Raindrops go ballistic in research on soil erosion

What happens when the water hits the soil is the micro-ballistic effect of displaced soil splattering around in all directions. In arid regions, such as central Arizona, this is an important process that shapes the landscape.

While it sounds elemental, it has only been recently that researchers, including one from Arizona State University, have studied these effects up close and in freeze frame.

The research team is led by Mark Schmeeckle, an assistant professor in ASU's School of Geographical Sciences and David Furbish, professor of earth and environmental studies at Vanderbilt Univ., Nashville, Tenn. They focused on the effect of raindrops hitting bare soil in a series of controlled experiments that included high-speed photography to capture the soil splattering process and its aftermath.

What they found were some violent confrontations, as water hit bare soil causing splatter effects of the soil. They also found that momentum plays a key role in slope erosion and gravity has a muted effect.

Schmeeckle and Furbish were joined by Katherine Hamner, Miriam Borosund and Simon Mudd, all of Vanderbilt, in the project. They report their findings in the current issue of the Journal of Geophysical Research (Jan. 16, 2007) in “Rain splash of dry sand revealed by high speed imaging and sticky paper splash targets.”

In the experiments, the researchers mounted a 20-foot long PVC pipe vertically and attached a syringe at the top of the pipe. The distance was great enough where raindrops, coming from the syringe, could achieve terminal velocity (the fastest speed they can fall through still air). The pipe blocks air currents from deflecting the drops.

“Without it we wouldn't be able to hit our target,” Schmeeckle said.

The drops were aimed at a sand target 2.5 cm in diameter by 2 cm deep set flush to a surrounding surface covered with sticky paper. Depending on the syringe needle size, the researchers could adjust drop size from 0.5 mm to 5 mm. A 5 mm raindrop traveling at terminal velocity would hit the sand target at a relative force of 20 mph.

When a drop hit the target, a high-speed camera operating at 500 frames per second recorded the dynamic interactions between the water and the sand. In addition, sand grains ejected by each impact stuck to the surrounding paper where they hit, allowing the researchers to precisely plot their positions.

The researchers did several experiments simulating raindrops hitting sand on flat surfaces.

“The raindrops splashed particles in all directions, resembling ballistic trajectories of particles going up and out and then down,” Schmeeckle said.

Then they angled the target to five inclinations (10, 15, 20, 25 and 30 degrees). With the target tilted, Schmeeckle said the researchers dispelled a 50-year old misconception about how rain splash transport works.

“We found that when the raindrop hits, very few particles actually move up slope and most of the particles move down slope,” he said. “It kind of bulldozes in the down slope direction and you get a large ejection of particles moving down slope.”

But gravity takes a back seat to momentum as the driver of this phenomenon.

“It's the momentum,” Schmeeckle said. “As the raindrop comes in, it already has downward momentum and that momentum gets transferred to the down slope momentum of the soil particles.”

This experimental result could have a big impact on soil erosion and add to the knowledge engineers use to devise systems to prevent such erosion on hills and mountains.

“The discovery is important for soil health,” said Schmeeckle, who primarily studies sediment movement in rivers. “In semi arid and arid regions like ours, where there is not a lot of vegetation on hills, raindrops directly and dramatically affect soil as they hit.

“A lot of material transport from hill slopes will eventually make it into the river systems,” he added. “This study will lead to a much better understanding of the processes of how soil is eroded and transported on hill slopes.”