Tidal Flat Morphodynamics

Abstract

The study of the morphology and evolution of tidal flats is particularly well suited in the context of morphodynamics in that 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 tidal flat morphology itself. When averaged over annual or longer timescales, tidal flat morphology typically approximates a dynamic equilibrium with external forcing. Over seasonal and shorter timescales, spatial asymmetries in the hydrodynamic energy associated with temporally alternating tide- and wave-dominance drive sediment landward and seaward, respectively. For flats exhibiting minimal spatial gradients in energy, net sediment transport can still occur in response to local, time-dependent asymmetries associated with finite depth and intertidal storage effects; negative feedback on the scale of entire tidal embayments may favor minimization of these local asymmetries. Assuming that morphological equilibrium is likewise associated with the minimization of spatial asymmetries in energy, dominance by tides results in a convex-up tidal flat profile, whereas dominance by waves results in a concave-up profile. These extremes lead to analytical solutions for tidal flat width, slope, and degree of curvature as a function of tidal range, wave height, and the critical velocity for profile stability. Although a tide- or wave-dominated static equilibrium theoretically exists at each of these extremes, natural tidal flats over annual timescales are better approximated by a dynamic equilibrium somewhere between these two asymptotes. Observations and models indicate that, within this range of morphologies, convex-up profiles are further favored by increased sediment supply, increased bioaggregation/adhesion, and external forcing by faster-rising tides; concave-up profiles are further favored by decreased sediment supply, increased bioturbation, and external forcing by faster-falling tides. Processes/properties associated with evolution toward a convex (vs. concave) profile include shoreward (vs. seaward) sediment transport, net deposition (vs. erosion), decreased (vs. increased) grain size, and a form that progrades seaward (vs. retreating landward). Because surficial grain size responds to energy gradients much more quickly than overall morphology can adjust, the common presence of tides without strong waves (alternating with much shorter periods of intense waves) leads to landward fining of surficial grain size most of the time.