Rotation, sedimentary deficit and erosion of a trailing spit inside ria of Arousa (NW Spain)

Based on the analysis of a large set of remote images, bathymetric studies and acoustic profiles, we studied the causes of erosion of a small island inside a ria of the NW coast of Spain. The island consists of a rocky sector to the south and a trailing, or comet-tail, spit to the north, which, until 1980, was in an equilibrium between the waves of the open ocean propagated inside the ria and the local wind waves. The development, in the 1970s, of a large park of floating rafts for mussel farming was identified as the factor triggering a period of disequilibrium and severe erosion on the island. The area covered by the floating rafts was the cause of the attenuation of local wind waves and the reinforcement of waves propagated from the open ocean. The spit underwent a period of approximately 38 years of disequilibrium characterized by a rotation movement with several phases. During the first period (1980-1989), a submarine lobe was formed, sequestering approximately 9000 m³ of sediment. Between 1989 and 2015, the spit maintained a rotation of 24° of amplitude and a phase of severe erosion, with rates of up to 6.6 m/yr, began in 2000. Since 2015, the spit has entered a new phase of stabilization, reducing the rotation to a small amplitude of 5°. The rates of erosion have also decreased, although they are still active.