Continental Shelf Research 31 (2011) 1515–1526

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Continental Shelf Research

0278-43

doi:10.1

n Corr

E-m

joao.dia

ana.pica

afortun

journal homepage: www.elsevier.com/locate/csr

Local sea level change scenarios for the end of the 21st century and potentialphysical impacts in the lower Ria de Aveiro (Portugal)

Carina L. Lopes a, Paulo A. Silva a, Jo~ao M. Dias a,n, Alfredo Rocha a, Ana Picado a,Sandra Plecha a, Andre B. Fortunato b

a CESAM, Departamento de Fısica, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugalb LNEC—Laboratorio Nacional de Engenharia Civil, 1700-066 Lisboa, Portugal

a r t i c l e i n f o

Article history:

Received 28 October 2010

Received in revised form

22 June 2011

Accepted 27 June 2011Available online 2 July 2011

Keywords:

Morphodynamics

Hydrodynamics

Local sea level change

Climate change

Ria de Aveiro

43/$ - see front matter & 2011 Elsevier Ltd. A

016/j.csr.2011.06.015

esponding author. Tel.: þ351 234370288; fa

ail addresses: carinalopes@ua.pt (C.L. Lopes),

s@ua.pt (J.M. Dias), alfredo.rocha@ua.pt (A. R

do@ua.pt (A. Picado), sandraplecha@ua.pt (S

ato@lnec.pt (A.B. Fortunato).

a b s t r a c t

Sea level change is an important consequence of climate change due to its impact on society and

ecosystems. Analyses of tide-gauge data have indicated that the global sea level has risen during the

20th century and several studies predict that the mean sea level will continue to rise during the 21st

century, intensifying coastal hazards worldwide. In Portugal, the Ria de Aveiro is expected to be one of

the regions most affected by sea level change.

The main aim of this study is to evaluate the potential impacts of the mean sea level change on the

hydrodynamics and morphodynamics of the Ria de Aveiro. With this purpose, local mean sea level

change was projected for the period 2091–2100 relative to 1980–1999, for different Special Report on

Emission Scenarios (SRES) scenarios developed by the Intergovernmental Panel on Climate Change

(IPCC). These projections revealed an increase in the mean sea level between 0.28 m under scenario B1

and 0.42 m under scenario A2.

The results obtained for sea level rise scenario A2 projection were used to force the morphodynamic

model MORSYS2D, previously implemented for the Ria de Aveiro. The modelling results were compared

with model forecasts for the present sea level. The residual sediment transport and its balance at the

lagoon inlet were computed and analysed for both situations. While the residual sediment transport is

generally seaward, sediments tend to deposit inside the inlet due to the weak sediment transport at its

mouth. The direction of the residual flux will not change with the sea level rise, but sediment fluxes will

intensify, and accretion inside the inlet will increase.

The rise in mean sea level will also affect the lagoon hydrodynamics. The tidal prism at the lagoon

mouth will increase by about 28% in spring tide. In the lower lagoon only a slight increase of the tidal

asymmetry is predicted.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Coastal regions are dynamic interface zones where land, waterand atmosphere interact in a dynamic balance that is constantlybeing changed by natural and human influence. Natural pressuresinclude storms, wind, tides, waves, sea level change and runoff,while anthropogenic pressures include all human activities suchas fisheries, coastal agriculture, tourism and dredging operations.Generally, coastal areas are extremely productive and accessible topeople. Therefore these areas are densely populated, intensifying

ll rights reserved.

x: þ351 234378197.

psilva@ua.pt (P.A. Silva),

ocha),

. Plecha),

the anthropogenic pressures. The natural pressures are also beingintensified as a result of climate change.

Sea level change is an important consequence of climatechange. Global mean sea level has been rising during the 20thcentury at a rate of 1.770.5 mm/year (Church and White, 2006)and it is expected to continue rising during the 21st century at anincreasing rate (Meehl et al., 2007a). For the 21st century, recentpredictions indicate a global rise between 0.18 and 0.59 m (Meehlet al., 2007a) and between 0.5 and 1.4 m (Rahmstorf, 2007).According to Chao et al. (2002) mean sea level change can becaused by thermal expansion (response of the ocean to globalatmospheric temperature rise), mass exchange (melting of moun-tain glaciers and ice caps and changes in Greenland and Antarcticice sheets), dynamic changes (result of density gradients) andland subsidence (vertical movements in the solid earth related totectonics and isostatic adjustment).While some of these causeshave global effects, long-term dynamic changes and land subsidence

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–15261516

affect mean sea level locally (Meehl et al., 2007a). Hence, the sealevel is not changing uniformly around the world, as both tide gaugeand satellite data confirm.

