Experiments on the sediment transport along pool-riffle unit

Abstract

This study examines channel dynamics and mixed-load bed load transport through a riffle and pool sequence forced by downstream channel width variations within an experimental flume. The experiment consisted of four runs across which we compare and contrast local and spatially-averaged bed surface texture and topography, sediment transport rates and sediment mobility at five locations across a pool-riffle pair. Sediment transport was measured using mini Helley-Smith (HS) samplers and particle tracers seeded in the monitored riffle and pool. In this study, “local” sediment transport rates were highly variable across the five monitoring locations. The lowest sediment transport rate was recorded at the riffle tail whereas the highest rates were measured at the riffle head and the pool centre. The texture of the bed surface and transported load do not explain measured bedload transport trends and depending on how the measurements are aggregated differing interpretations are supported. In general, the bed texture in the pool was finer than the texture in the riffle, however specific grain-size percentile classes derived from pooled population analysis suggests little to no difference between pool and riffle texture. The combined results highlight the importance of acknowledging and applying analysis techniques to better understand the inherent variability of bedload transport within channel reaches where morphology differs, such as pools and riffles.