Controls on sediment dynamics and medium-term morphological change in a barred microtidal beach (Cala Millor, Mallorca, Western Mediterranean)

Geomorphology 132 (2011) 87–98

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Controls on sediment dynamics and medium-term morphological change in a barred microtidal beach (Cala Millor, Mallorca, Western Mediterranean) Lluís Gómez-Pujol a, b,⁎, 1, Alejandro Orfila a, Amaya Álvarez-Ellacuría a, b, Joaquín Tintoré a, b a b

IMEDEA (CSIC-UIB), Mediterranean Institute for Advanced Studies, Miquel Marquès 21, 07190 Esporles, Balearic Islands, Spain SOCIB, Balearic Islands Coastal Observing and Forecasting System, Parc Bit, Ed. Naorte, Bloc A, 07121 Palma, Balearic Islands, Spain

a r t i c l e

i n f o

Article history: Received 30 March 2009 Received in revised form 18 April 2011 Accepted 24 April 2011 Available online 30 April 2011 Keywords: Carbonate beach sediments Beach morphology Wave climate Balearic Islands

a b s t r a c t This paper describes the sedimentological and morphological evolution of a microtidal beach over an eight-month period under varying hydrodynamic conditions. During the monitoring a set of transverse to crescentic bars migrated onshore welded to the upper beach and then they were flattened under energetic wave conditions. The grain size distribution of surficial sediments did vary consistently across the beach profile and temporal changes in the sedimentology were mostly related to the seasonal morphological response. From our results we can state that changes in the beach morphology resulting from erosion and deposition might induce, at least to some degree, concomitant changes in the beach when hydrodynamics exceed some intensity and duration levels (Hs N 1 m). Wave climate, rather than wave forcing is the major control on sediment and morphological change co-variation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction A major focus in beach geomorphology and nearshore research is to relate sediment transport rates to beach morphological change. Beaches respond rapidly to varying wave conditions by means of the redistribution of sediments resulting in spatial patterns of erosion and accretion. These changes modify the beach plan form as well as their cross-shore profile by the formation, modification, destruction and/or migration of secondary morphological features, such as berms, beach cups, rip channels and bars. In this sense, the existence of sediment textural gradients due to different hydrodynamic processes along the beach crossshore profile is well established from earlier studies (Bascom, 1951; Imman, 1953; Emery, 1978; Guillén and Palanques, 1996; Komar, 1998). For instance the role of sandbars on wave breaking and the resulting coastal erosion or accretion has been widely studied (Gallagher et al., 1998; Hoefel and Elgar, 2003). It has been observed that in a dynamically stable beach configuration, sandbars usually move offshore during storm episodes, when strong seawards currents (undertows) dominate the sediment transport phenomena. Onshore sandbar migration occurs between storm events when wave energy is lower. Nevertheless Aagaard et al. (1998) and lately Houser and Greenwood (2007) have documented the onshore sand bar migration under storm conditions. In any case, the

⁎ Corresponding author at: IMEDEA (CSIC-UIB) Mediterranean Institute for Advanced Studies, Miquel Marquès 21, 07190 Esporles, Balearic Islands, Spain. Tel.: +34 971 611 231; fax: +34 971 611 761. E-mail addresses: [email protected], [email protected] (L. Gómez-Pujol). 1 Current address: SOCIB, Balearic Islands Coastal Observing and Forecasting System, Parc Bit, Ed. Naorte, Bloc A, 07121 Palma, Balearic Islands, Spain. Tel.: + 34 971 439 906; fax: + 34 971 439 979. 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.04.026

waves breaking over the bar tend to remove finer sediments to the more quiescent trough where sediment are generally finer and less well sorted than sediments from the bars (Greenwood and Davidson-Arnott, 1972; van Houwelingen et al., 2006). Similarly, the step is the coarsest and most poorly sorted sediment unit of beaches due to the presence of energetic breaking wave conditions (Short, 1999). The textural characteristics of beach sediments are not constant, but change substantially over space and time. This is an important issue, since it is widely known that sediment size plays a key role in sediment transport processes and, hence, in beach morphological change. The sediment transport rate is directly related to the bed shear stress, which usually is assumed to depend on both the near bottom velocity and the friction coefficient controlled by the sediment size (van Rijn, 1993). Additionally, some authors relate the spatial gradient of grain size statistical parameters (grain size, skewness, sediment type, etc.) to transport (McLaren and Bowles, 1985; Gao and Collins, 1991; Le Roux, 1994; Plomaritis et al., 2008). Moreover, these authors consider that all changes in the beach morphology resulting from erosion and deposition might induce, at least to some degree, concomitant changes in the beach sedimentology. Nevertheless, sediment transport and beach response models often assume a constant value for the sediment and their attributes because the spatial and temporal variability of beach sediments are generally poorly understood. This assumption represents a limiting factor in beach evolution models (Soulsby, 1997) since uncertainties related to grain parameters or temporal changes in grain attributes may be significant. Different authors highlight that the consideration of grain size changes as well as in grain size statistical parameters in beach evolution models, will improve the forecasting of nearshore changes (Gallagher et al., 1998; Masselink et al., 2007, 2008a, 2008b).

