4.2. Assessing Observed Tidal and Salinity Changes Between 1987 and 2017
Figure 3 shows the evolution of amplitude and phase of
M2 constituent in monitoring sites between 1987 and 2020. To facilitate the understanding of the changes in the tidal propagation along each of the main channels of the lagoon, the analysis of the results obtained will be individualized for each domain previously defined. Regarding the Lagoon Entrance (Barra, Costa Nova, and S.Jacinto), the amplitude of the
M2 constituent increased by 9 cm in the period under analysis (
Figure 3a). It should be noted that the highest amplification (8 cm) occurred between 1987 and 2002. The phase changed slightly in the same stations (
Figure 3b). The major modification between these stations was found in Costa Nova, where a decrease of 5° (10 min) was detected between 1987 and 2002. Results for the stations located in the S.Jacinto Channel (
Figure 3c,d) reveal that the
M2 amplitude increased by 30 cm, 35 cm, 46 cm, and 53 cm in Puxadouro, Torreira, Varela, and Carregal, respectively. Moreover, the phase decreased by 20° (~40 min), 30° (~60 min), 40° (~80 min), and 55° (~100 min) in Torreira, Varela, Puxadouro, and Carregal, respectively. In Mira Channel, the
M2 amplitude (
Figure 3e) increased by 30 cm in Vagueira and 50 cm in Areão. Furthermore, the
M2 phase (
Figure 3f) decreased 25° (50 min) in Vagueira and remained unchanged in Areão. Again, it is noteworthy that changes in
M2 constituent occurred between 1987 and 2002. In the Ílhavo Channel (
Figure 3g,h), the
M2 amplitude increased 11 cm in all stations between 1987 and 2002 and after 2002 it slightly increased (~2 cm) in Vista Alegre and remained unchanged in Cais da Pedra. In addition, the phase decreased by 11° (22 min) between 1987 and 2002 in the two stations located further upstream (Vista Alegre and Cais da Pedra). In the Espinheiro Channel,
Figure 3 shows that the amplitude of the
M2 increased by 20 cm and 25 cm in Cais do Bico and Rio Novo, respectively. These stations further experienced a reduction in the
M2 phase (
Figure 3j). In detail, reductions of 12° (~25 min) and 25° (~50 min) were found for Rio Novo and Cais do Bico, respectively.
Figure 4 depicts the tidal asymmetry evolution, namely amplitude ratio and relative phase, throughout the lagoon channels between 1987 and 2020. The results evidence that the stations located in the entrance channel and at the beginning of the main channels present values of amplitude ratio (0.03–0.08) lower than those observed in the upstream stations (0.1–0.4), indicating that the tide is more distorted in the channels extremities than in the lagoon central area. Regarding the relative phase, it is verified that the upstream stations are flood dominated (35–180°), while the ones located in the central regions are ebb dominated (180–280°). In the stations located at the lagoon entrance, slight changes in the amplitude ratio and an increase in the relative phase of 40° were observed, denoting an intensification of the ebb dominance. In the S.Jacinto channel, the amplitude ratio decreased between 0.04 in Torreira and Varela and 0.06 in Manchão, Pardilhó, Carregal, and Puxadouro within the period analysed. This region is flood dominated, however as the relative phase increased since 1987, the flood dominance decreased. Regarding the Mira channel, it is notorious for intensification/weakening of the tidal distortion and flood dominance at the end/middle of the channel (Areão/Vagueira). The Ílhavo and Espinheiro channels experienced slight changes (lower than 0.02) in tidal distortion, but important changes in the relative phase. It is noteworthy that the Cais do Bico, Rio Novo, and Cacia stations changed from flood to ebb dominance between 1987 and 2002.
Table 4 presents the mean and maximum salinity values for the campaigns carried out in 1997 and the average of mean and maximum salinity values in June for the period 2012/2017. Results reveal that the mean values increased in all stations between 1997 and 2012/2017. The Vista Alegre and Varela stations experienced the highest increases (12.00 and 4.60, respectively) while Barra and Vagueira the lowest ones (1.33 and 1.10, respectively). The maximum salinity values also increased in all stations (6.82, 5.67 and 3.70 for Vista Alegre, Varela, and Vagueira, respectively), except in Barra where a slight decrease (0.43) was observed.
