Experimental and numerical study of the hydrodynamics of the western sector of Ria Formosa

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1 Experimental and numerical study of the hydrodynamics of the Western Sector of Ria Formosa 2011 Experimental and numerical study of the hydrodynamics of the western sector of Ria Formosa José Jacob, Sara Cardeira, Marta Rodrigues, Nicolas Bruneau, Alberto Azevedo, André B. Fortunato, Mónica Rosa and Alexandra Cravo CIMA, Universidade do Algarve Campus de Gambelas, Faro, Portugal Laboratório Nacional de Engenharia Civil, Av. do Brasil 101, Lisboa, Portugal British Antarctic Survey ABSTRACT Jacob, J., Cardeira, S., Rodrigues, M., Bruneau, N., Azevedo, A., Fortunato, A.B., Rosa, M. and Cravo, A., Experimental and numerical study of the hydrodynamics of the western sector of Ria Formosa. In: Conley, D.C., Masselink, G., Russell, P.E. and O Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp , ISSN The western sector of Ria Formosa, a lagoon system in the south of Portugal, represents approximately 90% of the total tidal prism of the lagoon and includes three inlets. Two sets of field campaigns to characterize the hydrodynamics of this sector in neap and spring tide conditions were conducted in the autumn of 2011 and spring The main findings related to the inlets hydrodynamics and water exchanges between the lagoon and the ocean along semi-diurnal tidal cycles are presented. To estimate the relative contribution of the three inlets to the water exchanges between Ria Formosa and the ocean, discharges were evaluated hourly along complete neap and spring semi-diurnal tidal cycles and the tidal prisms computed. In addition, two sea level time series measured in Faro-Olhão inlet and Faro commercial pier were harmonically analyzed. The results were compared with previous studies and used to validate the ELCIRC hydrodynamic model. This model provided additional information about the circulation and tidal prisms and distortion inside the western Ria Formosa. This study confirmed the Faro-Olhão inlet as the main inlet in terms of contribution for the total tidal prism. It is shown that the Ancão inlet lost hydraulic efficiency, contributing less than 6% to the total tidal prism in all situations and the Armona inlet gained efficiency in spring tide and lost efficiency in neap tide. Moreover, the Faro-Olhão inlet exhibits flood prisms higher than ebb prisms under neap and spring tides, suggesting a residual circulation towards the Ancão and Armona inlets. ADDITIONAL INDEX WORDS: lagoon system, tidal inlet, tidal prism, residual circulation, field campaign, hydrodynamic model. INTRODUCTION The Ria Formosa is a shallow lagoon system of about 80 km 2, 55 km long in the E-W direction, with 6 km of maximum width and an average depth less than 2 m, located in the south coast of Portugal (Figure 1). The Ria Formosa has six permanent connections to the ocean, providing a great water renewal. This is a meso-tidal system with a mean tidal range of approximately 2 m, varying from 1.5 m to 3.5 m, dominated by the semi-diurnal component of the tide. It is well-mixed vertically, with no evidence of persistent haline or thermal stratification due to reduced freshwater inputs (Newton and Mudge, 2003) and predominance of the tidal forcing in the water circulation inside it. The influence of the tide on the hydrodynamics of the Ria Formosa and on the dynamics of its inlets and the quantification of the tidal prisms and water exchanges with the adjacent ocean has been the subject of several studies such as the works described in Salles (2001), Soares et al. (2001), Williams et al. (2003), Salles et al. (2005), Brito et al. (2008), Dias et al. (2009), Pacheco et al. DOI: /SI received 07 December 2012; accepted 06 March Coastal Education & Research Foundation 2013 (2010) and Jacob et al. (2012). The knowledge and characterization of the hydrodynamics of the Ria Formosa, in particular the water circulation and the exchanges with the ocean through the inlets and the tidal influence are essential in order to understand the dynamics of nutrients, chlorophyll-a and suspended matter, which is the subject of the project Nutrients and particulate matter dynamics - exchanges between a COAstal LAgoon and the adjacent ocean through a multiple inlet system: the case of Ria Formosa, south of Portugal COALA that supports the present work. The COALA project aims to characterize the spatial distribution and the temporal scales of variability of nutrients, chlorophyll-a and particulate matter within the western sector of Ria Formosa. According to previous studies, such as Pacheco et al. (2010), the western sector of Ria Formosa (Figure 1) represents approximately 90% of the total tidal prism of the entire lagoon. This sector includes 3 inlets, the Ancão inlet artificially relocated, the Faro-Olhão inlet artificially opened and stabilized and the Armona natural inlet and several channels and creeks. The two main channels of this sector are the Faro channel connecting the Faro-Olhão inlet to the city of Faro and the Olhão channel connecting the same inlet to the city of Olhão.

