Bar dimensions and shapes in estuaries

Estuaries comprise fascinating patterns of dynamic channels and bars. Intertidal sandbars are valuable habitats, whilst channels provide access to harbours. We still lack a full explanation and classification scheme for the shapes and dimensions of bars in natural estuaries, in contrast with bars in rivers. Analytical physics-based models suggest that bar length in estuaries increases with flow velocity, tidal excursion length or estuary width, depending on the model. However, these hypotheses have never been validated due to a lack of data and experiments. We present a large dataset and determine the controls on bar shapes and dimensions in estuaries, with bar lengths spanning from centimetres (experiments) to tens of kilometres. To do so, we visually identified and classified 190 bars in real world estuaries, measured their dimensions (width, length, height) and local braiding index. For the estuaries used, data on estuarine geometry and tidal characteristics were obtained from governmental databases and literature on case studies. We found that many complex bars can be seen as simple elongated bars partly cut by mutually evasive ebb- and flood-dominated channels. Data analysis shows that bar dimensions scale with estuary dimensions, in particular with estuary width. Breaking up the complex bars in simple bars greatly reduces scatter. Analytical bar theory overpredicts bar dimensions by an order of magnitude in case of relatively small estuarine systems. Likewise, braiding index depends on local width-to-depth ratio, as was previously found for river systems. Our results suggest that estuary dimensions determine the order of magnitude of bar dimensions, while tidal characteristics modify this. We will continue to model bars numerically and experimentally. Our dataset on tidal bars enables future studies on the sedimentary architecture of geologically complex tidal deposits and enables studying effects of man-induced perturbations such as dredging and dumping on bar and channel patterns and ecological habitats.