The team's findings, published in the journal Physical Review Fluids, shed light on one of the most fundamental questions in geomorphology: How do river rocks get their characteristically smooth, round shape?
The answer, it turns out, lies in a combination of factors including the size and shape of the rocks, the speed and volume of the water flowing over them, and the amount of time they spend bouncing along the riverbed.
"We wanted to understand how the roundness of a rock evolves as it travels downstream," said Ian Phillips, a research scientist at the Institute for Geophysics (UTIG) at The University of Texas at Austin and lead author of the study. "We found that the roundness increases with the distance the rock travels, but at a decreasing rate."
In other words, the rocks start out relatively angular and become progressively rounder as they travel downstream, but the rate at which they round off slows down as they get smoother.
The team developed a mathematical model that describes this process and used it to simulate the evolution of rock roundness in a river. The simulations showed that the roundness of a rock depends on a number of factors, including the rock's size, shape, and density, as well as the water's velocity and discharge.
"Our model provides a new way to understand how river rocks become round," said co-author Jeffrey Banks, a professor of mathematics at The University of Texas at Dallas. "It can be used to predict the shape of rocks in a river at any given location, which can be useful for understanding the history of the river and the landscape it flows through."
The team's findings could also have implications for understanding how sediment is transported in rivers and how it affects the shape of riverbeds and the surrounding landscape.
"Our work is a first step towards understanding the complex processes that shape river rocks," Phillips said. "We hope that our model will inspire future research on this topic and help us to better understand the role of rivers in shaping the landscape."
