Residual circulation and sediment distribution in the Ria de Aveiro lagoon, Portugal

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Journal of Marine Systems 68 (2007) 507 – 528 www.elsevier.com/locate/jmarsys

Residual circulation and sediment distribution in the Ria de Aveiro lagoon, Portugal José Fortes Lopes ⁎, João Miguel Dias CESAM, Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal Received 13 September 2006; received in revised form 30 January 2007; accepted 2 February 2007 Available online 12 February 2007

Abstract A two-dimensional hydrodynamic and sediment transport models have been developed and applied to the Ria de Aveiro lagoon, Portugal. The application consisted in the study of the residual circulation, the residence time as well as the sediment dynamics in the lagoon. High residence time for particles situated at the far end of the main channels points out that they are retained for a long time and tend to remain there, with high probability of being deposited into the bed. Low residence time for the central areas implies that the particles are flushed out more rapidly toward the lagoon mouth, revealing that the water and the particles exchange are very effective there. Although the residual circulation due to the rivers induces an overall transport toward the lagoon mouth, the residual circulations due to the winds induce particular circulation patterns (clockwise or counter-clockwise eddies) at some locations of the lagoon, depending on the wind direction. It was found that the suspended sediment concentrations along the main channels are induced by tidal asymmetries (resulting in areas of ebb and flood dominance), as well as by the wind stress and rivers runoff effects. They contribute, in general, to increase the sediment export toward the ocean, although the wind stress may induce currents and circulations in the opposite direction of the tidal currents. High turbid zones are observed for strong tidal currents, associated with tidal asymmetries, as well as with high rivers runoff. The erosion–deposition budget indicates a tendency for sediment accumulation at important areas of the lagoon, namely the shallow ones. © 2007 Elsevier B.V. All rights reserved. Keywords: Cohesive sediments; Transport; Residual circulation; Residence time; Tides; Lagoon; Ria de Aveiro

1. Introduction Long-term residual currents play an important role in the transport of sediment, nutrients and organic matter from lagoons and estuaries toward coastal seas. Tidal residual flows in the coastal oceans are in general induced by non-linear interactions of tidal flow with

⁎ Corresponding author. Tel.: +351 234370821; fax: +351 234424965. E-mail address: [email protected] (J.F. Lopes). 0924-7963/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2007.02.005

variable bathymetry. Frictional torque and vorticity advection over an irregular seabed can also generate residual currents (Zimmerman, 1979). The generation process of overtides and compound tides, one of the dominant non-linear physical processes in many coastal areas, results from interactions between tidal flow and topography (Le Provost and Fornerino, 1985; Le Provost, 1991; Parker, 1991). These non-linear features may produce changes in the mean water surface elevation, flood-ebb asymmetries and long-term transport of materials, such as salt, pollutant, and sediments. Indeed, Allen et al. (1980), Uncles and Stephens (1989)

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and Dronkers (1986), among others, have observed that the asymmetries in the tidal currents, in which flood currents exceed ebb currents and high-water slack period exceed low-water slack period, play an important role in the resuspension of bottom sediments by tidal currents and in the formation of the turbidity maximum (ETM) in mesotidal and macrotidal estuaries. The immediate source of sediment to the water column within the ETM is usually from tidal resuspension of bottom sediments, which is enhanced during spring tide, resulting in large variations in the concentration of suspended sediments (Castaing and Allen, 1981; Geyer et al., 2001). A hydrodynamic model was previously applied by Dias (2001) and Dias et al. (2000b, 2003), to study the residual circulation inside the Ria de Aveiro, considering tide, wind and river runoffs as the main forcing. As the residual transport indicates the long-term fate of water and particles in a given sub-domain, this tool may be useful in the study of sediment transport in the lagoon. The study of the residual circulation improves not only the knowledge of the dynamics of the lagoon, but allows the qualitative estimation of the ultimate fate of passive particles released in the main areas of the lagoon. On the other hand, a sediment transport model which integrates the basic processes occurring in the water column and those resulting from the interaction between the currents and the bottom was applied by Lopes et al. (2006) in the study of the dynamics of cohesive sediments in the Ria de Aveiro lagoon. These two approaches have been, therefore, used in this study in order to have a better knowledge of the fate of sediments in the Ria de Aveiro lagoon. The main goal of this paper is, therefore, to study the influence of the main forcings, tide, river inputs and winds, in the sediment transport in Ria de Aveiro lagoon, in order to better understand its functioning and to assess the overall and the long-term transport of suspended particles. 2. The study area Ria de Aveiro (Fig. 1) is a lagoon situated on the north–west Atlantic coast of Portugal (40°38′N, 8°45′ W), 45 km long and 10 km large, supplied in freshwater by two main rivers, the Antuã river (5 m3 s− 1 average flow) and the Vouga river (50 m3 s− 1) (Dias et al., 1999). It has a very irregular and complex geometry, composed by long and narrow channels, with a high longitudinal development organized by successive ramifications from the mouth, as an arborescent network system. The bathymetry (Fig. 1) shows that Ria de Aveiro is a very shallow lagoon (average depth of 1 m). The deepest

Fig. 1. The geographic map and the bathymetry of the Ria de Aveiro lagoon.

areas of the lagoon are confined to the inlet channel and to small areas close to the lagoon mouth, at its western boundary, where the depths may reach values of the order of 20 m. In the navigation channels, that are frequently dredged, the depths are about 7 m. Elsewhere, the depths are, in general, smaller than 3 m, and most frequently close to 1 m, namely at the upper reaches of the lagoon. The lagoon regime is characterized by

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several periodic time-scales, the most important of which are the semi-diurnal (dominant tidal period) and the fortnight (one-half the lunar month and associated to the spring–neap cycle). The tidal period is the main period characterizing the sediment transport in the lagoon, but the lunar month determines the net erosion and deposition regimes, due to fortnight variability of the

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currents intensity, which reach extreme low and high values during neap and spring tides, respectively. Tides are the main forcing of the circulation in the lagoon. The estimated lagoon tidal prism is 136.7 × 106 m3 for maximum spring tide and 34.9× 106 m3 for minimum neap tide. The tidal prisms at S. Jacinto, Espinheiro, Mira and Ilhavo channels, relative to the tidal prism at the mouth

Fig. 2. Current speed field (m s− 1) in a spring tide situation (model forced with real tide): a) Ebb tide (high tide at the mouth); b) Flood tide (low tide at the mouth).

