8th International Coastal Symposium - Journal of Coastal Research

Evolution of Corsican pocket beaches

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Evolution of Corsican pocket beaches Balouin Yann†, Bélon Rémi‡, Anne Merour‡, Camille Riotte‡ †BRGM, French Geological Survey, Dir. Languedoc-Roussillon Montpellier, France [email protected]

‡ BRGM, French Geological Survey, Dir. Corse Bastia, France [email protected]

www.cerf-jcr.org

ABSTRACT Balouin, Y., Belon, R., 2014. Evolution of Corsican pocket beaches. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 096-101, ISSN 0749-0208. www.JCRonline.org

Pocket beaches are small beaches that are formed between headlands and within coves along rocky shorelines. They can be composed of a mix of boulders, pebbles, sand and mud and therefore have the attributes of a combination of shoreline types. Their evolution depends on wave forcing and morphological characteristics, and the most commonly observed dynamic is that of beach rotation due to prevailing wave direction. In this study, 44 pocket beaches off Corsica Island, in the Mediterranean were studied. The absence of significant tides and the generally weak coastal currents restrict significant beach morphology changes to the most important storm events though the variability of coastal evolution suggests that other factors played a role. To assess these evolution parameters, the morphological characteristics of the beaches as well as the historical evolution of the shoreline were studied. This analysis, based on aerial photos and DGPS surveys, allowed for the behavior of beaches over historical (1948-2012) period to be assessed. Over this period, 15 beaches are considered to be in erosion, 16 are stable and the others show shoreline progression. The historic analysis, through several increments of time, shows that beaches have been in perpetual movement. Moreover, some periods are particularly erosive (i.e., 1996 to 2002) for a number of beaches. Several parameters must be considered to explain the beach behavior changes such as: beach orientation, closure, geology, hydro-climatic regime, grain size and length. The wave backward modeling ANEMOC was used to analyze wave characteristics and the occurrence and direction of the most important storm events. The entire dataset was used to obtain a conceptual model describing the most important factors involved in the contrasting coastal evolutions observed, i.e., symmetric or asymmetric beach rotation, homogeneous retreat or progradation and rapid shoreline movements due to the presence of beach-cast Posidonia Oceanica seagrass litter. ADDITIONAL INDEX WORDS: shoreline, beach stability, sea grass litter.

INTRODUCTION A large majority of the world’s continental and island margins consist of rocky coastlines. Along these coasts, typical beach morphology is characterized by the presence of hard rock headlands (Short and Masselink, 1999). These embayed beaches, are usually small beaches known as curved, embayed, pocket and headland-bay beaches. Detailed studies of these pocket beaches have been made (Hsu and Evans, 1989, Klein et al., 2002, Bowman et al., 2009, Daly et al., 2011, van de Lageweg et al., 2013). It is widely accepted that the morphological behavior of these beaches is determined by beach planform and indentation ratio, beach type and slope, grain size and the presence or absence of nearshore bars. The morphological evolution of these systems is also driven by wave conditions which influence both the hydrodynamics at the coast (alongshore and rip-currents) (Short and Masselink, 1999) and beach rotation processes. The planform geometry approach (Hsu et al., 1989) has been widely used to obtain a first insight into the study of embayed coasts. This approach is based on an equilibrium form of the planview beach that fits either logarithmic, parabolic or power functions. This method thereby permits us to evaluate the equilibrium state of a given embayed beach, by evaluating the ____________________ DOI: 10.2112/SI70-017.1 received 30 November 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2014

relationship between indentation ratio (a/ro) of the beach and wave angle (β). Several studies have, however, shown that models using averaged environmental parameters were insufficient to clearly explain the morphological evolution of embayed beaches (Loureiro et al., 2009, 2013), with reflective and lower intermediate beaches showing more agreement with parametric approaches than intermediate beaches. Beach state classification, using parameters such as dimensionless fall velocity (Wright and Short, 1984), therefore provides additional information on the applicability of planform approaches and also gives an information on the nearshore circulation that is to be expected (Short and Masselink, 1999). The objective of this contribution is to evaluate the geometry, evolution and morphodynamics of a complete dataset obtained from 44 embayed beaches on Northern Corsica Island in the south of France.

