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Morphodynamic styles: a novel index of gravel-bed river behaviour
  • William Booker,
  • Brett Eaton
William Booker
University of British Columbia

Corresponding Author:[email protected]

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Brett Eaton
University of British Columbia
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Abstract

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.