**4. Conclusion**

22 Hydrodynamics – Natural Water Bodies

The effect of tidal regime on saline wedge propagation into the Mondego estuary can be assessed by comparing the saline front position at high or ebb tide achieved for the extreme tidal amplitudes (spring and neap tides). For the simulated conditions (scenarios SL2 and SL3) a difference of about 4 km in the estuarine saline wedge intrusion was observed: 12.5 km for a spring tide and only 8.5 km for a neap tide. Figure 21 depicts the differences on

Fig. 21. Effect of tidal regime on saline wedge reflux (ebb tide) (scenarios SL2 and SL3)

During the warm season (late spring and summer), the Alvo sluices are almost closed (scenario RT1). For this operational condition, the RT values near Pranto mouth station can quintuplicate when compared with those resulting from a Pranto river flow discharge of 15 m3.s-1 (scenario RT6), both under dry-weather conditions (low river Mondego inflows). Figure 22 shows this sensitive increase on flushing capacity of the Mondego estuary south

Fig. 22. Effect of Pranto river discharge on RT values distribution (scenarios RT1 and RT6) For the other hand, when the Alvo sluices remain closed the salinity and the RT values inside the southern arm are strongly influenced by tidal regime. Figure 23 illustrates the gradient of RT spatial distribution, which was mapped applying the *TemResid* module

Simulation results for these two tidal scenarios showed a RT values increase of 50% for a neap tide, when compared with a spring tide, both in the south arm and in the north arm reach, between N1 and N2 control points. This increase is smoothed in the northern arm inner areas, with the lowest increase (only 17%) at the Mondego estuary mouth. The

computing availability for the simulation of management scenarios RT2 and RT3.

**3.5 Hydrodynamic influence on estuarine residence time distribution** 

arm due to Pranto river discharges from Alvo sluices opening.

the saline wedge return (ebb tide) for these two extreme tidal regimes.

The analysis of the results obtained in the performed simulations allows the confirmation that there is a significant influence of bathymetry in the spatial variation of the RT along the Mondego estuary and consequently, the definition of typical (unique) values for each one of its arms becomes inadequate if they are not associated to local and specific hydrodynamic scenarios.

The results obtained from hydrodynamic modelling have shown a strong asymmetry of ebbing and flooding times in the inner estuary south arm areas due to their complex geomorphology (extensive wetlands and salt marsh zones, over 75% of its total area). This information allows a better understand of the estuarine circulation pattern, since tide is the major driving force of the southern arm flushing capacity, when the Alvo sluices remain closed. Indeed, the absence of the Pranto river discharge (a typical dry-weather condition) drastically increases salinity and RT values in the inner estuary southern arm and, consequently, the nutrients availability for algae uptake is higher, enhancing estuarine vulnerability to eutrophication.

From the analysis of the results obtained, it is possible to conclude that in both arms of this estuary, the tidal prism volumes are influenced by the bathymetry (extensive wetland areas), tidal regime and freshwater inputs. However, the influence of the tidal regime on the tide prism values is much greater than that of the freshwater inflows, and it is possible to verify that those values do not increase proportionally to the incremental values of the Mondego River flow rate.

The knowledge of the ebbing and flooding duration asymmetry is crucial for a more accurate tidal flow calculation, based on previous tidal prim estimation using mathematical modelling tools. With this new approach for mean tidal flow estimation the variation of cross section area can also be computed increasing the feasibility of the obtained results.

For the simulated conditions a difference of about 4 km in the estuarine saline wedge intrusion was observed: 12.5 km for a spring tide and only 8.5 km for a neap tide. However, a sensitive surface water elevation was monitored in the upper control section (N8), near the

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For medium typical tide, drought conditions and conservative constituents, simulation results showed that estuarine RT values range between 6 days (at both arms) and 4 days in the downstream reach of its two arms confluence (control point N1).

The development of integrated methodologies linking tracer experimental approach with hydroinformatic tools (based on 2D and 3D mathematical models) is of paramount interest because they can constitute a accurate and useful operational tool to establish better warning systems and to improve management practices for efficiently protecting water sources and, consequently, public health.

The MONDEST model developed and applied in this work allowed the evaluation and ranking of potential mitigation measures (like nutrient loads reduction or dredging works for hydrodynamic circulation improvement). So, the proposed methodology, integrating hydrodynamics and water quality, constitutes a powerful hydroinformatic tool for enhancing estuarine eutrophication vulnerability assessment, in order to contribute for better water quality management practices and to achieve a true sustainable development.