FIELD STUDY The study site is the Haute-Corse division on Corsica Island; located in the Mediterranean Sea (Figure 1). There, the coast is divided into a sandy coastal plain along the eastern coast and a long rocky coast in the north where 44 pocket beaches with variable exposure, morphologies and evolution patterns are observed. Mean tidal range is around 0.2 m and can reach 0.3 m during spring tides. Modal wave conditions are weak (Hs mean = 0.8 m; Ts mean ≈ 4.5 s) but important wave events and winter

Journal of Coastal Research, Special Issue No. 70, 2014


Figure 1. Localisation of the study area and the 44 pocket beaches of Haute-Corse. storms are rather frequent. The 1 yr return period height for waves is around 4.7 m, but significant wave heights of more than 8 m were observed. Most of the field sites are reflective beaches composed of fine sand to large gravel. For most of the sites, the erosion rate of the nearby cliffs is low and the sediment supply is thus very limited. The cliffs along the Cape are composed of ophiolites and glossy schists, while the western coastline is formed by high-magnesium and -potassium, granitic rocks.

METHODS Beach morphology and shoreline evolution An analysis of shoreline evolution was performed using aerial photographs available since 1948 (1948, 1951, 1960, 1983, 1985, 1996, 2002, 2007). Photographs taken before 1990 are at the scale 1:25000, while more recent ones are at the scale 1:5000. Both the vegetation line and the wet/dry line were digitised, but as the upper beach limit is very often marked by a cliff or human infrastructures, the wet/dry line was used in this study to evaluate

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the long-term shoreline change. Given the higher variability of this indicator, and the resolution of images, the extracted position of the shoreline has an accuracy of +/-10 m for the old photographs (before 1996) and +/- 5 m for the more recent ones. Some of the studied beaches have been monitored yearly since 2000 within the Coastal Monitoring Network of Corsica (ROL, Balouin et al., 2013), to evaluate the short-term changes in shoreline position. A DGPS survey of the shoreline for all studied beaches was performed in 2012. Beach profiles, sediment samples, photographs and geomorphological observations (e.g., of vegetation, seagrass litter and the presence/absence of a developed berm, beach cusps or nearshore bars) were also collected. All shorelines were digitized using ArcGIS® software. To measure the shoreline evolution with time and the evolution speeds, the DSAS module developed by USGS (Thieler et al., 2009) was used. It allows for the comparison of shoreline position for a large number of points to be carried out automatically (see example on Fig. 2). Pocket beach characteristics were evaluated both by aerial photographs and by field data analysis. Characteristics assessed included: beach orientation, slope, length, sheltering, degree of embayment, closure, geology and the presence/absence of seagrass litter. This information permitted a morphodynamic indicator (Wright and Short, 1984) and a stability indicator (Hsu and Evans, 1989) to be calculated. Finally, a description of the main morphological behavior was defined for the 44 beaches distinguishing between beach stability, unidirectional beach rotation, bi-directional beach rotation, homogeneous retreat and accretionary behaviours.

Hydrodynamics Wave measurements are scarce along Corsica shorelines, and most of the thus-far collected data has no directional information. In order to analyze the wave climate and storm characteristics, the ANEMOC database (© CETMEF / EDF R&D-LNHE 2010/2011) was used. ANEMOC (Digital Atlas of State Oceanic and Coastal Sea; http://anemoc.cetmef.developpement-durable.gouv.fr) was built from hindcasts over a period of 30 years from 01/01/1979 to 31/12/2008 for the Mediterranean coast. Five virtual buoys (MEDIT 4655, 6530, 6771, 7846, 8110) were used to analyze wave climate along the Haute-Corse shoreline (Fig. 3). The mean wave climate was obtained as well as storm characteristics: maximum wave height, cumulated energy, frequency, duration, maximum run-up and set-up using the formula of Stockdon et al. (2006). A storm event along this coastline is defined by a

Figure 2. Example of shoreline evolution from 1948 to 2012 at Pietracorbara beach on the eastern coast of Corsica.


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significant wave height above 2 m (more than two times the mean significant wave height).

