Potential impacts of climate change on NW Portuguese coastal zones

Potential impacts of climate change on NW Portuguese coastal zones Carlos Coelho, Raquel Silva, Fernando Veloso-Gomes and Francisco Taveira-Pinto

Abstract Coastal erosion is a common problem within Europe, which results from the dynamic nature of coastal zones, the anthropogenic influences, such as coastal interventions, littoral occupation and river sediment supply reduction, and the effects of climate change. Coastal changes occur at timescales that range from geological, like neotectonic events and sea level rise, to decadal or daily (storm events). Climate change has possible effects on the wave climate with direct implications for the potential alongshore transport. The likely increase of the occurrence of extreme events, the weakening of river sediment supply, and the generalised acceleration of sea level rise, tend to aggravate the coastal erosion phenomenon in a time horizon of decades. Erosion processes may cause serious damage, especially to people and assets in urban fronts, and they therefore merit special attention. To minimise these impacts, it is necessary to understand the various processes involved and assess different scenarios for coastal evolution prediction (medium to long term), to what numerical models may be useful. Maps representing vulnerability and risk to energetic environmental actions (waves, tides, winds and currents) are thought to be of high importance for coastal planning and management, rationalising decision making. In this paper, a numerical model will be used to assess potential impacts of climate change in generic situations and for a vulnerable coastal stretch of the Portuguese northwest coast. Keywords: Erosion, beach evolution, medium to long term, numerical model, scenarios assessment Carlos Coelho, Civil Engineering Department, University of Aveiro, 3810-193 Aveiro, Portugal. Raquel Silva, Fernando Veloso-Gomes and Francisco Taveira-Pinto Hydraulics and Water Resources Institute, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal. Corresponding author: Raquel Silva; Tel: (+351) 225081907; Fax: (+351) 225081952; E-mail: [email protected].

Introduction Coastal zones and beach evolution Coastal zones are dynamic regions responding at different temporal and spatial scales with geomorphological changes to natural and anthropogenic actions. Nature acts over coasts through waves, winds, tides and surges and man through resources exploitation, construction interventions and pollution. Both actors induce coastal adjustments which in turn will cause actions to change, Figure 1.

Coastal Zones

Natural Wind, Waves, Tides, Surges, River flows

Anthropogenic Resources exploitation, Construction Interventions, Pollution

Figure 1. Coastal zones actions and reactions. The assessment of beach evolution over time is very complex. At a given moment, a beach may be rapidly changing due to a short term disturbance, such as a winter storm, from which it can recover during calm wave conditions, maintaining equilibrium. At the same time, slight changes may be occurring due to sea level rise, and the coastal stretch may also be experiencing one of the several morphological states of a dynamic equilibrium, cycling at years or even decades. Abrupt irreversible changes may also occur triggered by very extreme events or internal instabilities. A major challenge in understanding beach evolution is the ability to identify and separate the processes beyond a given morphological change. A possible approach to this problem is the consideration of the processes grouped by beach response scale: short, medium and long term, Figure 2.

100 m 1 km 5 km 10 km 100 km …

hours years decades Short Term Medium Term

centuries

millennia

Long Term Longer term

Figure 2. Spatial and temporal scales for beach change. The medium term beach changes are relatively well understand, since they may be percept by direct observation and, for this reason, have been studied for almost a century. Conversely, some of the processes and mechanisms behind the short term changes are presently under research (e.g. under the topic of cross-shore sediment transport in the surf zone, the relation between bed forms and cross-shore sediment transport is being investigated (van Rijn et al., 2007)). Processes and beach response taking place at such small scales may not even be observable in nature (e.g. the intensive sediment transport taking place in a thin bottom layer with a thickness of the order of some cm’s, sheet flow transport, can only be measured in the laboratory (Nielsen, 2006)) and, therefore, they are not so well known. Long term changes also have several observational difficulties: data time series are usually not long enough to allow the identification of long term cycles, and they frequently have gaps. This is the case for the wave data series measured offshore of the Portuguese west coast; in Figure 3 the annual percentage of valid observations obtained by Leixões wave buoy between 1993 and 2003 is presented; extreme conditions are often not registered since the wave stations are frequently damaged or malfunctioning during storms.

