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Dispersion processes in weakly dissipative estuaries: Part 1. Single harmonic tide.
  • Annalisa De Leo,
  • Nicoletta Tambroni,
  • Alessandro Stocchino
Annalisa De Leo
University of Genoa

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Nicoletta Tambroni
University of Genoa
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Alessandro Stocchino
The Hong Kong Polytechnic University
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We report the results of an extensive experimental campaign dedicated to the analysis of turbulent dispersion owing to the circulations in tide dominated estuaries, characterized by a compound cross section (a main channel and lateral tidal flats). Following the classification suggested by Toffolon et al. (2006), we concentrate our attention on weakly-convergent and weakly-dissipative estuaries, where the internal waters communicate with the open sea through an inlet mouth. Particle Image Velocimetry is employed to measure two-dimensional surface velocity. Large scale macro-vortices, generated by vortex shedding during the flood phase from the inlet barrier, tend to occupy the entire tidal flats width and, irrespective of the controlling parameters, they are completely flushed out during the ebb phase. Flow decomposition based on averaging over the tidal period enlightens the presence of an intense residual current, with shape influenced by the large-scale flood vortices. The measured Eulerian surface velocity fields form the basis for a thorough Lagrangian analysis, which yields a clear picture of the dispersion regimes. The presence of large-scale vortices and of an intense residual current strongly influences the Lagrangian auto-correlation functions and the corresponding absolute dispersion time evolution. Looping auto-correlations are the signature of both the periodic forcing and vortices, ultimately, leading to super diffusive regimes. Moreover, an asymptotic Brownian regime is always found for the investigated range of parameters allowing for an estimate of the horizontal dispersion coefficients. For the latter, we suggest a simplified algebraic formulation that well fits the experimental estimates.