AUTOGENIC CONTROLS ON DEBRIS-FLOW FANS WITH LIMITED ACCOMMODATION SPACE: LABORATORY EXPERIMENTS INFORMED BY A FIELD EXAMPLE

Abstract

Decades of historic levels of urbanization and expansion of the built environment on to existing alluvial fans at the periphery of most cities has placed humans at risk of floods and debris-flows that are formative processes on alluvial fans. Understanding the evolution of these features is to understand risks to human lives and infrastructure in these locations. Therefore, there is a need to explore the myriad of factors affecting alluvial fan evolution. Here, physical modeling is used to explore the effect of limited longitudinal accommodation space on autogenically derived debris-flow fan evolution. Physical modeling has furthered our understanding of the formative processes of alluvial fans, in part, by allowing for the isolated control of any number of variables. Operating in a laboratory setting also allows researchers to overcome potential challenges posed by field work (site remoteness, hazardous environments, unpredictability of phenomena, etc.) while creating an environment for manageable data collection. Prior alluvial fan physical modeling has largely focused on fluvially generated fans rather than those dominated by debris flow deposition. Moreover, the studies that have considered the latter have only done so under the assumption of unlimited accommodation space (the area in which fans can prograde); an assumption that is frequently not representative of natural conditions. Here, two debris-flow fans are generated using a small-scale physical model in order to explore the influence of limited longitudinal accommodation space on autogenic avulsion patterns. Fan-toe erosion is simulated through the repeated removal of debris-flow material at a fixed distance from the fan apex. Aided by high-resolution terrestrial laser scanning (TLS) data, geomorphic change detection and topographic profiles are used to examine differences in fan evolution. Results from small-scale physical modeling experiments show that cycles of channelization, the formation and persistence of a stabilized channel, channel narrowing and overflow, and avulsion result in the formation of new fan segments on a debris-flow fan with limited accommodation space. These results provide evidence for an explanation of debris-flow fan evolution alternative to the most widely accepted theory which can be summarized as cycles of channelization, backfilling, and avulsion. Furthermore, these results are informed and supported by field observations of a debris-flow fan located in Chalk Cliffs near Nathrop, Coloradao, USA where the fan-toe is periodically eroded by Chalk Creek.