Fortnightly switching of residual flow drivers in estuaries

Residual flows in estuaries correspond to currents averaged over one or more tidal cycles. They determine the salinity intrusion length and the net circulation of dissolved and suspended material. Their knowledge is therefore of uttermost importance for water quality management and the study of ecological processes.

Based on the well-described large estuaries of temperate latitudes, it is customarily assumed that the residual circulation is dominantly driven by horizontal density gradients. In this project, we will determine the circumstances in which the residual circulation switches from being baroclinic (density-driven) to being barotropic (tidally-driven). Switching of forcing can produce reversal in residual circulation, thus affecting long-term transport processes. The study will test the assumption that such switching can be determined with a non-dimensional number that scales those two forcings: the internal tidal Froude number, ‘Fr0’.

Studies in temperate estuaries - where residual flows are dominated by baroclinic forcing - have shown that maximum net flows occur during neap tides, when tidal mixing is weakest. Investigations in (the less known) subtropical estuaries dominated by tidal forcing have shown that maximum residual flows occur in spring tides. Hence, depending on the system, either density gradients or tidal stresses are the drivers of the residual flow. The magnitude of residual flows may vary between spring and neap tides, but their direction (i.e. seaward/outflow or landward/inflow) and distribution across the channel remain constant.

Recently, the paradigm of a temporally constant residual flow forcing has been challenged by observations. These studies have shown that at some subtropical, semi-arid and temperate estuaries, both density gradients and tidal stresses can alternate at a fortnightly time scale to drive residual flows: tidal forcing drives residual flows in spring tides, while density gradient forces drive residual flows in neap tides. Such fortnightly switching of drivers depends of the tidal range at the mouth and affects drastically the patterns (i.e. magnitude, direction, spatial distribution) of the residuals. To date, these studies have been mainly suggestive due to their limited temporal extent and along-estuary resolution. Despite these sampling limitations, some authors have proposed that the competition between tidal stress and baroclinic forcing can be characterized locally through a non-dimensional internal tidal Froude number ‘Fr0’. The alternation of these forcings and the applicability of Fr0 remain to be addressed at different positions (or sections) along an estuary.

The overall objective of the SWITCH project is to characterize the fortnightly switching of residual flow drivers in a type of estuaries that has been largely unexplored. Two specific objectives will be addressed through the following questions:

1) What is the along-estuary variability of the fortnightly switching in residual flow drivers?

2) Does Fr0 provide a robust metric to diagnose the fortnightly switching at different locations along an estuary?

In order to address the objectives, we propose to combine measurements with numerical simulations at an estuary with semi-arid climate, the Guadiana. Measurements at this estuary are the most supportive, to date, of a fortnightly switching between dominant barotropic and baroclinic forcings.

Comprehensive field experiments will include tidal-cycle measurements and deployments of moored instrumentation, with appropriate spatial resolution to resolve along- and cross-estuary variations. The results, complemented with those derived from other estuaries, will be placed in a parametric space (Fr0 vs Ekman number) in order to represent the spatial and temporal variability of estuaries’ behaviour based on dynamic parameters. This approach will allow us to propose a new classification able to represent the dynamics of estuaries, to supplement previous classifications based on ad-hoc parameters (such as river discharge and tidal currents).

The scientific objectives will be addressed by an international team of experts in estuarine hydrodynamics. As residual flows control salt intrusion in estuaries, the project studies a phenomenon of growing concern for many coastal populations, such as at the case study region due to the increasing occurrence of severe droughts. Thus, it also proposed to address management questions related to salinity intrusion following participatory approaches designed to optimize stakeholders’ involvement. The inclusion of a partner specialized in science communication will also ensure efficient dissemination to the general public.

Project Main Goals:

To characterize the fortnightly switching of residual flow drivers in poorly described estuaries.

Arnoldo Valle-Levinson (University of Florida) | Maitane Olabarrieta (University of Florida) | Alejandro López-Ruiz (University of Seville) | Ana Ramos (Centro de Ciência Viva de Tavira) | André Fortunato (National Laboratory of Civil Engineering - LNEC, Lisbon) | Manual Diez-Minguito (University of Granada)