Data from 541 alluvial rivers—those with banks made up of sediment—from a variety of regions and environments form the basis of a new study by Trampush et al. The study seeks to tease out how water flows in modern stream channels.
In alluvial rivers, water interacts with soil or sand, sometimes with enough force to carve into the bank. However, there is no universal understanding of how the shapes of the channels relate to the forces on the sediment—a relationship useful for understanding the behavior of water in a channel based on just a snapshot in time.
Toward a universal understanding, the authors expanded on previous works that found empirical relationships between the size of the particle and the force, known as the “Shields stress,” that is required to carve a particle away from the bank in modern rivers. The authors started by doubling the number of rivers that had been used in the previous study. Then, for each river, they looked at more than 10 years of data on the depth of the banks, their slopes, and the grain sizes that make them up. Analyzing the data, they found that the relationship between particle size and Shields stress is related to the physical features of the channel.
This means that a broad view of the shape of alluvial channels can give scientists clues to grain size and the force required to carve sediment from its bank. In addition to quantifying flow characteristics of modern alluvial rivers, the relationship can quantify how water flowed in ancient channels, the authors say. For now, however, the relationship is restricted to channels on Earth. (Water Resources Research, doi:10.1002/2014WR015597, 2014)
—Shannon Palus, Freelance Writer
Citation: Palus, S. (2015), Shape of stream channels gives clues to sediment size, Eos, 96, doi:10.1029/2015EO025963. Published on 12 March 2015.