Linkages Between Channel Bed Topography Adjustment and Particle Displacements

Abstract:

In principle, changes in bed topography are driven by the displacements of individual sediment grains through their entrainment and deposition. However, the linkages between these topographic changes and displacements of individual particles has not been comprehensively addressed. In this study, we explore these links using paired bed load tracers and repeat topographic surveys across three gravel-bed environments: a small mountain stream, a large gravel-bed river, and a laboratory flume. In each environment, we compare the length scale of observed degradation-aggradation pairs with the observed displacement distances of particles. To make this comparison, we apply a 1-D Bayesian survival process model to the observed starting and ending locations of tracers. This model estimates the likelihood that a given segment of channel will "trap" an entrained particle and allows for the generation of starting-location-specific probability distributions of particle travel distance. We then apply the survival process model to the topographic data by using DEMs of Difference to estimate the accumulated volumes of sediment in transport along the channel. Our study shows that (1) most particles move shorter distances than degradation-aggradation pairs, highlighting the importance of compensating scour and fill in fluvial morphodynamics. (2) Aggradational locations along the channel serve as preferential depositional locations, and these locations can often be resolved using particle-tracing methods. (3) The channel environment—namely the width, flood magnitude, and overall morphodynamic activity—sets the strength of the linkage between the observed topographic changes and measured particle displacements.