Caracterización e dinámica dos coídos nas costas de Galicia

Abstract

Os coídos son praias de cantos e bloques que se atopan asociados aos cantís e ás plataformas rochosas, especialmente en áreas dominadas por rochas graníticas. Trátase de moreas de clastos de tamaño heterométrico, con predominio dos de tamaño bloque, que nalgúns lugares poden chegar a superar os 2 m de longo no seu eixe maior, sendo moi abundantes os que miden entre 1 e 2 m. Os mellores exemplos de coídos atópanse na chamada Costa da Morte, situada ao norte do cabo Vilán (Camariñas, A Coruña), na área de Corrubedo (Ribeira, A Coruña) e no sector de costa que se alonga entre o cabo Silleiro e o esteiro do río Miño (Baiona, Oia, O Rosal e A Guarda en Pontevedra). Polo deseño que debuxan en planta téñense diferenciado 5 grandes tipos: lonxitudinais, en punta dobre, en arco, en corredor e en punta simple. Desde o punto de vista xenético diferéncianse catro tipos: derivados de formas de exhumación granítica, derivados da fragmentación de plataformas e cantís mariños por arranque, derivados de desprendementos e derivados de depósitos de orixe fría. En moitos casos a súa xénese é múltiple. Para poder coñecer a mobilidade dos bloques empregouse un micro dron para realizar voos de baixa altitude sobre dous coídos, en Laxe Brava (Ribeira, A Coruña) e en Oia (Pontevedra). Os voos realizáronse en xullo de 2012, maio de 2013 e abril de 2014. Os voos permitiron a obtención de ortofotografías e modelos dixitais do terreo de alta resolución (MDT) que, integrados nun Sistema de Información Xeográfica (SIX), foron usados para controlar os cambios na posición de miles de cantos e bloques. A altura máxima das ondas de tormenta foi maior no inverno de 2013-2014 que no inverno 2012-2013, e isto reflectiuse nun aumento na proporción dos bloques movidos, dun 17 % a case o 48 % en Laxe Brava e do 53 % a case o 88 % en Oia.

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.