RESULTS Hydrodynamics ANEMOC hindcast database was use to obtain the mean wave climate along the northern Corsican coastline, but also the characteristics of storm events during the period 1979-2008. Particular storm wave conditions were extracted for all beaches depending on their orientation, aperture. Figure 3 illustrates the prevailing wave direction. More than 750 events with Hs > 2 m were analyzed. Most of the storms have significant wave heights around 3 m. However, at this site Hs can reach more than 6 m as observed in November 2004. Mean duration of these events is around 21 hours. However, the maximum duration was 113 hours in February 1990. At most of the beaches, there is a prevailing storm direction (see Figure 3). However, smaller events can occur with an opposite direction thus contributing to the alternation in the direction of sediment transport and beach rotation processes along the surveyed period. Moreover, even if the storm characteristics are comparable for all beaches with a slight higher energy on the west coast where the fetch is longer, the frequency of storm events is more important on the northern and the western coast than the eastern one. While less than 500 storm events (H s > 2m) were recorded on the east coast, more than 1000 were recorded on the northeast coast during the period 1979-2008 (see Figure 4). To calculate the mean wave climate, the prevailing direction was used (see Ω and incidence in Figure 4).

Shoreline evolution Evolution of shoreline position was calculated for the 44 beaches (see example on Figure 2). Over the long-term (from 1948 to 2012), very few beaches have been enlarged. The beaches of Galeria North and Cala Genovese (see location on Figure 1) show a respective advance of 20 and 15 m. Two beaches of the western Cap Corse, Albo and Nonza, present a very strong enlargement reaching more than 300 m between 1948 and 1960. This evolution is however anthropogenic and derive from the mining activities in the vicinity of these bays (Mine of Canari) and the human deposition of large quantities of material extracted from the mine. Sixteen beaches are stable, 11 in erosion and 4 in strong erosion: Tollare, Giottani and Negru where the retreat reaches 40 m and Saleccia where a 60 m retreat was observed in the southern part of the bay. The recent evolution, from 2007 to 2012, gives the trend of the current evolution, onto all the studied beaches, 4 are stable, 16 are in accretion and 23 are in erosion. This evolution is much more contrasted than the historic evolution, 13 beaches have an important accretion and 16 have marked erosion. The analysis of the historical shoreline evolution shows contrasted behaviors of beaches exposed to the same forcing parameters, evidencing the importance of local parameters. The short-term evolution (see Figure 4) shows the high temporal variability of the erosional process. This is particularly the case for the beaches where an important seagrass litter covers the beach (Figure 4). The quantity of seagrass litter is highly variable in time because the destruction of the meadow is linked to storm occurrence, and some of the beaches are “cleaned” before the summer touristic season. Some periods show a very rapid shoreline retreat, as during 1996-2002 along the eastern coast of the cape. The wave conditions during this period were not more energetic, but

Figure 3. Wave characteristics along the coastline of northern Corsica. Vectors indicate the frequency of wave direction while colours indicate the associated significant wave high (Hs, m). duration between two successive events was slightly shorter, preventing beach recovery, and could explain the observed strong retreat.

Morphodynamics To understand the differences in shoreline evolution of the Corsican pocket beaches, several parameters were analyzed (Figure 4). The first parameter was wave incidence at 10 m depth. Given the prevailing wave direction from north and south-east along the east coast and from east and north-east along the northern coastline, wave incidence is highly variable. Apart from their orientation, beaches on the east coast are much more exposed to highly oblique waves. In such cases, wave refraction is higher, and energy supposed to be lower at the coast. The number of storm events during the last 30 years was also lower than on the northern coast. This wave climate is in agreement with the main morphological behavior of these beaches which was characterized by beach rotation that could be either unidirectional or bi-directional based on the orientation and closure of the beach. On the west coast of the Cape, the most significant storm events reach the coast with a very low incidence. The main morphological behavior observed was a homogeneous retreat of the shoreline, except for the anthropogenic-driven beaches of Nonza and Albo. Along the rest of the coast, wave direction is mostly shore-normal, but beach orientation is much more variable so a general trend cannot be defined. In order to take into account the morphology of the bays, the stability state of the beaches using the indentation ratio (a/Ro) and the wave crest angle (β) (see Figs. 4 and 5) were examined. This concept was widely used in a first step to evaluate the equilibrium



state of the beaches. The results (Figure 5) indicated that most of the beaches of the eastern coast are unstable. The main reason for this is the high wave incidence, particularly during storms from

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the south-east. These beaches, and some others along the coastline usually have reduced lengths, large indentations and are known to have a large amount of Posidonia oceanica beach casts. Despite

