Channel-ocean exchange driven by tides and river flow: Espinheiro Channel (Portugal)

Abstract

A 3D baroclinic model (Mohid) was used to perform hindcast simulations in a tidal channel – Espinheiro Channel (located within Ria de Aveiro, north coast of Portugal) – in order to study its hydrography. The simulations were conducted for four distinct periods where markedly different river discharge and tides occurred and the outputs are compared with synoptic thermohaline data. The model qualitatively reproduces alongchannel thermohaline distributions at three different vertical levels during low-to-medium river inflow, underestimating the salinity stratification under high river inflow (higher than 100 m3 s -1). Once the numerical model is fully implemented, the hydrography of the channel was analysed in terms of two main forcing factors: river discharge and tide. Tidal currents and thermohaline data were analysed at the channel’s inlet and at a mid channel station during two tidal cycles. The stratification and water column stability were found to vary in these stations as a function of the estuarine Richardson number and the buoyancy frequency. Well mixed conditions were found at the lower and mid channel area when the river flow is weak (independently of the tidal regime), turning to highly stratified when the river flow is very high (during neap tide). The residual currents were also analysed revealing an ebb-dominated channel, with a more intense seaward current near the surface due to the freshwater inflow.