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are about 35%, 26%, 14% and 10%, respectively (Dias, 2001). The total estimated freshwater input is very small (about 1.8× 106 m3 during a tidal cycle) when compared with the mean tidal prism at the mouth (about 70 × 106 m3). The hydrodynamic pattern of the lagoon is, therefore, imposed by the tide, its effect being detected even at the far end of each channel (Dias, 2001). Rivers, in overall, have small contribution in terms of water inputs when compared with the tidal prism, but may have a long-term influence in the residual transport. Winds affect the hydrodynamical regime and the mixing processes in the lagoon by generating surface shear stress and waves. Along a land boundary located at the downwind end of the lagoon, the surface stress piles up the water generating surface currents and elevation of the water surface. Extreme conditions of strong wind may induce particular circulation patterns, mainly in shallow areas and wide channels. From a dynamical point of view, the most important channels are the S. Jacinto and the Espinheiro channels. They are directly connected to the lagoon mouth and have the strongest currents, reaching values of about 2 m s− 1 near the mouth, during spring tides and flood period (Fig. 2). Other channels are dominated by shallow and tidal flat areas, and are characterized by very irregular geometry, which contributes to a strong damping of the currents and an increase of the phase delay of the tidal wave. The hydrological characterization of the Ria de Aveiro (Dias et al., 1999; Dias, 2001) reveals that, excluding the areas close to the lagoon mouth, where some stratification patterns in the salinity and water temperature are observed, the lagoon is a well-mixed system. No significant gravitational circulation is expected in the lagoon (Dias et al., 2000a; Dias, 2001; Lopes et al., 2006), since the gravitational circulation should be suppressed by increased vertical mixing, mainly during spring tides, as explained by Walters and Gartner (1985). The mechanism of advection and shear-flow dispersion is therefore the dominant mechanism in Ria de Aveiro, as occurring in most of well-mixed and partially stratified estuaries (Fischer, 1972). The general circulation of the Ria de Aveiro has been studied by Dias (2001), Dias et al. (2000a, 2001) and Dias and Lopes (2006a,b) applying a two-dimensional hydrodynamic depth integrated model. In particular, each channel of the lagoon has been characterized from the hydrodynamic point of view. The results show that tides are strongly deformed in their propagation through the Ria de Aveiro, due to the channels geometry and bathymetry. The general characteristics of the tidal wave in the lagoon are those of a damped progressive wave. In the deep zones, close to the mouth, it behaves like a progressive wave. As the tide propagates towards the

shallow areas, the wave characteristics shift to that of a standing wave. The tidal amplitude of the wave decreases and the phase lag increases from the mouth toward the far end of the channels. The amplitude ratio between the M2 semi-diurnal constituent and the S2 semi-diurnal solar constituent is of the order of 4 in most areas of the lagoon (Dias, 2001), evidencing the dominance of the lunar tide. In order to assess the tidal asymmetry of the lagoon, the amplitude ratio RA between the fourth-diurnal M4 constituent (caused by non-linearities due to bottom friction and continuity constrains) and the semi-diurnal M2 constituent have been calculated (Dias, 2001). The results reveal, that near the lagoon mouth RA is very small (close to 0) and increase to values close to 0.2 toward the far end of the lagoon, whereas phase differences are close to 300°, for the first half of the main channels, and lower than 90°, for the second half of the main channels and toward its far end. Therefore, the tidal asymmetry increases from the lagoon mouth toward the upper reaches of the lagoon. On the other hand, the overtide constituent M4 becomes important when the ratio of the tidal amplitude to the mean depth, AM2 / h, is large. Flood-dominant estuaries are typically shallow (AM2 / h N 0.3) with small to moderate areas of tidal flats, as it is the case of the second half of the main channels of the lagoon. On the other hand, low velocities in intertidal marshes and flats also cause high tides to propagate slower than low tides. At low tide, in fact, marshes and flats are empty while channels are still relatively deep, allowing a faster exchange. Ebb dominance is thus favoured by large σs / σc ratios (where σs is the intertidal storage area occupied by tidal flats and marshes and σc is the channel area covered by water at mean low tide) and tend to be deeper (AM2 / h b 0.2), which is the case of the first half of the main channels of the lagoon. The nature and the distribution of sediments in the Ria de Aveiro are extremely variable. The bottom sediment distribution of the Ria de Aveiro consists of a mixture of mud and sand. The granulometric composition of sediments is distributed between 2 to 90% of sands, 10 to 80% of silts and 0 to 30% of clays. Meanwhile, there are some differences between the north and the south lagoon bed composition. Fine cohesive particles and sands, mainly, compose the northern channels, whereas the southern channels are almost composed by coarse particles and sands. A gradient of the bed composition can be, therefore, observed at each channel of the Ria de Aveiro, with sand near the mouth and a mixture of sand and mud near its far extremity. Dias (2001) has calculated, from measured vertical profiles of currents, the bottom roughness length, z0, of different channels.

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Typical values, varying from 0.02 cm at the far end of the channels, corresponding to a bed, composed predominantly by mud or a mixture of mud and sands, to 0.07 cm near the mouth, corresponding to a bed composed predominantly by sand, have been found. The presence of a mixture of sands, gravel, shells and mud in the main channels of the Ria de Aveiro denotes the ocean influence in the lagoon. Nevertheless, the lagoon bed can be considered cohesive in accordance to the critical mud content criteria of 4% (Torfs, 1995; Mitchener and Torfs, 1996). 3. The hydrodynamic and the sediment transport equations In this approach the classical two-dimensional vertically integrated hydrodynamic (Leendertse and Gritton, 1971) and transport equations (Mike21, 1996) are used: Af AðHU1 Þ AðHU2 Þ þ þ ¼0 At Ax1 Ax2
2  AU1 AU1 AU1 Af ssx1 −sbx1 A U1 A2 U1 þ U2 −fU2 þ g − þ þ U1 −Ah ¼0 2 2 Ax1 At Ax1 Ax2 qH Ax Ax2
21  s b 2 AV AU2 AU2 Af sx2 −sx2 A U2 A U2 þ U1 −Ah þ U2 þ fU1 þ g − þ ¼0 At Ax1 Ax2 qH Ax21 Ax22
Ax2  AC A HDij AHC AðHU1 CÞ AðHU2 CÞ Axi þ þ − ¼ SE −SD Axj At Ax1 Ax2

ð1Þ where U1 and U2 are the depth integrated current speed components in the x1 (eastward) and the x2 (northward) directions, respectively: U1 ¼

1 H

Z

f −h

u1 dz

U2 ¼

1 H

Z

f

−h

u2 dz

ð2Þ

ζ is the surface water elevation, h is the undisturbed water depth, H = h + ζ, u1 and u2 are the two components of the flow speed along the x1 and x2 axis, respectively, t is the time, f is the Coriolis parameter, g is the acceleration of gravity, ρ is the water density, Ah is the kinematic turbulent horizontal viscosity, τs and τb are, respectively, the magnitude of the wind stress on the water surface and the magnitude of the shear stress on the bottom caused by the flow of water over the bed. The horizontal diffusion term is of secondary importance, being kept only for numerical reasons (Dias, 2001). The diffusion coefficient Ah depends on the grid step. In this work values of 20 m2 s− 1 for Ah and of 100 m for the spatial step in both x1 and x2 directions were adopted. C is the concentration of the suspended particles expressed in mg l− 1, Dij represents the turbulent dispersion coefficients and has been assumed constant (5 m− 2s− 1),

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for the two directions of the space. The source terms SE and SD characterize, respectively, the erosion and the deposition processes in the water column. The relationship between the wind stress and the surface wind may be expressed by the following empirical formula (Dronkers, 1964): sbx1 ¼ qa CD W 2 sena

sbx2 ¼ qa CD W 2 cosa

ð3Þ

where W is the wind speed (generally measured 10 m above the sea surface), α the wind direction, ρa the air density and CD the drag coefficient, which depends linearly on the wind speed and on the roughness of the water surface (Bowden, 1983). The bottom shear stress is an important parameter in the study of sediment dynamics in the water column. It is assumed to be proportional to the square of the horizontal velocity: sbx1 ¼ qg sbx2

