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Sandball genesis from raindrops
Bertil Trottet1,2, Daisuke Noto1, Douglas J Jerolmack1,3
1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104.
Summary
Raindrops create "sandballs" that magnify soil erosion by rolling downhill. This post-impact process, unlike splash erosion, involves dense suspensions forming stable shapes and jamming.
Area of Science:
- Earth and Environmental Sciences
- Fluid Mechanics
- Granular Mechanics
Background:
- Soil erosion by raindrops is a critical component of the water cycle.
- Raindrop erosion is complex due to its three-phase nature at the fluid-granular mechanics boundary.
- Previous research focused on particle entrainment during raindrop impact (splash erosion).
Purpose of the Study:
- To investigate the post-impact phase of raindrop erosion.
- To uncover efficient mechanisms of soil entrainment beyond initial impact.
- To characterize the formation and morphology of raindrop-induced soil aggregates.
Main Methods:
- Laboratory experiments with controlled raindrop impacts on dry, sloping granular beds.
- Field observations of natural rainfall events.
- Analysis of sandball formation, growth, and morphology under varying drop conditions.
Main Results:
- Raindrops spontaneously form dense, rolling "sandballs" on granular surfaces.
- These sandballs efficiently entrain sediment, significantly magnifying soil erosion.
- Two stable sandball morphologies were identified: peanut-like (hydrodynamic instabilities) and toroidal (mechanical locking).
Conclusions:
- The post-impact rolling of sandballs is a surprisingly efficient and dominant soil erosion mechanism.
- Sandball formation and morphology are influenced by hydrodynamic instabilities and sediment loading.
- Findings have implications for understanding erosion, sediment transport, and related phenomena in diverse fields like bioengineering and snow physics.
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