2 2012 Jacob, et al. Figure 1. The western sector of Ria Formosa with the location of the three tidal inlets, the two main channels and the position of pressure transducers, FCP Faro commercial pier and DIP Deserta island pier. Two sets of field campaigns were conducted in different periods of the year, one in the autumn of 2011 and the other in the spring of 2012, in the three inlets of the western sector of Ria Formosa. These campaigns aimed to characterize the hydrodynamics and to evaluate the nutrients and particulate matter exchanges between the lagoon and the ocean in neap and spring tide conditions. In this work the main findings related to the hydrodynamics and water exchanges between the lagoon and the ocean through the three inlets, along these semi-diurnal tidal cycles are presented. To estimate the relative contribution of each one of the three inlets to the overall water exchanges between Ria Formosa lagoon and the ocean, discharges were evaluated hourly along complete neap and spring semi-diurnal tidal cycles and tidal prisms were computed. The results obtained were compared with the previous results of Pacheco et al. (2010) and were used to validate the ELCIRC hydrodynamic model. The application of this model has provided additional information about the circulation and tidal prisms and distortion inside the western sector of Ria Formosa. In addition, we present an harmonic analysis of the time series of sea level elevation obtained in two different locations on the Faro channel, one in the Desert island pier (DIP, Figure 1), near the Faro-Olhão inlet and the other at the Faro commercial pier (FCP, Figure 1), inside the lagoon near the city of Faro. These data were compared with the results of the numerical model ELCIRC (Zhang et al., 2004). MATERIAL AND METHODS Field Site and Campaigns As stated above, the western sector of Ria Formosa (Figure 1) includes the Ancão inlet (An), the Faro-Olhão inlet (FO) and the Armona inlet (Ar). The first set of field campaigns, carried out in these inlets in November and December of 2011, included six semi-diurnal tidal cycle samplings - two in each of the three inlets, in both spring and neap tidal conditions. The second set, conducted in March and May of 2012, included seven semidiurnal tidal cycle samplings, two in the Ancão inlet and two in the Armona inlet also on both spring and neap tidal conditions and three in the Faro-Olhão inlet, one in neap tide conditions and two in spring tide conditions. Only one of the two spring tide semi-diurnal cycles sampled in the Faro-Olhão inlet in the spring season was used in the present work. The date and the fortnightly tidal phase of the campaigns can be found in Table 1. The fieldwork in all the inlets was conducted under fair weather, fetch limited and swell protected conditions. Current velocity measurements were taken just once per hour along a cross section of each inlet (Figure 1) due to lack of time, since other measurements of physical and chemical parameters and water collection were additionally performed. The flow velocity was measured using a Sontek/YSI 1.5-MHz Current Surveyor Acoustic Doppler Profiler (ADP) with bottom tracking, sidemounted on a boat and synchronized with a global positioning system (GPS Garmin GPSMAP 78S). Bottom-tracking allows the ADP to measure both its velocity (speed and direction) over the Earth, and the depth of the water beneath the system. These data are used to remove vessel motion from measured water velocity to determine the true water speed and direction (Sontek, 2005). The cell size and blanking distance were set to 0.4 m, the ADP transducer draft to 0.25 m and the number of cells to an appropriate number that accounted for the maximum depth of each profile. The ADP was operated in continuous mode with an average interval of 5 s. The software Current Surveyor v4.6 was used to record hydrodynamic data, to measure the cross-section shape and dimensions and to analyse the hourly transect surveys. The signal-to-noise ratio (SNR) was set to 3 db to remove invalid data below the ambient noise level. Moreover, two pressure transducers (PT, Level TROLL) were placed in two different sites in the lagoon before each set of experiments as stated before (Figure 1) that have been collecting data for two months with a sampling interval of 10 minutes. According to Pugh (2004), for a satisfactory harmonic