Several studies based on tide gauge records showed that thesea level has risen along the Portuguese coast during the 20thcentury. Dias and Taborda (1988) estimated a relative sea levelrise (SLR) at a rate of 1.570.2 mm/year at Lagos during 1908–1987and 1.370.1 mm/year at Cascais during 1882–1987, while Araujo(2005) estimated a relative SLR at a rate of 1.1570.68 mm/yearat Aveiro during 1976–2003. Recently, Antunes and Taborda(2009) found an acceleration of the relative SLR rate during1977–2000 at Cascais and have estimated for this period a rateof 2.170.1 mm/year.

In general, the physical impacts of SLR on coastal areas areinundation of low-lying coastal areas, landward intrusion of saltwater in estuaries and aquifers, erosion of sandy beaches andbarrier island coasts and habitats loss (Gornitz, 1991; Nichollset al., 1999; Mclean et al., 2001; Nicholls et al., 2007; FitzGeraldet al., 2008). However, the response of each coastal region to SLRdepends on the physical features of the coastal system and on therate of local relative SLR. Thus, the effects of SLR should beevaluated locally in order to improve the tools for vulnerabilityassessment (Nicholls and de la Vega-Leinert, 2008).

Various studies have reported the effects of SLR in severalcoastal areas around the world (Baric et al., 2008; Kont et al.,2008). In Portugal, the areas that will probably be the mostaffected by an accelerated SLR are the Ria de Aveiro and the RiaFormosa coastal lagoons and the Tagus and Sado estuaries(Ferreira et al., 2008; Andrade et al., 2006).

The main aim of this study is to evaluate the potential effect ofsea level change on the hydrodynamics and morphodynamics ofthe inlet region of the Ria de Aveiro. With this purpose, scenariosof local sea level change during the 21st century (2091–2100)relative to present mean sea level (1980–1999) were determinedin order to force the 2-D morphodynamic model system MOR-SYS2D (Fortunato and Oliveira, 2004, 2007; Bertin et al., 2009a),previously implemented for the Ria de Aveiro by Oliveira et al.(2006) and Plecha et al. (2010).

Fig. 1. Variation in sea level (m) during 2091–2100 relative to 1980–1999 period,

for the SRES scenario A2, in the North Atlantic.

2. Sea level change scenarios for the Portuguese coast

Global sea level change in the 21st century will be mostlydetermined by two factors. Thermal expansion will be responsiblefor about 70% of this, while the exchange of water between theocean and others reservoirs will account for the remaining (Meehlet al., 2007a). However, local sea level change will also be affectedby long-term dynamic changes (whose global mean is close tozero) and land subsidence.

Hence, instead of using the projected global average sea levelpublicised by IPCC (Solomon et al., 2007), simulated by a setof seventeen Atmosphere Ocean General Circulation Models(AOGCMs), the local sea level change along the Portuguese coastwas estimated with the output of model GISS-ER (Russel et al.,

Table 1Summary characteristics of the SRES scenarios A2, A1B and B1 (adapted from Carter e

Development

Population Economy

A2 Continuously increasing Moderate growth

A1B 2050 peak then decline Accentuated growth

B1 2050 peak then decline Not so accentuated than A1

1995, 2000). This model was selected because, it is the only onewithin the seventeen AOGCMs used by the IPCC, that accounts forthe change in sea level due to thermal expansion, dynamic changeand mass exchange at every grid point (Katsman et al., 2007).

Climate GISS-ER model output from simulations of the past(pre-industrial run), present (20C3M run) and future climates(different scenarios runs) have been made available to the scientificcommunity by the World Climate Research Programme’s (WCRP’s)Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset (Meehl et al., 2007b). The 20C3M run starts in 1880and ends in 2003. The future climates were simulated imposingdifferent emission scenarios of greenhouse gases developed by IPCC.Three scenarios Special Report on Emission Scenarios (SRES) wereconsidered: A2, A1B and B1. The emission scenarios were quantifiedthrough the CO2 equivalent concentration (i.e., the CO2 concentra-tion that would cause the same time-integrated radiative forcing asa given type and concentration of greenhouse gas) based on a rangeof possible behaviours of society, economy and technology (Table 1).

With the sea surface height data provided by the GISS-ERmodel, the change in sea level relative to the present (reference)mean sea level simulated by the GISS-ER model was computed forthe 21st century (2091–2100 period), for each scenario consid-ered. The reference was considered to be the mean of the1980–1999 period. The mean sea level change in the North Atlanticfor scenario SRES A2 reveals a rise in sea level for the whole region,and a very weak spatial variability near the Portuguese coast (Fig. 1).

