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Experiments by Invited Researchers

 

Splitting nature at its seams: morphodynamic stability of river and tidal bifurcations

Project acronym: H+-HRW-02-Kleinhans
Name of Group Leader:
User-Project Title: Splitting nature at its seams: morphodynamic stability of river and tidal bifurcations
Facility: The Fast Flow Facility
Data Storage Report: data-storage-report-HyUU18-HydralabWallingfordBifurcat2018.pdf
Publications:

Here we target firstly river bifurcations and secondly the mutually evasive ebb- or flood-dominated channels that form around bars and are found in all sandy tidal systems in the world. Such bifurcations are critical elements that partition flow and sediment through the channel network, govern bar merging and splitting and are locations where bed steps form in shipping lanes, as in river bifurcations. Stability and equilibrium configurations are mostly unknown for tidal bifurcations except for one recent theory. We have no idea whether bifurcations in reversing tidal flow are unstable for similar configurations and conditions as in rivers. Here we mean configurations that are entirely free of topographic forcings on the flow: straight channels split into two channels over some length and depth. Our objective is therefore to experimentally investigate bifurcation stability in a range of sediment mobilities in unidirectional flow and reversing tidal flow ceteris paribus.

The basic idea of the experiment is to create a 30 m long flat sand bed with a splitter plate along part of the length of the bed to simulate a bifurcated channel with an infinitely narrow bar so as to prevent curvature-induced secondary flows. The most important measurement is bed elevation along the entire flume to build up enough detail and averaging statistics over the expected ripples to be able to detect growing instabilities. From repeated bed scans we expect to observe the very gradual beginning of the process of destabilisation by sedimentation in one of the parallel channels and erosion in the other, particularly in the upstream few meters of the bifurcated channels. As this develops we also expect to observe the development of a transverse slope upstream of the splitter plate, grading into the bifurcated channel beds. The length of this upstream effect is an important parameter in the theory.

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