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- Fortnightly switching of residual flow drivers produced by seawater intrusion in a long, non-rotating estuaryPublication . Garel, Erwan; Khosravi, Maziar; Valle-Levinson, Arnoldo; Díez-Minguito, ManuelTowed and moored ADCP and salinity measurements were collected in Summer 2023 at two cross-channel transects of the 80-km-long Guadiana Estuary, at 4 km and 20 km from the mouth, during low (< 10 m(3)/s) river discharge conditions. The dataset indicates that the lateral structure of axial residual flows changed from vertically sheared to laterally sheared with tidal forcing. These structures resemble theoretical expectations for a density-driven or eddy viscosity-shear covariance (ESCO) circulation at neap tide and for a tidally driven circulation produced by longitudinal advection (tidal stress) at spring tide. The tidal variability of the residual flow magnitude and of the contributions of unidirectional or two-layer vertical profiles indicate that the primary driver of the residual circulation changes between neap tide and spring tide. Seawater (i.e. with salinity > 35 kg/g) intrusion was several kilometres, equivalent to similar to 1/8 of the estuary's length. As a result, the horizontal density gradient was weakest near the mouth where the residual circulation was barotropic, produced by tidal stress at spring tide and possibly by an ESCO mechanism at neap tide. At 20 km upstream, the dominant driver switched from tidal stress at spring tide to a baroclinic (and potentially ESCO) circulation at neap tide. At this location, scaling analyses of the tidally averaged momentum equation using the densimetric tidal Froude number supported the idea of dominance of tidal stress at spring tide.
- Fortnightly variability of the lateral structure of residual flows along non-rotating estuariesPublication . Khosravi, Maziar; Fortunato, A. B.; Lopez-Ruiz, A.; Valle-Levinson, A.; Garel, ErwanResidual circulation in estuaries plays a crucial role in controlling salt intrusion, material transport, and ecosystem functioning, making its understanding essential for coastal management worldwide. Despite its importance, the mechanisms driving the transverse structure of residual flows remain insufficiently explored, particularly in relation to the fortnightly tidal variability. This study shows how the main driver of residual flow may switch from baroclinic dominance at neap tide to barotropic dominance at spring tide. Three-dimensional, process-oriented numerical simulations are carried out in an idealized, non-rotating, estuary–shelf system, represented by an 80 km-long semi-closed channel with a 1 km-wide Gaussian-shaped cross-section. This setting is inspired by the Guadiana Estuary (Portugal/Spain), where previous observations have reported a switch of the residual circulation between spring tide and neap tide. To mimic such fortnightly pattern, the model is forced by freshwater inflow at the head and M2 and S2 tidal harmonics at the ocean boundary. Harmonic amplitudes were selected based on a sensitivity analysis. The along channel and temporal variability of the lateral structure of axial residual flows is represented using a non-dimensional parameter derived from the residual inflow that may develop in the deep channel. The simulation results show residual flow transverse structures consistent with theoretical expectations for a baroclinic driver in neap tides and a barotropic driver in spring tides, as confirmed by analyses of the residual momentum equation terms. In these cases, the near-bottom residual flow in the channel is relatively strong and its direction indicates the dominant driver. The study also reveals a previously undocumented transverse structure described by inflows near the channel bed and over the shoals. This structure develops in relation to the Stokes drift when baroclinic and barotropic forcings are relatively balanced across a section.