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Previous studies have suggested that sediment size and beach morphological changes have a covariability which reinforces the dynamics of morphological features and sediment transport modes related to those features by feedback processes (Sherman et al., 1993; Rubin and Topping, 2001; Buscombe and Masselink, 2006; Masselink et al., 2008b; Austin and Buscombe, 2008). However, temporal changes and trends in sediment size are often hard to discern. Indeed this scenario results in a conflict between those authors who argue that changes in grain size are not correlated through time (e.g. Davis, 1985; Liu and Lazarillo, 1989), and those researchers who find that there are some conservative properties of sediment grain size along the profile (e.g. Losada et al., 1992). Nevertheless there is an intermediate point of

view in which, assuming the role of geological factors on sediment size supply, data analysis reveal that morphological change results in some variability of sediment size although there is some kind of temporal persistence and the observed variations fluctuate around a master timeaveraged grain size distribution (Medina et al., 1994; Guillén and Palanques, 1996). Therefore, the latter cases support the suggestion that grain size characteristics have a morphodynamic role that contributes to explaining morphological change. Additionally, recent contributions highlight the role of other controls rather than hydrodynamic forcing on beach morphodynamics (Jackson et al., 2005; Gómez-Pujol et al., 2007; Jackson and Cooper, 2009); reflecting that geological setting can be a major control

Fig. 1. Location of Cala Millor site at northeastern coast of Mallorca, and survey profiles.

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on coastal response attending to Quaternary sea level evolution, the nature of sediments and the physical framework. The aim of this study is to describe the morphological and sedimentological evolution of a mictrotidal carbonate beach over a monthly (medium term) field campaign and to elucidate whether the morphological changes are reflected in sediment characteristics or if there are other controls other than hydrodynamic forcing that contribute to sediment changes and nearshore morphology. 2. Materials and methods 2.1. Field site A field monitoring was performed from May 2004 to December 2004 in Cala Millor beach, Mallorca, Western Mediterranean (Fig. 1). Cala Millor is located in the eastern coast of Mallorca Island and bounded by two rocky headlands, Cap Pinar northwards and Punta de n'Amer southwards. It is a sandy beach near 1700 m long with a concave shape, backed by a boulevard, hotels and residential houses (Tintoré et al., 2009). From a morphodynamic point of view, Cala Millor is an intermediate beach with a configuration of transverse and crescentic bars (Gómez-Pujol et al., 2007). The beach is subjected to a wave climate of prevailing NE and ESE swell with an average significant wave height of 0.5 to 1 m, and typical significant wave heights during storms of 2.5 m accounting for only 2% of days of the year (Tintoré et al., 2009). Forcing by tides is almost negligible in the Mediterranean with a spring tidal range of less than 0.25 m, although combined changes between tides, atmospheric pressure and wind setup can account for sea-level elevations close to ±1 m from mean sea level (Basterretxea et al., 2004). The sediment consists of medium carbonate bioclastic sands, being the median sediment size (D50) on the beach around 1.8ϕ. At depths from 6 to 35 m the seabed is covered by a seagrass meadow of Posidonia oceanica, which acts as a cover to sediment exchange and as a friction obstacle to waves (Infantes et al., 2009). Tintoré et al. (2009)

