Spatial patterns in coastal lagoons related to the hydrodynamics of seawater intrusion

Abstract Marine intrusion was simulated in a choked and in a restricted coastal lagoon by using a 3D-hydrodynamic model. To study the spatiotemporal progression of seawater intrusion and its mixing efficiency with lagoon waters we define Marine Mixed Volume (V MM ) as a new hydrodynamic indicator. Spatial patterns in both lagoons were described by studying the time series and maps of V MM taking into account the meteorological conditions encountered during a water year. The patterns comprised well-mixed zones (WMZ) and physical barrier zones (PBZ) that act as hydrodynamic boundaries. The choked Bages-Sigean lagoon comprises four sub-basins: a PBZ at the inlet, and two WMZ's separated by another PBZ corresponding to a constriction zone. The volumes of the PBZ were 2.1 and 5.4 millions m 3 with characteristic mixing timescale of 68 and 84 days, respectively. The WMZ were 12.3 and 43.3 millions m 3 with characteristics mixing timescale of 70 and 39 days, respectively.