4.4. Assessing the Effect of the Lagoon Morphology on Tidal and Salinity Patterns
Figure 5 shows the amplitude and phase of the main tidal constituent
M2 for 1987 and 2020 configurations. For both simulations, a decrease is observed in
M2 amplitude from the inlet to the channel’s heads along with an increase in the phase. For example, for the 1987 bathymetry, a difference of nearly 0.75 m is observed between the lagoon’s mouth (~0.90 m) and the heads of S.Jacinto channel (~0.15 m). For Espinheiro, Ílhavo and Mira channels head the attenuation of the tidal amplitude is less pronounced, approximately 0.45, 0.50, and 0.65 m, respectively. Regarding the 2020 bathymetry, the damping of the tidal amplitude is nearly 0.30, 0.15, 0.50, and 0.60 m between the lagoon’s mouth and S.Jacinto, Espinheiro, Ílhavo, and Mira channels heads, respectively.
Concerning the phase of the
M2 constituent, it ranges from 95° at the lagoon mouth for both bathymetries to 240° (170°) at the end of S.Jacinto (Espinheiro) channel for the 1987 bathymetry and to 180° (140°) for 2020 bathymetry. For the Mira and Ílhavo channels head, the phase is approximately 95° and 65° for past and present bathymetries, respectively. The
M2 constituent patterns are consistent with previous results [
13,
14], demonstrating that the tidal wave in Ria de Aveiro has the characteristics of a damped progressive wave: During the propagation the amplitude decreases, and the phase lag increases.
The tidal asymmetry (amplitude ratio and relative phase) was computed for all the lagoon and is mapped in
Figure 6, for 1987 and 2020 bathymetries. Results indicate that the magnitude of the tidal distortion increases from the mouth (between 0 and 0.1) towards the channels heads for both configurations, reaching values higher than 0.3 in the Mira channel head (
Figure 6a,b). The results of relative phase evidence that the lagoon is ebb dominated from the inlet until the middle of the main channels and flood dominated upstream for both configurations (
Figure 6c,d). It is noteworthy that the ebb dominated region extends slightly upstream for 2020 bathymetry.
The tidal current velocity (
Figure 7), quantified through the
VRMS, is stronger in the inlet channel with values between 1.4 and 1.6 m s
−1 for both configurations, while for the S.Jacinto and Espinheiro channels the
VRMS ranges from 0.8 to 1.2 m s
−1. For the rest of the lagoon, the current velocity is lower than 0.8 m s
−1.
The mean salinity values for 1987 and 2020 bathymetries were also computed and are shown in
Figure 8. A typical estuarine pattern can be observed characterized by a longitudinal salinity gradient, with the highest salinity values observed closer to the ocean boundary (nearly 35) and decreasing with the distance from the inlet, as the fluvial input increases (with values between 6 and 8, and 8 and 15, at the end of the channels, for the past and present bathymetries, respectively.).
To assess the changes that occurred between 1987 and 2020, the differences between model results obtained for both periods were computed for
M2 amplitude and phase,
VRMS, tidal asymmetry, tidal prism, and salinity (
Figure 9,
Figure 10,
Figure 11,
Figure 12 and
Figure 13). Additionally, and to understand whether the changes that occurred over the years are due to the inlet or main lagoon channels deepening, the differences between 1987 and scenarios 1 (considering the deepening of the inlet), and 2 (considering the deepening of the main channels) are also represented (
Figure 9,
Figure 10,
Figure 11 and
Figure 12).
The results show that the tidal amplitude increased in response to the morphologic changes that occurred between 1987 and 2020, with the highest differences observed at the end of the channels, reaching 0.60 m in the S.Jacinto head (
Figure 9). At the end of Espinheiro, Ílhavo and Mira channels the amplitude increases between 0.30 and 0.45 m. Otherwise, the phase delay decreased, with the highest differences observed at the channel heads. Once again, the main changes are observed in S.Jacinto channel end, with a phase lead higher than 60°, which means that for the present the
M2 tidal constituent takes nearly 2 h less to propagate from the lagoon mouth to the head of S.Jacinto channel than in 1987. For Espinheiro, Ílhavo, and Mira channels the phase lead is approximately 40° (80 min). These patterns determined by numerical modelling fully agree with observations represented in
Figure 3, showing the good model performance for both configurations developed.
Regarding the differences between the 1987 configuration and the two synthesized scenarios (Scenario 1 and Scenario 2), two different patterns are observed. On one hand, Scenario 1, which reflects the deepening of the inlet, induces an increase of M2 amplitude and a decrease in the phase delay close to the inlet, while differences are almost negligible at the higher lagoon. The differences at the inlet are not higher than 0.15 m and 20° (~40 min), respectively. On the other hand, results for Scenario 2, which reflects the deepening of the main channels, reveal an increase of tidal amplitude and a decrease in phase delay from the inlet to the channel’s head. Changes are almost negligible at the inlet, but are significant towards the end of the channels, showing a pattern similar to that resulting from the differences between the 1987 and 2020 configurations. Namely at S.Jacinto channel, an increase between 0.30 and 0.45 m in M2 amplitude and a decrease between 40° (~80 min) and 60° (~120 min) in the phase can be observed, in a way consistent with observations.