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from the analysis of the morphodynamic state of the Cala Millor beach, highlight that, despite human and economic impacts, the beach remains a dynamic system in an equilibrium state. 2.2. Beach morphology Traditional elevation and bathymetric surveys were performed using a differential RTK for the subaerial and submerged beach (between the shoreline and 0.5 m in depth) with sub-metric resolution. Additionally, for the submerged beach, bathymetric data were obtained using a Biosonics DE-4000 echosounder with a GPS, mounted on a boat, which allows high resolution mapping from 0.5 to 10 m in depth (Orfila et al., 2005). Therefore, this data-set covered the area between the boulevard sea wall and the lower shoreface (Fig. 1). Beach morphology was measured monthly. Elevations were referenced to the Balearic Islands Ordnance Survey mean sea level, and the horizontal position was referenced to UTM coordinates systems. For each survey a Digital Terrain Model Terrain (DTM) was created. In addition 18 cross-shore beach profiles spaced 20 m apart were extracted from DTMs in order to explore beach profile evolution and morphology (Fig. 2). The beach morphology data were analyzed using a systematic approach that couples bulk statistics on the beach's DTM features and Empirical Orthogonal Function analysis, EOF (Kroon et al., 2008). For this reason, each DTM was interpolated into 1 m equidistant points and data and the different surveys arranged into a matrix of depths yi,j, where i states the cross-shore position and j the time. Prior to EOF analysis, basic statistical properties were calculated including the mean, the standard deviation and the range defined as the envelope of profile variation over the duration of the study (Miller and Dean, 2007) using the 18 cross-shore profiles extracted from DTMs. To further investigate the similarity of the variability of the beach morphological configuration, spatial EOFs were calculated from DTMs following the same approach as Principal Components Analysis

Fig. 2. Schematic diagram of the beach study approach. Circles indicate methods used for each dataset and rectangles related products.

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(Jackson, 1991). This is a technique of linear statistical predictors that have been widely applied in coastal geomorphology (e.g. Winant et al., 1975; Medina et al., 1994; Larson et al., 2003; Houser et al., 2008; Kroon et al., 2008; Álvarez-Ellacuría et al., 2011). The analysis was based on the covariance matrix, for which the overall average value for all DTM was subtracted prior to computing its eigenvalues and eigenvectors (Larson and Krauss, 1994, 1995). 2.3. Hydrodynamics Data provided by the Spanish Harbour Authority (Puertos del Estado) were used to analyze the wave climate. One point located near Cala Millor beach, “WANA 20760737”, in the northern part at 50 m depth offers daily wave forecast outputs from the WAM wave model (Günter et al., 1992); these values were calibrated and fit well with deep water buoys (Cañellas et al., 2010a, 2010b) . The wave records analyzed cover the field campaign from May to December 2004 and consist of significant spectral wave height (Hs), spectral peak or wave period (Tp) and mean wave direction (θ) reported every 3 h. Due to the location and the water depth of the WANA point, it is representative of offshore deep-water wave conditions at Cala Millor, and for this study nearshore wave conditions have to be computed. Wave conditions at 5 m depth, were obtained by means of a mild slope parabolic model which includes all the processes for the

transformation of waves from deep to shallow water (OLUCA, González et al., 2007). The model domain consists of two nested meshes obtained from matching bathymetric surveys and nautical charts. The first mesh covers the nearest coastal region with a resolution of 150 m, and the second is a nested grid with a resolution of 20 m, covering the area closer to the beach (Gómez-Pujol et al., 2007). Statistical analysis on the hydrodynamic data time-series propagated at 5 m depth was conducted using daily data sections. For each of these, significant wave height (Hs), significant wave period (Tp) and direction (θ) parameters were computed. 2.4. Sediment sampling and analysis To provide a time-series of sediment characteristics, at least five sediment samples were collected, between the subaerial beach and 6 m water depth at each one of the five cross-shore transects represented in Fig. 1. The location and depth of each sediment sample point was surveyed using a total station and each point was resurveyed every two months resulting in a dataset of 115 samples. Sediment was collected by dragging on the bottom a plastic bag inserted in an oval metallic frame, giving a vertical penetration in the sediment of about 2–4 cm. The weight of samples ranged from 200 to 500 g. After collection, samples were soaked in fresh water for 4 h. Samples were then carefully drained before being soaked for 24 h in a 5% sodium hypochlorite solution to

Fig. 3. Nearshore (at 5 m in depth) hydrodynamic conditions encountered during the field campaign. Gaps in direction correspond to protected exposures and calms.

