O sector costeiro do Algarve central entre as praias da Galé e Olhos de Água deve a sua fisiografia crenulada a um sistema cársico bem desenvolvido em rochas carbonatadas. As arribas possuem no sopé uma plataforma litoral rochosa ou uma praia arenosa. Os principais objectivos do projecto de investigação EROS são: (i) a quantificação da dissipação da energia das ondas ao longo de plataformas litorais e de praias arenosas com diferentes declives e exposição às ondas; (ii) a caracterização da refracção e difracção das ondas e da modificação das correntes forçadas pelo controle morfológico. A metodologia base centra-se na aquisição de dados de ondas e correntes utilizando transdutores de pressão e correntómetros colocados em transeptos normais à linha de costa, de modo a obter o espectro completo de ondas e correntes, desde a água profunda até aproximadamente à base das arribas, quer nas plataformas quer nas praias.FCT- Research Project PTDC/CTE-GIX/11123

Abreu, Tiago

Fortes, Conceição

Gabriel, Selma

Horta, João

Jacob, J.

Moura, Delminda

Silva, Paulo A.

Universidade do Algarve

2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012   
Erosion of rocky shores- protection promoted by sandy beaches and 
shore platforms 
D. Moura (1), S. Gabriel (1), J. Jacob (1), C.J.E.M. Fortes (2), P. A. Silva (3),  
J. Horta (1), T. Abreu (4)  
(1) Universidade do Algarve, Centro de Investigação Marinha e Ambiental (CIMA), Campus de Gambelas, 
8005-139, Faro. dmoura@ualg.pt 
(2) Laboratório Nacional de Engenharia Civil (LNEC), Av. do Brasil, 101. 1700-066 Lisboa, jfortes@lnec.pt. 
(3) Universidade de Aveiro, & CESAM, Campus Universitário de Santiago, 3810-193 Aveiro, psilva@ua.pt 
(4) Instituto Politécnico de Viseu & CESAM, ESTGV Campus de Repeses, Viseu, 3504-510, 
tabreu@estv.ipv.pt.  
 
Resumo: O sector costeiro do Algarve central entre as praias da Galé e Olhos de Água deve a sua fisiografia 
crenulada a um sistema cársico bem desenvolvido em rochas carbonatadas. As arribas possuem no sopé uma 
plataforma litoral rochosa ou uma praia arenosa. Os principais objectivos do projecto de investigação EROS são: 
(i) a quantificação da dissipação da energia das ondas ao longo de plataformas litorais e de praias arenosas com 
diferentes declives e exposição às ondas; (ii) a caracterização da refracção e difracção das ondas e da modificação 
das correntes forçadas pelo controle morfológico. A metodologia base centra-se na aquisição de dados de ondas e 
correntes utilizando transdutores de pressão e correntómetros colocados em transeptos normais à linha de costa, 
de modo a obter o espectro completo de ondas e correntes, desde a água profunda até aproximadamente à base 
das arribas, quer nas plataformas quer nas praias.  
Palavras chave: Algarve, costa rochosa, plataforma litoral, ondas, correntes.  
Key words: Algarve, rocky coast, shore platform, waves, currents.  
 
