Tidal flat morphodynamics: a synthesis

Tidal flats commonly occur along coasts where the tidal range is large relative to typical wave height. They can be found where hydrodynamic energy is high or low, where sediments are sandy or muddy, and where shorelines are prograding, retreating, or stable. The study of the morphology and evolution of tidal flats is particularly well suited in the context of morphodynamics since characteristics such as profile shape, bed slope, and grain size clearly and systematically vary as a function of sediment supply and wave and tidal forcing, and the nature of wave- and tide-induced velocities across tidal flats is, in turn, a direct function of the flat morphology itself. In this presentation, which summarizes and extends the findings of Friedrichs (2011), a tidal flat profile in dynamic equilibrium is defined as one where its shape remains more or less constant over some characteristic period of natural forcing. The concept of a dynamic equilibrium is a central point of this work. When considered as an average over a typical annual cycle (and when accounting for antecedent geology and anthropogenic effects), this paper argues that most tidal flats are characterized by predictable morphologies that are in an approximate dynamic equilibrium with their local climate of waves, tides, and sediment sources and sinks. This does not mean that they are static in space as they are more often than not in the process of advancing or retreating. Rather, it means that when averaged over annual timescales or longer, their characteristic shape and the spatial distribution of surficial grain sizes remain relatively fixed in the reference frame of the flat itself as it recedes or progrades