Circulation and transport processes in a tidally forced salt-wedge estuary: The Sao Francisco river estuary, Northeast Brazil

The Sao Francisco river flow is regulated by a series of dams that work in cascade for hydro-power generation, maintaining the flow 1000 m3 s-1 all year long. This caused the dampening of the flood peaks and perpetuated the salt intrusion, changing the function of the system as a material source to the coastal ocean. Here we present an assessment of the estuary salt intrusion, hydrodynamics, mixing processes, and the transport of scalars based on field observations. The mains goal is to evaluate its present role as a sink or source of materials to the adjacent sea. Three tidal surveys were carried out to gather data on the water level, cross-section discharge, current velocity and direction, salinity, temperature, concentration of suspended particulate matter (SPM), dissolved oxygen, and chlorophyll. Data were recorded from moored instrumentations in different locations, from boat-based sampling in a cross-section, and also in along-estuary surveys. The estuary shows a highly stratified salinity distribution, with the salt intrusion excursion of 7 km during the spring tide, reaching similar to 11 km at high tide. The salt intrusion is laterally asymmetric, influenced by the system morphology. There is a strong tidal attenuation along the estuary, even though the flow regime is sub-critical (Richardson Number 0.25). The estuary presented lateral variability of currents modulated by the tide, being horizontally sheared during spring tide and vertically sheared during neap. The transport of scalars was modulated by the tide, and the estuary exported SPM, dissolved oxygen, and chlorophyll both during neap (44.4, 12.9 and 3.6x10(-3) kg s(-1), respectively) and spring tide (31.8, 7.4 and 2x10(-3) kg s(-1)), despite the severe flow regulation, meaning that the system still functions as a source of materials to the coastal sea. (c) 2020 Elsevier B.V. All rights reserved.