Generally, the results for tidal asymmetry suggest that the tidal wave is less distorted nowadays than in the past for most of the lagoon (
Figure 10a). Exceptions are observed at the far end of the main channels and in a specific channel in the central lagoon, where a slight increase in the tidal distortion occurs. Additionally, in the inlet region and at the beginning of Mira channel, the amplitude ratio differences are negligible. Comparing the amplitude ratio between 1987 and Scenario 1, it is observed that the deepening of the inlet channel induces a reduction in the tidal wave distortion from the inlet to the middle of the main channels and an amplification of the wave distortion upstream (
Figure 10b). Moreover, when the deepening of the main channels is considered (Scenario 2), results are similar to the ones observed for the 2020 configuration, with a general decrease of the tidal distortion (
Figure 10c).
Regarding the relative phase, results show that the ebb dominance of the central lagoon extends further upstream from 1987 to 2020 (
Figure 10d) and no differences occur between 1987 and Scenario 1. Otherwise, for Scenario 2 the ebb dominance observed at the beginning of Mira channel extends further upstream, and at the beginning of Espinheiro and Ílhavo channels, the tide dominance changes from ebb to flood (
Figure 10f).
The current velocity was evaluated through the
VRMS, and the results suggest its general increase along the main channels of the lagoon between 1987 and 2020 (
Figure 11a), with the highest changes observed in S.Jacinto and Espinheiro channels (between 0.3 and 0.4 ms
−1). At the intertidal areas, the current velocity slightly increases (less than 0.1 ms
−1). When the deepening of the inlet is considered (Scenario 1), a decrease of the
VRMS occurs at the inlet channel (more than 0.5 ms
−1), while for the rest of the lagoon the differences are negligible. Additionally, the comparison between
VRMS for 1987 and Scenario 2 shows that the channels deepening induce a significant increase of the current velocity in the main channels and a slight increase in intertidal areas. This pattern is very similar to that resulting from the differences between the 1987 and 2020 configurations, suggesting that the deepening of the lagoon main channels is the main cause of the changes verified in the Ria de Aveiro.
The values of computed tidal prisms (
Figure 12) reveal a significant rise between 1987 and 2020 in all cross-sections analysed, and for both tidal conditions. In detail, at the inlet, the tidal prism increased 60% on average, from 153.0 × 10
6 m
3 to 242.2 × 10
6 m
3 (~58%) and from 59.4 × 10
6 m
3 to 96.9 × 10
6 m
3 (~63%) for maximum spring tide and minimum neap tide, respectively. Results evidence that the tidal prism rise is not uniform across the lagoon main channels. Indeed, Espinheiro and S.Jacinto channels experienced the highest increase rates (85% and 77%, respectively) while Ílhavo and Mira channels experienced the lowest ones (45% and 35%, respectively). It is noteworthy that the tidal prism changes observed in S.Jacinto and Mira channels were mostly due to the main channels deepening, whereas the changes observed in the Espinheiro and Ílhavo channels are explained by both the deepening of the inlet channel (~15% and ~17%, respectively) and the main channels (~50% and ~27%, respectively).
Differences between the 1987 configuration and the two synthesized scenarios (
Figure 13) are also presented to assess the main causes of the changes observed in the salinity patterns. Results show that saline intrusion increased in the entire lagoon between 1987 and 2020. As for tidal propagation, salinity changes are almost negligible in the inlet and the highest differences are observed at the channel’s heads, with the maximum expression at S.Jacinto channel. So, salinity changes range from values close to 0 at the inlet to values between 8 and 14 at far the end of S.Jacinto channel, between 2 and 4 in Espinheiro and Ílhavo channels, and between 0 and 2 in Mira channel head.
Regarding the changes induced by the deepening of the inlet region (Scenario 1), they are almost negligible in the entire lagoon, ranging from −2 to 2. Otherwise, the deepening of the main channels (Scenario 2) induces a salinity increase in the entire lagoon, except for the inlet region and Mira channel head, where the differences are negligible. At the end of S.Jacinto channel, the salinity increases between 8 and 14, while for Espinheiro and Ílhavo heads, the salinity increases between 2 and 4, as found from the differences between the 1987 and 2020 configurations.