Hydrodynamics of river plume intrusion into an adjacent estuary: The Minho River and Ria de Vigo

Minho River intrusion into the Rias Baixas has a twofold impact on estuaries. On the one hand, freshwater intrusion can modify the water exchange with the shelf affecting the residence time. On the other hand, it can fertilize the area and promote phytoplankton blooms and increase estuarine primary production. For these reasons, it is crucial to understand the frequency, duration, and relaxation of river freshwater intrusions. The frequency of intrusion events into the rias was calculated by means of in situ data from 2006 to 2016 using the difference in salinity between the inner part of the estuary and the southern mouth (ΔS) as a proxy. Minho River intrusions, which are characterized by ΔS > 0, were detected in 8.9% of the available measurements in the Ria de Vigo, in 8.4% in the Ria de Pontevedra, and in only 4.5% in the Ria de Arousa. The FLOW module of the Delft3D model, which was previously calibrated and validated for the study area, was used to analyze the hydrodynamics of the intrusion events. The Ria de Vigo, which is the closest one to the Minho Estuary, was chosen to analyze the duration and intensity of the freshwater intrusions. Several consecutive events were detected in early 2010 (January 11th to February 8th) with ΔS values ranging from 0.7 to 2.0. Velocity and density profiles were analyzed in the middle part of the estuary, where seven freshwater intrusions matched the wind peaks favorable to intrusion events with an approximate delay of ~12 h. In general, the duration of the events was on the order of 1.5 days, although in one particular case the event lasted >3 days. The dynamic behavior of the freshwater intrusions was represented using velocity and density profiles along the main axis of Ria de Vigo. Finally, the phases of an intrusion event were characterized: initial positive estuarine circulation pattern; development of the intrusion; well-developed negative estuarine circulation; relaxation of the intrusion and recovery of the initial positive circulation.