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neutralize organic material. Samples were oven dried at 105 °C during 24 h, and divided into sub-samples for sieving analysis. Laboratory analysis was undertaken to establish textural characteristics of the sediment by dry sieving using a series of sieves ranging in mesh size from −2 to 4ϕ at half phi intervals. Each sample of approximately 200 g was shaken for 15 min and each fraction was sieved, weighted and saved in separate bags. The calculation of grain size parameters was obtained through the Folk and Ward (1957) method by means of GRADISTAT© software (Blott and Pye, 2001). Thus, the grain size statistical parameters are reported logarithmically based on a lognormal distribution with phi size values. In order to address sediment variability, 10 random samples were sieved for 5 trials, the average coefficient of variation for that test was of 0.25% and 0.92% for the mean size and sorting respectively. Additionally, at least 4 subsamples of 10 different samples were compared for characterizing sampling representativeness. The average coefficient of variation associated with

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sampling replication was 1.7% and 1% for the mean size and sorting. According to the results of these errors tests, one subsample per sample was sieved in at least 3 trials. If the coefficient of variation was larger than 3%, the measure was repeated on a new subsample until the coefficient of variation was lower than 3%. 3. Results 3.1. Nearshore hydrodynamics The wave climate recorded during the study campaign experienced a high variability where calm episodes follow low energy events of around 0.5 m significant wave height (Fig. 3a). 55% of days experienced wave heights higher than 0.25 m and smaller than 0.5 m. Only 1% of wave records exceeded 1 m in height. The wave signal recorded exhibits three notable periods. The first, from May to late

Fig. 4. Morphological evolution of the surveyed profiles during the study period.

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Fig. 5. Elevation change between consecutively surveyed profiles for a representative survey line (profile 5). Dots indicate the sediment sampling positions.

September 2004, the significant wave height fluctuates from calm conditions or average heights around 0.25 m to events approximately of 0.5 m to 0.75 m being of a couple of days in duration. The second period is characterized by very calm conditions during most of October, and the third period during November exhibits an event with wave heights over 1 m. It should be noticed that this event duplicated in few days the mean significant wave height of the entire monitoring period. Otherwise, the wave period recorded throughout the study survey reflected these distinct periods with Tp fluctuating from small values up to 6 s until November and increasing further 9 s after this point (Fig. 3b). In addition to the wave height and period, the dominant wave direction indicated that waves related to heights larger than 05 m approached from the north-eastern and eastern sectors (Fig. 3c). The surf similarity parameter ξb = tanβ/(Hb/Lo) 0.5 where H and L are the inshore wave height and wave length, respectively, classifies the beach as intermediate and predicts the occurrence of spilling and plunging breakers and exceptionally surging waves related with the November event (Battjes, 1974). 3.2. Morphological response The nearshore profiles at Cala Millor beach are characterized by the presence of a quasi-permanent nearshore bar nearly at 1 m depth and between 100 and 150 m away from the shoreline. Although profiles of the 18 survey lines have a similar shape, resulting in a fragmented longshore bar and trough intermediate beach, a longshore spatial variability is apparent. Northern and southern survey lines exhibit well developed bars, whereas survey lines in the middle of the bay display more gentle and smoothed profiles (Fig. 4). Time-series of cross-shore profiles, from May to December, and differences in average profile elevation with respect to the previous profile exhibit changes in sediment volume and in bar location (Fig. 5). From May to September there is a clear runnel and bar configuration, although

from November to the last survey the profiles adopt a subdued topography and bars tend to disappear and to pass to wide, low gradient swash and surf zones. In terms of net profile change, most of the elevation change concentrates between the shoreline and depths of 2–2.5 m, with depth differences between successive beach profiles ranging from 0.30 to 1 m; whereas from depths deeper than 2.5 m the differences between successive beach profiles are within surveying accuracy and, therefore, they can be considered as negligible. EOFs were computed to investigate the characteristics patterns that governed spatial and temporal beach DTM response. The results of the EOF analysis show that the first few eigenfunctions explain the major part of the variance of the beach DTM. The first eigenfunction accounts for over 99% of the total variability, while the first three typically account for the 100%. The first spatial EOF (Fig. 5) can be interpreted as the mean or modal beach morphology. Cross-shore beach morphology exhibits an S-shape form reflecting the presence of a single bar. The temporal amplitude of the first eigenfunction (Fig. 5) is positive, although with some variations, indicating a major accentuation or attenuation of the modal beach morphology. The second EOF (Fig. 5), which shows large spatial gradients, can be related to the sediment supply provided by the bar system. This pattern is related to the morphological change and exchange of material across the profile during the study period. The temporal morphological change is mainly bounded within the bar-berm area (Fig. 5c, d). Seawards of the bar, about 200 m from the shoreline, the second EOF exhibits small variations. It is widely accepted that in a dynamically stable beach configuration, sandbars usually move offshore during storm episodes when strong seaward currents (undertow) dominate the sediment transport phenomena, and onshore sandbar migration occurs between storm events when wave energy is lower (Hoefel and Elgar, 2003). Thereby positive values of the temporal amplitude of the second eigenvector correspond to accretion processes associated with mild waves

Table 1 Summary of sediment grain size parameters (phi units).