Hs (m)

10

31%

47%

6%

55%

57%

76%

84%

32%

49%

77%

97%

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

5 0

Figure 3. Significant wave height time series, obtained from Leixões wave buoy observations between 1993 and 2003 (buoy station located offshore of the Portuguese northwest coast, owned and operated by Hydrographical Institute - IH). The short term beach change scale (100 m – 1 km; hours – years) comprises the instantaneous response to the direct action of waves, winds, tides and surges, including surf zone seasonal changing between storm and calm profile (Komar, 1998). In Figure 4 cross-shore profiles collected in an open sandy beach where a transversal curved groin structure was constructed are presented. The profiles were collected down-drift of the structure for a period of time longer than 2 years (June 2001 to December 2003). Until the construction, seasonal beach changes may be identified, with the profiles alternating between berm-type, in the maritime summer (April to September), and dissipative in the winter (October to March of the subsequent year). Afterwards the down drift beach experiences accentuated erosion and the profile becomes permanently dissipative. MSL Winter 2002

Summer 2001 Groin constructed

Winter 2001 Summer 2003

Summer 2002 Winter 2003

Elevation above MSL (m)

10 8 6 4 2 0 -2 -10-4

10

30

50

70

90

110

130

150

170

Distance to backdune reference (m)

Figure 4. Cross-shore beach profiles collected in a sandy beach, Areão, located in the Portuguese northwest coast, between June 2001 and December 2003, before and after the construction of a groin up drift, which was concluded in February 2003. At this small scale analysis, the nearshore waves and currents patterns which induce a spatial distribution in the sediment transport are the most important natural factors contributing to beach change. Artificial nourishments and surf zone structures interfere with these patterns. At a medium term scale (1 km – 5 km; years – decades) surf zone bar cycles, inter annual wave regime change, or recover from a major storm event, may be found. At this scale, coastline configuration evolution is usually consistent with estimates obtained from the balance of the involved volumes of sediments, based on the knowledge of the alongshore transport as induced by wave heights and angles at the breaking zone. Changes resulting from the influence of large surf zone structures or other coastal structures may also occur at this scale. Other medium term responses are related with changes in the available amounts of sediments, for instance from river sediment supply.

At a longer time scale (10 km – 100 km; decades – centuries) coastal inlet cycles may be detected. An example of shore evolution observable at this scale is given in Figure 5. In the figure different morphological states passed on the formation of the Aveiro lagoon are sketched. The process took several centuries, in 1787 the sand spit became a barrier that was artificially opened in 1808 and maintained until the present, being one of the most important morphological features in the Portuguese northwest coast (INAG/FEUP, 2001).

Figure 5. Morphological states of the evolution of the Aveiro lagoon, located in the Portuguese northwest coast (after Delgado and Choffat, in INAG/FEUP (2001)). The long term beach changes also reveal the effects of sea level rise and of regional wave climate change, together with evidences of negative impacts, as coastal erosion resulting from coastal (non) management. However, beach evolution at long term can only be understood under years of experience, sensibility and sense to reasonable and logical considerations.

Coastal Erosion, Risk, Planning and Management Coastal erosion is a common problem within Europe which can be observable at a medium term scale. Several reasons of different orders may be contributing to it. In one hand the dynamic nature of coastal zones and climate change and on the other the anthropogenic influences. To deal with the problem in an effective manner an adequate understanding of these reasons would be needed. However, the knowledge in coastal zones dynamics is limited, the effects of climate change over coastal zones are pointed out in a context of large uncertainty, and the effects of coastal interventions have not been systematically monitored. Nevertheless, if the causes are not clearly identified it is becoming quite clear that people and assets in some littoral urban fronts are endangered, and that serious damage and high costs should be expected. The situation claims for measures to be taken, being increasingly more important to make available to decision makers, scientific and technical elements to support their resolutions. To allow sustainable actions, plans should be conceived based on the coastal evolution assessment at medium to long term. Due to the inherent uncertainty, this assessment of future conditions can only be done on the basis of scenarios evaluation, for what numerical models, comprising the present state of knowledge, may be used as a tool. To help in hierarchy priorities of action the risk along the coast must also be assessed, and this can be accomplished crossing vulnerability and hazard information in risk maps depicting spatial analysis (as a recommendation from the EU, directive 2007/60/CE).

