Transient Circulation
Transient circulation describes the instantaneous flow, which in case of Douro is mainly controlled by tide and river discharge. The model was forced by tide and the average discharge of Douro River of 355 m3/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. During ebb, velocity is higher than during flood. The maximum velocity in spring tide exceeds 1m/s at the mouth of estuary, due to existence the Cabedelo sand bank.
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 Douro estuary mouth. This result was obtained by integrating the velocities in Douro Estuary over a period of 15 days. The figure shows two eddies near the estuary mouth adjacent to the ebb jet, one Northward of the jet with anti-cyclonic circulation and other Southward of the jet with cyclonic circulation. These eddies are located at estuary mouth, creating preferential paths for the water entering and leaving the estuary.
Figure 2 - Residual specific flux at mouth in Douro estuary.
Residence Time
To calculate residence time in estuary, the Computation of hydrodynamics forced by tide and mean annual river inflow (355 m3/s). The estuary was divided into 6 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 12 hours after realising only 70% of the initial volume remains inside the estuary and after 2 days only 10% of tracers still remain there. Defining residence time as the time required for 80% of water to leave an estuary, it can be concluded that, residence time in the estuary is around 1 day (orange line).
Figure 3 - Evolution of the ratio between the volume of lagrangian tracers inside the estuary and total estuary volume as a function of time.