Potholes as evidence of abrasion process: investigations on an Atlantic (Spain) and a Mediterranean (Italy) coast

Abstract

Potholes are cylindrical depressions in rocky substrates, the genesis of which is connected to the presence of two elements: sediments and water energy. Coastal potholes have been less studied than stream potholes, for which a model of enlargement triggered by the vortex motion of water was developed. The main factors that determine the formation of potholes are bedrock type, geological structure, the presence of sediments and the occurrence of mechanical erosion, namely abrasion, traditionally considered responsible for shaping the pothole itself. The aim of this work is to investigate the role of abrasion and the effect of other factors, such as the amount of sediments, in shaping potholes on shore platforms or other flat surfaces along rocky coasts. Study sites were selected in two different landscapes: the Atlantic coast of NW Spain and the Mediterranean coast of NW Italy. Five potholes were randomly selected in each study site. Abrasion results in an increase of mechanical strength of the rock that can be measured with a durometer such as the Equotip. Rock hardness was tested in the investigated potholes along cross-shore oriented transects and the amount of sediment content in each pothole was analyzed and related to the pothole morphometry. Abrasion was considered effective in specific tracts of the profile where hardness values proved to be particularly high. Data obtained in most of the potholes showed a common pattern: they underlined a change in hardness along the profile and showed the effectiveness of the abrasion process on the bottom and on the highest part of the walls, mainly on the landward side. It was also demonstrated that when a pothole is actively developing due to abrasion, an equilibrium between the amount of sediments and wave energy exists, that can be quantified through an index. Our work contributes to the quantitative knowledge of the process of abrasion as responsible of the genesis of coastal potholes.