An analysis, based on the spatial classification and weighting of the parameters though to be of importance for the evaluation of the vulnerability to sea action, has already been performed to Aveiro District, located in the Portuguese northwest coast (Coelho et al., 2006). Coastal stretches very vulnerable to sea action are not necessarily hazardous. An approach similar to the considered for the vulnerability analysis was established for hazards evaluation. In terms of hazards, the parameters considered of importance for classification were population density, economic activities potentially affected by erosion, and ecological, cultural and historical values exposed to sea action, Table 1. Spatial classification allows the creation of hazards maps, Figure 6. Risk maps will consist of the classification obtained from crossing vulnerability and hazards information, Figure 7. Table 1. Classification of parameters considered in hazards analysis for Aveiro District, located in the Portuguese northwest coast. Hazards Parameters Population Density (inhabitants/km2)

Very low 1

Low 2

Moderated 3

High 4

Very High 5

< 100

≥ 100 and < 200

≥ 200 and < 350

≥ 350 and < 500

≥ 500

Industrial zones

Zones of specific equipment

Economic activity (settlements)

Without edification or economic activity

Ecology

Non classified

Cultural and historical values

Non existent

a)

Rural zones with agricultural activities National Agricultural Reserve Non classified

Urban zones with associated economic activities National Ecological Reserve Traditional activities and edifications

b)

Ecological protected zones Regional historical edifications

c)

Figure 6. Maps resulting from hazards parameters spatial classification for Aveiro District: a) population density; b) economic activity; c) ecology (colour scale in Table 1).

Natural parks National historical monuments

Vulnerability

Hazard 1 1 I 2 I 3 I 4 II 5 III

2 I I II III IV

3 I II III IV V

4 II III IV V V

5 III IV V V V

Risk I – Very Low II – Low III – Medium IV – High V – Very high

Figure 7. Risk classification scale crossing global vulnerability and global hazard classification.

Portuguese NW Coast Portugal is located in the Iberia Peninsula in the southwest Europe, facing the Atlantic Coast. The Portuguese northwest coast at latitudes between 40ºN and 42ºN is a highly energetic coastal sector, as may be inferred from the inspection of Figure 8, representing estimates of the global average wave power obtained with data from the WAM model (Wave Analysis Model) archive of the ECMWF (European Centre for Medium-range Weather Forecasts), according to Cruz (2008).

Continental Portugal

Figure 8. Global mean annual power estimates in kW/m obtained with data from the WAM model archive of the ECMWF calibrated and corrected with Fugro OCEANOR against a global buoy and Topex satellite altimeter database (after Cruz (2008)). In the Portuguese coast, waves usually come from NW and are characterized by an offshore mean significant wave height ranging between 2 and 3 m and a mean wave period of 8 to 12 s. Storms are frequent during maritime winter, generated mainly in the North Atlantic and they can persist for up to 5 days, with significant wave heights that may reach up to 8 m. The tides are semi-diurnal ranging between 2 and 4 m in spring tides. The mean sea level is +2 m (CD, Chart Datum). The strong wave regime induces a southward directed high alongshore transport, at rates with mean values between 1 and 2 million m3/year. The Portuguese northwest coastal sector may be divided in two stretches according to their geomorphological characteristics (Figure 8): one between Caminha and Espinho

essentially composed by low rocky formations and another between Ovar and Marinha Grande which is mostly a low-laying open sandy shore, N21ºE oriented very vulnerable to sea action, backed by dunes already affected in some places. The most important morphological features placed in this coastal sector are the Douro river estuary and the Aveiro lagoon, Figure 8. Major cities, Porto and Aveiro, respectively, bring about strong human pressures to these features. The main sediment sources for the stretch Ovar – Marinha Grande are the Douro River and coastal erosion. The first in its natural regime used to supply about 1.8 million m3/year, but this value has decreased to about 0.25 million m3/year (Oliveira, 1997) causing coastal erosion to increase, since the potential alongshore transport has not been significantly altered. In the last two decades, shoreline retreat rates have become higher, reaching 7 m/year in some stretches of the Portuguese northwest coast (Veloso-Gomes et al., 2006).

Figure 8. Littoral drift and dominant incident wave directions in the Portuguese northwest coast (left); Hypsometric map of Continental Portugal (centre, IGP); Morphostructural scheme of Continental Portugal (right, IGP). The sediment supply reduction is related with dam construction, associated river flow regulation, and past sand mining. Coastal structures also contribute to the felt erosion situation. Although harbours are fundamental in terms of socio-economic development, their long breakwaters having the two major functions of sheltering the harbour entrance and its interior (achieved by changing the wave propagation conditions) and of fixating navigation channels (by changing the sedimentary dynamics), introduce severe perturbations in the littoral drift system. The refraction, diffraction and reflection wave patterns are transformed; currents, that push sediments offshore to depths where the waves are not able to bring them back to the beach, are induced; and the littoral drift is interrupted. Besides that, external works in some harbours located in the northwest Portuguese coast (Viana do Castelo, Leixões, Aveiro e Figueira da Foz) tend to aggravate the erosion phenomenon (e.g. sand extraction on up-drift beaches, sand retention, and channels dredging without reposition down-drift).