 
1. INTRODUCTION 
 
The ability and extent of waves on quarrying the 
cliff foot depends on the structure connecting the 
cliff with the sea floor (shore platform or sandy 
beach). Medium-term (centennial) cliff retreat has 
been reported as mainly controlled by the shore 
platform morphology on the cliff foot as well as by 
hydrodynamic processes acting on it (Duperret et al., 
2005; Walkden and Hall, 2005; Pierre, 2006; 
McGlashan et al., 2008). The widen of the shore 
platform driven by several processes including the 
cliff retreat, leads to the successively topographic 
elevation of the cliff-platform junction and to wave 
energy dissipation, and therefore to the decrease of 
the recession rate through a negative feedback 
(McGlashan et al., 2008). Where beaches develop at 
the cliff toe, the coastal equilibrium correlates 
positively with beach width, which responds very 
rapidly to changes on waves and tides characteristics 
(Morisawa and King, 1974). The ongoing research 
project named Erosion of Rocky Shores-differences 
in protection promoted by sandy beaches and shore 
platforms (EROS) aims, among other objectives: (i) 
to quantify the amount of the wave energy 
dissipation upon both shore platforms and sandy 
beaches; (ii) to characterize the wave and current 
fields and its dependency on the local beach 
morphology. Two study sites differently exposed to 
the incident waves are considered. This work 
presents the results of a field experiment, which led 
to the proposal of the EROS project. The data were 
collected in December 2009.  Furthermore, two 
recent field campaigns made during March 2012 are 
also described.  
 
2.  STUDY AREA 
 
The coastal zone of the centre Algarve between Galé 
and Olhos de Água (Fig.1) displays a complex 
karstic morphology leading to a strong 
morphological control of the hydrodynamic 
processes. Cliffs exposing near horizontal carbonate 
rocks back both shore platforms and beaches. 
Sandstones filling the palaeo relief into the 
carbonate rocks may reach the rocky cliffs 
lowermost portions. This crenulated shore with 
pocket beaches bounded by headlands and shore 
platforms, offers suitable conditions as a case study. 
It enables to quantify and compare the behaviour of 
waves and currents over both shore platforms and 
the adjacent beaches during the same time span and 
offshore conditions, but with different obliquity 
when approaching the coast. The waves incoming 
from WSW represent 72% of the year. Off the 
Algarve, the wave height ranges from 0.30 m to 1.8 
m, with rare exceptional heights surpassing 3.7 m. 
Such high waves are usually associated with storms 
from the SW sector, during which waves attain an 
averaged height of 2-3 m with a period of 7-8 s 
2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012   
(Costa et al., 2001). The Algarve coast experiences a 
semi-diurnal mesotidal regime ranging from 2.70 to 
1.36 m during neap tides and from 3.82 to 0.64 m 
during spring tides (Instituto Hidrográfico, IH, 
1990). 
 
 
 
 
Fig. 1. Location of the study area.  
 
3. FIELD CAMPAINGN DECEMBER 2009  
 
3.1- Methods 
Four pressure transducers (pt) were used in order to 
simultaneously acquire the wave heights (H) and 
periods (T) crossing two shore platforms at different 
angles to the incident waves. Thus, two pressure 
transducers were used in Galé (pt1 and pt2, Fig. 2A) 
and the other two in Olhos de Água (pt3 and pt4, 
Fig. 2B). The seaward transducers (pt1 at Galé and 
pt3 at Olhos de Água) were positioned in the lower 
intertidal zone at the spring low tide level, whereas 
the landward pt2 and pt4 at Galé and Olhos de Água 
respectively were placed close to the cliff foot (Fig. 
2). The height above mean sea level and 
geographical position were taken using a differential 
geographical position system (dGPS). The pt sensors 
were protected with stainless steel cages to prevent 
both the algae accumulation upon the sensors and 
the damage by gravel transported by waves. 
 
