Experimental and Numerical Studies of Sediment Transport: Particle-scale influences on channel-scale behaviors
The dynamics of sediment transport in streams have wide-ranging effects from local land-surface dynamics to the health of local ecosystems. Sediment transport in rivers, streams, and other channels has mainly been modeled under the assumption that averaged fluid driving forces dominate the rate of the transport and related details. Yet there is increasing evidence that grain-grain interactions play an important role in the dynamics, particularly in setting where there is a wide range of particle sizes. In a mixture of particle sizes, a slight variation in local size distribution of particles can significantly influence the local transport dynamics. Further, particles tend to sort by particle size, so this leads to a complex interplay between fluid forcing, local concentration and transport dynamics. In rivers and streams these particle-scale dynamics go far beyond the transport of the sediment to influence the ability of the bed to play a healthy role in the stream ecosystem, for example, when pore spaces become clogged with fine particles.
In this presentation, we focus on understanding some of these dynamics for bimodal sediment mixtures using flume experiments, computational simulations, and insight from theoretical models. We use the flume experiments to determine how the size of fine particles introduced to an active gravel bed influences the mobility and composition of the bed. We show how experimental results influence the relationship between relative particle size and: bed slope, mobilization of the bed, and infiltration of fine particles into a course bed. We use the computational results – discrete element method simulations – to help provide insight on the importance of grain-grain interactions in the mobilization and infiltration behaviors. We draw on theory from what is often called “granular physics” to understand how these grain-grain interactions may be used to improve models for sediment transport in mixtures. We discuss these results and their implications for what is needed for a more transparent framework for incorporating dominant particle-scale influences on models for river morphology, river bed infiltration behavior, and some related issues of ecological health in stream and river beds.