To minimise sampling problems associated with the modeluncertainty at small spatial scales, the behaviour of sea levelchange at eight points (represented in Fig. 1) surrounding theregion of interest was analysed.

Table 2 shows the sea level change time-average and thestandard deviation relative to the 1980–1999, for the present(20C3M simulation) and for the future (2091–2100 period) ateach point. Again, the weak spatial variability is evident.

t al., 2007).

Surrogatestabilisation scenario

Technology

Slowest development Does not stabilise

Balanced across all sources 750 ppm

B Clean and resource-efficient 550 ppm

Table 2Mean (Z) and standard deviation (s) of sea level change, relative to the 1980–1999

period, for the 20C3M simulation and for the 2091–2100 period at the grid points

presented in Fig. 1.

20C3M A2 A1B B1

P1 Z (m) 2.6�10�3 0.43 0.36 0.28

s (�10�2 m) 3.1 2.0 3.2 1.6

P2 Z (m) 4.3�10�3 0.40 0.34 0.27

s (�10�2 m) 2.6 1.1 2.7 1.7

P3 Z (m) 2.1�10�3 0.39 0.32 0.25

s (�10�2 m) 2.6 1.1 2.3 0.8

P4 Z (m) 3.4�10�3 0.41 0.35 0.27

s (�10�2 m) 2.4 1.4 2.0 1.1

P5 Z (m) 2.3�10�3 0.39 0.33 0.26

s (�10�2 m) 2.3 1.0 1.8 1.0

P6 Z (m) 2.7�10�3 0.44 0.37 0.29

s (�10�2 m) 2.7 1.9 1.8 1.4

P7 Z (m) 1.2�10�3 0.42 0.36 0.28

s (�10�2 m) 2.3 1.0 2.0 1.1

P8 Z (m) �3.0�10�4 0.46 0.40 0.30

s (�10�2 m) 2.7 1.8 1.8 1.5

Table 3Sea level change projections for the Portuguese

coast at the end of 21st century.

Scenario SLR70.02 (m)

A2 0.42

A1B 0.35

B1 0.28

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–1526 1517

A t-student test revealed that the mean sea level is significantlyhigher, at the 1% significance level, in the future climate whencompared to the present climate, at each of the eight points. Also, anF-test showed that, in the future climate, the sea level variances ateach of the eight points are not significantly (1% significance level)different, from those in the present climate.

Since the GISS-ER model data do not include the effect of landsubsidence, this contribution to sea level change was calculatedseparately based on Peltier (2004), which computed a rate of sealevel change of �0.07 mm/year due to Glacial Isostatic Adjust-ment (GIA) for the Portuguese coast. This value represents about5% of the total change and is therefore neglected henceforth.

The spatial standard deviation of the sea level change betweenthe eight points was found to be 0.02 m for all scenarios,confirming the weak spatial variability of this variable in thestudy area. Hence, the mean value of sea level change obtained atthe points analysed was considered as a good estimation for thePortuguese coast (Table 3).

In general, predictions provided by global climate models tendto exceed linear extrapolations based on observed data, whichindicates that the rate of relative SLR can be nonlinear dependingon the scenario considered. However, the projections obtainedhere are broadly similar with other estimates based on theextrapolation of observed data. Dias and Taborda (1988) pre-dicted a relative SLR for Portugal between 0.14 and 0.57 m by theyear 2100, according to an empirical method based on theextrapolation of SLR rate obtained from observed data recordedby tide gauges of Cascais and Lagos. Using a similar method,Antunes and Taborda (2009) projected a rise of 0.47 m with a 95%confidence interval between 0.19 and 0.75 m in the year 2100relative to 1990 mean sea level for Cascais. In contrast, theextrapolation of relative SLR rate at the Ria de Aveiro mouth,

1.1570.68 mm/year, indicates for 2100 an increase in mean sealevel between 0.05 and 0.2 m (Araujo, 2005). The inconsistencybetween these values and the projections reported before can bedue to the relatively short record (o50 years) of the Ria de Aveirotide gauge, which is unsuitable for long-term mean sea level trenddetermination (Douglas, 1991).

Hereafter, only the worst scenario is considered, the A2scenario (0.42 m), because it has the highest potential impactsin coastal regions (Nicholls and Tol, 2006).

