Three-dimensionality of alluvial bedforms
This is my NERC Fellowship project and will be the main focus of my research for the three years.
Background:
Bedforms are ubiquitous in fluvial channels and in estuary and marine environments. They occur in a wide range of flows and sediment sizes. However, despite significant attention over many years, even relatively simple, fully parameterized predictions of flow and sediment transport over bedforms are far from complete and present a major obstacle to the understanding of flow/form interactions.
Research, over the past 20 years in particular, has demonstrated that the presence of bedforms significantly influences both the nature of the mean and turbulent flow structure, exerting strong controls on bed shear stress and therefore the entrainment, transport and deposition of sediment. These investigations, coupled with advances in field, laboratory and numerical techniques have contributed to an improved description of the key flow features over dune bedforms.
However, all of these studies concerning flow and form interaction have concentrated on two-dimensional (2D) forms. As natural dunes are invariably three-dimensional (3D), this morphological simplification has imposed inherent limitations on the interpretation and understanding of dune form and flow dynamics. Dunes often have variation in crestline planform curvature, crestline height, and crestline continuity, with phase differences between successive crests producing variability in dune wavelength and steepness.
This three-dimensionality has significant implications for understanding the flow structure over these dune fields, since 2D forms do not account for the significant affect that lateral flows and secondary circulation may have on the lee side flow structure, turbulence generation, momentum transfers, shear stress and overall dune dynamics. There are also implications for interpreting the sediment record, where 3D dune forms have significantly different grain sorting patterns and variability in the dip angles of dune cross strata. Indeed, although the complexity of bedform morphology in alluvial environments has been known for many years, and the considerable complications that the 3D form can introduce into the flow structure over bedforms highlighted, only very recently have any of the 3D flow effects of bedforms been investigated in any detail.
These recent experiments of have begun to demonstrate that even a simple three-dimensionality in dune form can significantly influence the average flow velocity and flow distribution, secondary currents and lateral flows, and the size and intensity of the separation zone, turbulence and downstream wake.
The interaction of different scales of bedforms is a particularly important aspect of bedform dynamics, which we know very little about. There will be interplay between larger 3D bedforms, such as unit bars and large topographic features (mid-channel bars, point bars etc.), and the 3D form of smaller features, such ripples and dunes. However, we have neither the data nor any real understanding of this feedback interaction.
This project aims to improve understanding of the influence of dune three-dimensionality on bedform dynamics. Employing an integrated approach, it aims to investigate the linkages between dune form, flow structure, sediment transport and dune migration patterns in a three-dimensional context. These aims will be addressed through the following objectives.
Other Research Projects:
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