Figure 4. Morphological indicators obtained from the 44 pocket Beaches along Northern Corsica. Upper panel: dimensionless fall velocity Ω; wave incidence at 10 m depth and stability test results (Hsu and Evans, 1989); Middle panel: long-term (1948 to 2012) shoreline evolution, short-term (2002-2007) shoreline evolution and main morphological processes observed (both on the historical photographs and the yearly surveys undertaken on some of the beaches); Lower panel: grain size, presence and quantity of seagrass litter on the beach and number of storm events (with Hs > 2m) during the period 1979-2009. Journal of Coastal Research, Special Issue No. 70, 2013

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this apparent unstable state, they have shown a quite stable position of the shoreline during the last few decades.

DISCUSSION Results on the evolution and morphological changes of 44 embayed beaches located along northern Corsica Island reveal contrasting responses, even for beaches submitted to the same forcing conditions. Beach orientation drastically affects shoreline evolution, with higher erosion where wave incidence is the lowest. This is particularly true for the beaches of Tollare, Giottani, SaintFlorent, Lumio. However, several beaches, like Ostriconi, have a low incidence and are exposed to energetic storms yet they remain quite stable on the long term. As was physically observed by Dai et al. (2010) and numerically simulated by Daly et al. (2011), beaches with a stable shape experience greater degrees of beach oscillation (onshore or offshore movement) than of beach rotation. This was observed at several beaches (Meria, L’Ile Rousse, Corbara, Lumio). The directional spreading was very large at all beaches, with a bimodal directional spectrum at all sites (Fig. 3). Nevertheless, only 13 beaches displayed a clear rotation tendency on the observed shoreline. Despite the occurrence of opposite storms, some of these beaches showed an alternation of beach rotation direction, while others presented a unidirectional evolution, with one part of the bay always providing sediment to the other. Of course, this observation was taken along several shorelines and short-term evolution in relationship with storm events was not taken into account. This could point towards a significantly different behavior between storm evolution and the mid- and long-term tendency.

Stability analysis has shown that only 9 of the 44 studied beaches possessed a stable planform. Most of these beaches were on the western coast of the Cape and are characterized by a low wave incidence; they are among the few intermediate beaches of northern Corsica. Beaches expected to be stable (in green on Figure 4) are supposed to keep a stable planform shape. In Corsica those beaches have shown an erosional trend during the last decade, but according to the available dataset, did not present an important modification of their planform shapes. At those sites, erosion concerned the entire bay and no rotation processes were visible. According to the classification of Hsu et al. (2008), eight of the 44 beaches were in a state of dynamic equilibrium. As expected, these displayed long-term erosional trends and the evolution of their shorelines could be either via homogeneous retreat or via beach rotation processes that also yielded a decrease in beach surface. For beaches in stable and in dynamic equilibrium, the stability analysis gave expected results and comprehensively explained shoreline evolution, even if the rate of shoreline retreat was highly variable and dependent upon local morphological parameters (e.g., beach orientation, beach length). Unstable beaches represent the main part of the Corsican pocket beaches (27 out of 44). Their dynamics and evolution are highly variable. On the east coast, where storms are less frequent and wave incidence higher, the long term shoreline evolution involves either stability or moderate erosion, whereas short-term evolution indicates a high temporal variability. Even if storm events are less frequent and more refracted, these beaches have quite significant morphological responses during storms; as in 2003, when the coastal road in Porticciolo was partly destroyed by breaking

Figure 5. Relationship between indentation ratio (a/Ro) and angle between the incident wave crest at the wave diffraction point and the control line joining the updrift diffraction point to the near straight downdrift beach (modified from Bishop (1983), in Simeoni et al. (2012)). Black circles on data points from Corsica (this study) indicate a significant amount of seagrass litter on the beach. Journal of Coastal Research, Special Issue No. 70, 2013