U1 ðU12 þ U22 Þ1=2 CZ2

ð4Þ

U2 ðU12 þ U22 Þ1=2 ¼ qg CZ2

where CZ is the Chezi coefficient: p ffiffiffiffiffiffiffiffiffiffiffi 6 hþf CZ ¼ n

ð5Þ

and n is the Manning roughness coefficient (m1/6). In order to solve the transport equation for cohesive sediments, the source and the sink terms SE and SD, must be defined. These terms are related to the physicochemical processes involving cohesive sediments in the water column, namely the erosion, the deposition and the flocculation of particles (silt and clay particles). In the model the deposition process occurs only when the bottom stress τb is lower than a threshold stress τCD. When τb is greater than τCD there is no deposition and SD is set to zero. The deposition rate has the following formulation (Krone, 1962; Mehta, 1986):


sb SD ¼ WS Cb 1− sCD SD ¼ 0

 sb bsCD

ð6Þ

s NsCD b

where WS is the settling velocity in m s− 1, Cb the nearbed particles concentration in mg l− 1, τb and τCD are, respectively, the bed shear stress and the critical shear stress for the deposition in N m− 2.

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In general WS is not constant but depends on the concentration (hindered settling) (Dyer, 1986): WS ¼ k1 C m ð1−k2 ðC−Chs Þms Þ

ð7Þ

The erosion rate SE is associated with the erosion by currents and depends on the degree of the consolidation of the bed. In the model the erosion process occurs only if the bottom stress τb is greater than a threshold stress τCE (Parchure and Mehta, 1985). When τb is lower than τCE there is no erosion and SE set to zero. Hayter and Mehta (1986) have proposed for dense and consolidated beds:
b  s SE ¼ E −1 sb NsCE ð8Þ sCE SE ¼ 0 sb bsCE where E and τCE are, respectively, the erodability of the bed and the critical shear stress for the erosion. Specific values have been assigned to the constants and the parameters presented in Eqs. (6)–(8) in order to validate the model (Lopes et al., 2006). The following values have been chosen: E = 2 × 10− 5 kg m− 2s− 1, τCE = 0.2 N m− 2 and τCD = 0.1 N m− 2 (Dyer, 1986, 1995), k1 = 0.00001 m s− 1 mg− 1L1 and Chs = 1000 mg l− 1. Dyer (1986) proposed the following values: m = 1.0, ms = 4.65. In this study C is, in general, always small compared with Chs, and therefore k2 has been chosen to be null. The hydrodynamic equations (Eq. (1)) were discretized using a finite difference method. The difference equations were solved by the ADI method (Alternating Direction Implicit), using a space-staggered grid (Leendertse and Gritton, 1971; Dias, 2001). The problems of the staggered grid arise due to the spatial averaging of the Coriolis terms, in the momentum equation. The model approximates the velocity components at a given grid point, by averaging the velocities for the four neighbouring grids points (Leendertse and Gritton, 1971). This approximation may induce some uncertainties in the narrow passages of the lagoon. Nevertheless, as they are located at shallow areas, namely the central and the far end areas, the momentum balance results primarily from the non-linear interaction of the tidal currents with the local bathymetry. The main physical processes driving the flow at those areas are, therefore, the horizontal pressure gradients, the momentum advection, and the bottom friction. On the other hand, as the water depth becomes increasingly small, far from the lagoon mouth, the bottom friction tends to increase and the flow is increasingly damped. It results that the current intensity tends to be small, and

therefore, the Coriolis effect plays a minor role in the momentum balance at the narrow passages located at the shallow areas. With appropriate boundary and initial conditions, the system of equations constitutes a wellposed initial boundary value problem, whose solution describes the depth-averaged circulation in a tidal basin. A dynamic water elevation at the ocean open boundary was imposed by using pre-determined tidal harmonics constituents. Constant current velocity was imposed at the rivers' boundaries. The initial conditions were horizontal level and null velocity in all the grid points. Along the solid boundaries a null normal velocity was imposed and a free slip condition was assumed. The hydrodynamic model treats also the shallow water flats in a mass consisting way, which are sometimes dry and occasionally covered with water, during a tidal cycle. During the dry periods the grid cells representing these areas are taken out of the algebraic system and added later on, once the adjoining water level is higher than the water inside the dry grid cell. The specific implementation adopted here conserves the mass in each grid cell (Leendertse and Gritton, 1971; Dias et al., 2000a; Dias, 2001; Dias and Lopes, 2006a,b). As the sediment transport model (represented by the last equation of the system of equations (Eq. (1)) is coupled to the hydrodynamic model, concerning the sediment concentrations the dry and the wet cells behave in accordance to 4 and 6, and no specific treatment is needed. The conservation of the transported constituent is essential in the advection–diffusion equations of the transport equation (Eq. (1)), where production or Y Y dissipation rates are small. The term ∇dðH UCÞ represents the advective contribution of the transport Y equation, where the symbol ∇ corresponds to the operator divergence. In this case the advective term may be approximated by finite-differences in a conservative form (Roache, 1985). The transport equation in the model uses the Flux Corrected Transport algorithm (FCT) in order to discretize the advective term (Leonard, 1979). The hydrodynamic model used in this work has been calibrated for the Ria de Aveiro by Dias (2001) and Dias and Lopes (2006a,b). The calibration procedure of the hydrodynamic model was performed with the help of the bottom stress, through a mapping of the Manning roughness coefficients. It has consisted of adjusting the Manning coefficient in order to obtain a distribution of stress allowing a good agreement between calculated and observed water level for 21 stations. The comparison between the harmonic constants of the main tidal constituents, determined from simulated time series of water level, and those determined from observed data,

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for 21 stations distributed throughout the main channels of the lagoon, completed the calibration procedure. The validation process of the hydrodynamic model has been achieved by comparing simulated water level and currents with data. The 11 stations and data used in this procedure were independent from the calibration procedure. The results (Dias, 2001; Dias and Lopes, 2006a,b) allow concluding that the hydrodynamic model simulates very well the water level and the currents in the lagoon, with an RMS of around 5% of the local tidal range for almost all the stations. Concerning currents at deeper and larger channels an RMS of around 7% has been obtained. The parameters involved in the calibration of the sediment model were chosen in accordance with the qualitative information concerning the distribution of bottom sediments (Dias, 2001; Lopes et al., 2006). Indeed, the Ria de Aveiro bed is characterized by a nonuniform horizontal distribution of bottom sediments, the bed composition varying from a mixture of sands, gravel and shells (near the mouth) to a mixture of sands and mud (at the far end of the channels). Dias (2001) has deduced with the help of observed vertical velocity profiles, the drag coefficient values for different lagoon locations. For the drag coefficient, CD, he found a spatial variation from the lagoon mouth to the far end of the channels, from 0.0066 (ripped sands, corresponding to a roughness length, z0, of 0.6 cm) to 0.0022 (mud, corresponding to a roughness length, z0, of 0.02 cm). The CD is defined by a classical ‘law of wall’: 2

