The effect of debris-flow sediment composition change on avulsion behavior and debris-flow fan development
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Debris-flow fans are widespread and often densely populated landforms that occur in regions with high relief. Avulsions cause a major risk for inhabitants as they occur sporadic and with current knowledge they are relatively unpredictable. Recent studies on debris-flow fan development and avulsion behavior reveal that the magnitude-frequency distribution and the associated flow-magnitude sequence of a debris-flow fan exerts a significant control on the occurrence of avulsions. Additionally, multiple studies also confirm the influence of debris-flow sediment composition on debris-flow morphology. Here, the effect of a change in debris-flow sediment composition per debris flow on the avulsion behavior and debris-flow fan development is investigated. This is done by creating two experimental fans where debris-flow sediment compositions are randomly extracted from a heavy-tailed double-pareto distribution with an average around 25 volumetric percent gravel. The sequence of volumes is distributed along a thin double-pareto distribution, where the majority of flow volumes cluster around a mean value of 5 kilograms sediment weight, but significantly larger and smaller flow volumes still occur sporadically. Comparing the experimental fans created in this study to a fan in De Haas et al. (in review) with similar flow-volume sequence but with uniform composition gives insight into the effect of a change in sediment composition for a debris-flow fan that is simultaneously subjected to a change in volumes. The effect of a sediment composition change on avulsion behavior and debris-flow fan development is best seen on a timescale of a couple of debris-flow events. From this study, three main conclusions can be established. (1) Debris-flow sediment composition is able to enhance or diminish the mobility of a debris flow with a given volume. Here, the volume dictates the main mobility of the debris flow but the mobility is influenced by related debris-flow sediment composition. (2) The influence of debris-flow sediment composition on debris-flow behavior is affected by local topography, where a complex topography diminished the influence of sediment composition, and where a plano-convex surface close to the apex enhances the influence of sediment composition (especially the low mobility of high-gravel debris flows). (3) The increased erosion with high-gravel debris flows is able to effectively enhance channelization. In contrast, gravel-poor debris flows are more likely to create a channel plug or deposit a solid body thereby influencing subsequent debris flows. This study shows that on a timescale of a couple debris-flow events, the effect of a change in sediment composition on debris-flow fan development can be such that it initiates avulsions or enhances the speed of avulsions. Although the debris-flow fan magnitude-frequency distribution seems to be the major controlling factor in fan development, these findings can have important implications for hazard mitigation and therefore an increasing focus on sediment composition during debris-flow fan research is required.