed topographic surveys in the field, (3) to identify the processes of the development of sedimentary structures at the reach-scale in a experiment that uses the river as a natural laboratory, (4) to evaluate the quality of high resolution velocity measurements sampled in a step-pool, and (5) to understand the links between the morphology, the sediment transport and the flow dynamics in two step-pools with different morphologies.; The thesis is organized into six main chapters: one that provides the context and objectives of the study and five that follow with the results of the research. The first chapter of results presents an innovative approach to quantify sediment transport in gravel-bed rivers. The advantages of using passive integrated transponder (PIT) tags are examined. These advantages are associated with the high recovery rates (90%) after each transport event and the possibility of tracking individually a large number of particles. The second paper defines sampling guidelines for surveying the topography of the river bed to produce detailed maps of gravel-bed rivers. Digital elevation models allowed the quantification of bed roughness at the reach-scale and the identification of sedimentary structures. In the third paper, the passive transponders and the maps were used to characterize the processes of development and maintenance of sedimentary structures on three sections in a step-pool channel. The bed in one of the sections has been completely disrupted by breaking all sedimentary structures. Following successive sediment transport episodes, particles are rapidly reorganized into new sedimentary structures particularly around keystones. The next chapter is presented as an essay and deals with velocity measurements sampled at a high temporal resolution in a step-pool using an acoustic Doppler velocimeter. The performance of the instrument and the quality of the data are limited although average flow velocities are representative of the morphological unit characteristics. The last paper links the morphology, the sediment transport, and the flow in step-pool channels of different morphology. The variables that explain the mobility of the particles are mostly local while those explaining the distances of displacement characterize reach-scale processes. The differences observed between the reaches are related to their stability, the steepest channel being the most stable.; The originality of the thesis is mostly based on the combination of methodological and dynamic objectives. It is also associated to the use of different approaches. The experimen...