The misruled littoral occupation and coastal erosion are linked in the beach, squeezing it. The weakening or even destruction of dune systems inhibits their function of straightening of the active beach profile reducing the land extension of waves swash, and their role in acting as natural defences having a major protection role for low-laying inner lands. Possible effects of climate change would add erosion impacts at medium to long term to the already critical situation in this coastal stretch. The likely increase of the occurrence of extreme events would directly accentuate erosion periods, and potentiate over washes jeopardizing low-laying lands. Changes on the mean annual wave climate, wave height and direction, would have direct implications in the potential alongshore transport: if larger waves are to be expected the increase in the potential alongshore transport would result in augmented coastal erosion. The acceleration of the sea level rise is related to the coastal embayment’s infilling and sand removal from the beaches, as direct morphological responses to compensate mean water depth increase. The change in the precipitation regime, with the likely increase of droughts could result in reduced river sediments supply to the shore. Some simple calculations have allowed concluding that the continued high sedimentary deficit in the Portuguese northwest coast is primarily due to Douro River sediment supply reduction, with a cessation tendency. Due to the present critical state, the generalized sea level rise, due to climate change, will only have significant importance in 50 to 100 years. The measures to recover and preserve dunes and natural defences are also important, but more severe measures are needed to face the problem (Silva et al., 2007). Under the continued erosion situation defensive measures have had to be taken, and several structures were already built along this stretch of the Portuguese northwest coast. The management of this coastal zone is a present necessity and it has to deal with some past miss measures.

Long Term Configuration Model The Long Term Configuration model (Coelho, 2005) was especially designed for sandy beaches, where the main cause of the medium term shoreline evolution is the alongshore sediment transport, dependent on the wave climate, water levels, sediment sources/sinks, sediment characteristics and boundary conditions (Silva et al., 2007). The model inputs are the changing water level and the topo-hydrography of the modelled area which is changed during calculation. The volumes transported are estimated through the application of formulae dependent on the shoreline to wave breaking angle, the wave breaking height, the beach slope and the sediment grain size. The model assumes that each wave acts during a certain period of time (the computational time step). The wave transformation by refraction, diffraction and shoaling is modelled in a simplified manner, or, wave conditions may be imported from more complex wave models like SWAN (Simulating Waves Nearshore). Due to the importance of the boundary conditions in the model simulations, several options can be made: constant sediment volumes going in or out; constant volume variations in the border sections; or extrapolation from nearby conditions. Moreover, different coastal protection works combinations may be considered with almost no limitation for the number of groins, breakwaters and seawalls, the number of sediment sources/sinks sites or artificial nourishments. This model allows the assessment of some medium term scenarios of combined anthropogenic and natural actions, permitting modelling climate changes scenarios.

Climate Change and Erosion Scenarios The referred numerical model was used to evaluate potential impacts of some climate change effects and erosion scenarios, considering the critical erosion situation of the Portuguese northwest coast and assuming that river sediment supply reduction is its major cause. The scenarios for climate change effects were established based on the following. According to Nicholls et al. (2007) the range of potential drivers of physical climate change impacts in coastal areas may be summarized as in Table 2. Changes in these drivers are subjected to regional variations. In most cases any impacts will be the result of the interaction between these climate change drivers and other drivers of change. Increases of extreme sea levels due to rises in mean sea level and/or changes in storm characteristics are of widespread concern. Future wave climate is uncertain, although extreme wave heights will likely increase with more intense storms (Meehl et al., 2007). Changes in runoff driven by changes to the hydrological cycle appear likely, but the uncertainties are large. For the numerical assessment the considered drivers were the sea level, storm intensity, and wave climate. Table 2. Main climate drivers for coastal systems, their trends due to climate change, and their main physical effects (Trend: ↑ increase; ? uncertain; R regional variability) (adapted from Nicholls et al. (2007)). Climate driver (Trend) Sea surface temperature (↑, R) Sea level (↑, R) Storm intensity (↑, R) Storm frequency (?, R) Storm track (?, R) Wave climate (?, R) Run-off (R)

