Transient Circulation
The model was forced with tide and most frequent value of Mira River discharge (0.5 m3/s). Simulation includes both spring and neap tidal, and results are represented for flood and ebb conditions. 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 figure show that during ebb, the velocity is clearly higher than during flood and that the maximum velocity in spring tide exceeds 1 m/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 Mira estuary mouth. This result was obtained by integrating the velocities in Mira Estuary over a period of 15 days. This figure shows one large eddy near the estuary mouth adjacent to the ebb jet, with residual velocities of, 10 cm/s. This eddy shows that water leaving the estuary is projected far off the estuary mouth and has a tendency for recirculating northwards. The leg of this eddy close to the mouth moves southward, creating conditions for renewal of estuarine water along that side of the estuary. This dynamics minimizes the amount of ebb water re-entering the estuary, contributing for shortening residence time inside the estuary.
Figure 2 - Residual specific flux at mouth in Mira estuary.
Residence Time
To calculate residence time in estuary, the Computation of hydrodynamics forced by tide and mean annual river inflow (42 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 1 week after tracers releasing only about 40% of the initial volume remains inside the estuary and that after 2 weeks only 20% remain there. Defining the residence time as the time required for 80% of water to leave estuary, one concludes that, considering the most frequent river discharge and in the absence of a littoral current, residence time in the estuary is of the order of 2 weeks.
Figure 3 - Evolution of the ratio between the volume of lagrangian tracers inside the estuary and total estuary volume as a function of time.