Tributary-junction alluvial fans situated at the intersection of confined valleys with <100 km2 tributary catchments are of special interest to evaluate the heterogeneous consequences of extreme rainfall events in arid zones. These fans record the episodic sedimentological behaviour of the hillslope response to rainstorm events within tributary catchments, together with the influence on the main fluvial systems. In this paper, we benefit from the March 2015 event (23–26 March 2015), which produced 75–46 mm of precipitation over four days in the southern portion of the Atacama Desert. This storm event triggered several debris flows in El Huasco River watershed tributaries and, therefore, tributary-junction alluvial fans received a total of ∼106 m3 of sediments across 49 activated catchments. We find that the characteristic storm signature across the catchments can be synthetised in a conceptual fan formation model based on field mapping of facies (F1 to F6) present in the fans. The characteristic signature is a record of initially high sediment-to-water flows restricted to the fan environments (mainly debris flows) followed by later, more dilute (mainly hyper-concentrated to fluvial) flows that incise the tributary-junction alluvial fan deposits and link tributary catchments with the main river. These later-stage flood event deposits, locally, are capable of ponding and compartmentalising the main river where the longitudinal connectivity of the tributary-junction catchment is effective. This situation improves tributary-junction fan slope and main-trunk-channel linkages. This approach provides a reference framework for understanding the distribution and routing of effective runoff from similar rainfall events that control the aggradation and incision of the fluvial system, which is of great value when studying past stratigraphic arrangements in these arid alleys.

中文翻译：

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支流交汇处冲积扇对智利北部 El Huasco 流域 ENSO 降雨事件的响应

位于狭​​窄山谷与小于 100 平方公里的支流集水区的交汇处的支流连接冲积扇对于评估干旱地区极端降雨事件的异质后果具有特殊意义。这些扇记录了山坡对支流流域内暴雨事件响应的偶发沉积学行为，以及对主要河流系统的影响。在本文中，我们受益于 2015 年 3 月的事件（2015 年 3 月 23-26 日），该事件在阿塔卡马沙漠南部的四天内产生了 75-46 毫米的降水。这场风暴事件在 El Huasco River 流域支流中引发了几次泥石流，因此，支流交汇处的冲积扇在 49 个激活的集水区总共接收了约 106 立方米的沉积物。我们发现跨流域的特征风暴特征可以在基于扇中相（F1 到 F6）的场映射的概念扇形成模型中合成。特征特征是最初高沉积物到水流仅限于扇环境（主要是泥石流），随后是更稀（主要是超集中到河流）的流动，切割了支流冲积扇沉积物和连接支流集水区与主要河流。这些后期洪水事件沉积物在当地能够蓄积和分隔主要河流，其中支流汇流域的纵向连通性是有效的。这种情况改善了支流连接扇坡度和主干通道连接。