Transient Circulation

Transient circulation describes the instantaneous flow. The model was forced by tide and the average discharge of  Mondego and Pranto Rivers, respectively 60 and 8 m³s^-1. Simulations were carried out for both, spring and neap tide, conditions (click here) to see animation of instantaneous velocities). In figure colour represents velocity magnitude and arrows magnitude and direction. Scales are indicated on the left side of figure. The velocity in the northern channel is much stronger than in the southern channel, where longitudinal gradient is clear. This is because the southern channel acts like a bay due to the artificial closure of the upper communication between the two channels. The flow is transversally quite uniform, with velocity parallel to the channel axis and a very clear influence of the river discharge. The maximum velocity reaches 1m/s at the mouth of the estuary.

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 Mondego Estuary. The figure show that the residual circulation is conditioned by the Mondego River discharge. In front of the estuary mouth there are two clear recirculation eddies. In this simulation no discharge from the Pranto river was considered because this discharge is episodic.The velocity in the northern channel is above 10 cm/s. The high value of the velocity is due to the high value of the river discharge when compared with the average volume of the estuary. As a consequence of this ratio also the residence time is very low (or the order of days).

Figure 2 - Residual specific flux at Mondego estuary.

Residence Time

To calculate residence time in estuary, the Computation of hydrodynamics forced by tide and the average discharge of  Mondego and Pranto Rivers, respectively 60 and 8 m³s^-1. The estuary was divided into 5 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 that 2 days after the emission only 30% of the initial volume remains inside the estuary and after 4 days only less then 10% of the tracers still remain there.

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