3.2- Results and discussion 
 
Changes in the direction of propagation due to 
variations in the group velocity may result in the 
increasing of wave heights when approaching the 
coast. During the monitored tidal cycle, the offshore 
wave direction was 247º and the offshore significant 
height (Hs0) was 0.9 m (IH data, Faro buoy). 
According to the coastline orientation, the waves 
approached the shoreline obliquely with angles of 
incidence of 65º and 25º at Galé and Olhos de Água 
respectively. The deep-water waves shoal at the 
Galé outer edge (pt1) show a statistically significant 
increase (P<0.05) of the significant wave height (Hs) 
of 43%. In a sheltered position to the WSW waves, 
the Hs at the shore platform outer edge (pt3) at 
Olhos de Água, was 41% lower than Hs0 but that Hs 
decrease was not statistically significant (P>0.05). 
This difference may be explained both by the 
relationship between the bathymetric contours and 
the waves direction and the bottom morphology 
(e.g., Davidson-Arnott, 2010). The inner continental 
shelf shows a steeper morphology at Galé and waves 
from WSW are less oblique to the coastline than in 
Olhos de Água.  
The water depth on the platform changes with the 
tide being the main factor controlling the changes of 
the wave height. The highest Hs was observed four 
hours after the beginning of flood at the outer 
pressure transducers, and two and one hour later at 
the inner ones at Galé and Olhos de Água 
respectively (Fig. 3). The Hs modification along the 
shore platform was noted worthy different between 
the studied sites (Fig. 3).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
      
 
Fig. 2. Shore platforms and pressure transducers at A) 
Galé and B) Olhos de Água. The accuracy of images and 
profiles are quite different because at Olhos de Água they 
were produced through a laser scan survey whereas a 
dGPS survey was used at Galé. Elevations report to mean 
sea level. 
 
 
The averaged wave Hs was 1.18 m at the outer pt in 
Galé decreasing along the platform to 0.78 m as 
registered in the landward pt (pt2) close to the cliff 
foot, representing a statistically extremely 
significance (P<0.001) decreasing of 33%.  The ratio 
between Hs and the water depth (Hs/h) ranged 
landward from 0.83 (SD=0.27) to 0.74 (SD = 0.10). 
Considering these thresholds and the Hs values in 
pt1, 65% of the waves broke before reaching the 
inner position at pt2. The remaining 35% of the 
(m) 
(m) 
A) 
(m) 
(m) 
B) 
 pt1 
pt2 
pt3 
pt4 
Legend:                location of pressure transducers  
2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012  
unbroken waves crossed the shore platform breaking 
close to the cliff foot (pt2 position). The wave power 
at breaking (Pb) in the shore platform outer edge 
(pt1) was 10819 Kw/m at Galé decreasing to more 
than half (4915 Kw/m) near the cliff foot at pt2.  
In contrast to Galé, Hs increased from 0.53 to 0.68 
m between the platform outer edge (pt3) and the 
landward pt (pt4) near the cliff foot at Olhos de 
Água. However, the observed 28% of Hs increasing 
between pt3 and pt4 was not statistically significant 
(P>0.05). Thereby, the sheltered (to WSW waves) 
and flatter shore platform at Olhos de Água did not 
show the effectiveness on dissipating the energy of 
the waves as observed at Galé. The Hs/h value of 0.6 
was quite constant along the shore platform 
traducing it relative flatness with respect to the Galé 
site. Similarly to Galé, 64% of the incoming waves 
broke before attaining the inner portion of the 
platform. However, the remaining 36% of the waves 
shoaling along the platform represented a landward 
increasing of the Pb from 2165 Kw/m in pt3 to 3750 
Kw/m pt4 close the cliff foot. 
Those results show that, together with the tidal 
elevation, the platform slope has a major role on the 
wave energy dissipation as also observed by other 
authors (Marshall and Stephenson, 2011; Ogawa et 
al., 2011).  
 
