Erosion rates in rock salt exposures with diverse karren monitored by erosion pins, close-range photogrammetry and terrestrial laser scanner

Abstract

Published data on solutional erosion in rock salt exposures are scarce, hindering our ability to understand the evolution of emergent salt diapirs with a number of practical implications (e.g. long-term geostorage). The Bofia Gran karst depression at the actively rising Cardona salt diapir (NE Spain) has been used as a test site to assess chemical erosion on a salt outcrop with a wide diversity of karren. Denudation measurements obtained by using erosion pins, terrestrial laser scanner (TLS) and ground-based photogrammetry consistently indicate a robust relationship between rainfall and slope-normal erosion, with an average value of around 10 mm per 100 mm rainfall. The significant variability observed in the surface-normal erosion is mainly controlled by slope gradient and karren micro-topography. Surface-normal erosion increases linearly as the slope declines and the surface area that interacts with rainfall decreases. The comparison of high-resolution point clouds shows that the range of erosion values across specific karren increases with their complexity, with extreme situations found in planar solution bevels and rugged solution flutes. The empirical relationships between slope-normal erosion and both rainfall and slope can be used to model geomorphic evolution in rock salt exposures, as illustrated with the case of the Bofia Gran, where diapiric uplift overwhelms chemical erosion. The experience gained in this investigation suggests that photogrammetry using a conventional camera can be more effective than TLS for assessing erosion in areas on the order of square meters. It also illustrates the benefits of using erosion values normalized to precipitation to enable worldwide comparisons.