analysis the sea level time series should extend over a lunar month with a sampling interval of one hour (709 hours), or better over a year (8766 hours). These two locations are subject to different environmental conditions: one, DIP, exposed to the direct influence of the open ocean forcing and conditions like for example the coastal tide and swell; and the other, FCP, inside Ria Formosa 6 km upstream of the first and protected from the direct influence of the adjacent ocean. A classical harmonic analysis of the sea level data measured in both locations, FCP and DIP, was performed using the package T- TIDE, a set of scripts and functions implemented in MatLab (Pawlowicz et al., 2002). New time series were synthesized from this harmonic analysis using the MSF, Q1, O1, P1, K1, N2, M2, S2, K2, MN4, M4, MS4, M6 components. These new time series of sea level were subsequently compared with the sea level series obtained at the same locations through the application of the ELCIRC model to the Ria Formosa. The discharge was calculated through the integration along the channel cross section of the product between the velocity component normal to the cross section and the corresponding cross-sectional area. The numerical integration in the time domain of the hourly discharge values along the flood and ebb periods provided respectively the flood and ebb tidal prisms. The residual tidal prism or net transport of water was obtained as the difference between the flood and ebb tidal prisms or, equivalently, through the numerical integration of the discharge over the complete semidiurnal tidal cycle. Numerical Modelling Model Description ELCIRC (Zhang et al., 2004) is an unstructured grid model that simulates the baroclinic circulation across river-to-ocean scales. The model solves the shallow water equations and computes the free surface elevation and the fields of velocity, salinity and temperature. ELCIRC combines finite volume, finite difference and Eulerian-Lagrangian methods, providing simultaneously stability, accuracy and mass conservation. Detailed description of the model can be found in Zhang et al. (2004).

3 Experimental and numerical study of the hydrodynamics of the Western Sector of Ria Formosa 2013 Figure 3. Bathymetry (m, Mean Seal Level) of the Ria Formosa used in the present study. Figure 2. Computational grid of the Ria Formosa. Application to the Ria Formosa The model setup used was based on Portela et al. (2011). Barotropic, depth-averaged simulations were performed. The horizontal domain was discretized in a grid with about nodes and elements, and a resolution varying from 10 m and 6 km (Figure 2). Bathymetry, from 1980, was updated with recent data (2001, 2003, 2004, 2008 and 2010) and combined locally with information from aerial photographs (Figure 3). The model was only forced by tides, which were imposed at the oceanic boundary. This boundary was forced with 14 tidal constituents (Z0, MSF, Q1, O1, P1, K1, N2, M2, S2, K2, MN4, M4, MS4 and M6) from the regional model of Fortunato et al. (2002). The time step was set to 60s. Initial conditions of water levels and velocities were set from the repose and a warm-up period of 2 days was used. RESULTS AND DISCUSSION The tidal prisms obtained for the three Ria Formosa western sector inlets in the Autumn of 2011 and Spring of 2012 field campaigns are provided in Table 1 and illustrated in Figure 4. This figure shows the values of the flood and ebb tidal prisms relative to the total prism of the western sector obtained as the sum of the prisms for the three inlets considered in the present study. Data were also compared with those obtained by Pacheco et al. (2010), which presents the prisms for the same inlets calculated from data collected in campaigns carried out between 2004 and This comparison reveals a time evolution of the hydrodynamic conditions in the inlets and inside the lagoon. The results obtained confirmed the Faro-Olhão inlet as the main inlet in terms of contribution for the total tidal prism. The Ancão inlet contribution is less than 6% for the total tidal prism in spring and neap tide conditions in both seasons. In addition, this study shows that the Ancão inlet lost hydraulic efficiency in spring tide conditions for the Armona inlet, while the Faro-Olhão inlet prism remained constant. In neap tide conditions, the Armona inlet lost efficiency gained by