Salt-Wedge Response to Variable River Flow and Sea-Level Rise in the Microtidal Rjecina River Estuary, Croatia

A finite-volume model for two-layer shallow-water flow is presented and applied to study the dynamic response of a salt wedge in a microtidal estuary to changes in river flow rate and sea-level rise (SLR). First, the shape of the arrested salt wedge was computed for different hydrographic conditions. Next, the response of the salt wedge to highly variable river flow was investigated. Finally, this model was applied to predict the impacts of the SLR on salinity intrusions in the Rjecina River Estuary. To assess the model performance and to examine the salinity structure in the estuary, a field-sampling campaign was conducted during 2014 and 2015. Field observations revealed negligible longitudinal density variations in both freshwater and saltwater layers and highly stratified conditions for all considered river flow rates and sea levels. Furthermore, the maximum buoyancy frequency, computed as a measure of vertical stratification, was among the highest ever reported in field investigations. The behavior of the salt-wedge intrusion depends mainly on the river flow rate, although sea level and channel geometry become more influential under low-flow conditions. Computed salt-wedge shapes and propagation rates agreed well with field observations. The salinity structure in the Rjecina River Estuary is not expected to change in the future for tested SLR scenarios, although stronger salt-wedge intrusion is predicted by the numerical model.