**3.4 Hydrodynamic influence on estuarine salinity distribution**

The analysis of the salinity distribution in the estuary had, as a primary goal, the identification of the areas that, throughout the tidal cycle, present salinity values within the range of 17 to 22‰, defined by Martins et al. (2001) as the most favourable for algal growth in this specific aquatic ecosystem.

The Pranto river inflow in estuary southern arm has shown a strong influence on salinity distribution decreasing drastically its values to a range far from the one defined as the most favourable for this estuarine eutrophication process. Figure 20 shows the opening Alvo sluices effect on southern arm salinity gradients caused by Pranto river flow discharge of 30 m3.s-1, during the ending of ebbing and the beginning of tidal flooding periods (scenarios SL 3 and SL1) (Duarte & Vieira, 2009a).

Fig. 20. Effect of Pranto river flow discharge on estuarine salinity distribution (high tide)

A Hydroinformatic Tool for Sustainable Estuarine Management 23

minimum RT values (3.2 days) occurred in the Mondego estuary mouth (N0) and in the mesotrophic wetland zone of the south arm (near station S2). The maximum RT values (9.5 days) were obtained for the zone (near station S3) with higher eutrophication vulnerability. Concerning the periodicity of tidal regime recurrence, its effect could be very relevant for

estuarine biochemical processes with a time scale lower than 6 days.

Fig. 23. Effect of tidal regime on RT values distribution (scenarios RT2 and RT3)

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

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

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

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

**4. Conclusion** 

vulnerability to eutrophication.

Mondego River flow rate.

scenarios.

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

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