Science of sediment transport key to river conservation and protection

Their solution, outlined in a new paper in Nature, all boils down to the shape and particular features of a sediment grain.

"Sediment transport controls the morphology of the Earth's surface -- that includes the physical environments of all ecosystems, the beds of rivers and the ocean, and even terrestrial environments," says SFU professor Jeremy Venditti, founding director of the School of Environmental Science, whose team carried out the study's research activities in SFU's River Dynamics Lab.

"Despite this, accurately predicting sediment transport remains a stubbornly difficult problem. Our work examines the granular dynamics of sediment transported by fluid flows, and shows that grain shape plays an important role in sediment transport rates. The model we developed substantially improves our ability to predict sediment transport."

Bed load sediment transport involves wind or water flowing over a bed of sediment, causing grains to "roll or hop" along the bed. The researchers say sediment is critically important to the life cycle of rivers and understanding its movement has been "notoriously imprecise."

Researchers decided to look beyond size and density and focused on two particular properties connected to a grain's shape -- its resistance to flow, or its drag, and its internal friction, which plays a part in its ability to slip past other grains.

Both factor into a new mathematical formula which provided predictions that were successfully matched during a series of flume experiments in the SFU lab.

During the experiments a current of water was pumped into a small wooden flume to flow over a bed of sediment with various grain shapes, from round glass beads and chips, rectangular prisms and natural gravel. Measurements of sediment transport, drag, and internal friction of the bed were recorded.

In their paper, the research team notes: "Sediment transport is a part of life on Earth's surface, from the impact of storms on beaches to the gravel nests in mountain streams where salmon lay their eggs. Damming and sea level rise have already impacted many such terrains and pose ongoing threats. A good understanding of bed load transport is crucial to our ability to maintain these landscapes or restore them to their natural states."

Venditti has been leading research into the 2018 Big Bar Landslide that prevented salmon from getting to their Fraser River spawning grounds, to map its effects and mitigate future risks.