Tidal asymmetries in estuaries due to channel-flat interactions, a simple model

Tidal flats, i.e. areas in tidal embayments that fall dry during part of the tidal cycle, have a significant impact on the nonlinear ”shallow water” tides, which are asymmetries of the tidal signal in sea surface elevation and tidal velocity. Tidal asymmetry is important for net sediment transport and thereby also for the morphological stability of the channel. Indeed, it has been shown in previous studies that the importance of tidal flats as a temporal storage of mass is non-negligible. In this contribution the focus lies on a new aspect of tidal flats, viz. that they also act as a net sink term for the along-channel momentum. This momentum loss term is a nonlinear function of sea surface elevation and velocity, and is significant in areas where the tidal flat width, bf, is of comparable or larger size than the width bc of the main channel. The process of momentum loss will be analyzed and its efficiency in generating nonlinear tides and net sediment transport will be compared with other sources of nonlinearities on the basis of results from a numerical model.