Hydrodynamic and morphodynamic patterns on a mid-channel intertidal bar in an estuary

Abstract. Mid-channel bars that emerge during low tide are a common feature of estuaries. In contrast to fringing tidal flats, these bars are fully surrounded by channels, which may lead to different hydrodynamics and sediment dynamics. Exactly how is unclear, and whether simple models for the dynamics of fringing flats are applicable to mid-channel bars is likewise unknown. These insights are needed for large-scale morphological prediction of the effects of human interference and sea level rise. By using a unique dataset consisting of six months of current velocity data at 16 locations on a mid-channel bar, we analysed the hydrodynamics and calculated a proxy for sediment transport. Taking advantage of the large number of measurement locations, detailed flow patterns were derived. The data show that the spatially non-uniform morphology influences the hydrodynamics and sediment transport in multiple ways. At the deeper parts of the mid-channel bar the ebb- or flood-dominance is determined by the surrounding ebb- and flood channels. This causes changes in ebb or flood dominance over short distances. On the tidal flat itself, a higher area on the seaward bar head causes large-scale circulation cells. Here the flow bends around the emerged areas and causes larger cross-shore flows than expected for alongshore uniform tidal flats. These large-scale flow patterns also determine sediment transport patterns and possibly explain how the mid-channel bar can increase in height. Although the measured data show that velocity can veer over depth, this does not result in large changes in sediment transport direction. This suggests that depth-averaged models are reasonable approximations for predicting sediment transport, sedimentation and erosion on mid-channel bars. Nonetheless, the effects of veering on long-term morphological development of intertidal areas remain to be assessed.