Causes of the different behaviour of the shoreline on beaches with similar characteristics. Study case of the San Juan and Guardamar del Segura beaches, Spain

Highlights: • The erosion rates are different in beaches with similar characteristics. • Study of the shoreline evolution using aerial images since 1956 • Before nourishment the beach lost 50% of its surface area in the period 1956–1990. • The greatest erosion occurs at beaches where sediment particles are more fractured. Storms can alter the beach shape, relocating large volumes of sediments and generating drastic changes in the coastline. In the last 60 years, beaches shoreline behaviour has been different even though the energy of the waves was similar. Therefore, it is necessary to understand the factors that affect the sandy coasts for better future management. In this research, two beaches, with different erosion rate, located in the southeast of Spain (separated by only 40 km of distance) have been studied. The beaches: i) have similar orientations, ii) are open to waves with similar sand lengths of 9.8 km and 6.6 km, and iii) have similar median sediment size (D₅₀). For its study, shoreline evolution has been analysed from 1956 to 2017. From the results obtained, it can be seen that: i) Between 1992 and 2017, San Juan just lost 3% of its surface, while in the previous period (1956–1990) it was 50%, and ii) Guardamar surface lost in 1992–2017 was 18%, and in the previous period it was 14%. For the analysis of the agents involved in both beaches, cross-shore profiles (volume), marine climate, biocenosis and sedimentology studies were carried out. The results showed that the energy on both beaches was very similar. The biocenosis had not changed and, however, the morphology of Guardamar seabed had increased to 1 m deep in some places, which had caused part of the beach berm erosion. Furthermore, important differences were found from the sedimentological study, concluding that the content of calcites and the degree of homogeneity of the particles are the real factors that caused these two beaches to behave differently against erosion.