**2.5 Simulated management scenarios**

16 Hydrodynamics – Natural Water Bodies

The size of the elements to consider in the spatial discrimination of the simulated domain of numerical models must be established as a function of larger or smaller spatial gradients than those displayed by the variables (water level and velocity) in that domain. In the case of the Mondego estuary, since the south arm was the preferred object for studying, the network of finite elements was refined in that sub-domain, thereby reducing the maximum

In the *MONDEST model*, the hydrodynamic module provides flow velocities and water levels for the water quality module, whose results acts as input on the *TempResid* module, feeding the constituents concentration over the aquatic system. The post-processing and

The *TempResid* module was integrally developed in this research work aiming to compute RT values of each water constituent (conservative or not) and allowing to map its spatial distribution over all the estuarine system, considering different simulated management

RT value of a substance was calculated for each location and instant, as an interval of time that is necessary for that corresponding initial mass to reduce to a pre-defined percentage of that value. In this work, a value of 10% was adopted for the residual concentration of the substance, attending to the fact that the effect of the re-entry of the mass in the estuary during tidal flooding is considered (a significant effect for dry-weather river flow rates). The determination of the RT in several stations along the estuary, where the eutrophication gradient occurred, was carried out by applying the *TempResid* programme to the results of the simulations that were performed with the transport module of the MONDEST model. Figure 15 shows an example of the MONDEST model transport module results for the management scenario considered as the most favourable to macroalgae blooms occurrence (Duarte, 2005), due to low freshwater inputs and consequent reduction of estuarine waters

**0 24 48 72 96 120 144 168 192 216 240**

**Scenario** *RT1*

**RT criteria (10% )**

**Time (hour)**

Fig. 15. Residence time computation using *TempResid* module

**estuary mouth Gala bridge river Pranto mouth**

mapping of model results was performed using SMS package (Boss SMS, 1996).

area of its (triangular) elements to 500 m2 (Duarte, 2005).

scenarios.

renovation (scenario RT1).

**Concentration (%)**

For hydrodynamic modelling purpose, a wide range (sixteen) of management scenarios were judiciously selected covering a representative set of hydraulic conditions (Table 2), resulting from the combination of typical tidal amplitudes (0.60, 1.15, and 1.60 m) and freshwater flow inputs (from Mondego and Pranto).


Table 2. Simulated management scenarios for the hydrodynamic modelling

For the *Mondest* transport model calibration and validation, the salinity was adopted as a natural tracer. Several management scenarios (nine) were also carefully selected (Table 3) considering the most representative hydrodynamic conditions in order to estimate salt wedge propagation into the estuary and to identify the areas (in both arms) where favourable salinity values for macroalgae growth can potentiate the estuarine eutrophication vulnerability.


Table 3. Simulated management scenarios for the hydrodynamic modelling

For the RT values calculation using the *TempResid* module, the simulated management scenarios (fourteen) were defined considering not only the most critical hydrodynamic conditions, but also by carefully selecting distinct pollutant load characteristics (e,g. location, duration and type of the discharge event, instant of tidal cycle when the release occurs) and

A Hydroinformatic Tool for Sustainable Estuarine Management 19

asymmetry is influenced by the tidal regime and has a fast increase into the inner areas of this arm reaching 2.5 hours: 5 hours for flooding and 7.5 hours for ebbing time. In the northern arm, between the sections N1 and N4, there is a little delay of fifteen minutes in the

Fig. 16. Effect of tidal regime on ebbing maximum values of currents velocity magnitude

Figure 17(a) shows an example of the tidal regime effect in the mean velocity magnitude (MVM) variation, at section N4 (where maximum values of this parameter occurred). It should be noted that for a neap tide, the VMM during the tidal flooding period is almost an

For upstream estuarine sections, water surface levels in high tide are similar, but, in ebb tide, water surface level increases in the inner section due to the effect of the estuarine

Fig. 17. (a) Effect of tidal regime on ebbing maximum values of currents velocity magnitude (section N4); (b) Surface water level variation along the estuarine system (N1, N7, N8)

The velocities and water levels field data obtained from the sampling programme were used for model calibration and validation. Figure 18 shows an example of a specific procedure performed in section S1 (Gala bridge/Lota) for the parameter "surface water level (SWL)". Two different sensitivity analyses were carried out to define the accurate values to adopt for the main calibration parameters used in both (hydrodynamic and water transport) modules of *Mondest* model: one for the Manning bottom friction coefficient (n) and horizontal Eddy

high tide occurrence and a bigger delay in ebb tide (about two hours).

(scenarios H2 and H3)

half of the value reached for a typical sprig tide.

bathimetry (elevation of bottom level) (Fig. 17b).

**3.2 Model calibration and validation** 


constituent decay rates (Table 4) in order to assess and confirm the highest eutrophication vulnerability of the inner areas of the Mondego estuary south arm, due to the expected occurrence of higher RT values.

Table 4. Simulated management scenarios for estuarine residence time calculation

In this work only a few examples of the very large amount of MONDEST model results obtained for those different simulated scenarios can be presented. The main aim of the following item will be to highlight the evident influence of hydrodynamics (tidal regime and freshwater inflows) on estuarine residence time spatial variation, which can play a special role in estuarine eutrophication vulnerability assessment.
