The effects of tidal flats on the hydrodynamics of the Tagus estuary are analysed using an accurate highresolution shallow water model, supported by field data. Tidal flats act mainly as energy filters, rather than sinks, transferring energy from astronomic to non-linear frequencies. They also play a major role in determining the strong resonance mode that amplifies semi-diurnal constituents. We show that the upper estuary, with extensive tidal flats, has an increasing accretion rate. This "filling-up" results from a positive feedback between sediment deposition and the hydrodynamics: accretion in the upper estuary lengthens the duration of ebbs, decreasing sediment flushing and further increasing accretion. 0 Elsevier, Paris / Ifremer / Cnrs / Ird tidal flats / tidal propagation / numerical model / Tagus estuary RCsumC-Sur l'effet des estrans sur l'hydrodynamique de l'estuaire du Tage. Les effets des estrans sur l'hydrodynamique de l'estuaire du Tage sont analysCs B travers un modtile d'Ccoulement de haute r&olution, et des donnCes de terrain. Les estrans fonctionnent surtout comme des filtres d'Cnergie, et non pas comme des dissipateurs, transfkrant 1'Cnergie des frkquences astronomiques 5 des frkquences non-lintaires. Les estrans ont aussi un rale t&s important dans la ddtermination du mode de rCsonance qui amplifie les constituantes semi-diumes. Nous montrons que la partie sup&ieure de l'estuaire, dominCe par des estrans, a un taux de stdimentation croissant. Ceci est d0 B une r&o-action positive entre le dtp6t de stdiments et l'hydrodynamique : le dkp& dans la partie supt%ieure de l'estuaire augmente la dun% des reflux de marCe, rkduisant le transport de sCdiments vers l'extkrieur, et contribuant ainsi ti leur accumulation dans l'estuaire.
Abstract. The variability of estuarine residence times is elucidated through accurate, diagnostic numerical tracking of a large number of particles, complemented with statistical analysis. Residence times are shown to have strong spatial and temporal variability, which is accentuated by exchanges between the estuary and the coastal ocean due to chaotic stirring at the mouth. The concept of a single residence time per estuary, while convenient from both ecological and engineering viewpoints, is therefore shown to be an oversimplification. Maps of residence times and cumulative histograms are examined as alternatives. The former are attractive only for once-through tracers, while the latter appear useful over a broader range of tracer characteristics.
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