Impact of Freshwater Inflow From the Volturno River on Coastal Circulation

Roberto Sorgente,Alberto Ribotti,Federica Pessini,Antonio Cascella,Fabrizio Lirer,Fabrizio Lirer,Fabrizio Lirer,Angelo Perilli,Sergio Bonomo,Luciana Ferraro,Luciana Ferraro,Luciana Ferraro,Ines Alberico,Ines Alberico,Ines Alberico,Antonia Di Maio

doi:10.3389/fmars.2020.00293

Abstract

The coastal area located in front of the Volturno river’s estuary (Gulf of Gaeta, central-eastern Tyrrhenian Sea) has been synoptically sampled during seven surveys, between June 2012 and October 2014. The vertical profiles of temperature and salinity have been acquired on a high resolution nearly-regular grid, in order to describe the spatial and temporal variability of the water masses characteristics. Moreover, to provide a first assessment of the steady circulation at small scale, the three-dimensional velocity field associated to each survey has been obtained through the full momentum equations of the Princeton Ocean Model. The data analysis has shown the entire water column characterized by an evident thermal cycle and a vertical thermohaline structure dominated by three types of water masses: the freshwater of river’s origin, the saltier coastal Tyrrhenian waters and transitional waters, resulting from the mixing of them. The freshwater inflow strongly affected the density distribution, causing a strong temporal variability in the upper layer. Its impact was more evident in winter inducing also a temperature inversion along the water column. In case of rainy events, its effect was clear also in condition of high vertical temperature stratification, forming a surface-trapper layer characterized by high density gradients. Together with wind forcing, they contributed to the formation of small scale shallow features, such as longshore currents, cyclonic and anticyclonic eddies. The latter influenced the vertical stratification and modify the coastal circulation preserving the transitional waters from the surrounding saltier ones.

Highlights

The hydrodynamics of a river frontal zone is a complex issue due to the interaction between the freshwater of the river outflow and the coastal waters (MacDonald et al, 2013; HornerDevine et al, 2015)
We provide a first assessment of the steady circulation using the Princeton Ocean Model (POM) in a purely diagnostic mode (Ezer and Mellor, 1994; Zavatarelli et al, 2002)
We describe the characteristics of the waters involved in the area and analyze their variability in terms of TS diagram, mean vertical profile, standard deviation (σ ), and squared Brunt-Väisälä frequency (BVF)

Summary

Introduction

The hydrodynamics of a river frontal zone is a complex issue due to the interaction between the freshwater of the river outflow and the coastal waters (MacDonald et al, 2013; HornerDevine et al, 2015). It influences the sediment balance, the distribution of chemical and physical parameters, and the nutrient input, making the area a very productive habitat (McLusky and Elliott, 2004). The Volturno river is the sixth-longest river in Italy and the longest in the South, with a length of 175 km and an estimated mean discharge of 40 m3s−1 (Iermano et al, 2012) It provides the greatest contribution of freshwater to the area. In the northern part of the Gulf of Gaeta, the seabed morphology shows a double order of bars: the first deeper than 5 m and parallel to the coastline and the second shallower and transverse to the shoreline (Cocco and Pippo, 1988)

Methods

Results

Conclusion