Littoral 2002, The Changing Coast. EUROCOAST / EUCC, Porto – Portugal Ed. EUROCOAST – Portugal, ISBN 972-8558-09-0
Preliminary Evaluation of the Morphological Evolution of Contrasting Beaches of the Ria Formosa With Relation to Differences in Refracted Wave Height. C. Sá-Pires1, Ó. Ferreira2, A. Matias1 & J.A. Dias2 1
CIACOMAR, Universidade do Algarve, Av. 16 Junho s/n, 8700-311 Olhão, Portugal Phone: +351289707087; Fax: +351289706972 E-mails:
[email protected],
[email protected] 2 FCMA, Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugal Phone: +351289800900; Fax: +351289818353 E-mails:
[email protected],
[email protected]
Abstract The Ria Formosa is a barrier island system in the Algarve (southern Portugal) with an unusual delta shape. This system is highly dynamic, exposed to both wave and tidal currents. Due to the unusual delta shape of the system, the eastern part is protected from the dominant wave directions, while the western part is directly exposed. As a consequence, the beaches along the barrier islands range from reflective to intermediate at the western end, and from intermediate to dissipative at the eastern end. The main purpose of the present study is to determine time scales of volumetric changes and recovery episodes for two contrasting sites (Ancão Peninsula and Cabanas Island) and to understand the association with differences in wave energy and morphodynamic characteristics. The study period was from October 1999 to November 2000, with monthly topographic monitoring. The average beach volume and the average beach face changes were correlated with the average refracted wave height for the month and week before each survey and for the most energetic five days between surveys. Differences in wave energy at the breaking zone were observed when comparing Ancão Peninsula with Cabanas Island. For the study period, Ancão Peninsula had 15.9% of “calms” (significant wave height smaller than 0.12 m) while Cabanas had 67.0%. From correlation results, Ancão Peninsula showed a more rapid response to changes in wave climate than Cabanas Island, morphological changes being generally associated with the most energetic five days of the month prior to the survey. Volumetric and morphological changes at Cabanas Island were slower and a background behaviour seems to prevail over the short term changes. The results are in agreement with the dynamics of the system, where the refraction process is extremely important for the definition of wave characteristics at the breaker zone for both sides of the system. These differences must be considered in the future for planning monitoring programs. characteristics in association with the differences in the dominant physical process (Dias, 1988). The morphological content of all islands and peninsulas of Ria Formosa are described by Pilkey et al. (1989) and Andrade (1990). The beaches of the system consist in general of medium grain size sand. Their cross-shore extent varies along the system, being globally larger in the eastern part (Teixeira et al., 1989; Andrade, 1990). The offshore wave climate is dominated by westerly and southwesterly waves, with 67.8% of occurrences (Costa, 1994). Southeasterly waves associated with locally generated sea (“levante”) are also important, with 25% of occurrences (Costa, 1994). The mean wave energy is low to moderate; however, storm events (significant offshore wave height higher than 3 m) are frequent during winter, mainly induced by Atlantic swells approaching from the southwest (Costa, 1994).
1. INTRODUCTION In the Algarve, southern Portugal, the Ria Formosa is considered to be the dominant physiographic unit. The Ria Formosa is a barrier island system, composed by five islands and two peninsulas separated by six inlets, defining an internal lagoon body (Figure 1). According to Andrade (1990), this system extends longshore over 55 Km and the maximum distance between the system and mainland is approximately 6 Km near Cape Sta. Maria. This barrier island system, unusually, is not found adjacent to a coastal plain, and also has an unusual delta shape. Pilkey et al. (1989) consider that the barrier island system of Ria Formosa is exceptional in its their physiographic setting, and in the particular combination of environmental parameters responsible for its origin and evolution. The barrier island system is highly dynamic, exposed to both wave and tidal currents (Dias 1988, Teixeira et al., 1989, Andrade 1990). Each island or peninsula has different features and
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Figure 1: Location of the Ria Formosa barrier island system. Also shown are the locations of the two study areas.
A monitoring program was designed to determine morphological and volumetric changes induced by nourishment along the Ria Formosa, as well as to allow an understanding of naturalisation processes after replenishment. The main purpose of the work presented here is to understand the association between the morphodynamic characteristics, including beach response, and the differences in incident wave energy To compare volumetric changes and recovery episodes of contrasting sites of the barrier island system of the Ria Formosa, two sites were selected: Ancão Peninsula and Cabanas Island (see Figure 1).
