Transient Circulation
Transient circulation describes the instantaneous flow, which in case of Minho is mainly controlled by tide and river discharge. The model was forced by tide and the average discharge of 267 and 12 m3/s for Minho and Coura River, respectively. 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. The figure show a zoom of the estuary. During ebb, the velocity is clearly higher than during flood for spring and neap tides, although in flood, velocities can reach 0.75 m/s in the same track than during ebb. The maximum velocity in spring tides reaches 1 m/s neared estuary mouth. At the estuary mouth Insua Island splits the flow into two channels.
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 Minho estuary mouth. This result was obtained by integrating the velocities in Minho Estuary over a period of 15 days. The figure shows a continuous residual flux off the estuary, which is induced by the river discharges. Inside the estuary, the main transport patterns are close to the borders. North Insua Island a strong anti-cyclonic ebb jet can be observed.
Figure 2 - Residual specific flux at mouth in Minho 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 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 shows that 12 hours after tracer 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 the estuary, one concludes that, residence time in the estuary is around of 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.