Transient Circulation
Transient circulation describes the instantaneous flow. The model was forced by tide and the average discharge of Sado River (10m3/s). 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. Inside the estuary the flow is conditioned by the local topography, showing strong curvature, which is responsible for the formation of the sand banks forming the system of two channels in the lower estuary. During ebb, the velocity is clearly higher in both in the southern channel and at the entrance of Marateca, creating conditions for the existence of a very intense residual flow.
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 Sado estuary mouth. This result was obtained by integrating the velocities in Sado Estuary over a period of 15 days. The figure shows an important recirculation inside the estuary, right in front of the town of Setúbal. This recirculation shows that in the northern channel there is more water flowing during flooding than during ebb and that in the southern channel it happens the way around. Integrating the flux all over across section, one would get the river discharge, and the corresponding residual velocity would be very small (of the order of 1 mm/s). This eddy shows residual velocities of the order of 10 cm/s, showing that the tidal mixing is the most efficient mechanism for estuary water renewal. In a more linear system, both flood and ebb velocities would be lower. This eddy is quite important for the residence time (see below), generating much shorter residence times in the lower estuary, through the generation of intense mixing. Similar eddies further inside the estuary also generate high mixing of water from different parts of the estuary. Through that mixing, those eddies also contribute to reduce the residence time of the water inside the estuary.
Figure 2 - Residual specific flux at mouth in Tagus estuary.
Residence Time
To calculate residence time in estuary, the Computation of hydrodynamics forced by tide and mean annual river inflow (10 m3/s). The estuary was divided into 10 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 show that after 15 days more then 60% of the initial volume is still inside the estuary and that after 30 days still 40% of the tracers remain inside the estuary.
Figure 3 - Evolution of the ratio between the volume of lagrangian tracers inside the estuary and total estuary volume as a function of time.