Eulerian and Lagrangian Transport Time Scales of a tidal active coastal basin.

In this work the flushing features of a tidal active coastal basin, the Venice lagoon, have been investigated. The water transport time scale (TTS) has been computed by means of both an eulerian and a lagrangian approach. The obtained results have been compared in order to identify the main differences between the two methods. The eulerian water transport time (WRT) scale has been computed through the definition of the remnant function of a passive tracer released inside the lagoon whereas the lagrangian water transport time (WTT) scale has been computed tracking the trajectories of simulated particles released inside the basin. Both the methodologies rely on computer modeling. A 2D hydrodynamic model based on the finite element method has been used. The model solves the shallow water equations on a spatial domain that represents the whole Adriatic Sea and the Venice lagoon. Numerical computations show that the two techniques, when applied to a tidal active coastal basin, characterized by a complex morphology and dynamic, are differently influenced by the tidal variability. In particular, the type and the phase of the tidal forcing at the beginning of the computation strongly influence the WTTs distribution within the basin. On the other hand, the WRTs computation is not affected by the tidal forcing variability.