Mean size Sorting Skewness Kurtosis D10 D50 D90

Mean

St. Dev.

Minimum

Maximum

N

1.86 0.72 − 0.07 1.03 0.89 1.90 2.73

0.60 0.19 0.14 0.22 0.80 0.57 0.52

0.16 0.46 − 0.33 0.58 − 1.29 0.20 1.19

2.81 1.56 0.27 1.41 2.09 2.69 3.87

115 115 115 115 115 115 115

Table 2 ANOVAs for mean sediment size and sorting on different season sampling (28 or 29 samples × 4 samplings). Sediment parameter

df

MS

F

p

Mean size Sorting

3 3

1.55 0.07

4.78 1.88

0.004 0.138

Note: n = group size, df = degrees of freedom; MS = mean square variance; F = ratio of variance; p = significance.

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conditions and positive values and negative ones to erosion after storm events with offshore sandbar migration. Descriptive and EOF approaches highlight that the morphological response of the beach was very similar across and between profiles, implying that the observed morphological changes were mainly twodimensional chaining onshore–offshore bar migration and bar destruction jointly with a gentle rotation in bar alignment respect to the coastline (Fig. 5b). Indeed, not much morphological change occurred between June and August and from September to October (Fig. 4). During the survey intervals from May to June and from August to September a weak bar flattening and a trough infilling can be observed in addition to the smoothing of the subaerial beach. These episodes are related to the persistence and duration of the events with wave heights (at 5 m depth) higher than 0.5 m. Finally major morphological change took place after November and early December storms when wave heights were around 1 m, and cross-shore erosion occurred resulting in bar destruction smoothing the profile.

Fig. 6. First (a, c) and second (b, d) spatial and temporal eigenfunction from EOF analysis. The first eigenfunction can be interpreted as the main beach configuration or modal morphology and the second eigenfucntion relates to the topographic gains or losses of the cross-shore component with respect to the modal beach configuration.

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3.3. Sediment change During the field study 115 sediment samples were collected. A summary of the sediment size data is presented in Table 1. The mean sediment size ranged from 0.2 to 2.8ϕ; hence sediments fall between fine and coarse sands. The average mean sediment size was of 1.9ϕ and the associated standard deviation was 0.7ϕ, being sediments that are moderately sorted medium sands. Sediments are mostly carbonate skeletal fragments (over 90%). The sediment distributions are symmetrical or slightly negatively skewed, and the average skewness was −0.07. Figs. 4 and 5 the maximum profile depth variability is at c. 200 m offshore, within the trough-bar zone. The variability of D10, D50 and D90 along the beach as well as the mean values of those diameters are shown in Fig. 6 for the study period. The highest variability occurs for the coarser diameters. There is some evidence of spatial and temporal trends in the sediment size and sorting. The sediment size decreases across the survey profiles (Fig. 7) whereas the sorting does not show any significant cross-shore trend. In comparison, the only noticeable temporal trend in the grain size data is the coarsening that coincides

Fig. 7. Variability and mean value of D10, D50 and D90 and different offshore locations.

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Table 3 Tukey HSD comparisons for sediment mean size on different season sampling. Sampling (i)

April

July

September

December

Sampling (j)

July September December April September December April July December April July September

MD

1.17 0.20 5.42⁎ − 0.17 − 0.51 0.37⁎ − 0.12 0.05 0.42⁎ − 5.42⁎ − 0.37⁎ − 0.42⁎

Std. Error

0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15

p

95% confidence interval

0.66 0.85 0.00 0.66 0.98 0.04 0.85 0.98 0.03 0.00 0.04 0.03

Lower bound

Upper bound

− 2.19 − 2.70 0.149 − 0.562 − 0.441 − 0.023 − 0.510 − 0.339 0.028 − 0.936 − 0.765 − 0.816