Main physical effects on coastal systems Increased stratification/changed circulation; reduced incidence of sea ice at higher latitudes. Inundation, flood and storm damage; erosion; saltwater intrusion; rising water tables/impeded drainage; wetland loss (and change). Increased extreme water levels and wave heights; increased episodic erosion, storm damage, risk of flooding and defence failure. Altered surges and storm waves and hence risk of storm damage and flooding. Altered wave conditions, including swell; altered patterns of erosion and accretion; re-orientation of beach plan form. Altered flood risk in coastal lowlands; altered water quality/salinity; altered fluvial sediment supply; altered circulation and nutrient supply.

Projected global mean sea level rise under the SRES (2000) scenarios are summarised in Table 3. Local changes depart from the global mean trend due to regional variations in oceanic level change and geological uplift/subsidence; it is the relative sea level change that drives impacts and is of concern. Regional sea level change will depart significantly from the global mean trends referred in Table 3 (Nicholls et al., 2007). Table 3. Projected global mean sea level rise at the end of the 21st century for the six SRES marker scenarios (adapted from Meehl et al., 2007). SRES marker scenarios Best estimate (m) Range (m)

5% 10%

B1 0.28 0.19 0.37

Sea-level rise (relative to 1980-1999) B2 A1B A1T A2 0.32 0.35 0.33 0.37 0.21 0.23 0.22 0.25 0.42 0.47 0.44 0.5

A1FI 0.43 0.28 0.58

Dias and Taborda (1988) performed a mean sea level analysis based on elevation time series obtained from tidal gauges located in the Portuguese coast (Leixões, Cascais, Lisboa, Lagos and Angra do Heroismo) finding rising tendencies in all of them. Yet, the only stations with long enough time series for a sea level rise analysis were Cascais

(105 years) and Lagos (79 years). For these stations the values found for sea level rise were 1.3 ± 0.1 mm/year and 1.5 ± 0.2 mm/year, respectively. Using the Cascais data series the authors projected a rise between 0.14 and 0.57 m for the mean sea level rise at the end of the 21st century (about 1 to 5 mm/year), what is consistent with the global projections presented in Table 3.

Numerical Evaluation of Potential Impacts Generic Tests A generic situation was used to represent what has been happening in many stretches of the Portuguese northwest coast: a rectilinear long uniform sandy beach in equilibrium acted by a characteristic wave climate inducing a potential alongshore transport rate of the same order of magnitude as the river sediment supply to the stretch. A reduction on the sediment supply was considered and as a response the shoreline started to retreat. The coastal stretch was represented by a computational grid, 20 x 100 m2 resolution, extending over an area of 8 x 20 km2, with a regular bathymetry grid corresponding to Dean’s equilibrium profile for sediments with a median diameter, d50 = 0.3 mm, m = 2/3 and A = 0.125 (Dean, 1977), and a beach face slope of 3 %.

a)

b)

c)

d)

Figure 9. Shoreline evolution under a scenario of sediment supply reduction in 50%, at north: a) equilibrium; b) after 10 years of sediment supply reduction; c) 4 years after the construction of 2 groins to protect northern urban front; d) 10 years after the construction of 4 more groins to protect southern urban front. The wave climate was represented by an offshore constant wave coming from 80ºN, with 2 m height and having a period of 9.3 s. The sediment supply in an equilibrium situation corresponds to an income of 1.8 x 106 m3/year at north what is consistent with the estimated potential alongshore transport in the Portuguese northwest coast. In Figure 9 the shoreline evolution under a scenario of sediment supply reduction in 50% is shown, starting from the equilibrium situation (Figure 9.a)). In Figure 9.b), the beach configuration is represented after a 10 years simulation. The deficit in the sediment supply is compensated with coastal erosion, and the shoreline retreats jeopardizing the northern urban front. To protect this area an intervention is needed and 2 groins are constructed. After 4 years the southern urban front becomes in danger (Figure 9.c)) and 4 more groins are needed to protect it. After 10 years completing 24 years of continued erosion the shoreline presents the configuration shown in Figure 9.d). For comparison,

numerical simulations for the beach evolution under the same scenario were performed, considering that the coastal interventions have not taken place. In Figure 9.d) the difference between the two situations may be inferred. The shoreline configuration is far different, especially in a ray of 1 km centred in each groin, however, the urban fronts have been spared and a reduction in total land loss may been found when the interventions take place. The total lost area in the stretch due to coastal erosion, in each of the numerical tests, and the mean annual lost area are presented in Table 4. Table 4. Lost area in generic numerical tests, for a coastal stretch under a scenario of sediment supply reduction in 50% at north. Time elapsed 10 years Intervention Reference Lost area (m2) 1280165 Lost area rate (m2/year) 128017 (relative to Reference)