4. FIELD CAMPAINGNS MARCH 2012 
 
Aiming to further quantify the amount of the wave 
energy dissipation upon shore platforms and sandy 
beaches, two field campaigns were recently 
performed (March, 8 and 22). We deployed 
electromagnetic current meters (ecm, Infinity) and 
pressure transducers (pt, In Situ Level Troll 700) in 
normal transepts to the shoreline both in shore 
platform and beach (Fig. 4). Permanent fixings have 
been made into the rock in shore platforms using 
bolts and chemical dowels in order to locate the pt 
and ecm equipments. The near shore wave climate 
(currents and waves) was obtained through an 
acoustic doppler current meter (adp, Sonteck, Mini 
ADP 1.5 MHz) anchored at the 8 m bathymetric 
contour. The frequency of data acquisition was 2 Hz 
for pt and 4Hz for adp and ecm. A cell size of 0.25 
m and an averaging interval of 10 min with profiling 
interval of 5 min, that represents a 4 min of wave 
measurement, were adopted for the adp data 
acquisition. The experiences were performed during 
two tidal cycles, with simultaneous data acquisition 
in the shore platform and the adjacent beach, at Galé 
and Olhos de Água sites (Fig. 4). The present data 
set will be analysed in the scope of EROS project to 
give further insight of the waves dissipation both in 
the platform and beach at both study sites. 
 
 
 
5. FINAL REMARKS AND FURTHER 
WORK 
 
The role of shore platform in dissipating the wave 
energy is largely dependent of the coastal sector 
orientation and the platform slope. For the same 
offshore wave conditions, we observed that:  
1- The waves shoaling at the steeper shelf adjacent 
to the well exposed site of Galé represented 63% of 
the Hs increase with respect to the offshore Hs0, 
whereas the Hs increase at Olhos de Água, where 
the waves approached with a high obliquity, was not 
statistically significant; 
2- The shore platform at Galé was more efficient in 
energy dissipation than at the Olhos de Água 
sheltered site, where both the inner continental shelf 
and the shore platform are flatter; 
3- Hs/h ranged landward between 0.83 and 0.74 at 
Galé and showed a constant value of 0.6 across the 
platform at Olhos de Água. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.3. Measured Hs and h along the shore platforms at Galé and 
Olhos de Água (Fig. 2 for pt location).  
 
pt1- Galé
0.00
0.50
1.00
1.50
2.00
2.50
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Time (hour)
w
at
er
 d
ep
th
 (
m
)
pt2-Galé
0.00
0.50
1.00
1.50
2.00
2.50
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Time (hour)
W
at
er
 d
ep
th
 pt3-Olhos de Água
0.00
0.50
1.00
1.50
2.00
2.50
3.00
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Time (hour)
w
at
er
 d
ep
th
 (
m
)
pt4-Olhos de Água
0.00
0.50
1.00
1.50
2.00
2.50
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Time (hour)
W
at
er
 d
ep
th
 (
m
)
Legend 
Water depth (h);    Significant height (Hs);      Hs/h      
pt1 
pt2 
pt3 
pt4 
2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012   
Future work within the scope of the EROS project 
will involve: (i) the pt, ecm, adv and adp data 
processing, (ii) the comparison of the waves and 
currents behaviours both in the shore platform and 
adjacent beach in the study area and, (iii) the 
numerical and physical modelling. 
 
 
 
 
 
 
 
Fig. 4. Equipment position during the March field campaigns 
 
Acknowledgments 
The Research Project PTDC/CTE-GIX/111230/2009 
(EROS) is supported by the Portuguese Foundation 
for Science and Technology (FCT). The authors 
thank Comandante Cruz Martins, Capitão do Porto 
de Portimaão, the permission of field campaigns. 
REFERENCES 
 
