Influence of Channel Bend Curvature on Debris-Flow-Driven Avulsion on Alluvial Fans, Explored through Discrete Simulations

Abstract

Alluvial fan morphology is often influenced by channelization of fluvial flow and episodic instances of avulsion (channel rerouting). Under certain conditions and in response to dramatic shifts (i.e., significant vegetation loss) or significant weather activity (i.e., intense rainfall or snowmelt) in the upstream environment, debris flows can manifest and have devastating impacts on downstream environments and communities. During their transport downstream through meandering distributary channels, debris flows can incise into the channel bed and laterally into channel banks. Debris flows can also rise and overtop the banks of their confining channels. These overtopping events are especially prevalent along channel bends where increases in centrifugal forces influence manifestations in debris-flow superelevation. This study investigates the parameter of channel bend curvature for debris-flow-driven avulsion using a debris-flow flume housed in the ECU Geomorphic Modeling Laboratory and series of 3D-printed rectangular channels of differing sinuosity imprinted in a simulated alluvial plain. The results of this experiment suggest variability in channel curvature (sinuosity) influences variability in manifestations of debris-flow runout and inundation behaviors, including debris-flow avulsion location, volumes and distances of debris-flow runout, and channel bend and alluvial plain inundation. Specifically, greater volumes and surface area coverages of debris-flow runout are suggested to result from avulsions from sharper curves as opposed to wider curves. Zones of likelihood of inundation that incorporate these findings are presented for areas of intersect between debris flows and channel outer bend crests on the debris-flow flume. Sharper curves are also suggested to influence greater frequency of avulsion. Lastly, this study demonstrates the potential for debris-flow avulsions to occur in channels free of debris pileup and as direct results of flow superelevation.