10+4 years 2 groins Without 1720455 1752176 122890 125155 -4% -2%

10+4+10 years 4 groins Without 2058787 2821420 85783 117559 -33% -8%

From Figure 9 and Table 4 some comments may be addressed, in spite of other beach profile generic conditions (both emerged and submerged) conduce to naturally different erosion rates: the interventions do not seem to increase the total eroded area in the coastal stretch, contrariwise they seem to reduce it, for example the total lost area 4 years after the first intervention is 1 720 455 m2 and if the intervention had not take place 1 752 176 m2; the effect of the sediment supply reduction is attenuated in time with the shoreline adjustment to a new equilibrium, after 4 years the mean annual lost area is reduced in 2% and after 14 years it is 8% smaller in the natural evolution situation; the interventions also seem to be more effective in time, since after 14 years from the reference situation, the mean annual lost area, in the stretch where the 6 groins were constructed, has decreased 33%.

Application to South Aveiro Coastal Stretch The model was also applied to a coastal stretch south from Aveiro harbour, assuming a cessation tendency in the sediment supply and the six scenarios presented in Table 5 for offshore wave climate and sea level rise. Table 5. Scenarios considered for the numerical model application to a coastal stretch south from Aveiro assuming a cessation tendency in the river sediment supply. Scenarios 1 2 3 4 5 6

Offshore wave climate Typical Typical Typical Increase in storminess Rotation to the North Rotation to the South

Sea level rise 0 mm/year 1 mm/year 5 mm/year 0 mm/year 0 mm/year 0 mm/year

In this coastal zone, the south sand spit of the Aveiro lagoon is sometimes only 200 m apart from the ocean, Figure 10. Primary littoral dune is almost continuous, with a general orientation parallel to the coastline, corresponding to the aerial beach limit. The dune crest ranges from +7 m to +18 m CD, but it has been destroyed in some parts, showing several wind corridors and being highly vulnerable to sea over washes (Ferreira, 1998). Inner from the primary dune system there are same low laying plane zones (+5 to +8 m CD) occupied with urban settlements, Figure 10.

Figure 10. Coastal sector near Vagueira settlement included in the modelled stretch (cf. Figure 11) (Ortho-photomap from CNIG (1995)). The modelled coastal stretch has been suffering of continued erosion for 20 to 30 years, and it is already protected with several groins and longitudinal adherent structures, Figure 11 (left). Topo-hydrographic information available for the study site (a nautical chart (IH, 2000) as bathymetric support, and an aerial survey (IGP, 1998) as topographic support) was used to generate a regular digital terrain model with 50 m x 100 m resolution and 20 km x 35 km extension, Figure 11 (right). The numerical model applications consisted of 25 years simulations for each of the scenarios listed in Table 5. In scenario 1, the only expected future change was the river sediment supply cessation tendency, the wave climate remained unchanged according to the typical present situation (Figure 12) and the mean sea level was considered constant. The result of this simulation is represented over ortho-photomaps together with the initial shoreline. After 25 years, three critical situations of imminent sand spit disruption become clear in the simulation results: near Costa Nova, south from Costa Nova and south from Vagueira (Figure 13.a), in front of the dark circles).

Figure 11. Coastal stretch of about 35 km located south from Aveiro harbour to which LTC model was applied (left, the existent coastal structures are sketched in an exaggerated manner; initial shoreline); Digital terrain model (right). Significant wave height distribution

Directional wave distribution

(Leixões wave buoy observations (1981-2003, IH))

