Clast mobility within boulder beaches over two winters in Galicia, northwestern Spain

Abstract

A micro-drone was used to make low altitude flights over boulder beaches at Laxe Brava and Oia in Galicia, northwestern Spain. Flights were made in July 2012, May 2013, and spring 2014. High resolution digital terrain models and orthophotographs, combined with GIS mapping, were used to monitor changes in the position of thousands of boulders. Maximum storm wave height was higher in the winter of 2013–2014 than in winter 2012–2013, and this was reflected in an increase in the proportion of the boulders that moved in the two winters, from 17% to almost 48% at Laxe Brava, and from 53% to almost 88% at Oia. The greater mobility of the boulders at Oia can be attributed in part to their generally smaller size, although there was considerable overlap between the size of boulders that moved and those that did not move within and between the two beaches. There were mobile boulders in areas up to several metres above the high tidal level on both beaches, and boulder transport in the shore-normal and alongshore directions triggered some changes in the beach profiles, particularly in the middle to upper parts of the beaches. Estimates of threshold transport conditions, scaled to boulder mass, breaker height, and other variables, suggested that all but the very largest boulders on the two beaches should have been mobile, even during the summer months when the waves were much lower than in winter. Model over-prediction can be attributed to a number of factors, including: constraints on movement imposed by surrounding boulders; differences in boulder size and their effect on pivoting angles and on the degree to which boulders are exposed or sheltered from wave impact; and difficulties in assigning appropriate values to model coefficients.