Faro-Olhão inlet and the Ancão inlet prism remained stable. It should be noted that the results of Pacheco et al. (2010) show similar prism values for the Faro-Olhão and Armona inlet in neap tide conditions. This temporal evolution of tidal prisms can be justified by the morphological and bathymetric changes and evolution of the Ancão and Armona natural inlets while the Faro- Olhão inlet was stabilized and remains unchanged artificially. In this respect it is important to note the eastward migration of the Ancão inlet and the narrowing of the Armona inlet. A small contribution from a sand bank that is growing in the Faro-Olhão inlet flood delta to these changes can exist, which must be investigated. Moreover, the Faro-Olhão inlet exhibits flood prisms higher than ebb prisms under both neap and spring tides, suggesting a residual circulation towards Ancão and Armona inlets. The experimental results were also used to validate the ELCIRC hydrodynamic model. Figure 5 shows a comparison between the experimental data for tidal prisms and the values obtained by the ELCIRC model for the Faro-Olhão inlet. The experimental results were compared only with the results of ELCIRC for the Faro- Olhão inlet because this inlet is kept stable while the Ancão and Armona inlets suffered major changes and there is no recent bathymetry of those inlets that can be used in ELCIRC. Regarding tidal prisms ELCIRC is able to represent patterns observed during the field campaigns at the Faro-Olhão inlet, both in spring and neap tides (Figure 5). Table 1. Tidal prisms in m 3 at the inlets of the western sector of Ria Formosa and dates of the field experiments. Ancão Inlet Faro-Olhão Inlet Armona Inlet Spring tide Autumn Spring Autumn Spring Autumn Spring Date 22 nd Nov rd Mar th Nov nd Mar rd Nov st Mar 2012 Ebb Prism -2.95x x x x x x10 7 Flood Prism 3.50 x x x x x x10 7 Residual 5.51 x x x x x x10 6 Neap tide Autumn Spring Autumn Spring Autumn Spring Date 7 th Dec th Mar th Dec th May th Dec th Mar 2012 Ebb Prism x x x x x x10 7 Flood Prism 2.2 x x x x x x10 7 Residual x x x x x x10 6

4 2014 Jacob, et al. Figure 4. Relative tidal prisms at cross sections near the inlets of the western sector of Ria Formosa: Pacheco et al. (2010) results (white bars) versus COALA field experiments, Autumn 2011 (gray bars) and Spring 2012 (black bars). An Ancão inlet, FO Faro- Olhão inlet and Ar Armona inlet. Figure 6 shows the experimental and model results for the mean velocity at the Faro-Olhão inlet cross-section where the discharges and tidal prisms were computed in the Spring of 2012 during a spring tide (22/03/2012) and a neap tide (14/05/2012). Current velocities are properly reproduced by the model at the Faro-Olhão inlet, during the 2012 Spring campaigns, with a root mean square error (RMSE) of about 14 cm/s at the spring tide and of about 10 cm/s at the neap tide (Figure 6). The main differences observed between the data and the model results may derive from the bathymetry used in the simulations, which is not contemporary with our data. The RMSE of the sea level variation time series from the ELCIRC model relative to the field data are presented in Table 2. RMSE were computed for both the field data as measured and the synthesized data for the tidal constituents imposed at the model boundary. In both locations, the RMSE show a good correlation between the experimental data and the model results. Figure 7 describes the distribution of tidal amplitude and phase for M2, K1, M4, M6 e MSF harmonic constituents for the Spring 2012 model and experimental values (two months of sea level variation sampled every ten minutes). In general, there is a good agreement between the model results and the field data. The results depict a dominance of semi-diurnal and diurnal tidal constituents, which had already been observed in previous studies (e.g. Dias et al., 2009). The difference between predicted and observed M2 amplitudes is 1.5 cm in the Deserta island pier and 3.5 cm in Faro commercial pier. The phase difference for M2 is 2º Figure 5. Tidal prisms at the Faro-Olhão inlet: model results (ELCIRC) versus COALA field experiments, Autumn 2011 and Spring Figure 6. Velocity at the Faro-Olhão inlet in the Spring 2012: model results (ELCIRC) versus COALA field experiments during a spring tide (22/03/2012) and a neap tide (14/05/2012). Positive (negative) values represent flood (ebb).

