Extreme rainfall & erosion

These images of the same section of the Mameyes river (with the identical tree marked in each for reference) show how dramatically river flow can change. Yet the study shows that these events have a much smaller contribution to erosion than might be expected.

IT seems intuitive that the force of an intense rainfall would lead to massive erosion of a riverbed. But that intuition is incorrect according to research conducted by Douglas J. Jerolmack, a professor at University of Pennsylvania’s Department of Earth and Environmental Science, and his former graduate student Colin Phillips. According to the work, published in a recent issue of Science, although extreme precipitation events can greatly increase the amount of water travelling through a river, large storms only move about 50 per cent more sediment than a typical rainfall. With climate change expected to bring more intense precipitation in many regions of the globe, the findings indicate “that the forces of the biggest flood events were really only incrementally larger than the moderate events because river channels adjust their size to be close to the so-called ‘threshold of motion’, or the force required to move particles on the riverbed”, said Jerolmack.

The work began with an investigation of the Mameyes river in north-eastern Puerto Rico, where flash floods are common. The researchers placed radio-frequency identification tags inside 350 grapefruit-sized cobbles on the riverbed and then checked on their location after numerous rains during the course of two years. They also monitored the force of the river water using a U.S. Geological Survey (USGS) stream gauge near their study site. They found that rainfall events strong enough to move the cobbles occurred about 20 times a year. But, although the heaviest rainfalls could increase the river discharge 100-fold, these storms moved the tracers only moderately farther than a more typical rain.

Surprised by their findings on the Mameyes, Phillips and Jerolmack gathered and analysed USGS data from 186 rivers for which they had information on the width, depth and sediment type of the river and records of the river discharge through time. While some rivers experience flows at the threshold of motion many times a year and others only once every few years, the scientists found a repeating pattern for flows above the threshold. “Not only did we find that they were very rare but that the distribution of forces that exceed the critical force to move particles was the same for all of the rivers that we looked at,” Jerolmack said. The reason for this commonality, according to the researchers, is that the width and depth of river channels adjust to keep the forces of water flow near the threshold of motion.

“The argument is that if the sediment transport rate during a flood was too high the river bank would erode and the channel would widen. There is this regulation mechanism that ensures the river channel is just the size it needs to be,” he said.