2.2.4. Wave Energy

Waves influence the beach sediment budget and are the predominant process: waves and associated storm surges, move sediment longshore, offshore and onshore; the latter direction seldom produces sediment deficit unless sand is deposited in urbanized areas and not returned to the beach, e.g., if polluted by traffic [84]. Cross-shore [85] and longshore [86] variability in sediment transport rate is well-known and documented. Net sediment long-shore transport ranges from few to millions cubic meters per year among the different coastal sectors. But the same magnitude order differences can be found locally at different times. Episodicity in long-shore sediment transport was studied by, e.g., Seymour et al. [87] who found that in one day sediment transport can be more than 600 times the mean daily net transport.

Shi-Leng and LiuTeh-Fu [88], studying a sector of the Mauritania coast, found that long-term variation in sediment transport follow a Gumbel distribution, but it is not necessary to wait for extreme events to have huge volumes of sediments moved along the coast: the net long-shore transport near Oregon Inlet (Outer Banks, NC, USA) is between one-half million and one million m3/year [89]. Along the Mediterranean coast, a "protected sea environment" according to the Davies classification [90], a net sediment transport of more than 400,000 m3 was calculated for a point near the harbor of Ashkelon, Israel [91].

With respect to long-shore transport, Raynor [92] introduced the term "cascade of uncertainties" involving the triple interaction of air, sea, and sediment into geomorphological literature. This is very pertinent when echoed in the concept of beach equilibrium. Pilkey and Cooper ([93], p.579) were of the opinion that "Sand volumes perhaps should be expressed as broad categories such as small, intermediate or large in recognition of the fact that meaningful determination of net annual transport of sand is probably impossible

in this complex, dynamic and changing natural system." A case study for the Northwest coast of Portugal [94], showed that the annual littoral long-shore transport values exhibit a large variability with a maximum of 2.24 million m<sup>3</sup> year<sup>−</sup>1, which exceeds the long-term mean magnitude by 105%, and a minimum, 108,000 m<sup>3</sup> year<sup>−</sup>1, 10 times less than the mean value. Taborda et al. ([94], p.466) pointed out that, "The long-shore transport estimates reported in the literature for this coastal stretch were made through different techniques from cartographic comparison with mathematical modeling, whose results led to a wide range of values from 200,000 (Abecasis 1955) to 3.5 million m<sup>3</sup> year−<sup>1</sup> (Teixeira 1994), although most values converge toward a mean value of around 1 million m<sup>3</sup> year<sup>−</sup>1."

Storms are known to have strongly modified the coast in specific periods of the Middle ages, e.g., the 1634 storm surge divided the German island of Strand in two parts, now known as Pellworm and Nordstrand islands [95]. In South Wales, UK, extreme storm events have been recorded in monastic records since the 3rd century culminating in the 14th and 15th centuries that saw much coastal erosion along with washovers. The storm events peaked in the 16th century, when they became especially ferocious causing much flooding and massive washover sand amounts formed the coastal dune fringe of South Wales [96,97]. Extreme events have been studied over a 6000-year-long period in Australia [98] surveying and dating beach ridges, showing the importance of the association of wave height and storm surge to build up those morphologies.

Climate change occurring during the last decades has modified storm frequency and intensity, e.g., along the Atlantic coastlines of Europe [99] and future scenarios forecast a further increase in storminess. This is an additional variable to the future beach sediment budget. Changes in wave directional distribution will modify long-shore transport direction and possibly invert its resultant direction, shifting convergence points, or completely modifying coastal cell geometry.

2.2.5. Biogenic Production and Chemical Precipitation, e.g., Inside Posidonia Prairies, Mangroves

Although most coastal sediments come from inland erosion, in some areas the beach comprises a consistent percentage of shell fragment and skeletons of marine animals. In Western Australia, these components are in the majority [100], but the same can be found in mid Latitude coasts, such as, Sardinia, where on the northern granite coast, the beach of Pelosa has more carbonate sand than quartz [101], or even in Scotland, where Coral Beach, Hebrides, has an abundance of calcareous seaweed [102]. Their production is influenced by water temperature, nutrients richness, and turbidity, all of which are extremely variable. Since historical times, this material was quarried to amend acid soils, e.g., in Scotland [103].