Due to the unusual delta shape of the system, the eastern part is protected to dominant wave directions, while the western part is directly exposed. As a consequence, the beaches along the barrier islands range from reflective to intermediate at the western end (Martins et al., 1996), and from intermediate to dissipative at the eastern end (Matias et al., 1998). Mean tidal range is about 2 m, reaching up to 3.4 m at spring tides. There are several previous studies that considered the morphological evolution of beach profiles and their correlation with wave climate in the Ria Formosa. However, most of these studies use general offshore wave characteristics (Granja 1984; Teixeira et al., 1989; Andrade, 1990; Martins et al.,1996; Martins et al., 1997; Ferreira et al., 1997). The exception is a study of the morphological evolution of the Cacela Peninsula with relation to the breaking wave characteristics (Matias, 2000) This barrier island system is controlled by the Ria Formosa Natural Park (RFNP) who have a policy of soft intervention. One of the most recent interventions was a general tidal channel dredging program with associated beach and dune replenishment, between April 1999 and July 2000. The entire amount of dredged sediments was approximately 2 408 800 m3. These sediments were mainly distributed along the two peninsulas (Ancão and Cacela) and three of the islands (Armona, Tavira and Cabanas). The nourishment sediments were placed at the backshore or at the upper foreshore.
2. METHODS A monitoring program was performed from April 1999 until December 2000 to determine morphological and volumetric changes along the Ria Formosa beaches prior to and after nourishment. The monitoring net included 37 profiles, with monthly topographic surveys, in both replenished and natural sites, on islands and peninsulas that underwent intervention. Cabanas Island was nourished between, April and June 1999, and Ancão Peninsula was replenished from February to March 2000. The details of the five profiles monitored at each site are shown in Table 1. For each profile the volume (m3/m) above mean sea level was computed. For each study site, volume changes at natural and replenished profiles were respectively grouped. The cumulative volume changes through time (April 1999 to December 2000 – period of monitoring plan) for natural and replenished profiles were computed. 424
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Natural profiles
Nourished profiles
Figure 2: Cumulative volume changes for Cabanas Island. 150
3
Cumulative volume changes (m /m)
Since the offshore significant wave height can not be considered as representative of the actual conditions at the breaker zone for the entire system (Matias, 2000), the wave characteristics (significant wave height, peak period and direction) obtained at an offshore a wave rider buoy every 3 hours were refracted for both study areas, using the WBREAK refraction model (described in Matias, 2000). This program assumes a regular and parallel bathymetry in the nearshore, and normal incident angle to the beach must be zero. After refraction, the average significant breaking wave height for the month and week before each survey and for the most energetic five days between surveys was determined. These values were correlated with the average beach volume changes and the average beach face slope, for the period October 1999 to November 2000.
100
Storm
Nourishment
50 0 -50 -100 -150
Natural profiles
Nourished profiles
Figure 3: Cumulative volume changes for Ancão Peninsula.
The average beach face slope was determined for both natural and replenished profiles, for each site (Figures 4 and 5). On Cabanas Island (Figure 4) the natural and nourished profiles had some differences in absolute values of change, however, they had in general similar behaviour. A southeast storm occurred after the survey of April 2000, and an increase in beach face slope was observed (Figure 4). On Ancão Peninsula (Figure 5) the average beach face slope changes was essentially the same for natural and nourished profiles. Therefore, an average beach face slope was determine for each site, considering both natural and nourished profiles (Figure 6). 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
Natural profiles
N-00
D-00
S-00
O-00
J-00
A-00
J-00
A-00
Storm
O-99
Beach face slope
3. RESULTS The cumulative volume changes above mean sea level, for natural and nourished profiles are shown in Figures 2 and 3, for Cabanas Island and Ancão Peninsula respectively. The replenished profiles at both sites had a similar behaviour to the natural ones (Figures 2 and 3). After replenishment all profiles experienced seasonal changes with a net loss of sediments during winter, including erosion associated with a storm event (April 2000), and net gains of sediments during summer. Transition to a summer condition occurs after the April 2000 storm, with a quicker recovery after the storm on Ancão Peninsula and a slower one on Cabanas Island. Thus for the period from October 1999 to November 2000, a period with overlapping surveys for both sites, average volume changes were determined for each site, considering both natural and nourished profiles. The computed volumetric changes were higher for Ancão Peninsula, ranging from an erosion of 30.6m3/m in April 2000 to an accretion of 28.4m3/m in May 2000. Volumetric changes of Cabanas beach were between an erosion of 19.0m3/m in April 2000 and an accretion of 12.5m3/m in October 2000.
