Caracterización e dinámica dos coídos nas costas de Galicia

Abstract

Coídos are beaches composed of cobbles and boulders that are usually found in association with cliffs and rock platforms, particularly in granite-dominated areas. They consist of accumulations of heterometric, predominantly boulder-sized clasts, often between 1 and 2 m and even with their main axis exceeding 2 m in some areas. The best examples of coídos are found in the area known as Costa da Morte, located north of Cape Vilan (Camariñas, A Coruña); in the Corrubedo area (Ribeira, A Coruña), and along the stretch of coastline located between Cape Silleiro and the Miño river estuary (Baiona, Oia, O Rosal, and A Guarda, in Pontevedra). According to their shape from top view, five broad types have been defined: longitudinal, double-pointed, arched, corridor-like, and single-pointed. Genetically, four types can also be distinguished: those derived from granite exhumation processes; those generated by the dislodgement of rock fragments from shore platforms and sea cliffs; those produced by rockslides, and those originated by glacial deposition. Their genesis is often of multiple origin. In order to study boulder mobility, a micro-drone was flown at low heights over two coídos, one in Laxe Brava (Ribeira, A Coruña) and the other in Oia (Pontevedra). Flights were carried out in July 2012, May 2013, and April 2014. These flights yielded ortophotographs and high-resolution digital terrain models (DTM), which were integrated in a GIS and used to monitor thousands of cobbles and boulders for changes in their position. Maximum storm wave height was greater during winter 2013-2014 than during winter 2012-2013, resulting in an increase in the proportion of moved boulders from 17 % to almost 48 % in Laxe Brava and from 53 % to almost 88 % in Oia.