Costa, M., Silva, R., Vitorino, J. (2001). 
Contribuição para o estudo do clima de agitação 
marítima na costa portuguesa. 2as Jornadas 
Portuguesas de Engenharia Costeira e 
Portuária. Associação Nacional de Navegação. 
Sines, 20pp. 
Davidson-Arnott, R. (2010). Introduction to coastal 
processes and geomorphology. Cambridg 
University Press, 442pp. 
Duperret, A., Taibi, S., Mortimore, R.N., 
Daigneault, M. (2005). Effect of ground water 
and sea weathering cycles on the strength of 
chalk rock from unstable coastal cliffs of NW 
France. Engineering Geology, 78, 321-343. 
Instituto Hidrográfico, 1990. Roteiro da Costa de 
Portugal.  Lisboa, 41pp. 
MacGlashan, D.J., Duck, R.W., Reidi, C.T. (2008). 
Unstable boundaries on a cliffed coast: 
Geomorphology and British laws. Journal of 
Coastal Research, 24, 1A, 181-188.  
Marshall, R.J.E., and Stephenson, W.J. (2011). The 
morphodynamics of shore platforms in a micro-
tidal setting: interactions between waves and 
morphology. Marine Geology, 2888, 1-4, 18-31. 
Morisawa, M., King, C.A.M., (1974). Monitoring 
the coastal environment. Geology, 2, 385-388.  
Ogawa, H., Dickson, M.E., Kench, P.S. (2011). 
Wave transformation on a sub-horizontal shore 
platform, Tatapouri, North Island, New Zealand. 
Continental Shelf Research, 31, 1409-1419. 
Pierre, G., (2006). Processes and rate of retreat of 
the clay and sandstone sea cliffs of the northern 
Boulonnais (France). Geomorphology, 73, 64-77.  
Walkden, M.J.A. and Hall, J.W. (2005). A predictive 
mesoscale model of the erosion and profile 
development of soft rock shores. Coastal 
Engineering, 52, 535-563. 
 
 
 
 
 
 


Erosion of rocky shores- protection promoted by sandy beaches and shore platforms