3. Ria de Aveiro—hydrodynamic and morphodynamic aspects

The Ria de Aveiro (Fig. 2) is a shallow vertically homogeneouslagoon with a very complex geometry, located on the north-western Portuguese coast (401380N, 81450W). It is 45 km long and10 km wide and covers an area of 83 km2 (66 km2) at high (low)water spring tide (Dias and Lopes, 2006). It is characterised bynarrow channels and by large areas of mud flats and salt marshes.Its main channels are Mira, S. Jacinto, llhavo and Espinheiro. TheEspinheiro channel receives the main input of fresh water in thelagoon by the Vouga River. This input represents 2/3 of the totalfresh water flow into the lagoon (Dias, 2001). The inlet region(Fig. 2c) provides access to the Aveiro harbour. The navigationchannel runs from the lagoon mouth to the Aveiro harbour. Theaverage depth in the navigation channel (5–10 m relative to chartdatum) is greater than the average depth of the remaining lagoon(1 m relative to chart datum). There are two deeper areas in theinlet region (Fig. 2c): at the lagoon mouth between the twobreakwaters, and further upstream, in the zone of the tidal gaugeand extending up to the bifurcation. Between these two zones,there is a wider and shallower zone, which is delimited at thesoutheastern side by the Meia Laranja beach.

The lagoon hydrodynamics is dominated by tides, which aremixed, mainly semidiurnal. The lagoon form number F (Bowden,1983) is 0.102 at the lagoon mouth (Dias, 2001). The residualcirculation is determined essentially by the asymmetries betweenthe flood and ebb regimes. As the lower lagoon is ebb-dominantthere is a trend to export sediments to the ocean (Oliveira et al.,2006). The lagoon can generally be considered vertically homo-geneous, except in rare situations of strong fresh water flows whenthe upper parts of the lagoon can be stratified (Dias et al., 2000).

Human action has been the major factor controlling the lagoonmorphology (Silva and Duck, 2001). The most noteworthy humanintervention was the creation of an artificial inlet in 1808, inresponse to the persistent accretion of the natural inlet. In thefollowing years, several works were performed in order to improvethe access to the Aveiro harbour, namely the construction of twobreakwaters in the first half of the 20th century, whose extensiongrew until the last intervention between 1983 and 1987.

After the extension of the northern breakwater (1987/1988)the Hydrographic Institute of the Portuguese Navy collectedbathymetric data for the entire lagoon. More recently, the AveiroHarbour Administration has surveyed the lagoon mouth in June of2001, September of 2005 and September of 2007. An analysis ofthe bathymetric changes occurred between 1987/88 (after theextension of the northern breakwater) and 2005 revealed that thephysical changes induced by the extension of the northernbreakwater together with the regular channel dredging led to adeepening of the channels in the inlet region (Plecha et al., 2007).The northern breakwater extension has also affected the along-shore littoral drift, leading to sediments accumulation (erosion)north (south) of the inlet (Plecha et al., 2007).

The analysis of recent bathymetric data shows that the inletregion is extremely dynamic, experiencing strong bathymetricchanges in short periods of time. This evolution is illustrated in

Fig. 2. Maps of the study area (datum WGS84): (a) location of Ria de Aveiro lagoon; (b) numerical bathymetry of 2001 (relative to local chart datum), presenting the cross-

sections location where tidal prism was evaluated (from A to E); (c) zoom of the inlet bathymetry of 2001 showing the cross-sections where transport of sediments was

computed (from 1 to 6) and regions where sedimentation rates were computed (from I to V).

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–15261518

Fig. 3 that compares the bathymetric changes observed withintwo periods of time: 2001–2005 and 2005–2007. The evolution ofthe deeper zone located at the lagoon mouth between the twobreakwaters highlights that between 2001 and 2007 there waserosion to the east and to the west of this zone and accretioninside it.

Furthermore, there was accretion close to the northern break-water. The pronounced bathymetric changes observed within a stripof around 50 m close to this structure are probably due to thecollapse of structural rocks from the breakwater and to the

emergency works performed in 2004 to repair this structure. Indeed,these works deposited large amounts of rocks at the breakwater andnear its bottom, which were observed in recent side sonar images.

The morphological evolution in the remaining inlet region wasless uniform. The shallower area between the two deeper areasexhibited erosion between 2001 and 2005 and accretion between2005 and 2007. The Meia Laranja beach also experienced oppositebehaviours: strong deposition between 2001 and 2005 andintercalate areas of mild erosion and deposition between 2005and 2007. Upstream of the tidal gauge, the lagoon experienced

Fig. 3. Observed bathymetric changes (m/year) at the lagoon inlet area between:

(a) 2001 and 2005; (b) 2005 and 2007. The solids lines represent the bathymetries

of: (a) 2001 and (b) 2005.

Fig. 4. Observed sedimentation rates (m3/day) at the regions represented at

Fig. 2c from 2001 to 2005 and from 2005 to 2007.

