Residual circulation in the Ría de Muros (NW Spain): A 3D numerical model study
Introduction
The Ría de Muros is the northernmost of the Rías Baixas, four large embayments jutting into the NW Atlantic coastline of the Iberian Peninsula between 42° and 43° N. From a geographical and geomorphological standpoint, the Rías Baixas are characterised as rías (Cotton, 1956) or as ‘young estuaries’ (Vidal-Romaní, 1983). From a thermohaline and dynamical standpoint, they are classified as partially mixed estuaries with a positive circulation (Souto et al., 2003). The Ría de Muros covers an area of 90 km2, with a mean volume of 2060 hm3; its main axis extends 13 km in a general NE–SW direction (Fig. 1). The inner part of the ría, east of Pt. Testal, consists of two narrow branches oriented towards the north and the south-east. The northern branch in turn divides into two branches, into which the rivers Tambre and Tines, the main tributaries to the ría, empty. West of Pt. Testal the north bank forms a succession of bays with a general ENE–WSW orientation, whereas the south bank, less indented, has a NE–SW direction. As a result, the ría broadens from a width of 850 m at Pt. Testal to over 6000 m at its mouth.
The bathymetry (Fig. 2) presents roughly triangular transverse sections in the outer and middle ría, with depths along the axis decreasing steadily from ~ 45 m at the mouth to less than 10 m in Freixo Bay. Upstream water depths are also below 10 m, with tidal flats constituting a significant part of the inner ría surface. The largest freshwater inflow is provided by the River Tambre, with an annual mean flux of 54 m3 s− 1. The contribution of the River Tines, 0.61 m3 s− 1(annual mean flux), is relevant only in the vicinity of its mouth. The average tidal range is 2.5 m (mesotidal). The amplitudes and phases of the seven major tidal constituents, obtained from the LeProvost global data base (LeProvost et al., 1994, Le Provost et al., 1995), are shown in Table 1. With a form number F = 0.09, the tidal forcing is clearly semi-diurnal. Basically the main tidal constituent (M2) is modulated by the S2 and N2 constituents over the spring–neaps cycle. Typical depth-averaged tidal velocities are on the order of 0.05–0.2 m s− 1 in the outer and middle ría; higher velocities, up to 1.5 m s− 1, are attained in the shallow and narrow inner ría (Iglesias et al., in press).
The western coast of the Iberian Peninsula is the northern limit of the NW Africa coastal upwelling system. In this area the regime of shelf winds results from the large-scale climatology of the NE Atlantic. Northerly winds, which favour upwelling, predominate from March–April to September–October, whereas southerly winds, which favour downwelling, are predominant the rest of the year (Wooster et al., 1976, McClain et al., 1986). During the upwelling period, the Eastern North Atlantic Central Water (ENACW), cold and nutrient-rich, ascends onto the shelf and subsequently enters the rías. During the downwelling period, warm and salty surface waters accumulate on the shelf. This thermohaline cycle brings about the high oceanic productivity of the Rías Baixas, among the highest in the world, which forms the basis of an important aquaculture and fishing industry (Blanton et al., 1987). In particular, the Ría de Muros produces cockles, mussels and clams, with twelve fishing ports; its environmental value is confirmed by the three special protection areas included in the Nature 2000 Network (Fig. 1).
The importance of residual flows for aquaculture, fisheries, and in general the marine environment in a coastal embayment cannot be easily overstated; they may be generated by the tide, winds, freshwater inflows, density gradients and – in the case of multiple inlet systems – external water level gradients. The influence of the tide has long been acknowledged (Ianniello, 1977, Fischer et al., 1979, Zimmerman, 1981, Wei et al., 2004, Lopes and Dias, 2007). Other authors have dealt with the influence of river inflows and winds (Garvine et al., 1992, Dyer, 1997, Jay and Flinchem, 1997, Lopes and Dias, 2007). In the context of the Rías Baixas, the residual circulation has been studied in the Ría de Vigo, albeit without considering tidal residual currents (Torres López et al., 2001, Souto et al., 2003) , and in the Ría de Pontevedra, including the tide-induced residual circulation (Ruiz-Villarreal et al., 2002). In this article, a three-dimensional baroclinic hydrodynamic model of the Ría de Muros is presented and applied to the analysis of its residual circulation. All the relevant forcings are taken into account: the tide, winds, river inflows, and salinity and temperature at the open boundary. Two types of numerical simulations are conducted. In the first, which is used for validation purposes, the forcing of the model represents the real conditions during a specific period of time (the validation period). In the second type of simulations, which is used for obtaining the response of the Ría de Muros to the different driving forces, the model is forced by each of the relevant hydrodynamic agents in turn—the tide, winds, and river inflows and density gradients.
This article is structured as follows. First, the materials and methods section comments on the field data gathered and on the model itself—equations, computational grid, boundary conditions, validation, etc. Second, the residual circulation driven by the different forcings – the tide, two wind conditions characteristic of the Ría de Muros, and the river inflows and density gradients corresponding to a high river runoff situation – are described. These results are subsequently compared and analysed in the discussion, which is followed by the conclusions.
Section snippets
Field data
An Acoustic Doppler Current Profiler (ADCP, Nortek AWAC AST) was deployed at a point close to the north bank of the ría (42° 45′ 27.48″ N, 9° 3′ 21.00″ W) (Fig. 1) at the 15 m depth. In the configuration used the water column was divided into 4 cells and the sampling interval was 30 min. This instrument is also equipped with a pressure gauge, which measured the water level variation with the same sampling interval. The Nortek AWAC AST functioned from 23 July 2007 (14:00 GMT) to 31 July 2007
Residual circulation induced by the tide
Upon validating the model, simulations were conducted to determine the residual circulation induced in the Ría de Muros by different forcings. First, the semi-diurnal M2 tidal constituent was considered, with the amplitude and phase shown in Table 1. The transient horizontal Eulerian velocity components, u and v, computed by the numerical model with this forcing may be decomposed into periodic components, up and vp, and residual components, ur and vr:
There are basically
Discussion
The residual circulation in the four cases analysed, which correspond to the relevant forcings of the ría hydrodynamics, present significant differences. Only in the tide-driven case do the surface and bottom layers show a similar flow pattern throughout the ría. Both the wind and river inflows bring about a two-layer estuarine circulation over a large part of the ría, either positive or negative (inverse).
The tide produces significant residual flow velocities (of the order of 10− 2 m s− 1) only
Conclusions
A 3D baroclinic finite-difference hydrodynamic model was successfully implemented in the Ría de Muros and applied to the investigation of residual flows, a fundamental aspect of its biological productivity. Field data of flow velocity and direction, water level, wind velocity and direction, river discharge, and salinity and temperature were used. For validation the model was run forced by the tide, wind, river inflows and thermohaline conditions. There was excellent agreement between simulated
Acknowledgements
The authors are grateful to three anonymous reviewers, whose comments have helped to improve the manuscript. This work was funded by the European Union under the Interreg IIIC Program [PROLIT/SP1.E.194/03] and by the Galician Regional Government [PGIDIT04TAM008E]. The authors are grateful to Prof. A. Suárez and Prof. J. Cremades of the University of A Coruña for their help during the field campaigns.
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