Future projections of a salt-wedge estuary under a changing climate: impact of the sea level rise and evaluationof a nature-based-solution

Abstract

Estuarine zones are particularly vulnerable coastal areas as consequence of the changing climate. The river flow decrease, RD, and the sea level rise, SLR, are leading to: (i) salinization of the surface and subsurface catchment waters, (ii) salt-wedge intrusion SWI moving more and more inland, with a non-linear response to the main drivers of the estuarine dynamics.  The current study uses a one-dimensional two-layer estuary box model, the so-called CMCC EBM (Verri et al 2020; 2021) which solves the estuarine water exchange by means of two conservation equations for volume and salt fluxes averaged over the diurnal tidal cycle, plus two parametric equations estimating the SWI length and the along-estuary diffusivity.  The EBM has been applied to the Po di Goro branch of the Po river delta, which is characterised by a river-dominated estuary flowing into the micro-tidal Northern Adriatic Sea. A strength of the EBM here proposed is the extremely low computational time which makes it particularly suitable for climate purposes by bridging the gap between available hydrology and marine hydrodynamics projections which reach at most the mesoscale with high computational costs and without representing the estuarine transitional areas. Additional assets are the minimal data storage and no need to postprocess the results as the SWI length is among the model outcomes. On the other hand, a proper tuning of the parametric equations is required and this was made possible by an accurate in-situ monitoring and a site specific “learning dataset” built upon the outcomes of a 3D unstructured modelling of the Po delta system.   Considering that there are few studies devoted to the impacts of the local SLR on the SWI and the salinity of estuaries in micro-tidal environments, one of the aims of this study is to expand the knowledge on this topic by proving future projections for the selected test-case.     Moreover, the increasing salinization of the Po di Goro estuary threats the local economy and the ecosystem health. Thus, the second aim of this study is to evaluate a Nature-Based-Solution, NBS, to mitigate the SWI, i.e. we assess the salt uptake capability of the Atriplex portulacoides within our modelling study. Three climate experiments with the EBM have been carried out over 1991-2100 with a ‘mechanistic’ approach: (i) Exp1 is a full-forcing experiment with the river inflow (volume flux at the estuary head) and the seawater inflow (volume flux, salinity and sea level at the estuary mouth) provided by a regional climate model RCM considering RCP 8.5; (ii) Exp2 is a twin experiment of Exp1 but neglecting the sea level among input forcings of the EBM; (iii) Exp3 is a twin experiment of Exp1 but with a reduced salinity of the seawater inflow by assuming that 20% of the estuary water volume interacts with the halophytes planted along the estuary banks. We propose a discussion on the relative role of the SLR and the RD in determining the future projections of the Po di Goro estuarine dynamics and the potential effect of a site-specific NBS.