Relative frequency

50% 45%

(Leixões wave buoy observations (1981-2003, IH)) N NNW 50%

40% 38%

40% 35%

30%

WNW

20% 10%

13% 13%

15% 10% 5% 0%

40%

NW

32% 30%

30% 25% 20%

6% 6%

7%

5% 4%

1% 1%

0.5

1.5

2.5

3.5

4.5

5.5

Hs (m) Storminess increase

0%

W

Typical

2%

6.5

1% 1%

0% 0%

0% 0%

7.5

8.5

9.5

E

WSW SW SSW Rotation N

S Rotation S

Typical

Figure 13. Typical wave climate (from Coelho (2005) based on observations of Leixões wave buoy (IH) between 1993 and 2003) and wave climate scenarios (Table 5) considered in the application of the LTC model to south Aveiro coastal stretch. The scenarios 2 and 3 in Table 5 concern the sea level rise (SLR) at rates of 1 mm/year and 5 mm/year, respectively, for an unchanged wave climate and a cessation tendency in sediment supply. In 25 years both of them result in an increase of less than 5% in the mean shoreline retreat rate, Figure 13.b).

The remaining three scenarios (4, 5 and 6 in Table 5) are related to wave climate changes. An increase in wave storminess was represented by an increase of 6% in the frequency of occurrence of higher waves relative to the more frequent ones, Figure 12 (left). Wave climate changes appear to have higher impacts in shoreline change than SLR, with an increase in the mean shoreline retreat rate of about 15%, Figure 13.c). A slight rotation in most frequent waves to the north and to the south (Figure 12 (right)) result in a maximum increase of 10% in the mean shoreline retreat rate, but the major effects of these scenarios appears to be related with a shoreline configuration rotation adjusting to the changes, Figure 13.d).

a)

b)

c)

d)

Figure 13. 25 years LTC simulation results for the evaluation of the effects of six different scenarios in a coastal stretch located south from Aveiro harbour. The obtained results should be taken as indicative and used for comparisons. Tidal variations were not considered and the mean sea level was admitted to be +2 m (CD). Near the north and the south boundaries of the modelled area, the sediment transport rates were extrapolated from the nearby conditions. The CERC (1984) formula was adopted for the calculation of the potential alongshore transport and the Hallermeier (1981) formula was used to estimate the closure depth of the active cross-shore profiles.

Conclusions The Portuguese NW coast suffers from a continued high sedimentary deficit that is primarily due to Douro River sediment supply reduction. The preferable solution to the erosion problem would be artificial sand nourishment, but this solution is not feasible due to the high amounts of sediments in deficit, the high wave climate energy and the costs involved. A solution may be achieved through the construction of coastal defence structures to protect urban sea fronts and the passive acceptance of erosion in intermediate stretches (Veloso-Gomes et al., 2006). The situation claims for measures to be taken, being increasingly more important to make available to decision makers scientific and technical elements to support their

decisions. To allow sustainable actions, plans should be conceived based on the coastal evolution assessment at medium to long term. Due to the inherent uncertainty, this assessment of future conditions can only be done on the basis of scenarios evaluation, for what numerical models, comprising the present state of knowledge, may be used as a tool. To help in hierarchy priorities of action, the risk along the coast must also be assessed and this can be accomplished by risk maps (as a recommendation from the EU, directive 2007/60/CE). A numerical model for medium term shoreline evolution was used to evaluate potential impacts of some climate change effects and erosion scenarios, considering the critical erosion situation of the Portuguese northwest coast and assuming that river sediment supply reduction is its major cause. A generic situation was used to represent what has been happening in many stretches of the Portuguese northwest coast where interventions had to be made. The performed generic numerical tests intended to compare the shoreline evolution with and without the interventions. From these tests some remarks were addressed: the interventions do not seem to increase the total eroded area in the coastal stretch, contrariwise they seem to reduce it; the effect of the sediment supply reduction is attenuated in time with the shoreline adjustment to a new equilibrium; the interventions also seem to be more effective in time. The model was also applied to a coastal stretch south from Aveiro harbour to evaluate scenarios concerning wave climate change and sea level rise. In 25 years of sediment supply reduction, critical situations of imminent sand spit disruption are expected and eventual connection between the sea and the lagoon. The scenarios of sea level rise are less important than the scenarios of wave climate change in 25 years. A slight increase in the relative frequency of higher waves has higher impacts than the consideration of a pessimistic sea level rise rate.

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Acknowledgements The author is acknowledged to Sonia Rey for the beach profiles collected under the Project CROP supported by the Portuguese Foundation for Science and Technology (PDCTM/P/MAR/15265/1999). Raquel Silva is acknowledged to the Portuguese Foundation for Science and Technology (PhD grant ref. SFRH/BD/19090/2004).