A river's connective tissue: Lab observations of particle pathways and riffle formation during floods

Abstract

In rivers it is difficult to quantify bedform dynamics during storm events. Direct observation of sediment pathways would provide insight into the mechanisms that underly bedform formation and destruction. In the current study, our objective was to visualize these processes in a meandering pool riffle system with partial bed cover. Observing erosive and depositional patterns, as well as the locations of active sediment transport, provides insight into the validity of various pool-riffle maintenance theories. We used a physical 1:40 scaled model of Toronto’s Wilket Creek to simulate storm events during which riffles formed as connective bedforms between alternate point bars. Exported sediment was weighed and sieved to measure the grain size distribution, while the bed’s pre- and post- storm topography was quantified using Structure-from-Motion techniques. Sediment pathways were observed using a novel technique, where regions of interest were filmed at 60 frames per second under ultra violet light, illuminating painted tracers. Three paint colors were used for different size tracers, which allowed us to apply image segmentation and create separate videos for three size fractions of the sediment. Pathways were then extracted using Lagrangian tracking software. Results show that the area of active transport is limited to a narrow portion of the channel width that increases with flood stage. At low flow, transport is routed along the toe of point bars, while no particles travel into the region of the pool, where the bed is uncovered. Riffles are rarely observed at these stages. As the flow increases, the lateral extent of active transport expands to include the higher parts of the bars, while connective riffles grow in areal extent and height. Erosion and deposition was found to occur more readily along the active sediment transport zones. Pathways varied by particle size so that smaller particles traveled higher over the point bar and large particles tended to collect in the riffle. These results indicate that sediment-routing is a dominant mechanism behind the formation and maintenance of riffles in meandering rivers. Future work to quantify these processes will increase the effectiveness and longevity of river remediation design through targeted sediment augmentation instead of bedform reconstruction.