What is a beach?
Coastal scientists use to say that beaches are waves and sediments. But what they really mean is that beaches are wave-deposited accumulations of sediment located at the shoreline. In fact, they require a geological basement to rest, sediment to form them and waves to shape them. Additionally most beaches are also affected by tides.
The beach extends from the point where waves feel and interact with the bottom and shoal across the nearshore, until they break and swash in the beachface. In doing so, sea waves are transformed by shoaling, breaking and swash induced by the interaction with seabed. This interaction determines the beach morphology and shape.
Figura 1. Scheme of zones along the beach.
Waves, types and origin
Waves are an important part of the coastal scenery. The sea surface is not a flat horizontal surface. Waves are generated by wind blowing over the sea surface, although there are other punctual sources such as tectonic seabed movements (i.e. tsunami). The stronger the winds, the longer it blows and the longer stretch of sea over which it blows (fetch), the larger the waves are.
We can distinguish between sea and swell waves. Sea waves are generated by wind and consist of short, steep, high, slower waves that tend to topple over and break nearshore. Sea waves have a broad spectrum of directions. But, once the wind stops blowing, the wave that it has generated travel away and leave the area of origin with minimal loss of energy. This type of waves is known as swell, and consists of lower, longer, faster and uniform on direction waves.
Figura 2. Picture of breaking waves reaching the shoreline (Cala Millor).
Waves properties
Waves are defined by their height (H) –which is the vertical distance from the crest to the through–, by length (L) –which is the horizontal distance from crest to crest)–, and period (T), understood as the time between successive crests. There exists a fundamental relation between wave period and wave velocity: the longer the period the longer and faster the waves. Roughly, for a period of 6 s, wavelength is 10 m and wave velocity 56 m/s, whereas for a period of 3 s, wavelength is 5 m and wave velocity 39 m/s.
When waves enter in shallow water the velocity is controlled by depth (d) and the gravitational constant (g). For this reason waves slow down as they move to the shore. For instance, 10 s period waves travelling at 56 km/h at deep water slow down to 7 km/h in 5 m water depth.
Figura 3. Wave parameters.
Beach sediments
Beaches consist of sediment that can range in size from sand and gravels up to cobbles and boulders. The finer grain sizes result in low gradient beaches (around 1º) whereas the coarser beaches may reach steep profiles, as much as 20º in slope. Most of the beaches are composed of quartz grains derived from cliff erosion, fluvial-terrestrial contributions and carbonate sediments (commonly composed of skeletal fragments of corals, coralline and green algae, foraminifers and molluscs shells). Sediments may therefore be derived from terrestrial environments and delivered via rivers, glaciers or shoreline erosion, and from marine organisms from sea.
Figura 4. Near-shore zone sedimentarian system.
Beach zonation and processes
A universal beach zonation can be identified according to three main zones of wave transformation (shoaling, breaking and swash).
The shoaling wave zone consists of a low gradient concave profile with small ripples generated by waves. Generally the sediment transport direction is onshore. Waves, getting close to the coast, shoal and interact with the seabed, reducing velocity and increasing in steepness and height.
The surf zone is one of the key zones in beach morphodynamics because most of the sediment transport and processes related with beach accretion or erosion account there. Waves break when the water depth is approximately 1.5 times the wave height, and in doing so wave potential energy is transformed in kinetic energy. Surf zone current can transport sediment onshore, longshore and offshore contributing to the build up of sand bars and through. The position of these bars and their displacement respect of the coast, are the main mechanism of regulating wave energy by means of sediment transport in the nearshore. A roughly approximation can state that fair or low energy and regular wave conditions displace bars landwards, whereas energetic and irregular wave conditions induce offshore bars movement. One of the returning water backs out to sea, and a conduit to transport seaward eroded sediment during high seas are the rip currents. Rip currents are a major hazard to beach users and responsible for most beach rescues and beach safety incidences.
Figura 5. Aerial image of propagating waves, along the shoaling, the breaking and the swash zones.
The swash zone corresponds with the area where the broken wave reaches the base of the wet beach and runs up the beach face. Wave run up and backwash produce a relatively steep seaward sloping (1 to 20º). When sediment is deposited in this swash zone it can build a berm (a subhorizontal landward-dipping sand surface). The swash zone may also contain some undulations known as beach cups.