M-00
3
M-00
2
2
-150
J-00
3
Cabanas Island
-100
F-00
Ancão Peninsula
0 -50
D-99
PROFILES
Storm
A-99 M-99 J-99 J-99 A-99 S-99 O-99 N-99 D-99 J-00 F-00 M-00 A-00 M-00 J-00 J-00 A-00 S-00 O-00 N-00 D-00
PROFILES
Nourishment
50
N-99
NOURISHED
100
A-99 M-99 J-99 J-99 A-99 S-99 O-99 N-99 D-99 J-00 F-00 M-00 A-00 M-00 J-00 J-00 A-00 S-00 O-00 N-00 D-00
Table 1: Natural versus nourished profiles on Ancão Peninsula and Cabanas Island. NATURAL
150
3
Cumulative volume changes (m /m)
The beach face slope was also computed for each profile, and for each site the natural and replenished profiles were respectively grouped.
Nourished profiles
Figure 4: Beach face slope average changes for natural and nourished profiles on Cabanas Island.
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0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
Table 2: General offshore wave climate for the Algarve, during the study period (October 1999 to November 2000). Values represent annual averages and extreme monthly averages. Hso: significant offshore wave height; Tp: peak period. ANNUAL AVERAGE
D-00
S-00
O-00 N-00
J-00 A-00
A-00
M-00 J-00
F-00 M-00
N-99
D-99 J-00
Storm
Natural profiles
Nourished profiles
MAXIMUM
MONTHLY AVERAGE
0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
N-00
D-00
S-00
O-00
J-00
A-00
J-00
A-00
M-00
M-00
J-00
F-00
D-99
N-99
O-99
71.0
46.6 (Nov99)
93.7 (Out99)
Hso (m)
0.8
0.6 (Jul00)
1.4 (Apr00)
Tp (s)
9.1
6.1 (Jul00)
12.8 (Feb00)
% occurrence
27.3
6.3 (Out99)
48.7 (Nov99)
Hso (m)
1.1
0.6 (Out00)
1.7 (Apr00)
Tp (s)
5.6
3.1 (Jun00)
6.6 (Mar00)
After wave refraction Ancão Peninsula beach had 15.9% of “calms” and Cabanas 67.0% during the study period, thus showing the differences in wave energy at the breaking zone at the two sites (Table 4).
Storm
Cabanas Island
% occurrence
SE-E
Figure 5: Beach face slope average changes for natural and nourished profiles for Ancão Peninsula.
Beach face slope
MINIMUM
W-SW
O-99
Beach face slope
The average beach face slope changes for Cabanas Island and Ancão Peninsula were different in absolute values but very similar in behaviour (Figure 6). Ancão Peninsula was more reflective with an average beach face slope of 0.12, and frequently had beach cusp systems. Cabanas Island was less reflective, with an average beach face slope of 0.08.
Ancão Peninsula
Figure 6: Beach face slope average changes for Cabanas Island and Ancão Peninsula.
Table 3: Absolute observed maximums were recorded during the April 2000 storm. Duration of the storm; Hso: average significant offshore wave height; Tp: average peak period and average direction are shown.
During the study period the offshore wave climate (from the Faro waverider buoy) was dominated by west-southwest incidence directions with 71.0 % of occurrences (Table 2). The highest monthly averaged significant wave height was observed in April 2000, with 1.7 m and highest monthly averaged periods were approximately 12.8 s, in February 2000. Absolute observed maximums were recorded during the April 2000 storm (Table 3), with daily maximum significant offshore wave height of 3.6 m and an associated peak period of 9.5 s, incident from the southeast. Periods of low incident wave energy at the breaking zone were designated as “calms”. These “calms” were defined as waves with less than 0.12 m on Ancão Peninsula and 0.13 m on Cabanas Island, corresponding to the smallest value recorded for each site. Wave directions with an offshore approach angle greater than 90o, therefore impossible to refract, were also considered as “calms”.
DURATION HSO (M)
TP (S)
DIRECTION
2 - 3 April 2002
17h
3.3
9.6
SW
7 April 2002
17h
3.6
9.5
SE
9 April 2002
10h
3.2
9.5
W
Table 4 shows general results of the wave climate at the breaking zone for Ancão Peninsula and Cabanas Island, for the study period (October 1999 to November 2000), resulting from the refraction of offshore records.
