Coupling between downstream variations of channel width and local pool-riffle bed topography

A potential control of downstream channel width variations on the structure and planform of pool-riffle sequence local bed topography is a key to the dynamics of gravel bed rivers. How established pool-riffle sequences respond to time-varying changes in channel width at specific locations, however, is largely unexplored and challenging to address with field-based study. Here, we report results of a flume experiment aimed at building understanding of how statistically steady pool-riffle sequence profiles adjust to spatially prescribed channel width changes. We find that local bed slopes near steady-state conditions inversely correlate with local downstream width gradients when the upstream sediment supply approximates the estimated transport capacity. This result constrains conditions prior to and following the imposed local width changes. Furthermore, this relationship between local channel bed slope and downstream width gradient is consistent with expectations from scaling theory and a broad set of field-based, numerical, and experimental studies (n=88). However, upstream disruptions to coarse sediment supply through actions such as dam removal can result in a transient flipping of the expected inverse correlation between bed slope and width gradient, collectively highlighting that understanding local conditions is critical before typically implemented spatial averaging schemes can be reliably applied.

Automated summary

The control of downstream channel width variations on the structure and planform of pool-riffle sequence local bed topography is crucial to understanding the dynamics of gravel bed rivers. However, the response of established pool-riffle sequences to time-varying changes in channel width at specific locations is not well-studied. This study conducted a flume experiment to explore how statistically steady pool-riffle sequence profiles adjust to spatially prescribed channel width changes. The findings reveal an inverse correlation between local bed slopes and downstream width gradients, providing insights into the conditions before and after local width changes.