32 j CD ¼ 4 h i5 ln zz1000

ð9Þ

where z100 is the roughness length based on a velocity measured 100 cm above the bed. It is, therefore, more accurate to define the parameters involved in the physical process related to the sediments as a function of the horizontal co-ordinates, namely the erodability E(x,y) and/or the critical shear stress τCE. Smaller values of erodability (0.5 × 10− 5 kg m− 2s− 1) have been assigned to the southern subdomains of the lagoon, close to the lagoon mouth, whereas for the northern sub-domain the erodability was allowed to vary between 0.7 × 10− 5 kg m− 2 s− 1 and 1.5 × 10− 5 kg m− 2 s− 1, with maximum values assigned to the northern and eastern end. Annual mean suspended concentration observed from 1989 to 1993, for 12 stations distributed throughout the main lagoon channels (Alcântara et al., 1992), was used for the calibration of the sediment

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transport model. The main calibration parameters are the erodability, which reflects the spatial variability of the bed, and the settling velocity. The best values for the critical stress for erosion, τCE, and for deposition, τCD, are 0.2 N m− 2 and 0.1 N m− 2, respectively (Lopes et al., 2006). A fine tuning of the erodability and the settling velocity was needed to better achieve the calibration. 4. The residual circulation The residual circulation [U1r, U2r] can be obtained from the eulerian depth integrated velocity, calculated with the help of the hydrodynamic model (Eq. (1)). As the total eulerian depth integrated velocity [U1, U2] can be decomposed in the sum of a periodic component due to the tide [U1t, U2t] and a residual component [U1r, U2r] then: ½U1 ; U2  ¼ ½U1t ; U2t  þ ½U1r ; U2r 

ð10Þ

There are two different approaches to determine the residual circulation, using numerical models (Heaps, 1978): – filtering or time averaging the transient solution computed by the model (Marinone, 1997; Umgiesser, 1997); – solving numerically the residual velocity evolution equations, which are obtained by time averaging the transient velocity evolution equations (Nihoul and Ronday, 1975; Salas-de-León et al., 2003). The first approach is largely adopted in the literature, and, therefore, it will also be used in this study, as it easy to implement, and it only requires a statistical treatment of the model outputs. On the other hand, the second approach requires the development of a set of complex equations and the inclusion of a new term defined by Nihoul and Ronday (1975) as the ‘tidal stress’. Furthermore, it also requires the development of a numerical model in order to solve the set of new equations. Using the first approach, the residual velocity is calculated by time averaging the transient solution obtained from the hydrodynamic eulerian model: ½U1r ; U2r  ¼

1 T

Z

T =2 −T=2

½U1 ; U2 dt

ð11Þ

T is the tidal period, here the cut-off period of the averaging operator, such that most of the variability due to tidal motion is cancelled out.

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5. Results 5.1. Residual circulation The residual circulation in the Ria de Aveiro lagoon has been obtained by applying the numerical hydrodynamic depth integrated model previously calibrated and validated by Dias and Lopes (2006a,b), considering the following forcings: tides, typical winds and rivers inflow. The residual circulation is, therefore, obtained from the hydrodynamic eulerian model transient solution through the application of Eq. (11). The main goal of this section is to compare the influence in the residual circulation of a semi-diurnal M2 tide, representing a theoretical tide at the lagoon mouth, to the influences of the two major forcings, the winds and the river freshwater input. 5.1.1. Residual circulation induced by tides The semi-diurnal M2 tide, with an amplitude of 0.96 m and a period of 12 h 25 m has been imposed at the open boundary. Fig. 2 evidences the flood and the ebb regime, inside the lagoon. The tidal currents are stronger along the S. Jacinto and the Espinheiro channels. They also present high values along narrow channels and entrances, namely at Varela and Laranjo entrances. The residual circulation was computed by time averaging the transient values over a period of 12 h 25 m. The residual currents induced by tides presented in Fig. 3 are about two orders of magnitude lower than the tidal currents. They tend to flow downstream toward Barra and are strong along the first half of the S. Jacinto and the Espinheiro channels, but their intensities weaken toward the lagoon far ends. The strength of the residual currents intensity at the S. Jacinto and the Espinheiro channels reflects some important feature of these areas: tidal asymmetries, ebb dominance, highwater depth and strong tidal currents intensity. 5.1.2. Residual circulation induced by rivers The residual circulation induced by rivers was computed by forcing the hydrodynamic model with a semi-diurnal M2 tide, of amplitude of 0.96 m and period of 12 h 25 m, and considering a situation of high river runoff, which is characteristic of the winter or the autumn seasons. The wind stress was set null at the lagoon surface. As stated previously, the freshwater inflows imposed for the Vouga, the Antuã and the Boco rivers were 350 m3/s, 35 m3/s and 2 m3/s, respectively. At the far end of the Mira channel a freshwater inflow of 35 m3/s was imposed.

Fig. 3. Residual currents (m s− 1) induced by the tide (model forced with M2 tide at the open boundary).

The residual currents induced by river freshwater inputs, presented in Fig. 4, are very low when compared to the tidal currents and are directed downstream to the Barra, contributing to a net water export toward the ocean. When compared to other lagoon areas, these currents are stronger at Laranjo, and along the Espinheiro channel, due to the proximity of the Antuã

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currents are very small, due to the shallowness of the area, the low intensity of the currents, as well as the low river freshwater input. Along the Mira and the Ilhavo channels a northward current is also present, even though small. Dias (2001) calculated the residual transport induced by rivers along the main lagoon channels (S. Jacinto, Espinheiro, Ilhavo, Mira and the inlet channel of the lagoon mouth), at sections (5, 6, 7, 8) located in areas close to the lagoon mouth. The residual flows induced by the Vouga and the Antuã rivers were of the order of 8 m3s− 1 and 5 m3s− 1, respectively, at the Espinheiro section. On the other hand, the residual river flow at the lagoon mouth section was found to be of the order of 15 m3s− 1, denoting small contributions of the other rivers. 5.1.3. Residual circulation induced by winds The residual circulation induced by winds was computed by forcing the hydrodynamic model with characteristic winds for the Aveiro region. The tide used in the simulation was the semi-diurnal M2 tide, with an amplitude of 0.96 m and a period of 12 h 25 m.