Sapientia

2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012   Erosion of rocky shores- protection promoted by sandy beaches and shore platforms D. Moura (1), S. Gabriel (1), J. Jacob (1), C.J.E.M. Fortes (2), P. A. Silva (3),  J. Horta (1), T. Abreu (4)  (1) Universidade do Algarve, Centro de Investigação Marinha e Ambiental (CIMA), Campus de Gambelas, 8005-139, Faro. dmoura@ualg.pt (2) Laboratório Nacional de Engenharia Civil (LNEC), Av. do Brasil, 101. 1700-066 Lisboa, jfortes@lnec.pt. (3) Universidade de Aveiro, & CESAM, Campus Universitário de Santiago, 3810-193 Aveiro, psilva@ua.pt (4) Instituto Politécnico de Viseu & CESAM, ESTGV Campus de Repeses, Viseu, 3504-510, tabreu@estv.ipv.pt.   Resumo: O sector costeiro do Algarve central entre as praias da Galé e Olhos de Água deve a sua fisiografia crenulada a um sistema cársico bem desenvolvido em rochas carbonatadas. As arribas possuem no sopé uma plataforma litoral rochosa ou uma praia arenosa. Os principais objectivos do projecto de investigação EROS são: (i) a quantificação da dissipação da energia das ondas ao longo de plataformas litorais e de praias arenosas com diferentes declives e exposição às ondas; (ii) a caracterização da refracção e difracção das ondas e da modificação das correntes forçadas pelo controle morfológico. A metodologia base centra-se na aquisição de dados de ondas e correntes utilizando transdutores de pressão e correntómetros colocados em transeptos normais à linha de costa, de modo a obter o espectro completo de ondas e correntes, desde a água profunda até aproximadamente à base das arribas, quer nas plataformas quer nas praias.  Palavras chave: Algarve, costa rochosa, plataforma litoral, ondas, correntes.  Key words: Algarve, rocky coast, shore platform, waves, currents.    1. INTRODUCTION  The ability and extent of waves on quarrying the cliff foot depends on the structure connecting the cliff with the sea floor (shore platform or sandy beach). Medium-term (centennial) cliff retreat has been reported as mainly controlled by the shore platform morphology on the cliff foot as well as by hydrodynamic processes acting on it (Duperret et al., 2005; Walkden and Hall, 2005; Pierre, 2006; McGlashan et al., 2008). The widen of the shore platform driven by several processes including the cliff retreat, leads to the successively topographic elevation of the cliff-platform junction and to wave energy dissipation, and therefore to the decrease of the recession rate through a negative feedback (McGlashan et al., 2008). Where beaches develop at the cliff toe, the coastal equilibrium correlates positively with beach width, which responds very rapidly to changes on waves and tides characteristics (Morisawa and King, 1974). The ongoing research project named Erosion of Rocky Shores-differences in protection promoted by sandy beaches and shore platforms (EROS) aims, among other objectives: (i) to quantify the amount of the wave energy dissipation upon both shore platforms and sandy beaches; (ii) to characterize the wave and current fields and its dependency on the local beach morphology. Two study sites differently exposed to the incident waves are considered. This work presents the results of a field experiment, which led to the proposal of the EROS project. The data were collected in December 2009.  Furthermore, two recent field campaigns made during March 2012 are also described.   2.  STUDY AREA  The coastal zone of the centre Algarve between Galé and Olhos de Água (Fig.1) displays a complex karstic morphology leading to a strong morphological control of the hydrodynamic processes. Cliffs exposing near horizontal carbonate rocks back both shore platforms and beaches. Sandstones filling the palaeo relief into the carbonate rocks may reach the rocky cliffs lowermost portions. This crenulated shore with pocket beaches bounded by headlands and shore platforms, offers suitable conditions as a case study. It enables to quantify and compare the behaviour of waves and currents over both shore platforms and the adjacent beaches during the same time span and offshore conditions, but with different obliquity when approaching the coast. The waves incoming from WSW represent 72% of the year. Off the Algarve, the wave height ranges from 0.30 m to 1.8 m, with rare exceptional heights surpassing 3.7 m. Such high waves are usually associated with storms from the SW sector, during which waves attain an averaged height of 2-3 m with a period of 7-8 s 2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012   (Costa et al., 2001). The Algarve coast experiences a semi-diurnal mesotidal regime ranging from 2.70 to 1.36 m during neap tides and from 3.82 to 0.64 m during spring tides (Instituto Hidrográfico, IH, 1990).     