On steep alluvial fans, debris floods happen rarely but are often catastrophic. Debris floods and their associated hazards are well documented, but the time between flood events has generally been considered a period of ‘dormancy’, and thus ignored. Here, we present results from a series of four alluvial fan experiments in which we examine the inter-flood period processes and their influence at both event and fan evolution timescales. We built each fan from the same number of debris floods, and the same volume of sediment, but varied the duration of the inter-flood period. This duration had a fundamental influence on fan morphology: in particular fan slope and fan area varied between the experiments. In addition, longer inter-flood periods led to increased flow channelization and channel incision near the fan apex. Our findings challenge the notion that inter-flood periods on steep alluvial fans may be considered dormant. Moreover, the results suggest that longer inter-flood periods may act to contain subsequent debris flood events, shift the locus of debris flood hazard, and reduce their severity.

In gravel-bed rivers, deterministic approaches to predicting bedload transport use the mean bed shear stress (termed one-dimensional or ‘1D’ equations) or integrate across the frequency distribution of shear stress (2D equations). At low flows, incorporating a range of shear stress values increases prediction accuracy, but at relatively high flows the 1D and 2D approaches are similarly accurate. We contribute to an understanding of the stage-dependent relationship between morphology and bedload transport, and specifically why the mean shear stress characterises transport capacity at formative discharges. We performed physical modelling using a generic Froude-scaled model of a steep laterally-constrained gravel-bed river and captured digital elevation models to perform 2D hydraulic modelling. Both 1D and 2D Meyer-Peter Müller equations were highly accurate across two distinct channel morphologies. In alternate bar channels, transport capacity was controlled by negative feedbacks between flow depth and local bed slope that resulted in a relatively homogeneous distribution of bed shear stress. In plane-bed channels, which lacked the degrees-of-freedom available for large-scale morphologic adjustment, transport capacity was controlled by a spatially variable migrating surface texture. The contrasting spatial patterns of morphology, hydraulics, and surface texture between the two channel morphologies highlight the potential for the same correlation between mean shear stress and transport capacity to emerge through different mechanisms. We suggest that nonlinear feedbacks explain why simple bedload transport equations can be highly effective above a certain flow stage across a range of channel morphologies, and further work should examine whether lateral adjustment confounds this result.

Alluvial fans form through primary and secondary geomorphic processes. Primary processes act to transport sediment from the watershed to the fan while secondary processes re-mobilize and rework the fan surface. While primary processes on alluvial fans are well studied, secondary processes and their relationship to fan flood hazards have received little attention. The experiments described herein isolate the role of secondary processes in determining alluvial fan behaviour and morphology. We conducted four experiments, in which alluvial fans were allowed to evolve under alternating primary and secondary process periods, with different durations of secondary processes. While the secondary process duration changed, the total primary process duration remained constant keeping the total volume of sediment constant for each experimental fan. Experiments with longer durations of secondary processes generated fans with larger areas and gentler gradients. In addition, longer secondary process durations led to increased flow channelization and centralization between flood periods. These morphologic changes resulted in fewer avulsions, that occurred later during primary process periods. These results indicate that changes to the relative duration of primary and secondary process periods caused by climate change can affect fan morphology and flow behaviour.

The interaction between form and process within a river produces the variety of morphodynamics we observe in channels. This poster presents a method using a simple index of channel behaviour that quantitatively represents the style of deformation a river reach undergoes. We term this index the throughput ratio (ζ), and it is calculated by comparing the volume of morphologic change recorded during an event to the volume of sediment transported during the event. The ratio of these two volumes represents a change in behaviour from exchange-based deformation of the channel (ζ < 1) to a more resilient throughput channel state where material is sourced from upstream, does not interact with the reach in question and is transmitted through (ζ > 1). A pair of experiments that developed different morphodynamics whilst sharing the same initial width, slope, discharge and grain size were used to demonstrate this methodology and interpretation of the results. The difference in morphodynamics between the channels was due to the presence of inerodible banks in one experiment, and a freedom to widen in the other. The inclusion of fixed banks prevented the system from being able to adjust its channel cross-section as freely, and maintained a high but variable sediment throughput over the experiment. In the system with mobile banks, the channel widened and exhibited a greater capacity to store sediment inside and outside of the active channel, causing the sediment transport rate to decline to zero during the experiment. In both, the rate of morphologic change tended to zero despite their marked differences in sediment transport over time. As a result, the throughput ratios depict two contrasting evolutions of channel behaviour. The differences in trajectory are due to the processes available to each system and their feedback with channel form. This approach provides a new method of representing channel character that may act to supplement existing analyses of river behaviour.