Preliminary Analysis of Sea Intrusion in the Isonzo river delta Inlet (North-East Italy)

Hydrodynamic processes in river mouths represent a complex interplay of fluvial discharge, tidal influence, and saline intrusion. Standard marine flow and discharge measurement techniques face significant limitations in estuarine zones, particularly near river mouths, due to complex bidirectional flow patterns and the influence of tidal and saline forces. The peculiar characteristic of the microtidal regime in the northern Adriatic Sea makes the understanding of the hydrodynamic processes at the river mouth extremely complex.  Moreover, the Isonzo basin climate condition varies from alpine to sub-Mediterranean. Thus, can introduce large variations in the flow rate of the river (related to precipitation), amplifying an already pronounced intra- and inter-annual variability due to the torrential regime and prolongated drought season.

In this work, we focus on the monitoring system of the Isonzo Current meter, an ADCP (Acoustic Doppler Current Profiler) station fixed at a depth of 13 meters from the river surface, located 7 km far from the Delta Inlet in the Northern Adriatic Sea. The station, operative since 2005, acquires and transmits data every 10 minutes. Continuous measurements of river flow velocity, flow direction, temperature and calculation of the flow rate, have enabled the identification of two distinct river flow regimes: 1) normal condition of low freshwater discharge related to high rising tide and/or drought period, and 2) exceptional high discharge related to freshwater flood events.

In normal and drought conditions we observe that the river water column present an average direction upstream related to: a) marine water intruding the river, b) variation of the river high coinciding to the diurnal tidal modulation, c) discharge fluctional related to the maximum and minim tide, d) stratified water column in the central portion with thin mixing layer at the bottom and surface of the river section, and e) a high temperature (related to the marine water temperature) at the riverbed.

In opposition, during flood events, the flow direction becomes homogeneous in the entire water column, discharging downstream and pushing the tidal force outside the river. The onset of a flood event records a sharp thermal drop, indicating the replacement of the marine water by freshwater throughout the channel and the flow rates can exceed 2500 m³/s.

The presence of a semi-stationary salt wedge intrusion in the river was identified and wide. However, it is not clear how the tide and the river runoff interact in the water mass exchanges, or what are the water mixed or stratified condition in different hydrodynamic regime. 

These results are essential for understanding estuarine dynamics, particularly in a climate context marked by increasingly frequent extreme events.