5 Experimental and numerical study of the hydrodynamics of the Western Sector of Ria Formosa 2015 in DIP and 1º in FCP, which corresponds to a delay of about 4 min and 2 min between the predicted and observed tide respectively. For K1 the difference in amplitudes is 0.15 cm in DIP and there is no difference between observed and predicted values in FCP. As expected, the amplitude of the fourth-diurnal tidal component M4 is greater inside the lagoon (FCP) than in the inlet (DIP). The observed differences could arise from bathymetric differences as previously explained and from the boundary conditions. Future developments of the model will include updating the bathymetry based on the latest data, as well as the upgrading of boundary conditions. This application will serve as a basis for the implementation of the baroclinic model SELFE (Zhang and Baptista, 2008) in the Ria Formosa. The SELFE model will be used in conjunction with a particle model to study the chlorophyll-a and nutrients exchanges between the Ria Formosa lagoon and the ocean. CONCLUSION This study has provided additional quantitative information about the hydrodynamics of the western sector of Ria Formosa. The results obtained confirm that the Faro-Olhão inlet is the main inlet in terms of water exchanges. Its contributions ranged from 59% to 71% for the tidal prisms of this sector, followed by the Armona inlet contributing 25% to 37% and the Ancão inlet with less than 6% in all the tidal situations sampled. The comparison of the present tidal prisms with older values (Pacheco et al., 2010) reveals a time evolution of the hydrodynamic conditions in the inlets and inside the lagoon. Ancão inlet lost hydraulic efficiency in spring tide conditions for the Armona inlet while Faro-Olhão inlet tidal prism remained constant. Armona inlet lost efficiency in neap tide conditions for the Faro-Olhão inlet while the Ancão inlet prism remained stable. As the Faro-Olhão inlet was artificially stabilized and is relatively unchanged, the temporal evolution of the tidal prisms may be due to morphodynamic and morphological changes in the Ancão and Armona natural inlets. In addition, Faro- Olhão inlet exhibits flood prisms higher than ebb prisms under Table 2. Root mean square error (RMSE) of the sea surface elevation from the ELCIRC model with respect to measured values, for the two sea level stations, DIP and FCP. RMSE (cm) DIP FCP Time series of sea level synthesized Autumn Spring Aut.2011+Spr Time series of sea level as measured Autumn Spring both neap and spring tides, suggesting a residual circulation towards Ancão and Armona inlets. The application of the ELCIRC model to Ria Formosa has confirmed the information about the circulation and tidal prisms and distortion inside its western sector obtained from the experiments conducted in the framework of the project COALA. The comparison of the experimental values of the tidal prisms and mean velocities in the Faro-Olhão inlet with the corresponding numerical values from ELCIRC application to Ria Formosa shows a good agreement between the two. These results can be improved with the use of an up-to-date bathymetry in the numerical simulations. The study of the sea level variation in two different locations of the Faro channel (DIP and FCP) shows a good correlation between the experimental data and the model results in both locations, with RMSE values of the same order of magnitude or lower than values obtained in previous studies (Dias et al., 2009). To better understand the hydrodynamics of Ria Formosa it is also important to study the relative influence of the wind and other atmospheric and oceanic forcings in the circulation of the lagoon. It is planned to do this with the coupling of the results of a mesoscale atmospheric model to the hydrodynamic model. Figure 7. Distributions of the tidal amplitude and phase for the M2, K1, M4, M6 and MSF harmonic constituents: black barsmeasurements; white bars - model results.

