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

Abstract. Many estuaries have mid-channel bars that emerge during low tide. In contrast to fringing tidal flats, these bars are fully surrounded by channels. This may lead to different hydrodynamics and sediment dynamics, but 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. Using 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. 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, the presence of a higher area on the seaward bar head causes large-scale circulation cells. 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, suggesting that depth-averaged models are reasonable approximations for predicting sediment transport on and morphological change of intertidal areas even on mid-channel bars.