Characterization of intertidal flat hydrodynamics

The paper reviews the different physical forcings that control tidal flat h ydrodynamics. Tidal propagation and cross-shore or long-shore currents, tid al asymmetry, wind-induced circulation, wave propagation and drainage proce sses are successively considered. Some simple methods are described for est imating cross-shore currents and wave-induced bottom shear stresses, and th e results obtained are compared to held measurements on three contrasted si tes in Europe. In particular the cross-shore current is shown uniform in th e lower part of the flat, and decreasing towards the shore. Bottom friction -induced wave attenuation is simply formulated on gently sloping beds, lead ing to a maximum wave height that a flat can experience; it is proportional to the water height according to the ratio between the slope and the wave friction factor, The maximum related shear stress occurs at high water and is also proportional to the water depth. Maximum tidal velocities are very similar in the three sites where bottom sediment is muddy, suggesting a rel ationship between physical stresses and sediment characteristics. The conse quences of physical forcings on sediment transport are listed, The bottom s hear stress is suggested as the relevant parameter for comparing tidal and wave effects. In general, tide induces onshore sediment transport, whereas waves and drainage favour offshore transport. The processes leading to a po ssible tidal equilibrium profile are analysed: they involve the intrinsic a symmetry that favours net deposition at high water, and an ebb dominance ge nerated by the resulting bottom profile convexity, Eroding waves are likely to upset such a balance; this equilibrium then reduces to a trend for the system. (C) 2000 Elsevier Science Ltd. All rights reserved.