Nonlinear Baroclinic Dynamics and Salt-Wedge Response to Transient River Discharge in the Microtidal Neretva River Estuary

Highly stratified microtidal estuaries are governed by nonlinear baroclinic processes and delayed responses of estuarine circulation and salt-wedge propagation to transient river discharge. This study applies a two-layer numerical model to the lower 21.2 km of the Neretva River Estuary (Croatia), using 3456 hourly time steps and 93 cross-sections. Model calibration and validation draw on 2004 field observations comprising 30 CTD profiles collected over eight survey days and seven ADCP transects. The simulated salt-wedge toe exhibits a peak cross-correlation lag of 38 h relative to the flash-flood response; this lag is specific to the simulated event and 2004 boundary conditions. Bulk Richardson-number diagnostics identify the most persistent low-Rib conditions (Rib < 0.25) within a 4.2 km transition zone during modelled discharges of 180–280 m3/s, although intermittent low-Rib conditions also occur elsewhere. Baroclinic pressure gradients support landward intrusion at low flow (QA < 80 m3/s), while higher upper-layer momentum and interfacial shear promote downstream wedge displacement above 280 m3/s. Complete flushing of the modelled 21.2 km reach requires approximately 487 m3/s under the simulated 2004 geometry and forcing. The results provide a process-based interpretation of salt-wedge variability while identifying the limitations that must be addressed before operational application.