Fig. 1. Location of the study area.   3. FIELD CAMPAINGN DECEMBER 2009   3.1- Methods Four pressure transducers (pt) were used in order to simultaneously acquire the wave heights (H) and periods (T) crossing two shore platforms at different angles to the incident waves. Thus, two pressure transducers were used in Galé (pt1 and pt2, Fig. 2A) and the other two in Olhos de Água (pt3 and pt4, Fig. 2B). The seaward transducers (pt1 at Galé and pt3 at Olhos de Água) were positioned in the lower intertidal zone at the spring low tide level, whereas the landward pt2 and pt4 at Galé and Olhos de Água respectively were placed close to the cliff foot (Fig. 2). The height above mean sea level and geographical position were taken using a differential geographical position system (dGPS). The pt sensors were protected with stainless steel cages to prevent both the algae accumulation upon the sensors and the damage by gravel transported by waves.  3.2- Results and discussion  Changes in the direction of propagation due to variations in the group velocity may result in the increasing of wave heights when approaching the coast. During the monitored tidal cycle, the offshore wave direction was 247º and the offshore significant height (Hs0) was 0.9 m (IH data, Faro buoy). According to the coastline orientation, the waves approached the shoreline obliquely with angles of incidence of 65º and 25º at Galé and Olhos de Água respectively. The deep-water waves shoal at the Galé outer edge (pt1) show a statistically significant increase (P<0.05) of the significant wave height (Hs) of 43%. In a sheltered position to the WSW waves, the Hs at the shore platform outer edge (pt3) at Olhos de Água, was 41% lower than Hs0 but that Hs decrease was not statistically significant (P>0.05). This difference may be explained both by the relationship between the bathymetric contours and the waves direction and the bottom morphology (e.g., Davidson-Arnott, 2010). The inner continental shelf shows a steeper morphology at Galé and waves from WSW are less oblique to the coastline than in Olhos de Água.  The water depth on the platform changes with the tide being the main factor controlling the changes of the wave height. The highest Hs was observed four hours after the beginning of flood at the outer pressure transducers, and two and one hour later at the inner ones at Galé and Olhos de Água respectively (Fig. 3). The Hs modification along the shore platform was noted worthy different between the studied sites (Fig. 3).                                        Fig. 2. Shore platforms and pressure transducers at A) Galé and B) Olhos de Água. The accuracy of images and profiles are quite different because at Olhos de Água they were produced through a laser scan survey whereas a dGPS survey was used at Galé. Elevations report to mean sea level.   The averaged wave Hs was 1.18 m at the outer pt in Galé decreasing along the platform to 0.78 m as registered in the landward pt (pt2) close to the cliff foot, representing a statistically extremely significance (P<0.001) decreasing of 33%.  The ratio between Hs and the water depth (Hs/h) ranged landward from 0.83 (SD=0.27) to 0.74 (SD = 0.10). Considering these thresholds and the Hs values in pt1, 65% of the waves broke before reaching the inner position at pt2. The remaining 35% of the (m) (m) A) (m) (m) B)  pt1 pt2 pt3 pt4 Legend:                location of pressure transducers  2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012  unbroken waves crossed the shore platform breaking close to the cliff foot (pt2 position). The wave power at breaking (Pb) in the shore platform outer edge (pt1) was 10819 Kw/m at Galé decreasing to more than half (4915 Kw/m) near the cliff foot at pt2.  In contrast to Galé, Hs increased from 0.53 to 0.68 m between the platform outer edge (pt3) and the landward pt (pt4) near the cliff foot at Olhos de Água. However, the observed 28% of Hs increasing between pt3 and pt4 was not statistically significant (P>0.05). Thereby, the sheltered (to WSW waves) and flatter shore platform at Olhos de Água did not show the effectiveness on dissipating the energy of the waves as observed at Galé. The Hs/h value of 0.6 was quite constant along the shore platform traducing it relative flatness with respect to the Galé site. Similarly to Galé, 64% of the incoming waves broke before attaining the inner portion of the platform. However, the remaining 36% of the waves shoaling along the platform represented a landward increasing of the Pb from 2165 Kw/m in pt3 to 3750 Kw/m pt4 close the cliff foot. Those results show that, together with the tidal elevation, the platform slope has a major role on the wave energy dissipation as also observed by other authors (Marshall and Stephenson, 2011; Ogawa et al., 2011).   