Airborne observations of frontal systems in the inlet channel of the Ria de Aveiro, Portugal

Abstract

Flow from the Atlantic Ocean into the inlet channel of the Ria de Aveiro, Portugal, delimited by two west-converging breakwaters, moves a tidal prism of up to 110Mm³. The associated velocity field controls the formation and movement of foam lines and turbid water masses. Observation by airborne 35mm photography and direct measurements of surface water temperature, salinity and suspended solids concentration give a view of the circulation in the littoral zone, which is important as this area is soon to receive effluent from an outfall located c.3 km to the north. On the flood tide turbid water is diverted from the north-south direction of the littoral drift into the channel. On the ebb this turbid water is pushed offshore by a jet flowing west and south-west from the channel. The surface water temperature inside the channel mouth is lower on the north side where the salinity is less reduced by dilution with lagoon water. During the flood tide the water flowing from the north rotates to the east, forced by flow from the south-west along the axis of the channel. This motion holds the less dense water on the south side and causes intense vertical mixing on the N side. The frontal convergence of these two water masses is usually marked at the surface by a clear foam line.