waves. Evolution of these beaches is poorly predicted by currently used classifications. However, the characteristics of these beaches and their exposures were quite comparable to the others; the only common characteristic being the presence of seagrass litter along the shoreline and, for some of the sites, upon the entire beach. This biogenic beach litter has a very important role on the erosion trend: when an large quantity of leaves is present on the beach and in the nearshore zone, it protects the beach from storm waves. However, a storm can have a split impact on the quantity of leaves on the beach: large waves can induce the destruction of the seagrass meadows, favoring the deposition of leaves on the beaches; meanwhile, the most energetic storms can erode the beach cast, enhancing the vulnerability of beaches to subsequent storm events. For these beaches, the stability classification of course does not reflect the real beach state. The seagrass leaves have a very particular behavior. When only a few leaves grow at the shoreline, they tend to me moved easily by waves, but when they form a banquette, their structure is more solid and only more energetic events can move them. Moreover, when leaves are dispersed in water close to the shoreline, they give the water a more viscous behavior that attenuates wave energy at the coast. In this study, the available dataset was not sufficient to clearly understand the role of the seagrass litter and its dynamics (when it is deposited/eroded and why?). However, real-time monitoring using video systems could be useful to determinate the dynamics of these beaches which represent the main part of Corsican beaches.

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Hsu, J.R.C., Silvester, R. and Xia, Y.M., 1989. Generalities on static equilibrium bays. Coastal Engineering, 12, 353-369. Klein, A.H.F., Filho, L.B. and Schumacher, D.H., 2002. Short-term beach rotation processes in distinct headland bay beach systems. Journal of Coastal Research, 18, 442-458. Loureiro, C., Ferreira, O. and Cooper, J.A.G., 2009. Contrasting Morphologic behavior at embayed beaches in Southern Portugal. Journal of Coastal Research, SI56, 83-87. Loureiro, C., Ferreira, O. and Cooper, J.A.G., 2013. Applicability of parametric beach morphodynamic state classification on embayed beaches. Marine Geology, 346, 153-164. Short, A.D. and Masselink, G., 1999. Embayed and structurally controlled beaches. In: Short A.D. (ed.), Handbook of beach and shoreface dynamics. Chichester, England: John Wiley and Sons, 392p.. Simeoni, U., Corbau, C., Pranzini, E., Ginesu, S., 2012. Pocket Beach. In: Angeli, F. (ed.), Dinamica e gestione delle piccole spiagge, 171 p. Van de Lageweg, W.I., Bryan, K.R., Coco, G. and Ruessink, B.G., 2013. Observation of shoreline-sandbar coupling on an embayed beach. Marine Geology, 344, 101-114.

CONCLUSION A dataset on 44 pocket beaches located along Corsica island in France was obtain, describing the mid- and long-term shoreline evolution, local beach characteristics and wave exposures. Hsu et al. (1989) stability analysis was applied to evaluate the pertinence of such a method in Corsica. The study indicates that this methodology gives quite interesting results for predicting the behavior of stable and dynamic equilibrium beaches. However, for most of the beaches, the method is not useful because of the presence of seagrass litter on the beach which fully modifies the morphodynamics. Further studies on the impact of seagrass litter on coastal evolution are needed to be able to anticipate the evolution of Corsican pocket beaches.

ACKNOWLEDGEMENT The work described in this publication was supported by the Direction Départementale du Territoire et de la Mer de HauteCorse. For this study, backward modeling results from the ANEMOC database (EDF R&D and CETMEF) were used.

LITERATURE CITED Balouin, Y., Stepanian, A. and Belon, R., 2013. The Corsican coastal monitoring network. In: Cipriani, L.E. (ed.), Coastal erosion monitoring – a network of regional observatories. Nuova grafica fiorentina. Bishop, C.T., 1983. A shore protection alternative: artificial headlands. Proc. Candian Coastal Conf., NRCC, 305-319. Bowman, D., Guillen, J., Lopez, L. and Pellegrino, V., 2009. Planview geometry and morphological characteristics of pocket beaches on the catalan coast (Spain). Geomorphology, 108, 191-199. Daly, C.J., Bryan, K.R., Roelvink, J.A., Klein, A.H.F., Hebbeln, D. and Winter, C., 2011. Morphodynamics of embayed beaches: the effect of wave conditions. Journal of Coastal Research, SI64, 1003-1007. Dai, Z., Liu, J.T., Lei, Y.P. and Zhang, X.L., 2010. Patterns of sediment transport pathways on a headland beach – Nawan Beach, South China: a case study. Journal of Coastal Research, 26(6), 1096-1103. Hsu, J.R.C. and Evans, C., 1989. Parabolic bay shapes and applications. Proc. of the Institution of Civil Engrs., 87, 556-570.
