Coastal mixing in multiple-mouth deltas: A case study in the Po delta, Italy

Abstract

Satellite imagery provides evidence of complex mixing dynamics in the coastal zone in front of multiple mouth deltas. One peculiar feature, identified in front of the Po Delta (Italy), consists in warmer water bulges present in some periods in the coastal zone between the river mouths. Such features are evident during both high and low river discharge. Through an integrated approach based on the analysis of satellite imagery, in situ field data and a high-resolution oceanographic model, representing the whole river-delta-sea system, we investigated the relative contribution of the different forcing in controlling coastal mixing of riverine waters. The results evidence that the occurrence of these warmer saltier water bulges is due to upwelling induced by the combined action of tides and wind regimes aligned along coastline. Winds from the land and along the coast drive the upwelling through the well-known mechanism described by Ekman. The presence of river discharge enhance the water column stratification, creating the conditions in which tidal action follows the tidal straining theory. Both processes are identified in modeling results. The occurrence of these localized coastal waters with peculiar thermohaline characteristics, detectable on satellite imagery of the area, can be relevant in the definition of the freshwater areas of influence and the mechanisms of riverine water mixing in the near coastal zone. This can shed some light, eventually, on characterizing the sediment dynamics, as well as the thermohaline properties of waters in the area, and also to identify eventual impacts on the local ecosystems and fishery.

Saltwater Intrusion in a Mediterranean Delta Under a Changing Climate

“…The model reproduces the tidal dynamics in the area, the correct distribution of river discharge in the branches, the water flows at the lagoons’ inlets and water temperature and salinity structures within the lagoons and in the prodelta (Bellafiore et al. 2019; Maicu et al., 2018). However, because of the different model set‐up and forcing conditions, extensive validation has been performed of the numerical model in this study.…”

“…A similar behavior is also seen for Canarin and Scardovari lagoons. Moreover, tidal straining arising from the interaction between a vertically sheared tidal flow and a horizontal salinity gradient (as in the expSEA run), can affect the salinity patterns within the lagoons, with corresponding changes in the circulation (Bellafiore et al., 2019). On the other hand, in the lagoon of Marinetta‐Vallona SLR induces a salinity increase corresponding to the 18% of the change because of river discharge decrease and varied surface fluxes.…”

“…(2018) and Bellafiore et al. (2019) for more details about the modeling application to the Po Delta.…”

“…The coastal area in front of the delta is characterized by mixing dynamics related to the interaction of the coastal currents and the river plumes (Bellafiore et al. 2019; Braga et al., 2017; Brando et al., 2015).…”

Downscaling With an Unstructured Coastal-Ocean Model to the Goro Lagoon and the Po River Delta Branches

“…In the Adriatic Sea, the two dominant wind regimes, the Bora (NE), and Sirocco (SE) force the general circulation (Orlić et al, 1994;Ursella et al, 2006;Jeffries and Lee, 2007). The morphology of the Po Delta coastline significantly modifies the coastal current in front of the Goro Lagoon (Falcieri et al, 2013;Maicu et al, 2018;Bellafiore et al, 2019). The Bora strengthens the SW coastal current and piles up the water on the coastline south of the delta (Figure 6A).…”