Dune dynamics under unsteady flows

Hu, Hao

Abstract

River bedforms keep translating and deforming perpetually during their migration. During flow field unsteadiness they also change in size and shape over time and in space. However, our knowledge of how bedforms adapt to changing flows remains inadequately understood. Therefore, how do dunes adapt to different changing flows (floods and tides), and how does the coupled sediment transport affect dune morphology and dynamics is urgent to be investigated.

Large-scale flume experiments were conducted to simulate dune dynamics during carefully controlled floods with various hydrographs, while field surveys were undertaken in both the middle reach, close to the backwater zone, and within the estuary of the Changjiang (Yangtze) River, in order to examine the combined effect of the flood and tide on dune evolution.

The result indicates that the sediment transport mechanisms dominate how dunes adapt to unsteady flows. The analysis of dune three-dimensionality reflects that the generation of the larger dunes is the main factor controlling sediment transport and thereby the bedform adaptation. Moreover, the processes of bedform adaptation to changing flows will be varying under different sediment transport mechanisms due to diverse sediment redistribution over and between dunes.

In the tidally influenced area where riverbed is composed of fine sediment, our result implies that clay content is a first-order control on bedform aspect ratio and the specific sediment composition of the riverbed, in some extent, affects the mechanism of sediment transport related to the exchange between suspended sediment and riverbed.

This work extends our knowledge on how dunes generate and develop under variable flows and has delineated how variations in transport stage can be coupled with the variation in the dominant sediment transport mechanisms. Moreover, these developments will provide fundamental knowledge that is of significance for a wide variety of purposes, such as improving morphodynamic modelling over large spatio-temporal scales, environmental and engineering management, and more reliable flood predictions.

Citation

Hu, H. Dune dynamics under unsteady flows. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4221728

• Thesis Type: Thesis
• Deposit Date: Aug 2, 2019
• Publicly Available Date: Feb 23, 2023
• Keywords: Geography
• Public URL: https://hull-repository.worktribe.com/output/4221728
• Additional Information: Department of Geography, The University of Hull
• Award Date: Jan 1, 2019