MIT researchers develop better method to gauge sediment flow

Most people have heard the predictions that cities on the American coasts are in danger of being swallowed by the sea. A new finding by researchers at the Massachusetts Institute of Technology (MIT) could be key to helping engineers manage river restoration and coastal erosion.

The team has developed a formula that offers a more accurate method of calculating sediment flow in rivers. The findings were published in the journal Nature, MIT said. 

Understanding the rate at which sediment moves is crucial for engineers in planning river restorations and dam removals, as it helps predict the downstream impact of such projects. The existing models used for estimating sediment flow often have significant inaccuracies. The MIT team's formula focuses on the shape of sediment grains as a key factor, the university said.

The shape of the sedimentary grains has been found to have a significant influence on how they move in flowing water. One of the takeaways shows that while a smooth, round stone might seem like it would skip across a riverbed faster than an angular pebble, the angular one experiences greater pressure from the flowing water, potentially offsetting the advantage of the round stone.

The MIT researchers found a solution by analyzing grain shape based on two properties: friction and drag. The drag represents the resistance of the grain to fluid flow, while friction measures its resistance to sliding past other grains. By incorporating these properties into a standard model for bed load transport, the team achieved predictions that aligned with experiments conducted in the lab, the article said.

The researchers – Eric Deal, Santiago Benavides, Qiong Zhang, Ken Kamrin, and Taylor Perron from MIT, along with Jeremy Venditti and Ryan Bradley from Simon Fraser University in Canada – emphasized the significance of understanding bed load transport, which involves the movement of sediment by a fluid, such as air or water, across a sediment bed. 

This process plays a crucial role in maintaining and restoring various terrains affected by dams and rising sea levels. It also drives the migration of rocks in rivers and the skipping of sand grains in deserts. Accurately estimating bed load transport is essential in projecting the effects of urban flooding and coastal erosion. Since the 1930s, a formula based on the Shields parameter has been widely used for calculating bed load transport. However, this formula did not account for grain shape. The existing model has been known to be inaccurate by a factor of 10 in its predictions of sediment flow. The inclusion of grain shape in the new formula improves accuracy, MIT said in its review of the findings.

Researchers previously avoided accounting for grain shape due to the complexity of this characteristic. However, the researchers behind the latest project managed to mathematically represent grain shape by considering the ratio of drag to friction.

To validate their formula, MIT explained, the researchers conducted flume experiments using different types of sediment with varying grain shapes, such as round glass beads, smooth glass chips, rectangular prisms, and natural gravel. They measured the amount of sediment transported through an inclined tank within a specific time frame and determined the effect of grain shape by assessing drag and friction. With the new formula, scientists were able to successfully predict the measured bed load transport for each sediment type. 

The improved model offers scientists and engineers a valuable tool for assessing how riverbeds will respond to various scenarios, including severe weather events or dam removals. The ability to make precise predictions is crucial for planning and mitigating potential risks associated with these activities. The researchers believe that their new formula will significantly enhance the accuracy of sediment flow estimations, leading to better-informed decision-making in river management and restoration projects, MIT said.