6 2016 Jacob, et al. ACKNOWLEDGEMENTS The authors acknowledge Commanders Guilherme Marques Ferreira and Conceição Duarte, Captains of Faro and of Olhão ports respectively, for providing the conditions to conduct the field work. We also would like to thank Dr. Óscar Ferreira for lending the ADP to measure the current velocities, António Brás and Mr. Alves for supporting the placement and maintenance of the PT in Desert island pier and the Port Authority of southern Portugal for authorizing the placement of the 2 PT's in its installations and for supporting their placement and recovery. This work was financially supported by Portuguese Foundation for Science and Technology (FCT) under the project ref: PTDC/MAR/114217/2009-COALA. Zhang, Y., Baptista, A.M. and Myers, E.P., A cross-scale model for 3D baroclinic circulation in estuary plume shelf systems: I. formulation and skill assessment. Continental Shelf Research, 24 (18), LITERATURE CITED Alcântara, P., Cravo, A. and Jacob, J., Variability of nutrients and chlorophyll-a in Ancão inlet during a spring tidal cycle in April Ciencias Marinas 30 (1B), Duarte, P., Azevedo, B., Guerreiro, M., Ribeiro, C., Bandeira, R., Pereira, A., Falcão, M., Serpa, D. and Reia J., Biogeochemical modelling of Ria Formosa (South Portugal). Hydrobiologia. 611, Dias, J.M., Sousa, M.C., Bertin, X., Fortunato, A.B. and Oliveira, A., Numerical modeling of the impact of the Ancão Inlet relocation (Ria Formosa, Portugal). Environmental Modelling & Software, 24 (6), Fortunato, A.B., Pinto, L.L., Oliveira, A. and Ferreira, J.S., Tidallygenerated shelf waves off the western Iberian coast. Continental Shelf Research, 22 (14), Jacob, J., Cardeira, S., Rodrigues, M., Bruneau, N., Azevedo, A., Fortunato, A., Cravo, A. and Rosa, M., Contribuição para o estudo da propagação da maré no setor oeste da Ria Formosa. 2as Jornadas de Engenharia Hidrográfica (Instituto Hidrográfico, Lisboa, Portugal), pp , ISBN Newton, A. and Mudge, S.M., Temperature and salinity regimes in a shallow, mesotidal lagoon, the Ria Formosa, Portugal. Estuarine, Coastal and Shelf Science, 57, Pawlowicz, R., Beardsley, B. and Lentz, S., Classical tidal harmonic analysis including error estimates in MatLab using T_TIDE. Computers and Geosciences, 28, Pacheco, A., Ferreira, Ó., Williams, J.J., Garel, E., Vila-Concejo, A. and Dias, J.A., Hydrodynamics and equilibrium of a multiple-inlet system. Marine Geology, 274, Portela L., Azevedo, A and Fortunato, A.B., Valorização hidrodinâmica da Ria Formosa e mitigação do risco nas ilhas barreira, Relatório 3: Valorização hidrodinâmica da laguna, 3ª fase. Relatório 395/2011-NEC, Lisboa. Pugh, D., Changing Sea Levels: Effects of Tides, Weather and Climate. Cambridge University Press, 265p. Salles, P., Hydrodynamic Controls on Multiple Tidal Inlet Persistence. Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, PhD thesis, 272p. Salles, P., Voulgaris, G. and Aubrey, D., Contribution of nonlinear mechanisms in the persistence of multiple tidal inlet systems. Estuarine Coastal and Shelf Science, 65, Soares, C., Onofre, J. and Grade, N., Caracterização Hidrodinâmica do Sistema de Barras da Ria Formosa. Actas das 2as Jornadas Portuguesas de Engenharia Costeira e Portuária (Sines, Portugal), Associação Internacional de Navegação, CD-ROM, 14p. Sontek, Current Surveyor System Manual: Software Version San Diego, California, 160p. Williams, J.J., O'Connor, B.A., Arens, S.M., Abadie, S., Bell, P., Balouin, Y., Van Boxel, J.H., Carmo, A.J., Davidson, M., Ferreira, O., Heron, M., Howa, H., Hughes, Z., Kaczmarek, L.M., Kim,,H., Morris, B., Nicholson, J., Pan, S., Salles, P., Silva, A., Smith, J., Soares, C. and Vila-Concejo, A., Tidal inlet function: field evidence and numerical simulation in the INDIA project. Journal of Coastal Research, 19(1), Zhang, Y. and Baptista, A.M., SELFE: a semi-implicit Eulerian- Lagrangian finite-element model for cross-scale ocean circulation. Ocean Modeling, 21 (3-4),