Erosion after an extreme storm event in an arid fluvial system of the southern Atacama Desert: an assessment of the magnitude, return time, and conditioning factors of erosion and debris flow generation

Abstract

The contribution of an individual extreme storm event to long-term erosion rates has been estimated for the first time in the Atacama Desert. A mean erosion of 1.3 mm has been calculated for the March 2015 event that impacted the southernmost part of the Atacama Desert. The estimated erosion is consistent with millennial erosion rates and the previously reported return times of high-sediment-discharge events in the study area. This is significant because erosion rates, related to events of high sediment discharge in arid fluvial systems, are difficult to measure with sediment loading due to destruction of gauges by devastating flash floods and therefore have not been directly measured yet. During the March 2015 storm, debris flows were reported as the main sediment transport process, while gullies and channels erosion were the main source of sediments that generated debris flows reaching the tributary junctions and the trunk valleys. Sediment yield at tributary outlets is highly dependent on the ability of catchments to store sediments in stream networks between storms. The largest tributary catchments, the high hydrological hierarchy, the low topographic gradient and the gentle slopes are the most determining factors in generating debris flows capable of reaching alluvial fans in any storm event from large sediment volumes stored in the stream networks. Our findings better assess the susceptibility to debris flow of arid catchments, which is significant for the southernmost valleys of the Atacama Desert because human settlements and industries are mostly established in alluvial fans.