Fig. 5. Flowchart of MORSYS2D procedure every morphodynamic time step

(extracted from Bertin et al., 2009a).

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–1526 1519

mostly deposition between 2001 and 2005, while between 2005and 2007 it experienced intercalate areas of mild erosion anddeposition.

Based on the surveys data, the sedimentation rates in theregions (I–V) represented in Fig. 2c were computed for bothperiods (Fig. 4). The inlet region experienced mostly accretion,which was more intense between 2001 and 2005. The strongbathymetric changes observed in the study area together with thedifferent behaviours observed in the two periods analysed showsthe morphodynamic complexity of this system.

4. The modelling system MORSYS2D

The modelling system MORSYS2D (Fig. 5) simulates the non-cohesive sediment dynamics and bottom changes in estuaries, tidalinlets and coastal regions, driven by tides, waves, wind and riverflows. This system was applied successfully to the Obidos lagoonreproducing the main morphological changes observed at the inlet(Bertin et al., 2009c; Bruneau et al., submitted for publication). Themodel was also applied to Ria Formosa lagoon, reproducing success-fully the migration of the Anc~ao inlet (Bertin et al., 2009b).

The system integrates the hydrodynamic model ELCIRC (Zhanget al., 2004), the wave model SWAN (Boij et al., 1999) and thesediments transport and bottom update model SAND2D (Fortunatoand Oliveira, 2004, 2007, Bertin et al., 2009a). Because the

applications presented in this study neglect the wave effects onlythe modules ELCIRC and SAND2D are used and described herein.Waves are expected to play a minor role in the transport ofsediments in the Ria de Aveiro inlet region, because the break-waters protect the inner lagoon from the swell (Plecha et al., 2010).

The hydrodynamic model ELCIRC solves the shallow waterequations (conservation of mass and momentum) using a finitevolume technique for volume conservation and a natural treatmentof wetting and drying. A semi-implicit time stepping algorithm andthe Lagrangian treatment of the advective terms ensure stabilityat large time steps. The horizontal domain is discretized with atriangular mesh, and the model in this application is used in 2Ddepth-averaged mode.

SAND2D simulates sediment transport due to currents, andcomputes the resulting bed changes by solving the Exner equa-tion, using a node-centred finite volume technique based on atriangular unstructured grid:

Dhi ¼1

1�lrQi

n

where Dhi is the bottom variation over a morphodynamic timestep, l is the sediment porosity and Qi

nis the sediment flux

integrated over a time step, which is a function of instantaneoussediments fluxes, coefficient diffusion, porosity, water depth,velocity and tidal elevation.

Instantaneous sediments fluxes are computed using an empiri-cal or semi-empirical formula selected from a wide range imple-mented in MORSYS2D.

The computation of sediments fluxes is one of the mostimportant sources of errors in morphodynamic modelling (Pintoet al., 2006). Errors in sediments flux estimations are generallyhigh because formulas are approximations and many of thecomplex aspects of sediment transport are not yet fully under-stood and therefore not included in formulas (Yang, 2006).Furthermore, the empirical sediment transport formulas are verysensitive to errors associated to sediment grain size and flowvelocity (Pinto et al., 2006). Thus, small errors in sediment grainsizes and velocity (from the hydrodynamic model) can inducesignificant errors in sediments transport computation.

4.1. MORSYS2D application to the study area

MORSYS2D was previously implemented and calibrated for Riade Aveiro lagoon by Oliveira et al. (2006) in order to study thesediment dynamics at the inlet. ELCIRC was implemented forthe entire lagoon, while SAND2D was only implemented for theinlet evolving area (from off-shore till the cross-sections 5 and 6

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–15261520

represented in Fig. 2c). Consequently, MORSYS2D solves thehydrodynamics for the entire lagoon, but the sediment fluxes andthe bathymetry updates are only evaluated in the SAND2D gridarea. The ELCIRC grid has 18,851 nodes while the SAND2D grid has1812 nodes. Resolution varies from 0.5 km near the open boundaryto 1 m in the narrow channels. The ELCIRC calibration was basedon a comparison of model results with amplitudes and phasesobtained from the harmonic analysis of elevation data measured in1987/88 at 22 stations scattered throughout the lagoon. The M2

amplitude constituent errors vary from 5% at the inlet region to20% at some stations located in the upper reaches of the lagoon(Oliveira et al., 2006). The same work concluded that the accuracyof ELCIRC is similar to a previous model implemented for Ria deAveiro lagoon, suggesting that limitations in the data, in particularbathymetric data, may explain most of the errors.

