Bar Morphology in a sinuous bedrock flume

Abstract:

The extent and morphology of bars in sinuous bedrock channels should be a function of the bedrock planform, and the size and flux of sediment. Difference in bar morphology should alter the extent of bare bedrock and the relative rates of lateral and vertical erosion. As part of a series of experiments to explore the interaction between sediment supply, bar morphology, and erosion of bedrock bed and banks, we conducted an experiment in a 6.2 m long flume with 4 low-amplitude bends. The water discharge and sediment supply in the ~30 cm wide flume were held constant 0.016 m³/s and 47 kg/hr, respectively. The uniform sediment had a median diameter of 4 mm. Bedrock and bar topography were measured using images taken from three cameras mounted on a movable cart processed using Photoscan and flow velocity was measured using an ADV. During the first 12 hours of the experiment, bars developed in the 3 downstream bends while the majority of the bed in the upstream-most bend was exposed. There was very little bedrock erosion during this period. The bar morphology, sediment flux, and water surface slope stabilized after 12 hours. The slope for the remainder of the experiment was 0.007 with an average flow depth of about 13 cm. The bars were 4-10 cm thick. Sediment covered about 79% of the channel bed area. Exposed bedrock surfaces occurred at each bend and extended from just downstream of the bend apex on the outside of the bend to just upstream of the apex of the inside of the next bend downstream. Except for at the very upstream end of the flume, transported sediment did not contact exposed bedrock, but rather transported along the alluviated portions of the bed. During this equilibrium phase (12-32.5 hours), we did not observe any lateral or vertical erosion in the flume, likely due to the small impact force of the relatively fine sediment. The next steps in these ongoing experiments is to run the same sediment at a lower sediment feed to compare the bar dimensions and extent of alluvial cover. We will then feed a coarser sediment sufficient to erode the bed at two distinct feed rates to observe the changes in bar morphology and erosion of the bed and banks as the sediment cover adjusts to the feed.