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

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.

A modeling methodology to study the tributary-junction alluvial fan connectivity during a debris flow event

“…As described by Cabré et al (2020a), F 1 and F 2 correspond to debris flow deposits associated with matrix-supported flows with high fine sediment concentrations. F 1 is the thickest deposit (above 1 m) and reaching near 7000 m 3 of deposited sediment volume (Aguilar et al, 2020). Deposition occurred in a narrow area in the upper portion of the fan without reaching the main river.…”

“…Deposition occurred in a narrow area in the upper portion of the fan without reaching the main river. F 2 is a wider but lower thickness deposit (10 -30 cm) that overlays F 1 and covers all the fan's length, from the apex to the distal toe, with near 5000 m 3 of deposited sediment volume (Aguilar et al, 2020). F 1 and F 2 deposits overlain pre-event sediments, indicating that the associated flows had none or negligible erosion capacity (Cabré et al, 2020a).…”

“…The sediment stored in the catchments is transported towards tributary-junction alluvial fans and main rivers during these events. Tributary-junction alluvial fans play a crucial role in the transference of sediment from source areas to the main river (Fryirs, 2013;Aguilar et al, 2020). The efficiency of this sediment transference depends on the degree of connectivity within the fluvial system (Brunsden and Thornes, 1979), where tributaryjunction alluvial fans can modulate the sediment transference towards the main river by buffering or bypassing the sediment discharge, creating different coupling conditions (Heckmann et al, 2018;Savi et al, 2020) that depend on the combination of the fan's geomorphological configuration and the flow properties.…”

Sensitivity and identifiability of rheological parameters in debris flow modeling

“…This valley is characterized by perennial rivers that only exist in the trunk valleys, while tributaries only show ephemeral streams. In these areas, heavy rainfall events may induce catastrophic debris flows and mud floods that greatly contribute to erosion (Aguilar et al, 2020). The Acerillas creek (15 km 2 basin area; Fig.…”

“…Over the last decades, numerical models have emerged as a powerful tool to understand the behavior and magnitude of debris flow events, since they allow for the quantification of key variables used by engineers and decision-makers for risk management (Quan Luna et al, 2011;Frey et al, 2016;Calvo and Savi, 2009) and urban planning (Hürlimann et al, 2006;Lucà et al, 2014;Naef et al, 2006;Arattano et al, 2006). However, the application of debris flow models requires several assumptions and simplifications that make results diverge from reality at various levels (Sosio et al, 2007).…”