Fig. 4. Residual currents (m s− 1) induced by the rivers (model forced with M2 tide at the open boundary).

and the Vouga rivers, respectively. Two distinct branches are evident: the current issued from the Vouga river flows directly toward the lagoon mouth, along the Espinheiro channel; the current issued from the Antuã river flows westward, reaches the S. Jacinto channel and then turn southward toward the lagoon mouth. North of Torreira and Varela, the residual

5.1.3.1. The spring and summer situations. N and NW winds are the most characteristic winds for the spring and the summer season for the Aveiro region. This wind regime may extend throughout late summer and autumn. The N wind mean intensity was found to be of the order of 16.9 km/h for spring and 13.1 km/h for autumn, whereas for summer an NW mean intensity of 14.7 km/h has been observed (Dias, 2001; Dias et al., 2000b). These winds induce northward currents, which start at S. Jacinto and flow along the S. Jacinto channel. These currents are also observed along the western coasts of the channel, near Torreira (Fig. 5). A southward current is observed flowing along the eastern coast, from Varela and Torreira, then turning to the east toward Laranjo. On the other hand, along the Espinheiro channel, a southward current is generated and tends to flow toward the lagoon mouth. Some eddy structures, generated by the interaction of the currents with the geometry and the bottom topography, are observed at particular areas of the lagoon. At the northern far end of the S. Jacinto channel an eddy is noticeable and seems to be generated by the shear between the two currents reaching this area, the one from the S. Jacinto channel and the other from Torreira, generating a closed anti-clockwise circulation. At S. Jacinto (the southern part of the S. Jacinto channel), another clockwise eddy circulation is noticeable and seems to be generated by the interaction of two currents, the one flowing from the lagoon mouth and the other flowing toward the Espinheiro channel. Indeed, this area

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Fig. 5. Residual currents (m s− 1) induced by the wind (model forced with M2 tide at the open boundary): a) Autumn wind (13.1 m/s; N) b) Summer wind (14.7 m/S; NW c) Spring wind (16.9 m/s; N) d) Winter wind (14.7 m/s; SE).

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or reinforce the eddy. At the Mira channel a clockwise eddy can be observed near the Costa Nova and a southward residual current flowing from this location toward the far end of the channel. The eddy structures are, therefore, generated by the interaction of the currents with the bottom topography and are strongly influenced by the morphology of the area. Dias (2001) calculated the residual transport induced by spring and summer winds at the same sections as in the case of rivers. The residual flows induced by a NW wind were of the order of 5 m3s− 1 toward the north and 3 m3s− 1 toward the south, at the Espinheiro and the S. Jacinto sections, respectively. These values are relatively high when compared to those of the remaining sections (of the order of 1 m3s− 1).

Fig. 6. Contour of tidal-mean residence time (days) for passive particles released in each grid cell.

has a complex geometry and topography, and it is located near a sand bar (Dias, 2001) that covers and uncovers along the tidal cycle, deflecting the flow, originated from the mouth, eastward and then northward. As the flow encounters land at the eastern side, a close counterclockwise circulation is established. It should be pointed out that the flood residual current flowing along the S. Jacinto channel and the ebb residual current flowing along the Espinheiro channel may, also, contribute to generate

5.1.3.2. The winter situation. S and SE winds are the most characteristic winds for the winter season in the Aveiro region. Their mean intensity is of the order of 14.7 km/h (Dias et al., 2000a,b; Dias, 2001). They induce southward currents, which start from Torreira and flow along its western coast and then follow the western coasts of the S. Jacinto channel, toward S. Jacinto (Fig. 5). On the other hand, westward residual currents generated at Laranjo, flow westward and then turn to the north toward Torreira, where they flow along its eastern coast. Along the Espinheiro channel, a north– eastward current is generated and tends to flow toward the far end of the channel. As in the case of the N–NW winds, eddy structures are also observed. At the northern far end of the S. Jacinto channel an anticlockwise eddy is apparent and seems to be generated by two currents crossing there, the one flowing toward the S. Jacinto channel and the other coming from Laranjo and veering toward Torreira. At S. Jacinto a counter-clockwise eddy can be observed. In this case the flood residual current flowing along the Espinheiro channel and the ebb residual current flowing along the S. Jacinto channel contribute to generate or reinforce the eddy. At the Mira channel, a counter-clockwise eddy can be observed near the Costa Nova and a northward residual current flowing from the far end of the channel toward this location. Dias (2001) calculated the residual transport induced by winter winds at the same sections as in the case of rivers. The residual flows induced by a SE wind at the Espinheiro section and the S. Jacinto section, respectively, were of the same order as for those induced by the NW wind, with the directions reversed. 5.2. The residence time Residence time is defined by Dronkers and Zimmerman (1982) as ‘‘the time it takes for any water parcel of a

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Fig. 7. Concentration of suspended cohesive sediments (mg l− 1) at stations located in the main lagoon channels, and for different particles size: P1) 0.15 mm; P2) 0.25 mm (model forced with a real tide alone).

sample to leave the lagoon through its outlet to the sea’’. It is, therefore, the time that a water parcel starting from a specified location within a waterbody remains in the

system before exiting from the domain. Residence time provides a fair method for quantifying spatial heterogeneity in the distribution of particles and how long those

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particles spend in the system before exiting (Monismith, 2002), reason why this approach is applied to the Ria de Aveiro, in order to better understand the particles fate from its original areas. The residence time was computed using a Lagrangian particle tracking module coupled to the hydrodynamic model (Dias et al., 2000b; Dias, 2001; Dias et al., 2001, 2003). Two-dimensional particle trajectories were computed from eulerian velocity fields, using a Lagrangian approach, and a fourth-order Runge-Kutta scheme for the time integration of the trajectories. In this model, particles were considered passives, that is, besides the advection, no other physical or chemical processes (e.g. sinking, dissolution, aggregation) were allowed. As the hydrodynamic model calculations are performed in a discrete grid, but the particles are free to move to any position within the grid cells, the velocities obtained from the hydrodynamic model must be interpolated to the instantaneous particles positions, before the integration of the trajectories is performed. The interpolation is carried out by averaging the velocities computed at the nearest four grid points, weighed by the reverse of the distance from the particle position to each one of the grid points. Fig. 6 presents the computed residence times, determined by averaging the results over 12 separated model runs, with particles released at consecutive lunar hours, and under the single influence of the tides. For each run, one particle was released at each grid cell and tracked for up to 14 days. The mapping of the residence time allows a classification in three main areas: (1) the areas close to the lagoon mouth, including the S. Jacinto and the Espinheiro channels, which have the lowest values (approximately 1 day); (2) the central part of the lagoon, which presents values varying between 2 and 6 days; (3) the extreme end of the channels which can reach values close to two weeks. In most of the lagoon areas, namely those close to the lagoon mouth and the central areas, the particles have a short-term trend to escape the domain, whereas in the areas situated at the extreme end of the channels the particles are retained for long time and tend to remain there. 5.3. Tide, rivers and wind induced sediment transport The cohesive suspended sediment model has been applied to simulate the tidal evolution of suspended sediments in the lagoon, for a period covering four months (from 1st January 1994 to 1st Mai 1994). In order to assess the independent contribution of tides, rivers input and wind stress in the cohesive

