Ramon Blanco-Chao

Characterization of abrasion surfaces in rock shore environments of NW Spain

Despite the recent upsurge in rock coast research, many aspects of abrasion and their relationships to other processes remain poorly understood. In this paper, mechanisms subsumed under the general term abrasion were investigated at the beaches of Oia and Sartaña along the Galician coast of NW Spain, in particular at the micro- to meso-scale (mm–cm). Relationships between abrasion and mechanical rock strength served to explore feedbacks between weathering and abrasion on rock coasts, based on measurements of rock surface strength by means of the Equotip (Proceq) method, and stereomicroscope analyses of rock surfaces undergoing varying degrees of abrasion. The results suggest that (1) abrasion along near-vertical rock surfaces leads to a decrease in rock strength with elevation above the top of the basal sediment layer, (2) abrasion processes encompass two different modes, namely, the wave-induced sweeping and dragging of sand and gravel, and the projection of clasts against rock surfaces, each mode depending predominantly on the grain size of the abrasive agent, and (3) the two abrasion modes produce different rock surfaces whose roughness is strongly influenced by the properties of diverse minerals, in particular fracture and cleavage.

Late-Holocene storm imprint in a coastal sedimentary sequence (Northwest Iberian coast)

A combination of sedimentological, geomorphological, and pedological methods has been used to study a late-Holocene sedimentary sequence in a rock coast sector from NW Spain, with the aim of relating it to storm events and their morphodynamic effects. The sequence contains two coarse beach layers at an elevation of 2.8–3.5 m above the present highest astronomical tide (HAT). Radiocarbon dating revealed that their deposition had begun during 1735–1590 cal. yr BP and has continued until the present. The entire beach system experienced considerable morphological change after 1320–1230 cal. yr BP, with a westward displacement of the beach and a retreat of the sedimentary cliff in the eastern section. The two beach layers seem to have been formed by vertical aggradation of clasts and sand during high-energy storm events, highlighting the role of these events in the formation of sedimentary sequences on the rocky coasts of mid-Atlantic Europe. The effects of a recent storm event, recorded in March 2008, and the results of wave calculations suggest that long swell waves were needed for the accretion of the clasts. Using a hindcast model of wave data, we found a positive correlation between the winter North Atlantic Oscillation (WNAO) index and the winter monthly mean wave height and peak period. While the 14C chronology of beach sedimentation coincides with known climatic periods dominated by a positive NAO index, these results point to the importance of high-energy events and the synergies between past and present processes in the recent evolution and the morphodynamics of rock coast environments.

Chapter 6 The rock coast of continental Europe in the Atlantic

Abstract Rocky coasts occur along more than one-third (37%) of the Atlantic continental European coastline, approximately 3666 km, often forming vertical cliffs and characteristically gently sloping shore platforms. The continental European Atlantic rocky coasts show a great variability of rock types and structural contexts, as well as different wave climates and tidal ranges. Through a review of previously published data on cliff retreat rates and shore platform erosion measured on monthly, seasonal, annual and decadal timescales, this paper highlights the different processes and agents, their magnitude and frequency in shaping rocky coasts. In particular, the links between cliff retreat, shore platform evolution, present dynamics and inheritance (understood as whether platform and other rock coast features were shaped by a higher sea level than the present) comprise one of the major contributions from continental European Atlantic rocky coasts to a global understanding of rock coast coastal geomorphology.