BEDLOAD SHEETS IN A SAND-GRAVEL MIXTURE

The development of bedforms increases sediment transport rates within fluvial channels, directly impacting channel morphology, reservoir sedimentation and sediment fluxes. Bedforms known as bedload sheets exhibit typical heights around that of the sediment mixture D₉₀, exhibit lengths up to 300 times their height, and are documented to enhance sediment transport rates. Although sediment transport rates related to bedload sheets are known to be elevated, the extent has not been fully elucidated for sediment mixtures with comparable sand and gravel proportions. This study investigated the impacts of bedload sheets by assessing changes in their spatial extent as well as their relative abundance within the bed under differing flow shear stresses. To do this, a series of flume experiments was conducted using a sand-gravel mixture containing 56% sand with a D₅₀ of 1.9 mm and D₉₀ of 5.2 mm. This mixture was subjected to several flow shear stresses during which the height, length, width, frequency and migration rates of the sheets were measured. Preliminary results indicate that the variability in the size of bedload sheets increases with shear stress. Additionally, bedload sheets exhibited larger concentrations of coarse grains as the stress increases. Bedload sheets formed when all size fractions of the bed material were mobilized as well as when the size distribution of the bedload was approximately equal to that of the bulk mixture. These results suggest that the spatial extent of bedload sheets as well as the grain sizes they are composed of scale with increases in flow shear stress over the range of flows they are capable of forming under. Understanding the ramifications of flow shear stress on bedload sheets will assist with the future experimentation seeking to investigate other controlling factors such as sediment supply dependency and sediment supply composition.