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sediment distribution in the lagoon, the model was applied to study the following cases: – forcing by real tide, considering no river input and no wind stress; – forcing by real tide and rivers, considering no wind stress. The rivers input were set to a value twice the rivers mean flow, as could be the case during the very rainy spring season. – forcing by real tide and wind stress, considering no river input. Monthly mean values of the surface wind intensity, obtained from meteorological stations situated at S. Jacinto and at the University of Aveiro, show that the Aveiro coast is characterized by a north or a north–west wind, during spring. In this study the wind direction was kept constant from north–west, but the intensity was considered variable during the day, from 12 km h− 1 at 9 h to 21 km h− 1 at 21 h. Initially all points of the lagoon have been set to a concentration of 10 mg l− 1. The concentrations at the river boundaries were set to 100 mg l− 1, whereas those at the open boundaries, considering the dilutive effect of the ocean, were set to 1 mg l− 1. The temporal series of suspended sediment concentrations induced by rivers was obtained subtracting the model results considering the tidal forcing alone, to the model results considering rivers input and tidal forcing together. The suspended sediment concentration temporal series induced by winds were obtained using the same methodology. 5.3.1. The tidal influences Analysing the temporal series of suspended sediment concentrations at each individual station (Fig. 7), and the maps of the suspended sediment concentrations field in the Ria de Aveiro (Fig. 8), it can be observed the tidal pattern in the evolution of the suspended sediment concentrations in the lagoon. The semi-diurnal and fortnight patterns are well represented in the temporal series of suspended sediment concentrations, with sediment concentrations reaching relative maximum and minimum values twice a day and the absolute maximum and minimum values twice a month. The most outstanding picture of the sediment concentration maps is the presence of a turbid zone that covers a large area between the C. Bico and the Laranjo and at S. Jacinto channel, which is very noticeable during the spring tide. It is characterized by a relatively high turbid core and a smooth concentration gradient in the east–west direction. This core is not stationary, being advected in the eastern–western

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Fig. 8. Suspended sediment concentrations field (mg l− 1) in a spring tide situation (a) and neap tide situation (b), with no rivers input and no wind stress (model forced with a real tide).

direction following the tidal currents cycle, and it is the result of the conjunction of two factors: – the funnelled narrow entrance connecting the C. Bico to the Laranjo, that generates strong currents during both the flood and the ebb tides; – the tidal asymmetry due to the shallow area between C. Bico and Laranjo, resulting in a strong tidal currents during ebb (Fig. 2), enhancing, therefore, the erosion.

During the neap tides the turbid zone formed between the C. Bico and the Laranjo is still present, but less important, and almost vanishing during slack waters. A similar pattern is observed along the Ovar channel, in the very shallow area between Varela and Torreira, but in a smaller scale. The S. Jacinto and the Espinheiro are the deepest channels and experience very strong currents. Sediment

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Fig. 9. River and wind relative influence in the monthly temporal evolution of suspended sediment concentrations (mg l− 1) at stations located in the main lagoon channels (model forced with a real tide).

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concentration gradients are generated in these channels and propagate along them, following the tidal cycle. The suspended sediment concentrations in the lagoon are, in general, much higher during the spring tide due to the high intensity of the currents. Meanwhile, it is worth to point out the presence, during ebbing, of a high turbid zone at Torreira, Varela and C. Bico, generated by the strong ebb currents. 5.3.2. The rivers influence The temporal series of ‘relative concentration’ (Fig. 9) and the map of suspended sediment concentration field (Fig. 10), show the influence of river runoffs on the suspended cohesive sediment concentration. The ‘river time series’ of Fig. 9 were constructed using the difference between the suspended sediment concentrations, corresponding to simulations forced by tide and river, and those corresponding to simulations forced by tide alone. The term ‘relative concentration’ means, therefore, that the tidal influence in the sediment concentrations was filtered out from the time series. The evolution of the concentration pattern follows the tidal cycle and shows that the tidal currents are the main forcing in the transport of suspended sediments. The rivers' influence is more important near the river mouths (stations 1, 2, 3), but during spring tides and ebbing, when the currents and the transport increase their intensity, a large amount of sediments is mobilised to the water column and transported toward the S. Jacinto channel and the lagoon mouth. The Vouga river influence, concerning the suspended sediment transport, is mainly restricted to the Espinheiro channel. Indeed, this river does not input directly into the Espinheiro channel but is rather deflected northward and southward into a complicated ramified structure with several intertidal zones, contributing to a deceleration of the river flow and to a decrease of the river sediment input to the lagoon. During spring tides, the Vouga river contribution is enhanced, as can be observed in Figs. 9 and 10. In particular, during the ebb, a large amount of sediments is exchanged with the ocean through the plume that emerges from the mouth of the lagoon. The persistence of the outflow plume at the lagoon mouth, at the beginning of the flooding, in the situation of high river input, is an evidence of the ebb dominance of the first half of the S. Jacinto channel. The Antuã river flows into the Laranjo but has a negligible contribution in terms of water and sediment input to the lagoon, except near the river mouth, at the Laranjo and the C. Bico areas. Meanwhile, in situation of spring tides and high rivers inputs, the contribution of the Antuã river is enhanced

and may affect areas far from it. The concentration at the Laranjo follows the tidal cycle; during the flood cycle, the turbid front originated at the C. Bico penetrates into the Laranjo through the funnelling narrow entrance between the C. Bico and the Laranjo, whereas during the ebb period the front leaves the Laranjo toward the C. Bico. In both cases a strong convergent flow, associated

Fig. 10. Suspended sediment concentrations field (mg l− 1) in a spring tide situation, with rivers input and no wind stress (model forced with a real tide).

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with the tidal asymmetry induces important bed erosion, generating a high turbid zone, covering an area from Laranjo toward the C. Bico, during both flood and ebb. Meanwhile, in a situation of spring tides and during ebbing the high turbidity zone extends far westward, reaching the S. Jacinto channel, evidencing the ebb dominance of C. Bico. Halfway the S. Jacinto channel a high turbidity zone develops and follows the tidal cycle in the north–south direction. Even though the intensity of currents at the S. Jacinto channel is important, this high turbid zone is only evident in situations of spring tides and high river runoff. The extension of this turbid zone northward of the S. Jacinto channel evidences the flood-dominance pattern driven by tidal asymmetries on the second half of S. Jacinto channel. 5.3.3. The wind stress influence The temporal series of ‘relative concentration’ presented in Fig. 9 and the map of suspended sediment concentration field (Fig. 11) show the influence of the NW wind on the suspended cohesive sediment concentrations. Similarly to the ‘river time series’ the ‘wind time series’ of Fig. 9 were constructed using the difference between the suspended sediment concentrations, corresponding to simulations forced by tide and wind, and those corresponding to simulations forced by tide alone. The temporal evolution of the sediment concentrations reveals a trend modulated by tides, with the ‘relative concentration’ reaching alternatively negative and positive values, namely at the S. Jacinto and the C. Bico. The wind stress effect may, therefore, contribute to decrease the suspended sediment concentrations, as has been observed when analyzing the wind induced residual circulation. This result can be explained considering that when currents generated by the wind are in the opposite direction of the tidal currents, wind stress may contribute to decrease the tidal induced advection of the suspended sediment particles. This situation can occur at the S. Jacinto during the flood period, as well as at the C. Bico during the ebb period when the wind direction is from north–west. When the wind induced current is in the same direction as the tidal current, as it is the case during the ebb period at the S. Jacinto, there is an increase of the bottom erosion as well as of the net advective transport, leading to an increase of the concentration of suspended sediments. Ruessink et al. (2006) showed that the primary effect of wind in shallow waters is to add a mean flow to the tide which interacts with the tidal flow, distorting the principal M2 component and modifying the M4 and M6 generation mechanisms. Davies and Lawrence (1994) found that

Fig. 11. Suspended sediment concentrations field (mg l− 1), in a spring tide situation, with no rivers input and an NW wind (model forced with a real tide).

