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

Abstract

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 barsoften develop either flood- or ebb-dominance which remains unexplained.Tidal experiments are rare and have not reproduced these phenomena todate because these are usually highly ebb-dominated. Here we reportpreliminary experiments in a novel setup that produce tidal bars from aninitial situation with flat bed and exactly symmetrical ebb- andflood-related sediment mobility. The bars initially develop in asaw-tooth pattern with mutually evasive ebb- and flood channels. Thetips of the bars evolve further into lobate bars built up from sedimentfrom excavating channels. The bar tips form steep and high obstructionsfor the opposing flow direction, which then diverges into evasivechannels. The bars develop into a diamond shape as channels bifurcateand deepen around it. We further found that bar height correlates wellto bar width but not with bar length, whilst bar length correlates wellto the tidal excursion length. As the experiment progressed scour holesdeveloped that affected the channel and bar pattern and are consideredartifacts of the experiment. We conclude that the mutually evasive ebb-and flood-dominance of channels emerges simultaneously with the onset oftidal bar formation, and that these preliminary data confirm resultsfrom linear stability analyses. The results are proof of principle forthe novel experimental setup for tidal systems.