Bed Morphology and Sediment Dispersion: A Particle Tracing Study in the Field and Flume

Abstract:

Studying the movement of individual grains on a streambed offers unique insights into the local mechanisms of sediment transport, which can lead to better estimations of bulk transport rates, as well as improve prediction of morphologic responses to changing flow and sediment supply conditions. Although the influence of channel morphology on bed particle dispersion has been widely recognized in the literature for decades, there continues to be limited understanding of the mechanics of this process. In order to closely examine the influence of morphology on particle travel dispersion, long-term displacement data was collected from 1,400 tracer particles since 2004 in East Creek – a small channel with both riffle-pool and rapid morphologies. A set of flume experiments was devised to complement the field data in a controlled environment. Herein, particles were traced through a scale model of the field site using two tracing techniques in both plane bed, as well as riffle-pool bed morphologies and under varying flow conditions. Field data shows that channel morphology influences both particle travel distance and vertical mixing. Furthermore, detailed mapping of the channel bed displays a localized spatial pattern of particle mobility that reflects changes in both channel morphology and particle composition. The flume experiments allowed for the exploration of the conditions and timing under which a shift in the travel distance distribution occurs from a stochastic distribution (a Gamma distribution), to a distribution controlled by the local bed morphology.