Flow Strength and Bedload Sediment Travel Distance in Gravel Bed Rivers

Abstract The quantification of bedload sediment transport in rivers is possible from statistics of individual particle displacements. However, there is a lack of empirical basis for a universal relation between particle displacement distance and hydraulic drivers. Previous work suggests that a simple linear relation exists between the energy of a flood and the mean travel distance of bedload particles. Such a relation would be advantageous, but a consistent model able to collapse the data from different rivers has not been developed. Here, we develop a predictive relation from a unique data set collected in three watersheds from a single region but contrasting hydrologic regimes due to urbanization and storm water management. Additional data from two rivers from outside the region are used to validate the model. We show that the mean of an exponential distribution of surface particle travel displacements can be reliably predicted from either the cumulative discharge or stream power exceeding the mobilization threshold, which is calibrated using field data. The strength of the relation decreases after large flood events that appear to cause tracer burial due to vertical mixing. This result indicates that the relation is most applicable for the entrainment phase of transport in which tracers are dispersing over the bed surface. Tracer movements become more challenging to predict over a long series of events due to burial and eventual tracer slowdown, but the relation remains valid for the particles located on the bed surface, making it suitable for analyzing the impact of climate and landscape changes over time. Plain Language Summary Coarse (bedload) sediment transport models are notoriously challenging to build because of how many different factors affect the movement of individual grains in rivers. Many different models have been proposed that relate the strength of the flow to the distance sediment grains travel in a particular river. However, a universal model that applies across all rivers has not been built. We propose that a large portion of the variation between models is due to inconsistency in how flow strength and grain movements are measured and calculated between studies. Following a consistent methodology that aims to minimize many of the complexities of bedload transport, we developed a model between the flow energy in a flood and the distance coarse grains travel that applies to three rivers with very different hydrologic settings. We use data from two additional rivers in different parts of the world to further demonstrate the success of the model. The universality of this model makes it ideal for analyzing the impact of climate change and land‐use change on sediment transport. The results provide some evidence of a universal relation between flow and sediment transport and call for a more thoughtful and consistent methodological approach between studies. Key Points Mean tracer travel lengths normalized by the channel width can be modeled as a linear function of the cumulative excess stream power A consistent tracer data set analysis methodology allows for a collapse of the model between rivers with different hydrologic regimes Transition between entrainment and diffusion bedload transport regimes due to tracer burial is confirmed using a field data set