Morpho-sedimentary dynamics of pool-riffle sequences in a gravel -bed river: Bedload transport reversal and pool -riffle maintenance.

摘要

Hydraulic and morpho-sedimentary differences between pools and riffles have always intrigued fluvial geomorphologists. Surprisingly, earlier explanations of pool-riffle morpho-sedimentology dynamic focus too exclusively on hydraulic patterns and neglect analysis of sediment transport. Understanding the mechanisms governing pool-riffle morpho-sedimentary dynamics, such as the sediment transport patterns represent a big challenge in fluvial geomorphology, considering the stochastic nature of bedload transport in gravel-bed rivers. The main objective of this thesis is to understand the two-dimensional relationship between hydraulic patterns, sediment transport patterns and morpho-sedimentary changes in pool-riffle sequences and to use these insights to understand how pool-riffle sequences maintain their form over time.;This thesis improves the understanding of the morpho-sedimentary dynamics of pool-riffle sequence; it argues that a purely hydraulic view of pool-riffle sequence is insufficient to understand their stability. It explains pool-riffle maintenance as a result of a bedload transport reversal caused by the spatial heterogeneity of the sediment transport patterns, and raises the importance of acquiring two-dimensional sediment transport data to improve two-dimensional bedload transport formulae.;This thesis is organized into four chapters. These chapters analyze hydraulic, sediment transport and morpho-sedimentary measurements collected on a riffle-pool-riffle sequence of the Des Coeurs river during seven controlled experimental floods. Chapter 2 tests the velocity reversal hypothesis (Keller, 1971), with results demonstrating that no velocity reversal was observed in the field, for peak flow up to 123% of the bankfull discharge. Chapter 3 demonstrates the existence of a bedload transport reversal: below 60% of bankfull discharge, sediment transport on the riffle exceeds transport in the pool; as discharge increased, more sediment was exported from the pool than imported, thereby maintaining the pool. Chapter 4 demonstrates that the bedload transport reversal is caused by the spatial heterogeneity of the sediment transport patterns (sediment supply, particle displacements and sediment sorting), which underlies the importance of developing two-dimensional bedload transport formulae. Chapter 5 tests the two-dimensional applicability of well-known bedload transport formulae developed in a one-dimensional environment. None of the formulae could be applied with accuracy in a two-dimensional environment. However, site-specific calibration considerably improves bedload transport rate and grain-size distribution prediction.