0.562 0.510 0.936 0.219 0.339 0.765 0.270 0.441 0.816 − 0.149 0.023 − 0.028

Note: MD = mean difference; p = significance. ⁎ mean difference significant at 0.05 level.

with November sea storm events. The inspection of the complete grains size distribution – obtained averaging the size distributions of samples taken at each survey, rather than the summary statistics – reveals that there are two specific grain size spectra that do not exhibit the same modal size and tail extension (Table 2). May, June and September have a modal fine sands grain size but characterized by a coarser tail. The coarser tail is 1–2ϕ and relates to the presence of angular shell fragments. Additionally, the December averaged size distribution shows a different mode (a 1 phi shift), whose sorting and tail shape is quite comparable (Fig. 8). A parametric one-way ANOVA test has been applied to evaluate significant temporal differences in mean size parameters. Table 3 shows the critical levels and the significance of the test for samples grouped by seasonal surveys. Results enable rejection of the hypothesis that average mean size is equal across temporal sampling groups, whereas one cannot reject that for sediment sorting attributes. Additionally, post-hoc comparisons (Tukey's HSD) between coupled seasons for sediment mean size and sorting highlight that sediments sampled after the November sea storm event differs from the rest in size but not in sorting. This reflects than the sediment parameters experienced a coarsening from mean size values of 1.5 (April) to 2.1ϕ (December), whereas sorting just ranges from 0.7 to 0.8ϕ, respectively. Sediment data were taken from six locations at five survey profiles over four bimonthly field campaigns. In order to further examine cross-shore sediment change, those sediment samples were separated according to the variability of the second spatial EOF discerning between: (a) a dynamic sector of the beach, where the second EOF upcrosses and down-crosses zero, and (b) a non-dynamic morphological domain where the second eigenvector shows negligible changes (Fig. 5). The complete grain distributions for both domains show that there are differences in grain size spectra between morphological regions. In the dynamic sector of the beach, the December complete grain size distribution has a modal distribution coarser than those corresponding to May–July–September (Fig. 8). Nevertheless the samples show similar sorting and a coarse tail shape. The differences between complete grain distributions between samples are largely

Table 4 ANOVAs for mean sediment size and sorting on different beach morphological domains (dynamic vs non-dynamics sector). Sediment parameter

df

MS

F

p

Mean size Sorting

1 1

12.11 0.21

47.96 6.09

0.000 0.015

Note: n = group size, df = degrees of freedom; MS = mean square variance; F = ratio of variance; p = significance.

Table 5 ANOVAs for mean sediment size and sorting on different season sampling and beach morphological domains. Season sampling at EOF1

Mean size Sorting

Season sampling at EOF 2

df

MS

F

p

df

MS

F

p

3 3

1.40 0376

5.27 1.68

0.002 0.180

3 3

0.30 0.01

2.40 0.53

0.08 0.67

Note: n = group size, df = degrees of freedom; MS = mean square variance; F = ratio of variance; p = significance. EOF1 = morphology dynamic sector; EOF2 = non-dynamic morphologic sector.