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Table 5: Correlation values between average incident wave heights obtained for different periods and average beach face slope or average beach volume changes. Values in bold are significant at a 0.05 level. Hb w: average breaking wave height for the week before each survey; Hb m: average breaking wave height for the month before each survey and Hb 5: average breaking wave height for the most energetic five days between surveys.
Table 4: General breaking wave climate for Ancão Peninsula and Cabanas Island, for the study period (October 1999 to November 2000). Values represent annual averages and extreme monthly averages. Hb: average breaking wave height; Tp: average peak period. ANNUAL AVERAGE
MINIMUM
MAXIMUM
MONTHLY AVERAGE
Ancão Peninsula 15.9
3.0 (Apr00)
33.3 (Nov99)
Hb (m)
0.8
0.5 (Jul00)
1.5 (Apr00)
Tp (s)
8.5
5.9 (Jul and Aug00)
11.9 (Feb00)
% calms
67.0
93.3 (Out00)
Hb (m)
0.5
0.2 (Out00)
0.7 (Nov99 and Mar00)
Tp (s)
6.5
5.8 (Nov99 and Jan00)
7.5 (Apr00)
% calms
Hb m
Hb 5
-0.51
-0.48
-0.74
-0.76
-0.86
-0.88
0.13
-0.79
-0.72
0.26
-0.74
-0.48
Ancão Peninsula Volume (m3/m) Beach face slope Cabanas Island Volume (m3/m) Beach face slope
Cabanas Island 47.9 (Nov99)
Hb w
4. DISCUSSION From the results of the cumulative volume changes it is possible to observe a faster recovery after the storm in April 2000 at Ancão Peninsula than at Cabanas Island. This trend is expected since the southwest facing beaches are more reflective than the southeast ones. In fact, the western part of the system is in general more energetic being exposed to the open Atlantic southwest waves, and therefore changes occur more rapidly than in the eastern part. These results also agree with those obtained by Martins et al. (1996) for Ancão Peninsula and by Matias (2002) for Cacela Peninsula. These authors found similar trends in response to erosion, with significant losses of sediments in a few hours or few days. However, the recovery period was quite different with a maximum beach recovery of 22 m3/m/day in Ancão Peninsula (Martins et al., 1996) while at Cacela Peninsula the maximum recovery was of 13m3/m/week (Matias, 2000). On the contrary, Teixeira et al. (1989) and Andrade (1990) considered that recovery rates are similar for the entire system. However, they analysed monthly profiles without correlating them with wave data at the breaking zone. Given the same offshore wave regime, the wave energy at the breaking zone can be quite different in both sectors of the barrier island system due to the differences in coastline direction (Table 2 and 4). While the westsouthwest waves are more frequent, therefore being responsible for the majority of the observed beach changes, the occurrence of “levante” can be also important. The smaller fetch southeast waves are less frequent than west-southwest direction waves, however their significant wave height is generally higher.
Correlation values between average incident wave heights (for the month and week before each survey and for the most energetic five days between surveys) and average beach volume changes and average beach face slope were calculated for both sites using a statistical significant level of 0.05 (Table 5). Significant values are above 0.754 considering the number of elements that are compared. Negative correlations mean that higher waves induce reduction on beach volume changes and smaller beach face slope. According to the results in Table 5 the beach face slope at Ancão Peninsula has a significant correlation with the wave height for all periods considered, with a higher value for the correlation with the most energetic five days of the month prior to the survey. This period also shows a good, but not significant, correlation with the average beach volume changes. At Cabanas, a significant correlation is obtained for the relationship between volume changes and wave height for the month prior to the survey. The beach face slope and the wave height have a high, nearing significant, correlation for the same period. The correlation between the mean average wave height and the beach volume changes or the beach face mean slope for Cabanas Island are the weakest of all obtained values. 427
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This contrasting behaviour for the two sides of the Ria Formosa barrier island system seems to be related to the different breaking conditions to which they are exposed. The differences in morphologic behaviour described in this study must be considered in the future, for planning monitoring programs.
