Coastal sedimentological studies allow the understanding of both natural processes and anthropogenic interventions, e.g., coastal dynamics and required management actions. Beach sediments characteristics play an essential role in coastal ecology but also in the choice of a pleasant site by beach users [1
]. The most important characteristics of the sediment are texture (grain size and sorting), mineralogy, colour and morphology of grains (roundness and sphericity). These characteristics should be preserved during anthropogenic actions since their modifications can cause undesired impacts. First, sediment colour and texture directly influence the perception of beach users [2
] and coastal scenery characteristics [3
]: beach sediment colour is one out of the 18 natural parameters considered in the probably most commonly used landscape assessment method, i.e., the Coastal Scenic Evaluation [4
]. This methodology, applied in >1000 coastal sites around the world [5
], including the study area of this paper [6
], is based on enquires that revealed that golden or white sand is better valued than dark sand. In this way, beach users also prefer sandy sediments to gravel or pebble ones [4
]. Second, beach nourishment can cause ecological and physical impacts on the coast. For example, nourishment works can degrade different beach habitats and affect several marine species [7
]. Addition of new sand volumes can involve a change in sand colour, which can modify sand temperature with consequences on biological processes of different species, e.g., the sex of marine turtles [1
]. Finally, beach nourishment can also modify beach characteristics such as beach profile and/or water colour. Consequently, the modification of beach colour due to sediment injection can cause discontent between local beach users and the economic impacts to the local economy [8
]. Sardinia (Italy) constitutes an example of this socioeconomic problem, where a court case was opened stimulated by stakeholders because a natural white beach was replaced by a black one [10
]. On the contrary, nourishment works can sometimes improve the natural sand colour making the beach more attractive to tourists, this way incrementing the economic value [11
Previous studies carried out along the study area of this paper, were focused on biological, mineral, and textural characteristics [12
]. However, no attention was focused on the chromatic analysis of native sand and its modification after beach nourishment. Such studies on beach colour are relatively recent and they have important implications for coastal conservation and beach management. Sand colour characteristics can be used to create a catalogue of sand compatibility and improve nourishment works [3
]. This catalogue should be useful to identify the origin of sediments, as well as to detect the occurrence and level of oil contamination, e.g., after the beach oiling in NW Spain linked to the sinking of the Prestige [19
]. In Tuscany (Italy), compatibility studies in sand colour [20
] have been carried out to avoid future management issues. Further, the chromatic analysis of sand in combination with the study of other sedimentological parameters such as the granulometry or mineralogical composition can be used for didactic activities or as an interesting geo-sedimentary resource to outreach geoscience among the general public [21
The colour of solid objects mainly depends on three factors: the light source, the observer, and especially the characteristics of the material surface [24
]. The colour of a stimulus can be assessed in the CIEL*a*b* colour space, recommended by the Commission Internationale de l’Eclairage [25
] and is appropriate for sediment colour comparison [26
]. On the one hand, the coordinate L* is the Lightness component, and it ranges between 0 and 100 (from black to white, respectively). It is directly associated with the visual sensation of luminosity. On the other hand, the coordinates a* and b* are denominated “Chromaticity”. The a* coordinate defines the deviation of the achromatic point corresponding to Clarity, to red if a* >0 and to green if a* <0. Similarly, the coordinate b* defines the deviation to yellow if b* >0, and to blue if b* is <0 [24
]; both can theoretically shift from –200 to +200, with values external to ±100 being unusual [3
]. Several authors have quantified the colour of beach sand from different countries (e.g., Italy, Belgium, Cuba, Portugal, New Zealand and Japan) in the CIEL*a*b* colour space [3
The most important characteristics of coastal sediments are particle size distribution, mineralogy, and colour (which is generally the least studied). For this reason, the main objective of this study was to carry out the first chromatic characterization of beach sands from the south-eastern coast of Spain. Samples from natural and altered (nourished) beaches were taken in order to analyse the influence of the origin of sediments on the actual beach colour and sediment chromatic characteristics were compared with the mineralogical composition of the studied sands. Colour studies in sandy beaches are very simple to perform compared to complex mineralogical studies, so it is mandatory to advance in this type of research in order to establish other objective criteria to adequately choose borrowed sand for nourishment projects. This study is the starting point for future research on sand colour characteristics in the study area in order to establish objective criteria for designing future anthropogenic actions on the coast attempting to maintain original beach sediment characteristics.
