We report the results of an extensive experimental campaign dedicated to the analysis of turbulent dispersion owing to the circulations in tidal environments, characterized by a tidal inlet and a channel with lateral tidal flats. We focus on weakly-convergent and weakly-dissipative estuaries or tidal embankments, where the internal waters communicate with the open sea through an inlet mouth. Tides are reproduced as single or multiple harmonics waves. Particle Image Velocimetry is employed to measure two-dimensional surface velocity field. 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 are completely flushed out during the ebb phase. In all experiments an intense residual current, with shape influenced by the large-scale flood vortices, is observed. The presence of large-scale vortices and of a residual current strongly influences the Lagrangian auto-correlation functions and the corresponding absolute dispersion time evolution. Looping auto-correlations are the signature of both periodic forcing and vortices, ultimately, leading to super diffusive regimes. An asymptotic Brownian regime is always found for the investigated range of parameters allowing for an estimate of the horizontal dispersion coefficients which turn out to decrease with the friction parameter and tend to be enhanced when the semi-diurnal constituents prevail. Finally, multiple particle statistics show multiple regimes depending on particle separations, compared to a typical injection length scale that seems to coincide with the inlet mouth dimension.

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.

In the present study, we extend the analysis of the dispersion processes induced by tidal flow in weakly-dissipative estuaries discussed in the companion paper. Here we focus the attention on the flow induced by more realistic tidal waves provided by different combinations of semi-diurnal and diurnal constituents. We employ a large-scale physical model of a system composed by a large basin (open sea) and a compound tidal channel, where tides are produced as volume waves with prescribed shapes. Two-dimensional superficial velocity fields are used to study the main Eulerian and Lagrangian properties of the flow, in terms of absolute and relative particle statistics. The results suggest that the mixed character of the tides strongly influences the shape of the macro-vortices generated at the tidal inlet, whereas the overall residual currents seem to be less sensitive. Moreover, for the present tidal setting, longitudinal dispersion, the dominant dispersion process, is enhanced when the semi-diurnal constituents prevail. Finally, multiple particle statistics show regimes typical of non-local dynamics for particle separation larger than a typical injection length scale, which is the size of the tidal inlet. Non-local dynamics imply that the dispersion is dominated by flow structures larger than the mean separation length, i.e. the tidal wavelength and the size of the macro-vortices. The present results together with those discussed in Part 1, offer a thorough insight in the main dispersion processes induced by tidal flows, which are extremely relevant in the case of estuarine dynamics.