The simulations presented here were forced by tides at theocean open boundary, using ten tidal constituents (MSF, O1, K1, N2,M2, S2, MN4, M4, MS4 and M6) and the mean sea level (Z0) takenfrom the regional tidal model of Fortunato et al. (2002). Theeffects of wind and river flow were neglected relative to tidalforcing.

The median sediment grain size (d50) used is variable in space,following the work of Plecha et al. (2010), and is based on datapublished by Freitas et al. (2005) and on sediment data collectedin 2006. The porosity was set to 0.35.

The Engelund and Hansen (1967) formulation is adoptedfollowing Plecha et al. (2010). These authors applied MORSYS2Dto the Ria de Aveiro considering only the tidal forcing, andperformed a sensitivity analysis on the transport formulae bestsuited to describe the bathymetric evolution at the inlet region.They concluded that the Engelund and Hansen (1967) is the one

Fig. 6. Residual sediment flux (m2/s) field at Ria de Aveiro inlet for present mean sea lev

scenario A2: (b) 2001 bathymetry, (d) 2005 bathymetry and (f) 2007 bathymetry.

that best describes the morphodynamic changes driven by tidesin Ria de Aveiro inlet. They also concluded that the modelcorrectly reproduces the erosion and deposition patterns,although it underestimates the sedimentation rates.

The impact of relative SLR in the lagoon was studied byperforming simulations with MORSYS2D imposing differentamplitudes of Z0 (mean sea level) in agreement with the SLRprojections presented above. One-year simulations were per-formed, initialised with the bathymetries of 2001, 2005 and2007, for both the present mean sea level and for the projectedSLR scenario A2. The three bathymetries were used to initialiseMORSYS2D in order to assess the variability associated with theinitial conditions. A future bathymetry could be projected con-sidering the sedimentation rates computed from the differencebetween observed bathymetries (see Fig. 4). However, as dis-cussed before, the sedimentation rates for region I (Fig. 2c) shouldnot be used to extrapolate the bathymetry because they areaffected by the rocks that have collapsed from the breakwater.Moreover, the extrapolation of these rates is inadequate, becauseit ignores the future dredging operations and possible changes inthe inlet configuration, and the system behaves non-linearly.Thus, it was decided to evaluate the sensitivity of the system todifferent bathymetric configurations.

4.2. Morphodynamic results

The residual sediment fluxes were computed in the study areaby averaging the instantaneous fluxes during one lunar month(two MSF constituent periods), for each bathymetry and for boththe present mean sea level and the SLR scenario A2, projectedpreviously (Fig. 6).

el: (a) 2001 bathymetry, (c) 2005 bathymetry and (e) 2007 bathymetry and for SLR

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–1526 1521

The residual fluxes are nearly seaward for each model config-uration and for most areas; the exceptions are at the head of thesouth breakwater (where they are northward) and in the upperpart of the navigation channel close to the north breakwater(where they are landward). These patterns suggest a tendency foraccretion close to the bifurcation, as well as near the inlet,considering that there is an accentuated decrease of the sedimentfluxes in these regions. Furthermore, downstream the cross-section 1, a strong increase in the sediments fluxes suggestserosion. These patterns are strictly related to the bathymetry(Fig. 2c): in the accreting regions, depth increases suddenly and inthe eroding region near the lagoon mouth the opposite occurs, asudden depth decrease. The SLR numerical results show for all thebathymetries an intensification of the residual fluxes.

The net residual sediment transport through the cross-sectionsrepresented in Fig. 2c were computed for each bathymetry bothfor present mean sea level and for SLR scenario A2 (Fig. 7).

The net residual sediment transport is seaward (negativevalues), except at the cross-section 2. SLR intensifies the netresidual transport, without affecting its direction. Results suggest

Fig. 7. Net residual sediment transport (m3/day) through the cross-sections represente

SLR scenario A2.

Fig. 8. Predicted bathymetric changes (m) at Ria de Aveiro inlet for present mean sea lev

scenario A2: (b) 2001 bathymetry, (d) 2005 bathymetry and (f) 2007 bathymetry.

that there is accretion in the inlet region, because the export ofsediments through cross-section 1 is smaller than the inputof sediments through cross-sections 5 and 6. The deposition ofsediments will increase with the SLR, because the input ofsediments at cross-sections 5 and 6 increases more than theexport of sediments at cross-section 1. An underlying assumptionin these simulations is the availability of sediments coming fromthe inner lagoon into the inlet region.