the magnitude of the non-linear effects was smaller in situations when the wind-induced flow exceeded the tidal current amplitude than in situations when the tidal current amplitude was the dominant flow component. Ruessink et al. (2006) observed for Terschelling, (Netherlands) that when the wind-driven flow is in the flood direction, wind and tide interact in a non-linear

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way, resulting in the reduction of the tidal amplitude and the distortion of the tidal shape, therefore, leading to an increase of the surface friction (at the interface between air and water) during flooding and its decrease during ebbing. Surface friction is, though, enhanced during the flood phase more than it is reduced during the ebb flow, increasing, therefore, the overall surface friction over a tidal cycle. Thus, when wind blow in the flood direction, the wind-driven flow increases the surface friction, resulting in a change of the tidal flow regime, which advances in time the maximum flood flow and delays the maximum ebb flow. The non-linear effects induced by the winds are, therefore, more pronounced in shallower water, namely at northern areas situated along the S. Jacinto channel, but also at those situated between C. Bico and Laranjo. Considering the north– west wind, during flooding, it can be concluded that between C. Bico and Laranjo (Fig. 2), not only the maximum flood flow will be advanced in time but also its intensity will be increased, whereas along the S. Jacinto channel, during flooding, the opposite situation occurs. On the other hand, during the ebb situation at the S. Jacinto channel (Fig. 2), the maximum ebb flow will be forwarded and the flow intensity increased. As has been referred in Section 2, the non-linear interactions with the bottom induce an increase of the tidal asymmetry, resulting in a decrease of the amplitude of the M2 constituent and the generation of overtides. The amplitude ratio, RA, between the fourth-diurnal M4 constituent (caused by non-linearities due to bottom friction and continuity constrains) and the M2 constituent will be, therefore, reduced. In the shallow areas of the lagoon, these non-linearities may result in an increase of the asymmetry between the flood and the ebb bed frictions, amplifying the non-linear interaction at the bottom. As the bed friction increase during the flood phase more than it is reduced during the ebb flow, it results in an overall increase of the bed friction over a tidal cycle. 6. Discussion and conclusions In coastal systems the transport of conservative substances is a long-term process. The water circulation and transport in estuaries and lagoons are mainly tidal dependent, showing a back and forth flow. The residual circulation is an averaged flow, mainly resulting from the asymmetries between the flood and ebb regimes and the influence of the winds and the river runoff. Its intensity is one or two orders of magnitude lower than the tidal currents intensity. Non-linear effects can produce residual currents due to the oscillation of the

tides. The currents are influenced by advective acceleration, frictional damping and Coriolis effects (Tee, 1976; Greenberg, 1989). The importance of the wind residual circulation has been demonstrated by several authors, namely Cheng and Casulli (1982) and Cheng et al. (1993) for the South San Francisco Bay and Proctor and Greig (1989) for the Hauraki Gulf. Wind induced residual circulations produce currents that interact with topography, resulting in topographic stress and, therefore, in a rectified mean current (Lessa, 1996). Furthermore, the residual circulation pattern in Ria de Aveiro lagoon evolves continually in time as a result of the bottom sediment dynamics and of the anthropogenic activities related to the dredging operation in several channels, as well as the exploitation of the lagoon sediments. The tide, river and wind residual transport in Ria de Aveiro are at least two orders of magnitude lower than the tidal transport (Dias, 2001). Residual currents and transport are strong at S. Jacinto and Espinheiro channels as well as at the lagoon mouth (Barra), and are directed downstream toward the open ocean, contributing to net water and particles exports. At the far end of the channels (namely at Varela), the tidal and the residual currents are in general null or very small, leading, therefore, to a significant deposition of particles into the bed. The residual circulation induced by river freshwater inputs may experience an important increase during the rainfall season, due to the increase of the input from the main freshwater tributaries, influencing therefore, the lagoon channels dynamics. On the other hand, wind intensity and direction, show diurnal and seasonal variabilities which influence the overall transport inside the lagoon (Dias, 2001). The response of the residual currents to the wind forcing seems to be almost linear: when the wind direction changes, the residual current direction changes as well. SE wind residual circulation seems to induce a rising of the water level at the lagoon mouth. A residual circulation pattern is therefore generated at S. Jacinto and flows northward along the S. Jacinto channel, whereas along the Espinheiro channel it flows toward S. Jacinto. On the other hand, N–NW wind seems to correspond to a dropping of the water level at the lagoon mouth. This induces a reverse pattern comparatively to the previous case. The generation of the eddy structures may be explained by the differences in the bottom friction due to the local bathymetry. As the wind blows over a surface of a closed variable-depth basin, it accelerates the shallow waters more strongly than it does for the deep waters. The immediate effect is the production of closed circulation loops in the basin, with the transport direct downwind for shallower depths and upwind for higher

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depths (Csanady, 1973, 1976). Dias (2001) points out that the eddy structures present at S. Jacinto, Torreira and Costa Nova may impact the water exchange between the lagoon and the ocean, as they tend to disturb the flow exchanged with the lagoon mouth. On the other hand, the wind-driven flow may disturb the tidal flow regime, advancing in time the maximum flood flow and delaying the maximum ebb flow, as well as increasing the flood flow intensity when the flow is in the flood direction. The river residual transport contributes to increase the ebb flow and the water export to the ocean. Even though only the M2 component has been considered in the study of the residual circulation, it should be pointed out that the S2 tidal component generates a modulation of the frictional setup of the lagoon mean water level during the spring and neap tides Dias (2001). During flood tide, as the lagoon fills with waters, the water depth inside the lagoon channels increases and hence the frictional resistance to inflow decreases. On the other hand, during ebb, the reverse situation occurs, resulting in an increase of the frictional resistance to the water flow. Liu (1992) and Liu and Aubrey (1993) found that the generation of tidal residual currents in a highly non-linear channel is most sensitive to the mean sea-level difference, less sensitive to the tidal amplitude difference, and much less sensitive to the tidal phase difference between the two ends of the channel. In their case study, Liu and Aubrey were able to relate the long-term sediment transport pattern through a multiple tidal inlet system to residual currents that were largely caused by the mean sea-level difference and, to a lesser extent, by the tidal differences between the two open boundaries of the channel. On the other hand, the flooding-drying process over the intertidal areas also tends to enhance the asymmetry of tidal currents over a tidal cycle, during the spring–neap cycle, resulting in a relatively large residual flow along the estuary. This mechanism amplifies the tidal asymmetries inside the lagoon, enhancing the residual currents. The neap–spring cycle may, therefore, play an important role in the long-term sediment transport in the central areas of the lagoon. In these areas, the residence time is small and the ebb currents are strong, resulting in a residual transport toward the lagoon mouth. The residence time is a very important tool for the assessment of the particles exportation and their fate within a given sub-domain of the lagoon. Monsen et al. (2002) has shown for the Mildred Island that maps depicting spatial variability of the residence times can as well provide strong clues about the importance of transport processes in shaping the spatial patterns of non-conservative quantities (such as sediments, temperature, specific conductivity, Chl a, and dissolved oxygen).