Finally, the data shows the current colour of beach sand in different locations, which can be very interesting in applied environmental studies (e.g., in terms of the beach quality assessment, tourism or engineering activities). For instance, this study can be particularly interesting to identify the type of sand (natural or nourished), to nourish beaches attempting to employ natural-like sand colour, and even to detect future changes in the sand colour or the presence of contaminants.
This research is a new contribution to the diverse geological heritage of the southeast of the Iberian Peninsula. Sand colour of natural beaches is influenced by the geological context in which they are located: External and Internal zones of the Betic Cordillera, and the Bajo Segura basin. These three main source areas of sediments determine natural sand composition and thus, the native beach colour. Because of this, the resulting colour of each beach is the consequence of their mineralogical composition linked to the source and hydrographic basin. In this sense, natural beaches related to the geological context of the Bajo Segura basin are clearer than the rest due to the high content in quartz grains, especially the southernmost ones. On the contrary, the darkest beaches are located in Tabarca Island, whose rocky outcrops mainly belong to the Internal zones of the Betic Cordillera. Therefore, although geology is the basis of the native beach colour, human factors can modify sand characteristics, as in the case of several studied beaches, especially those related to the geological context of the External Zones of the Betic Cordillera.
To measure the colour in sandy sediments it is adequate to apply CIEL*a*b* methodology to objectively quantify the chromatic appearance of each sample. In the study area, generally nourished/altered beaches differ from the native ones. The alteration of the sand colour by anthropic actions transformed the natural colour increasing green and blue tones (a* and b* colour components) and Lightness (positive values in L*). Despite this, a big group of natural beaches was identified with the highest a* and b* values (towards red and yellow tones). Colour and mineralogical parameters are related among them, such as the Lightness and the content of leucocratic components. Many beaches with native sand present a high content of melanocratic components (>60%), while most of the nourished beaches normally show less than 60% of such components. Principal Component Analysis shows good groupings associated with the mineralogical composition (related to the geographical location) and anthropic vs natural beaches. The position of the samples in the PCA graph is very useful to compare the sand colour from different beaches and also to interpret the limited information about nourishment works carried out in the past.
The 66 investigated samples differ in colour parameters and the beaches that receive more tourists have been nourished and tend to be clearer (e.g., “Ponent”, “El Postiguet”, “San Juan”). Results show that a few nourished beaches, e.g., “Playa del Cura”, “Llevant”, “Arenal Bol”, “El Postiguet”, and “Ponent”, have a similar colour to the nearby native ones. The rest of the nourished beaches have lower values of a* and b* colour parameters, moving towards green and blue tones, e.g., “Punta Prima”, “Marineta Cassiana”, “Playa Lisa”, “Bernabeu II”, “Calas del Cuartel”, etc. Despite this, native clear samples generally presented higher values in the a* and b* parameters, this giving a more golden visual appearance, e.g., Ferri Beach, Cervera Beach, “Los Locos”, “Les Ortigues”, “La Mata” or “Bon-nou” among others. Today, the beach colour is not taken into consideration in some nourishment works, at least in the study area where “El Puerto” in the south or Fustera Beach in the north are clear examples.
This study showed with analytical data a post-nourishment change in sand colour, a common consequence also produced in other coastal sites such as Italy, Belgium or Cuba. Such changes may not be appreciated by national and international tourists as well as by old local beachgoers. The CIEL*a*b* colour space used to characterise sediment colour and Lightness is a useful tool to avoid significant changes in sand colour that can affect coastal scenery, beachgoers preferences, and the biological processes of some species. Native sediment colour is a natural characteristic of the coastal landscape and for this reason it should be maintained, at least in rural beaches and natural parks. It is essential to preserve the most natural coastal areas such as the dunes of Guardamar or the Island of Nueva Tabarca, considered by the Spanish Geological Survey as places of geological interest, mainly due to their geomorphological, sedimentological and stratigraphic particularities. In urban beaches, where all is artificial, a change of sand colour can be accepted in order to improve the beach attractiveness. Regarding future research prospects on the colour of beach sand, it would be interesting to analyse the sand properties against time in order to detect possible changes in the coastal environment.
Sand is a limited and increasingly valuable resource. Its use and relocation generates strong environmental, economic, and social impacts, especially on the coast. For this reason, it is preferable to opt for dune restoration and the elimination of unnecessary structures/constructions (on the coast or inland) that promote beach erosion instead of proposing nourishment projects that involve large economic investments and that may affect different coastal and marine habitats as well as associated species. Finally, the method used in this paper is easy to apply and can be very useful to carry out appropriate erosion management actions by means of beach nourishment works in the future. Consequently, the current conclusions are of particular interest to future actions to preserve the natural environments and original characteristics of native deposits.