Interplay of flow and bed morphology caused by pool-riffle sequence formation without upstream sediment supply in gravel-bed rivers

Abstract

Pool-riffle sequences (PRSs) are widely distributed in gravel-bed rivers, but their formation regimes under conditions with no sediment supply have not yet been explored. This study selected one field section of PRSs in the East River and measured the bed surface and bedload longitudinally and used these observations to design scaled, variable width flume experiments where we measured feedback of flow and bed morphology under different flows and slopes without a sediment supply. Experimental results show that there exists the critical value of width-depth ratio during PRS formation, corresponding to the aggregated velocity distribution. Meanwhile, the peak value of turbulent kinetic energy easily occurs in the middle riffle under flood discharge conditions, promoting the removal of fine sediment and its transport downstream. The formation and destruction of the periodic armoring layer in the PRS become common without sediment supply. In fact, downstream sorting effect of the armoring layer is the dominant factor to lead the formation-destruction of bed surface, and causes discontinuous surface GSD (grain size distribution) longitudinally in the field. The increase in the flow and slope can effectively promote PRS development, expanding the variation in the lateral sedimentation but shortening the length of pools and riffles. Furthermore, self-maintenance of adjacent riffles and pools response to the lack of sediment supply is faster and more frequent than that with sediment supply, and higher flow can promote incipient sediment and bedload transport on the bed surface. The PRS with variable-width largely increases the influence of sediment supply on the bedload transport, e.g., the increase of bed load transport rate with incoming sediment supply of the PRS is much higher than that of constant-width channel.