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The residence times presented in Section 5.2, provide, therefore, strong clues about the importance of the transport processes inside the lagoon and are valuable to define the spatial variability of sediments in the domain. The results show that the residence times are extremely variable, depending on the location, and range from 1 to 14 days. The residence time is high for particles released at the lagoon far ends, not only because the travel times for those particles to exit the domain are longer, but also due to the morphology and shallowness of these areas, as well as to the low currents intensity observed there. As it would be expected, locations near the lagoon mouth have shorter residence times than those situated at the lagoon far ends. Particles tend to remain and be deposited at the extremity of the main channels, in zones where the residual currents are low and the residence times are high. In particular, the Mira channel and the far end of the S. Jacinto channel (near Torreira and Varela) as well as Laranjo, have weak currents and high residence times and, therefore, the deposition is more likely to occur there. Dias (2001) and Dias et al. (2001, 2003) have shown that the residence times, calculated for the different areas of the lagoon, are consistent with the observed salinity patterns during most part of the year. This result confirms that the transport inside the lagoon is essentially dominated by the longitudinal advection induced by tide. Tidal pumping produced by asymmetries, resulting from the complex network configuration and the intertidal morphology of Ria de Aveiro, seems to be the primary source of water column turbidity and the effective sediment trapping mechanism along the main channels and areas of the Ria de Aveiro. This process, which is enhanced during the spring tides, originates the resuspension of a readily erodable particle pool and is the main responsible for the high turbid zones observed along the S. Jacinto channel and between the C. Bico and the Laranjo areas, as well as for net sediment transport inside the lagoon. The zone of maximum turbidity, found along S. Jacinto, Espinheiro, and Ovar channels, as well as at the C. Bico, are, therefore, mainly induced by tidal currents and by the flood and ebb asymmetries (Lopes et al., 2006). The location and the intensity of the tidal turbidity maximum vary in function of the tidal cycle and the current intensity. Along the S. Jacinto channel, it varies longitudinally, as the turbidity front travel in the north– south direction, moving up and down the channel, whereas at the C. Bico it varies through the narrow entrance along the east–west direction. Rinaldo et al. (1999), point out that the longer slack before ebb and the higher peak velocity during the flood are typical of a flood-dominant system, favouring the predominance of

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flood erosion and generating a net upstream transport. In the upper tidal reaches of the estuary, however, the tidal wave and currents progressively damp out, the flood dominance disappears, and net sediment transport is directed seaward by the flowing river-like waters. A tidal sediment trapping zone (ETM) is thus created at the upstream limit of tidal influence, even in the absence of residual density circulation. ETM, however, is generally located farther upstream than the head of salt wedge intrusion (Allen et al., 1980). The ETM at the upper tidal reaches of Ria de Aveiro, namely that located between C. Bico and Laranjo seems to be ebb dominated, with a predominantly ebb erosion and a net sediment transport toward S. Jacinto channel. Along the S. Jacinto and the Espinheiro channels, net erosion and longitudinal transport seem to occur during both flood and ebb situations. Nevertheless, the progressive damping of the tidal wave and currents, as it propagates from the mouth toward very shallow areas of the lagoon, as well as the outflow plume at the mouth, resulting from the convergence of the ebb flows from the S. Jacinto and the Espinheiro channels persists even during flood. This fact reveals a net seaward residual transport, and therefore, an ebb dominant zone, for at least the first half of these channels. The upper tidal reaches of those channels are typically flood dominated, evidencing net flood erosion and upstream transports. As can be observed in Fig. 10, along the Espinheiro channel and between Laranjo and C. Bico, when the river input is high and during the spring tide, the increased suspended load leads to a significant sediment export. Castaing and Allen (1981) have shown that in the Gironde estuary the suspended sediment is higher concentrated in the water column during spring tides, where it is carried seaward by the net surface outflow. They also noted that tidal dispersion increases the export of sediment during high flow and spring tide conditions, when the turbidity maximum is pushed closer to the mouth. Geyer et al. (2001, 2004) noted that the timing of the spring freshet with respect to the spring–neap cycle may be as important as the actual magnitude of the freshet in determining whether sediment is exported during high flow events. The baroclinic effects have been neglected in this study. Indeed, the hydrological characterization of the Ria de Aveiro (Dias et al., 1998, 1999; Dias, 2001) reveals that, excluding the areas close to the lagoon mouth, where some stratification patterns in the salinity and the temperature vertical profiles are observed, it can be characterized as a well-mixed system. On the other hand, the horizontal pressure may have some importance, close to the river mouth or along the Espinheiro channel that connects the major river Vouga to the

lagoon mouth, during the rainy season. Nevertheless, due to the strong influence of the ocean in the lagoon, its shallowness and mixed structure, no significant baroclinic circulation is expected to occur in most areas of the lagoon and during most part of the year.

Fig. 12. Simulated net erosion–deposition field (g m− 2), cumulated after 120 days (model forced with a real tide alone): erosion (negative values); deposition (positive values).

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Water level spectral density computed for the lagoon mouth (Dias, 2001) shows that most of the spectral energy is concentrated in the semi-diurnal and less in the diurnal frequency, the other frequencies of the spectrum show insignificant contributions. As referred by Anthony and Orford (2002), strong tidal currents associated with large tidal ranges commonly characterize shallow seas and wide, low-gradient shelves. Both settings favour tidal amplification. Wave generation conditions and fetch may also become restricted in semienclosed coastal seas, while shallow shelf seas favour wave energy dissipation. Therefore, concerning the Ria de Aveiro lagoon the conjunction of situations of wave and tidal conditions leading to a wave-tidal dominated coastal lagoon is unlike to happen. Fig. 12 shows the simulated net deposition field (g m− 2) at the Ria de Aveiro, which represents the amount of sediments accumulated in the lagoon bed during the simulation period, that is, after 120 days. This figure refers to a forcing with real tide, with no wind stress or river inputs. Negative values of deposition indicate effective erosion, whereas positive ones indicate effective deposition. Area dominated by effective erosion are situated along the S. Jacinto and the Espinheiro channels and at C. Bico, where intense currents induce high bottom stress, whereas the remaining areas of the lagoon are characterized by net deposition. The erosion–deposition budget indicates that sediment deposition is a dominant process in the main areas of the lagoon, which may suggest that sediment accumulation is most likely to occur in the very shallow areas of the lagoon, in agreement with the calculated residual circulation and residence times. In conclusion suspended sediment transport in Ria de Aveiro is dominated by tidal currents. The lagoon characteristics (shallowness, very complex network structure, composed by several sub-systems, dominated by tidal asymmetries, presenting areas of ebb and flood dominance, and weak gravitational circulation due to its well-mixed structure), determine the tidal transport characteristics. Although the wind stress in general increase the suspended sediment concentrations along the main channels, due to the increase of the bottom erosion and the advective transport of sediments, the residual circulation may contribute to decrease the suspended sediment concentrations, in some phase of the tidal cycle, when currents generated by the winds are in the opposite direction of the tidal currents. Rivers contribute to increase the sediment export toward the ocean. High turbid zones are particularly observed during spring tides at several shallow areas of the lagoon, and are induced by strong tidal currents, associated with tidal asymmetries, as well as rivers' runoff.

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Aknowledgements This work was supported through the MODELRIA and SIMCLAVE projects.

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