4. FIELD CAMPAINGNS MARCH 2012  Aiming to further quantify the amount of the wave energy dissipation upon shore platforms and sandy beaches, two field campaigns were recently performed (March, 8 and 22). We deployed electromagnetic current meters (ecm, Infinity) and pressure transducers (pt, In Situ Level Troll 700) in normal transepts to the shoreline both in shore platform and beach (Fig. 4). Permanent fixings have been made into the rock in shore platforms using bolts and chemical dowels in order to locate the pt and ecm equipments. The near shore wave climate (currents and waves) was obtained through an acoustic doppler current meter (adp, Sonteck, Mini ADP 1.5 MHz) anchored at the 8 m bathymetric contour. The frequency of data acquisition was 2 Hz for pt and 4Hz for adp and ecm. A cell size of 0.25 m and an averaging interval of 10 min with profiling interval of 5 min, that represents a 4 min of wave measurement, were adopted for the adp data acquisition. The experiences were performed during two tidal cycles, with simultaneous data acquisition in the shore platform and the adjacent beach, at Galé and Olhos de Água sites (Fig. 4). The present data set will be analysed in the scope of EROS project to give further insight of the waves dissipation both in the platform and beach at both study sites.    5. FINAL REMARKS AND FURTHER WORK  The role of shore platform in dissipating the wave energy is largely dependent of the coastal sector orientation and the platform slope. For the same offshore wave conditions, we observed that:  1- The waves shoaling at the steeper shelf adjacent to the well exposed site of Galé represented 63% of the Hs increase with respect to the offshore Hs0, whereas the Hs increase at Olhos de Água, where the waves approached with a high obliquity, was not statistically significant; 2- The shore platform at Galé was more efficient in energy dissipation than at the Olhos de Água sheltered site, where both the inner continental shelf and the shore platform are flatter; 3- Hs/h ranged landward between 0.83 and 0.74 at Galé and showed a constant value of 0.6 across the platform at Olhos de Água.                                     Fig.3. Measured Hs and h along the shore platforms at Galé and Olhos de Água (Fig. 2 for pt location).   pt1- Galé0.000.501.001.502.002.500.0 2.0 4.0 6.0 8.0 10.0 12.0Time (hour)water depth (m)pt2-Galé0.000.501.001.502.002.500.0 2.0 4.0 6.0 8.0 10.0 12.0Time (hour)Water depth pt3-Olhos de Água0.000.501.001.502.002.503.000.0 2.0 4.0 6.0 8.0 10.0 12.0Time (hour)water depth (m)pt4-Olhos de Água0.000.501.001.502.002.500.0 2.0 4.0 6.0 8.0 10.0 12.0Time (hour)Water depth (m)Legend Water depth (h);    Significant height (Hs);      Hs/h      pt1 pt2 pt3 pt4 2as Jornadas de Engenharia Hidrográfica                                                                     Lisboa, 20-22 de junho de 2012   Future work within the scope of the EROS project will involve: (i) the pt, ecm, adv and adp data processing, (ii) the comparison of the waves and currents behaviours both in the shore platform and adjacent beach in the study area and, (iii) the numerical and physical modelling.        Fig. 4. Equipment position during the March field campaigns  Acknowledgments The Research Project PTDC/CTE-GIX/111230/2009 (EROS) is supported by the Portuguese Foundation for Science and Technology (FCT). The authors thank Comandante Cruz Martins, Capitão do Porto de Portimaão, the permission of field campaigns. REFERENCES  Costa, M., Silva, R., Vitorino, J. (2001). Contribuição para o estudo do clima de agitação marítima na costa portuguesa. 2as Jornadas Portuguesas de Engenharia Costeira e Portuária. Associação Nacional de Navegação. Sines, 20pp. Davidson-Arnott, R. (2010). Introduction to coastal processes and geomorphology. Cambridg University Press, 442pp. Duperret, A., Taibi, S., Mortimore, R.N., Daigneault, M. (2005). Effect of ground water and sea weathering cycles on the strength of chalk rock from unstable coastal cliffs of NW France. Engineering Geology, 78, 321-343. Instituto Hidrográfico, 1990. Roteiro da Costa de Portugal.  Lisboa, 41pp. MacGlashan, D.J., Duck, R.W., Reidi, C.T. (2008). Unstable boundaries on a cliffed coast: Geomorphology and British laws. Journal of Coastal Research, 24, 1A, 181-188.  Marshall, R.J.E., and Stephenson, W.J. (2011). The morphodynamics of shore platforms in a micro-tidal setting: interactions between waves and morphology. Marine Geology, 2888, 1-4, 18-31. Morisawa, M., King, C.A.M., (1974). Monitoring the coastal environment. Geology, 2, 385-388.  Ogawa, H., Dickson, M.E., Kench, P.S. (2011). Wave transformation on a sub-horizontal shore platform, Tatapouri, North Island, New Zealand. Continental Shelf Research, 31, 1409-1419. Pierre, G., (2006). Processes and rate of retreat of the clay and sandstone sea cliffs of the northern Boulonnais (France). Geomorphology, 73, 64-77.  Walkden, M.J.A. and Hall, J.W. (2005). A predictive mesoscale model of the erosion and profile development of soft rock shores. Coastal Engineering, 52, 535-563.       

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Erosion of rocky shores- protection promoted by sandy beaches and shore platforms

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