Turning the tide : Mutually evasive ebb-and flood-dominant channels and bars in an experimental estuary

Tidal bars in estuaries are poorly understood compared to fluvial bars. There is limited theory that predicts tidal bar dimensions. Moreover, where fluvial channels bifurcate around bars, tidal channels around bars often develop either flood-or ebb-dominance which remains unexplained. Tidal experiments are rare and have not reproduced these phenomena to date because these are usually highly ebb-dominated. Here we report preliminary experiments in a novel setup that produce tidal bars from an initial situation with flat bed and exactly symmetrical ebb-and flood-related sediment mobility. The bars initially develop in a saw-tooth pattern with mutually evasive ebb-and flood channels. The tips of the bars evolve further into lobate bars built up from sediment from excavating channels. The bar tips form steep and high obstructions for the opposing flow direction, which then diverges into evasive channels. The bars develop into a diamond shape as channels bifurcate and deepen around it. We further found that bar height correlates well to bar width but not with bar length, whilst bar length correlates well to the tidal excursion length. As the experiment progressed scour holes developed that affected the channel and bar pattern and are considered artifacts of the experiment. We conclude that the mutually evasive ebb-and flood-dominance of channels emerges simultaneously with the onset of tidal bar formation, and that these preliminary data confirm results from linear stability analyses. The results are proof of principle for the novel experimental setup for tidal systems