Effect of a broad shallow sill on tidal circulation and salt transport in the entrance to a coastal plain estuary (Mira-Vila Nova de Milfontes, Portugal)

We describe the tidal circulation and salinity regime of a coastal plain es tuary that connects to the ocean through a flood tide delta. The delta acts as a sill, and we examine the mechanisms through which the sill affects ex change of estuarine water with the ocean. Given enough buoyancy, the dynami cs of tidal intrusion fronts across the sill and selective withdrawal (aspi ration) in the deeper channel landward appear to control the exchange of se awater with estuarine water. Comparison of currents on the sill and stratif ication in the channel reveals aspiration depths smaller than channel depth during neap tide. During neap tide and strong vertical stratification, sea water plunges beneath the less dense estuarine water somewhere on the sill. Turbulence in the intruding bottom layer on the sill promotes entrainment of fluid from the surface layer, and the seawater along the sill bottom is diluted with estuarine water. During ebb flow, salt is effectively trapped landward of the sill in a stagnant zone between the aspiration depth and th e bottom where it can be advected farther upstream by flood currents. Durin g spring tide, the plunge point moves landward and off the sill, stratifica tion is weakened in the deep channel, and aspiration during ebb extends to the bottom. This prevents the formation of stagnant water near the bottom, and the estuary is flooded with high salinity water far inland. The neap-sp ring cycle of tidal intrusion fronts on flood coupled with aspiration durin g ebb interacts with the sill to play an important role in the transport an d retention of salt within the estuary.