As a consequence, to characterise the main processes in the barrier island system of the Ria Formosa, breaking wave height should be considered instead of the offshore wave height. The results from Table 5 seems to indicate that on Ancão Peninsula breaking wave height has a stronger influence on beach morphology changes (beach face slope) than on volumetric changes. Ancão Peninsula also seems to have a quicker response to changes in wave climate than Cabanas Island, with morphologic changes generally associated with the most energetic five days of the month prior to the survey. This more rapid response was already noted from the results of the volume variations analysis. At Cabanas Island the profile response seem to be dependent on monthly wave characteristics, having a very low dependence on the last week wave conditions. Volumetric changes at Cabanas Island beaches are slower with a background behaviour influencing the beach response to the changes in breaking wave height. These differences in profile response to the same offshore wave regime should be considered for planning monitoring programs. To correctly describe the morphological evolution of a beach, longer and sparser time series should be considered for the eastern part of the system. On the contrary, survey periodicity should be increased for the western part.
ACKNOWLEDGEMENTS This work was partially funded by the projects “Monitorização das dragagens da Ria Formosa” (protocolo IMAR – Instituto do Mar / ICN – Instituto de Conservação da Natureza) and CROP (PDCTM/P/MAR/15265/1999). Ana Matias was supported by the Fundação para a Ciência e a Tecnologia, grant number SFRH/BD/1356/2000. The authors kindly acknowledge all the people that helped during field work, and all colleagues at CIACOMAR, especially Célia Duarte who performed this monitoring program in its early stages. The authors also want to thank Rui Taborda, for allowing the use of the WBREAK model and to the Instituto Hidrográfico for access to the wave data from the Faro waverider buoy. REFERENCES Andrade, C., 1990. O ambiente de barreira da Ria Formosa. Algarve - Portugal. PhD Thesis, Universidade de Lisboa, 645 p. Costa, C.L., 1994. Final report of sub-project A. Wind wave climatology of the Portuguese coast. Report PO-WAVES 6/94-A, IH/LNEC, 80 p. Dias, J.A., 1988. Aspectos geológicos do Litoral Algarvio. Geonovas, 10, 113-128. Ferreira, Ó., Martins, J.T. and Dias, J.M.A., 1997. Morfodinâmica e vulnerabilidade da Praia de Faro. In: Seminário sobre a zona costeira do Algarve, 67-76. Granja, H., Froidefond, J-M., and Pera, T., 1984. Processus d∋evolution morpho-sedimentaire de la Ria Formosa (Portugal). Morphosedimentary evolution process of the Ria Formosa lagoon (Portugal). Bull. Inst. Bassin d∋Aquitaine, 36, 37-50. Martins, J.T., Ferreira, Ó., Ciavola, P. and Dias, J.M.A., 1996. Monitoring of profile changes at Praia de Faro, Algarve: A tool to predict and solve problems. In: J. Taussik and J. Mitchell (Eds.), Partnership in Coastal Zone Management, Samara Publishing, 615-622. Martins, J.T., Ferreira, Ó., and Dias, J.M.A., 1997. A susceptibilidade da Praia de Faro à erosão por tempestades. Livro de Comunicações do 9º Congresso do Algarve, 206-213.
5. CONCLUSIONS The morphodynamic behaviour of two opposite sites in the Ria Formosa barrier island system (Ancão Peninsula and Cabanas Island) was studied and compared with breaking wave height. The breaking wave height regime was different on either side of system, for the same offshore wave height. Therefore, breaking wave parameters should be considered for coastal characterisation instead of offshore characteristics. The morphological changes observed during the studied period were different, being more rapid in the western part of the system and less rapid in the eastern part. On the Ancão Peninsula, morphologic response was more coherent with breaking wave height than the volumetric response. The observed changes were better correlated with the most energetic five days of the month prior to the survey. At Cabanas Island the volumetric response seems to be more dependent on monthly breaking wave height. This is probably related to some degree with the background influenced behaviour for this side of the system.
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Pilkey Jr, O.H., Neal, W.J., Monteiro, J.H., and Dias, J.A., 1989. Algarve barrier islands: a non coastal-plain system in Portugal. Journal of Coastal Research, 5 (2), 239–261. Teixeira, S.B., Alves, F.J., Andrade, C.F. and Romariz, C., 1989. Dinâmica morfológica das praias das ilhas barreira da “Ria Formosa” (Algarve-Portugal). Geolis, vol. III, fasc. 1 e 2, 238-254
Matias, A., Ferreira, Ó., Mendes, I. and Dias, J.A., 1998. Monitorização da alimentação artificial da Península de Cacela. Proceedings of Seminário sobre Dragagens, Dragados e Ambientes Costeiros, EUROCOAST, Lisbon, Portugal, 47-56. Matias, A., 2000. Estudo Morfossedimentar da Península de Cacela. MSc Thesis. Universidade do Algarve. 246 p.
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