Transient Circulation

Transient circulation describes the instantaneous flow. The hydrodynamic simulations were forced imposing tide elevation in the open boundary of the system. A tidal gage located offshore from Culatra Island was used with their phases corrected to the open boundary. Due to the low runoff and irregularity of the flow no fresh water inputs were considered. Simulations were carried out for both, spring and neap tide, conditions (click here) to see animation of instaneous velocities).

In figure colour represents velocity magnitude and arrows magnitude and direction. Scales are indicated on the left side of figure. There is a strong difference between spring and neap tide conditions. The velocity magnitude in spring tide is larger and the intertidal areas get almost completed covered by water.

For spring conditions, during flood strong currents can be found in the main channels, while the flow over the intertidal areas is weak. Strong jets with velocities of more than 1 m/s are observed in the inlets. During flood the main path for the water entering the Ria is Barra Grande while during ebb the outflow is done predominantly by "Barra Nova".

In neap tides it is possible to see an asymmetry; wich is a typical characteristic of Ria Formosa, being better observed in the residual circulation fields.  In the past this circulation was stronger. In the last decades its importance has decreased due to the continuous silt up of Barra Grande. The Main inflow and outflow path to the city of Olhão is the Olhão Channel since the Marim Channel is very narrow. The main inflow path to the city of Faro is from Barra Nova along the Faro Channel. The outflow is divided between this path and Barra de S. Luis. This last bar was recently reopened improving the circulation in the West part of Ria Formosa.

Residual Circulation

Residual circulation represents the local average of transient circulation, giving information on preferential transport in the estuary.  Figure 2 shows residual flux derived from residual velocity at Ria Formosa. This result was obtained by integrating the velocities in Ria Formosa over a period of 15 days. The residual flow has a predominant flood direction in Barra Grande and a transport from Barra Grande to Barra Nova inside Ria Formosa. A strong anticyclonic eddy is formed west from Olhão Channel. A small cyclonic eddy is also formed over Barra Grande, showing the preferential entrance path in the east part of the bar and preferential exit path in the west part of the bar. It is also possible to see that Barra Nova has an ebb characteristic and that Faro Channel is the principal path for the water arriving Faro City. The figure also shows that Barra de S. Luis is the principal exit way for the water arriving Faro from Faro Channel. The recent dredging and reopening of this bar has significantly improved the circulation of this area.

Figure 2 - Residual specific flux in Ria Formosa.

Residence Time

 The estuary was divided into 7 boxes, which are filled with lagrangean tracers. The total volume of the tracers in the estuary, at the beginning of the simulation, is equal to the total volume of the estuary. It is important to keep in mind that the total volume of the estuary varies with time, due to daily tidal oscillations and to the spring-neap tidal cycle (click here) to see animation of lagrangean tracers).

Figure 3 shows the estuary’s water fraction as a function of time is shown. It is computed in each instant as the volume of water present inside the estuary at the beginning of the simulation that still remains inside of it divided by the total volume of water in the estuary in that instant. As explained in other estuaries the residence time is computed when a residual fraction of 20% is reached. For Ria Formosa as a whole the value obtained is about 4 days.

Figure 3 - Evolution of the ratio between the volume of lagrangian tracers inside the estuary and total estuary volume as a function of time.