Reversing the tides: experiments on tidal bars and ebb- and flood-dominated channels in tidal systems

Abstract

Tidal bars and ebb- and flood-dominated channels are characteristic for tidal inlet systems and estuaries. The dynamics of tidal bars are poorly understood whereas fluvial bars are well understood due to linear stability analyses, numerical modelling and field observations. Ebb- and flood-dominated tidal channels flank tidal bars. For both tidal bars and ebb- and flood- dominated channels theory is hardly theory available and scale experiments on bars and tidal channels have not yet been performed. Main objective of this thesis is to ascertain whether tidal bars and channels form in physical scale experiments, investigate characteristics of tidal bars and channels and compare with previous literature. The experimental setup consisted of a tilting basin that was 3.2 meters long and 1 meter wide. Tides were created by tilting the basin over the diagonal. In addition braided rivers bars were investigated by unidirectional flow for comparison with the formation of tidal bars under reversing flow conditions. The results show that both tidal bars and ebb- and flood-dominated channels initiated spontaneously due to small perturbations in the sediment bed and leaded to the formation of bar-channel couples. These bar-channel systems were only morphological active during one phase of the tides. Two types of bars emerged in the system: either large round/diamond shaped bars or elongated sharper-edged bars. Tidal excursion length was the main forcing that determined the shape and size of bars and channels. The length of the bars is significantly correlated to tidal excursion length, which is in agreement with previous modelling studies. Unlike fluvial bars, tidal bars exhibited no net migration and moved back and forth symmetrically. The formed ebb- and flood-tidal channels were similar to observations in nature. The braided river formed in the experimental setting was characterized by multiple channel systems, irregular bar patterns and bars that varied in shape and size.