Drivers of Spatial Dispersion and Residence Time of Coarse Sediment in Gravel-Bed Rivers
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Transport of coarse sediment in gravel-bed streams alternates between mobile and immobile phases. Mobilized sediment disperses primarily in the streamwise and vertical directions. While travel times of sediment in the channel are brief, rest times between periods of mobilization can be relatively extensive. Both phases of transport are investigated with a focus on physical drivers and interactions between dispersive behavior and properties related to sediment, morphology and hydrology. This novel approach illustrates transport mechanisms to be either static or dynamic. Uniquely identified tracer stones are used in field and flume experiments to study dispersive characteristics of coarse sediment in gravel-bed channels. This study interrogates tracer data from previous flume experiments and three field studies, the Allt Dubhaig, Monachyle Burn and East Creek, to investigate dynamics of vertical mixing and streamwise travel distances with particle characteristics, morphologic features and flow conditions.Sorting on stone shape during vertical mixing is negligible while streamwise travel distance is significant after sufficient cumulative flooding durations. Sorting occurs dynamically on shape geometry where differences in axis ratios are prominent with significance order C/A>C/B>B/A. Sorting on stone size is static along vertical and dynamic in streamwise directions where sorting is ordered from large to small stones into the channel substrate and downstream. Speed and strength of linear dynamic response to sorting on stone shape and size is highly variable. Channels with less-developed morphology undergo complicated, dynamic vertical mixing patterns where flow has strong interaction with sediment and morphologic features. As channel morphology develops, influence on vertical mixing shifts from flow-channel interaction to morphologic features. Flood duration controls how far travel distances spread while channel energy controls how it is distributed along the channel. Stream power and vertical mixing have high short-term correlation that diminishes over time. Residence time distributions, which characterize the immobile phase of sediment transport, are easily computed from bed elevation time series, and have common distributional form. Travel distance is not correlated with burial depth. However, since residence time is related to vertical mixing and correlated with travel distance through virtual velocity, it may be a key factor linking streamwise and vertical dispersion.