Abstract

The Tongoy bay (30 degrees S/70 degrees 30'W) and the related catchment system in the western fringe of the semiarid Chilean Coastal Cordillera consists of an inherited landscape subject to present-day geomorphic processes. It is an interactive morphological system, formed by coastal range watersheds, marine terraces and sandy beaches. Taking into account the geological setting, a detailed geomorphological survey has shown a particular landscape, which is interpreted as inherited from past tropical climatic conditions. Rock chaos and tors on granitic slopes, as well as the presence of palaeosol horizons on glacis and alluvial terraces, are evidences of this palaeoclimatic heritage. The current average annual rainfall in the area is ca. 70 mm; however, during ENSO events, a daily similar rainfall amount is reached. Thus, the area can be considered as affected by rainfall events of low frequency but of high intensity and magnitude. Consequently, the deeply dissected and eroded paleo-landscape system is further shaped during El Nino-related events. Hence, local hazard conditions are generated, especially in slope gullies, alluvial fans and glacis. The geomorphic dynamics related to these events consists of flooding, debris flows and linear/areal erosion processes, occurring especially in micro-catchments situated on the scarps of the highest marine terraces.