attenuated in the non-dynamic morphological domain. Modal distributions are quite similar although the December spectrum has a larger coarser tail (Fig. 8). For this reason, a one-way ANOVA was also used in order to discern significant differences between these zones in terms of sediment mean size and sorting. The results (Table 4) clearly show that there are significant differences in sediment properties between cross-shore domains, the samples of the morphological variable sector being coarser than those from the less dynamic domain (average mean size and sorting of 1.5 vs 2.2ϕ and 0.7 vs 0.8ϕ respectively). Otherwise temporal contrasts between zones defined by the second spatial EOF show that sediment size exhibits significant differences between December survey and the rest of the year over the dynamic sector of the profile, whereas sediment sorting did not experience any significant change (Table 5). Both sediment size and sorting did not exhibit significant differences between surveys in the non-dynamic morphological domain. 4. Discussion The field measurements presented in this paper describes the morpho-sedimentary variation of a microtidal carbonate sand beach over an 8 months survey. During the monitoring a set of transverse to crescentic bars migrated onshore, welded to the upper beach and then flattened under energetic wave conditions (Fig. 8). From May to September, coinciding with the summer conditions (Hs from 0.25 to 0.5 m), the balance of the mass movement of the sandy material – calculated from differences in bathymetries – was of 30,130 m 3 or of c. 17 m 2 per m of beach length. Whereas for winter conditions (Hs roughly or larger than 1 m), experienced from October to December, the mass balance resulted in — 33,707 m 3 or c. 20 m 2 per m of beach length. These wave conditions are the classical pattern of sea storms in this sector of the Balearic Sea (Cañellas et al., 2007). The annual beachwide balance is considerably small, 3.570 m 3, that represents only minor changes, c. 2 m 2, per m of beach length. The grain size distribution of surface sediments did vary across the beach and temporal changes in sedimentology were mostly related to shifts in environmental energy. Changes occurring in the beach shape and sediment distributions are clearly seasonal and the wave conditions, characterized by a marked seasonally, provided the main forcing function for this nearshore morphological change. Therefore, the coarsening of the sediments since November, correlated with wave conditions, can be explained in terms of removal of fine material due to the combination of cross-shore and longshore transport gradients (McLaren and Bowles, 1985). Referring to the measured morphological change, it is evident from the sediment volumes (Fig. 9), that most of the gross morphological change is constrained between the beach and upper shoreface. During summer conditions bars move alongshore and cross-shore according to wave pulses, resulting in a general beach accretion. It can be appreciated that most of this bottom variation was constrained in a 200 m wide band from the emerged beach. The comparison between sediment attributes between different summer bathymetries shows that there are not significant differences in mean grain size and sorting. In comparison, matching up with winter wave conditions,

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Fig. 8. Grain size distribution for Cala Millor beach during the study period. The distributions were obtained by averaging the size distributions of the samples taken in each survey.

bars and beach slope were flattened and significant differences in grain size can be observed between summer and winter samples. From our results we can state that, at the medium-term scale, changes in the beach morphology resulting from erosion and deposition might induce, at least to some degree, concomitant changes in the beach sedimentology; and vice versa. These findings correspond with previous studies (i.e. Moutzouris, 1988; Medina et al., 1994; Austin and Buscombe, 2008), although depart from others which adopt time-analysis scales that are shorter or longer. For example Masselink et al. (2007, 2008b) relate shortterm intertidal beach response to changes in sedimentology in the intertidal zone of a macrotidal beach. In only a few instances morphology and sedimentology covaried, but most commonly sedimentological changes were unrelated to the morphological change. In a similar way Guillén and Palanques (1996), despite observing some proportionality with the intensity if the accretion or erosive processes in the beach with grain size evolution from 1988 to 1991, conclude that the temporal evolution of the sediment size in deltaic beaches is controlled by processes not related to a seasonal temporal scale. This combination of short to long-term approach empirical studies leads to some consideration related to temporal scales and the wave energy in the beach system. At monthly scales there are differences in morphology but not significant differences between sediment size and sorting despite of the registration of different peaks (Hs = 0.5 to 0.75 m). Besides, when waves achieved significant heights around 1 m or larger (winter season), then the differences arise. According to this, it can be envisaged that below certain energetic levels the sediment removal is constrained to the

inner morphological zone of the submerged beach, transporting sediment volumes of the same texture from one place to another. When the wave energy increases above these thresholds, fines are removed and also, because the depth of closure is deeper, sediments from other morphological domains are added to the beach. In these cases, a coarsening of the beach sediments occurs parallel to the morphological change. This leads to the conclusion that wave climate rather than wave forcing is the major control on sediment and morphological change co-variation. In doing so, short-term studies can underestimate events that have the capacity to mobilize large amount of sediments, adding or removing different sediment grain sizes to the system, and their impact on scenarios related to former energetic conditions. Additionally long-term studies introduce other controls such as sediment starvation, human impact, very lowfrequency energetic events, etc. The medium term shares some aspects with the long-term approach related to correlation between morphological and sediment change, and introduce other controls on beach morphodynamics different from wave forcing (Jackson et al., 2005). For instance one of the key points of interest of this study is the beach framework or the boundary conditions. Most of the works investigating sediment dynamics relate to deltaic beaches (Guillén and Palanques, 1996); or coastal cells with an important sediment contribution from river or estuarine mouths and tide effects (Liu and Lazarillo, 1989; Medina et al., 1994; Austin and Masselink, 2006; Masselink et al., 2007; Austin and Buscombe, 2008) and with sediment predominantly siliciclasticterrigenous. In contrast the Cala Millor study site corresponds to an individualized microtidal coastal cell where the fluvial component is

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Fig. 9. Cala Millor morphological change between May–December 2004.