Beach variability
Beach shoreline and profiles have been observed to change over a range of spatial and temporal scales. The primary forcing of beach change are wave conditions, which are themselves driven by non-stationary and episodic meteorological processes. Although there are some, a priori, cyclical changes, such as seasonal variations, there are observational evidences that stress the understanding of beaches as a variable and dynamic system.
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Figura 6. Example of beach profile variability.
This issue is of particular interest in urban beaches as those related with touristic stations, because they cannot be understood as constant sandy solariums and dry beach variability is an intrinsic feature of these environments, such as the weather and the rainfall during the year.
Beaches at the Balearic Islands
The shoreline of the Balearic Islands is of 1,723 km in length. Most of the coastline (70%) is cliff like or rocky, whereas gravel and sandy beaches just represent the 10% of the coastline.
Figura 7. Beach location along Balearic Islands coastline (red).
At Balearic Islands average values of wave height (H) vary throughout the year presenting high values, above 1 m, in autumn and winter seasons. During spring and summer seasons mean height drops below 1 m. North and north-eastern wave directions are the predominant wave direction in the northern and eastern coasts, whereas in south-western wave directions prevail in the southern and south-eastern shores. It is important to notice that during summer season exists a thermal sea breeze regime in the major part of the coast with intensities between 5 and 10 m/s that can generate some waves and associated rip currents.
Figure 7. Location of beaches along the coastline of the Balearic Islands
At Balearic Islands most streams are ephemeral and exhibit very low mean annual discharges near their mouths. The river loads in the higher-discharge wet season mainly comprise decalcified clays with some sand but without any coarser material. As the streams have a low competence, the main source of terrestrial sediments is therefore likely associated with cliff erosion. Therefore, sandy beaches of the Balearic Islands consist primarily of medium to coarse moderately sorted marine biogenic carbonate sands (up to 70% of the bulk sediment). Reworked biogenic sediments, molluscs and snail shell fragments and foraminifers are the most abundant grains among the biogenic sediments at Balearic Islands beaches. For this reason Balearic Islands beach waters are clear and transparent.
Figura 9. Zoomed image of sediments
Reference and recommended reading
Balearic Islands specific references:
Álvarez-Ellacuría, A., Orfila, A., Olabarrieta, M., Medina, R., Vizoso, G., Tintoré, J. 2010. A neashore wave and current operational forecasting system. Journal of Coastal Research, 26: 503-509.
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.
Álvarez-Ellacuría, A., A., Orfila, A., Olabarrieta, M., Gómez-Pujol, L., Medina, R., Tintoré, J. 2009. An alert system for beach hazard management in the Balearic Islands. Coastal Management, 37: 569-584.
Gómez-Pujol, L., Orfila, A., Álvarez-Ellacuría, A., Terrados, J., Tintoré, J. 2013. Posidonia oceanica beach-cast litter in Mediterranean beaches: a coastal videomonitoring study. Journal of Coastal Research. Spec. Issue., 65: 1768-1773.
Gómez-Pujol, L., Orfila, A., Álvarez-Ellacuría, A., Tintoré, J. 2011. Controls on sediment dynamics and medium-term morphological change in a barred microtidal beach (Cala Millor, Mallorca, Western Mediterranean). Geomorphology, 132: 87-92.
Gómez-Pujol, L., Orfila, A., Cañellas, B., Álvarez-Ellacuría, A., Méndez, F.J., Medina, R., Tintoré, J. 2007. Morphodynamic classification of sandy beaches in a microtidal low marine energetic environment. Marine Geology, 242: 235-246.
Gómez-Pujol, L., Roig, F.X., Fornós, J.J., Balaguer, P., Mateu, J. 2013. Provenance-related characteristics of beach sediments around the island of Menorca (Balearic Islands, western Mediterranean). Geo-Marine Letters, 33:195-208.
Jaume, C., Fornós, J.J., 1992. Composició i textura del sediment de les platges del litoral mallorquí. Bolletí Societat d’Història Natural de les Balears 35: 93-110.
Tintoré, J., Medina, R., Gómez-Pujol, L., Orfila, A., Vizoso, G. 2009. Integrated and interdisciplinary approach to coastal management. Ocean and Coastal Management, 52: 493-505.