Fig. 8 represents the bathymetric changes for each modelconfiguration after one year of simulation. The patterns of erosionand deposition are fairly independent on the bathymetry used asinitial condition. The model predicts erosion eastward of thedeeper area at the lagoon entrance and deposition westward. Inthe shallower area between the two deeper areas, the modelpredicts intermediate areas of erosion and deposition. North ofthis region, close to the northern breakwater, the model alsopredicts erosion, which can affect the stability of the northernbreakwater. The bathymetric changes simulated in the MeiaLaranja beach are negligible and upstream of the tidal gaugemainly close to the bifurcation the model predicts deposition.

d in Fig. 2c for the three bathymetries and both for present mean sea level and for

el: (a) 2001 bathymetry, (c) 2005 bathymetry and (e) 2007 bathymetry and for SLR

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–15261522

A comparison between the bathymetric changes simulated forthe present mean sea level and for the SLR scenario A2 indicates thatthe rise in mean sea level intensifies the patterns of erosion anddeposition. The SLR will contribute to a deepening of the deeper areaat the entrance and to an increase of the deposition in the inlet area.

The sedimentation rates were computed for each modelconfiguration (Fig. 9). Comparing these rates with those estimatedfrom the observations (Fig. 4) shows that the model underestimatesthe sedimentation rates. The erosion predicted in region I can beconsidered reasonable, because the observed sedimentation rate inthis region includes rocks that have collapsed from the northernbreakwater. The divergence between predictions and observationsat the Meia Laranja beach (region III) can be partially explained bythe omission of wave forcing. The waves interact with the break-waters and generate complex wave patterns that may affect trans-port of sediments in this area.

Fig. 9. Calculated sedimentation rates (m3/day) at the five regio

Fig. 10. Tidal prism in each cross-section fo

The sedimentation in the study domain doubles when the sealevel increases by 0.42 m.

4.3. Hydrodynamic results

The tidal prism and tidal asymmetry changes resulting fromthe rise in mean sea level were also evaluated in the lower lagoon.As for the morphodynamic analysis, the sensitivity to the differ-ent bathymetries (2001 and 2007) was also considered. The tidalprism was computed at the cross-sections represented in Fig. 2bfor each model configuration, for spring and neap tide conditions(tidal ranges of 3.2 and 0.4 m, respectively). The cross-section A isrepresentative of the inlet, and the cross-sections B–E representthe mouth of the four main channels of the Ria de Aveiro. Theresults obtained are presented in Fig. 10.

ns, both for present mean sea level and for SLR scenario A2.

r spring tide and neap tide conditions.

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–1526 1523

For the present mean sea level the results show a decrease ofthe tidal prism of about 10% for all cross-sections (except section E)between 2001 and 2007, which is probably due to the sedimenta-tion occurred at the inlet region in this period. Araujo et al. (2008)indicates that a reduction in the inlet depth decreases the tidalamplitude in the inner lagoon and the tidal prism.

The tidal prism for each of the main channels relative to itsvalue at the inlet is about 13% for the Mira channel, 45% for the S.Jacinto channel, 9% for the llhavo channel and 25% for theEspinheiro channel. The projections for the SLR scenario A2indicate that the tidal prisms will increase by about 28% onspring tide and 35% on neap tide, for all cross-sections.

The tidal asymmetry changes induced by the SLR were alsoinvestigated in the lower lagoon along three channel axes, fromthe lagoon mouth until: the cross-section B (Mira channel), thecross-section C (S. Jacinto channel) and the bifurcation pointwhich originates the llhavo and Espinheiro channels (Navigationchannel). The tidal asymmetry was not evaluated upstream ofthese limits due to the unavailability of updated bathymetric datafor the remaining lagoon.

The amplitude ratio ðAR ¼ AM4=AM2Þ and the relative phase

ðj¼ 2yM2�yM4

Þ were computed along each of these three chan-nels for each model configuration. The results (Figs. 11 and 12)show that in the lower lagoon the tidal asymmetry is low (theamplitude ratio is very small). This area is ebb-dominated,because the relative phase is always in the range from 1801 to3601. This trend does not change with the SLR scenario A2.

Fig. 11. Amplitude ratio (m) along Mira channel (first column), S. Jacinto channel (seco

(first line), for SLR scenario A2 (second line), and the difference between present and S

Nevertheless, the amplitude ratio and the relative phase increaseslightly with the mean sea level, in the three channels.

The numerical results also show that the bathymetric changesthat occurred between 2001 and 2007 did not induce significantchanges in tidal asymmetry in this region. The amplitude ratioincreases more strongly in the first 600 m, which coincides withthe beginning of the second deeper area at the inlet region, andthen the amplitude ratio growth is softer. The amplitude ratioevolution in the Mira channel shows a significant increase afterthe bifurcation (at about 1500 m from the mouth). This pattern isexpectable because the Mira channel is very shallow and narrow,consequently the friction increases suddenly after the bifurcation.