weak. Permanent rivers are absent and continental sedimentation inputs are negligible (Fornós and Ahr, 2006). Additionally, regional studies of beach and shelf sediments highlight that the predominant biogenic sediment is derived from communities that thrive in the seagrass meadows of Posidonia oceanica (Alonso et al., 1988; Jaume and Fornós, 1992; Canals and Ballesteros, 1997; Fornós and Ahr, 1997)

which are the main constituents (ca. 95%) of beach and dune sediments. All this background has different implications for our study. First of all, the input velocities of new sediment to the beach sediment budget are larger than seasonal or yearly time-scale (Canals and Ballesteros, 1997; de Falco et al., 2003). Therefore, disturbances related with new sedimentary budgets or with tides can be subtracted

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from morphological and sediment change co-variation analysis. Secondly, the minor presence of red algal and coralligenous bryozoans fragments, as well as of rhodolites (maërl), among dominant bulk components of foraminifera, echinoids and molluscs highlights a weak sediment intrabasinal contribution from benthic biota habitats between 20 and 50 m in depth (Fornós and Ahr, 2006). It is known from the analysis of 44 years of wave data to the beach (Gómez-Pujol et al., 2007) that the waves necessary for carrying these coarse sediments inshore only have probabilities of occurrence during some winter gales (Cañellas et al., 2007). The results from this work are in agreement with those authors who argue that changes in grain size remain correlated with time or, in other words, with morphological change (Davis, 1985; Liu and Lazarillo, 1989; Austin and Buscombe, 2008). Nevertheless, aspects of beach composition or similar modal distributions that become slightly shifted apart after an energetic event and/or season, cannot discount the existence of a spatial and temporal averaged master grain size distribution in the sense of Medina et al. (1994) and Guillén and Palanques (1996). 5. Conclusions This study has demonstrated that in carbonate microtidal beaches with negligible fluvial contribution, the spatial distributions of individual sediment grain sizes can reflect the beach morphological change when hydrodynamics exceed some intensity and duration levels (Hs N 1 m). When waves do not exceed this threshold there is evidence of morphological change constrained between the emerged beach and the rear of the bar, and sediment volumes of the same textural properties are transported from one place to another. This leads to the conclusion that wave climate rather than wave forcing is the major control on sediment and morphological change covariation. Additionally the sediment nature and composition, as well as the physiographical constrain for the study site, highlight geological factor as a secondary control on the beach morphodynamics at the medium-term temporal scale. Acknowledgements We would like to thank Pau Balaguer, Gotzon Basterretxea, Benjamín Casas, Tomeu Cañellas, Antonia Fornés, Marta Fuster, Antoni Jordi, Rosario Simonet and Guillermo Vizoso for their assistance in the field. This research was sponsored by the “Conselleria de Medi Ambient” from the Government of the Balearic Islands and by the project CTM2006-12072 from the MICINN. Collaboration from Puertos del Estados is also gratefully acknowledged. LGP is indebted to the “Consejo Superior de Investigaciones Científicas” (CSIC) for the funding provided in the JAE-Doc Program. We would like to thank two anonymous reviewers and Dr. A. Plater whose comments improved the manuscript. References Aagaard, T., Nielsen, J., Greenwood, B., 1998. Suspended sediment transport and nearshore bar formation on a shallow intermediate-state beach. Mar. Geol. 148, 203–225. Alonso, B., Guillén, J., Canals, A., Serra, J., Acosta, J., Herranz, P., Sanz, J.L., Calafat, J., Catafau, E., 1988. Los sedimentos de la plataforma continental Balear. Acta Geol. Hispanica 23, 185–196. Álvarez-Ellacuría, A., Orfila, A., Gómez-Pujol, L., Simarro, G., Obregón, N., 2011. Decoupling spatial and temporal patterns in short-term beach shoreline response to wave climate. Geomorphology 128, 199–208. doi:10.1016/j.geomorph.2011.01.008. Austin, M.J., Buscombe, D., 2008. Morphological change and sediment dynamics of the beach step on a macrotidal gravel beach. Mar. Geol. 249, 167–183. Austin, M.J., Masselink, G., 2006. Observations of morphological change and sediment transport on a steep gravel beach. Mar. Geol. 229, 59–77. Bascom, W.N., 1951. The relationship between sand size and beach-face slope. Trans. Am. Geophys. Union 32, 866–874.

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