Slope and sediment size in the development of pebble and cobble beaches with low permeability

Abstract

The relationship between beach slope, sediment size and therefore its permeability has been the subject of study for decades. Particular attention has been paid to sand and gravel beaches, but less research has focused on coarse beaches. In this work, we analyse coarse beaches characterized by a low thickness, around 35–50 cm, composed of surface pebble and cobble clasts (D50 of the B ‐axis between 41 and 102 mm) with a mixed matrix of sand and gravel (D50 between 0.76 and 12.33 mm). The most notable element is the presence of an impermeable underlying cohesive sediment up to 1 m of thickness, which gave a radiocarbon date much older than the time when sea level reached its present level. In spite of the occurrence of an impermeable layer at a low depth that reduces its permeability, the slope of the beach ranges from 7.13° to 11.3°, and the correlation between the beach slope and the bulk matrix and sand sizes of the infilling sediment corresponds to what was expected, and the underlying cohesive sediment reached the same slope as the beach. These results suggest that the development of the present coarse‐grained beaches is that the erosion of cohesive sediments occurred contemporaneously with beach formation, with abrasive clasts playing a central role in shaping the equilibrium profile.