These results are in agreement with the results presented byDias (2001), Oliveira et al. (2006) and Picado et al. (2010). Theseauthors analysed tidal asymmetry in the upper lagoon, and con-cluded that the lagoon shifts from mildly ebb-dominant at themouth to strongly flood dominant in the upper reaches of thechannels. This fact suggests that the upper lagoon acts as asediment sink and the lower lagoon acts as a small sedimentexporter. This tendency to export sediments from the lower lagoonagrees with the morphodynamic results analysed previously.

5. Conclusions

This work reports the potential effect of relative sea levelchange on the morphodynamics of the Ria de Aveiro inlet and on

nd column) and Navigation channel (third column), for the present mean sea level

LR scenario A2 (third line).

Fig. 12. Relative phase (1) along Mira channel (first column), S. Jacinto channel (second column) and Navigation channel (third column), for the present mean sea level

(first line), for SLR scenario A2 (second line), and the difference between present and SLR scenario A2 (third line).

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–15261524

tidal prism and tidal asymmetry in the lower lagoon. To evaluatethese effects, local projections of relative sea level change wereperformed for the2091–2100 period using the GISS-ER modeloutput. The analysis showed that the effect of land subsidencedue to the GIA does not change significantly the mean sea level inthe Portuguese coast, which depends essentially of the thermalexpansion, mass exchange and long-term dynamic changes. Theprojections presented here indicate a local rise between 0.28 and0.42 m, which corresponds to the scenarios B1 and A2, respec-tively. The hydrodynamic response of the lower Ria de Aveiro andthe inlet morphodynamic changes to the relative SLR wereevaluated only under the worst scenario.

The recent morphological evolution of the inlet between 2001and 2007 showed that the interior of the inlet region is dominatedby accretion, in contrast with the deeper area at the entrance,which experienced erosion. The difference between bathymetrieshighlighted the strong and rapid bathymetric changes in thisregion. Moreover, some regions within it showed opposite beha-viours concerning the erosion and accretion trends between 2001and 2005 and between 2005 and 2007, demonstrating the morpho-dynamic complexity of this system. Furthermore, the study areaprovides access to the Aveiro harbour, being therefore constantlyaffected by human influences. Hence, predicting the bathymetricchanges is a very complex and hard task.

The impact of relative SLR on the morphodynamics andhydrodynamics of Ria de Aveiro lagoon was established throughthe numerical results of MORSYS2D.

MORSYS2D results revealed that the residual transport ofsediments at the inlet region is mostly seaward both in thepresent and future configurations. However, the seaward sedi-ment fluxes at the lagoon mouth are weak when compared tothose coming from the Mira and navigation channels. A rise inmean sea level will intensify the sediment fluxes and conse-quently the bathymetric changes will be more pronounced. Theerosion at the entrance will increase, as well as the accretioninside the inlet.

The analysis of the hydrodynamic results showed a decrease ofthe tidal prism at the lagoon mouth of about 10% between 2001and 2007, which can be explained by the observed accretion inthis period. However, an increase of 0.42 m in the mean sea levelwill potentially enhance the tidal prism by about 28% on springtides and 35% on neap tides. The tidal asymmetry was analysed inthe lower lagoon. In this region the lagoon is ebb-dominant, andthis trend will not change with the rise in mean sea level.However a slight intensification of the tidal asymmetry with theSLR is expected. While this work did not quantify the response ofthe tidal asymmetry to the SLR in the upper lagoon, it is clear thatthe sedimentation in this region will also increase.

Acknowledgements

This work was been supported by FCT in the framework of theresearch projects GRID/GRI/81733/2006—G-Cast: Application of

C.L. Lopes et al. / Continental Shelf Research 31 (2011) 1515–1526 1525

GRID-computing in a coastal morphodynamics nowcast–forecastsystem, PTDC/AAC-CLI/100953/2008—ADAPTARia: Climate ChangeModelling on Ria de Aveiro Littoral—Adaptation Strategy for Coastaland Fluvial Flooding and PTDC/MAR/107939/2008—DyEPlume:Estuarine Dynamics and Plume Propagation in the Portuguesecoast—Impacts of Climate Change, co-funded by COMPETE/QREN/UE.

We acknowledge the modelling groups, the Programme forClimate Model Diagnosis and Intercomparison (PCMDI) and theWCRP’s Working Group on Coupled Modelling (WGCM) for theirroles in making available the WCRP CMIP3 multi-model dataset.Support of this dataset is provided by the Office of Science, U.S.Department of Energy.

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