Reconsidérer les interactions entre l’écoulement, le transport de sédiments en charge de fond et la morphologie en rivière à lit de graviers : approches, échelles et analyses

The overarching objective in fluvial geomorphology is to explain river morphology and its temporal and spatial evolution. The multiplication of studies led to the realization that geomorphic systems are complex and difficult to understand. The observed river morphology is more than the sum of the individual processes which govern them because of the superimposition of process interactions and non-linear feedback loops at multiple scales. In this context, the goal of this thesis is to explore new avenues of research in order to comprehend more accurately the complexity of gravel-bed river dynamics by using methodological and analytical approaches focusing on the interactions between the flow, transport of sediments as bedload and the bed morphology. This orientation of the thesis is in line with the fact that current paradigms in fluvial geomorphology have not succeeded in explaining adequately the natural variability of the sediment transport and its consequences on the river bedforms. Five avenues are developed in the form of articles based on case studies:  1. The study of the scales of variation of the flow allows us to integrate the turbulent flow structures into flow pulsations at larger scales and to potentially improve our understanding comprehension of bedload sediment transport variability.  2. The quantification of the rates of change of the flow magnitude (acceleration /deceleration) during a flood explains as much the variability of bedload transport fluxes as the flow intensity itself.  3. The use of complementary measurement techniques of bedload processes can reveal new river bed dynamics in gravel-bed rivers: the dilation and contraction of the bed resulting from a flood.  4. The investigation of the generally accepted fact that bedload sediment transport is correlated with the intensity of the morphological changes reveals that the two processes do not relate directly due to their different characteristic scales.  5. The systemic approach of river dynamics based on the use of multivariate analytical techniques is better suited for the study of complex linear and non-linear interactions and feedbacks in the evolution of a river channel. This analysis has shown the importance of the recent history of the morphological changes in response to a flood.  The conceptual orientation of this thesis results from a deep reflection on the traditional approaches previously used for several decades in fluvial geomorphology. It is based on a single very extensive data set collected during 21 flood events in a small gravel-bed river, Béard creek (Quebec). The experimental protocol aimed at the simultaneity of processes measurements in order to focus the analysis on the interactions between the flow, the channel morphology and the bedload sediment transport rather than on the individual processes, an approach seldom used in fluvial geomorphology. Each chapter reports on a new concept or a new approach making it possible to resolve some of the issues met in fluvial geomorphology. This work has important implications for the understanding of river bed dynamics and fluvial habitats and is a starting point for new developments.

Keyphrases: Fluvial Geomorphology, Scales, Gravel-bed rivers, Flow, Flood, Bedload sediment transport, Bed morphology