Formation of erosive structures in a sediment layer

with Éric Clément, Florent Maloggi, Peter Lee, José Lanuza

erosion patterns — Different erosive patterns observed on the plate pulled from the bath.

Without the erosion by water, the face of our planet would be very distinct from its present look. Erosion is responsible for the shape of mountains, canyons, sea coasts, rivers, and, at a smaller scale, for the patterns which can be observed on the beach at low tide, or on hills deprived of a protective vegetative cover. Despite this ubiquity, our understanding of the physical mecanisms of erosion and granular transport is far from satisfactory.

phase diagram — "Map" of the different patterns observed as the sediment layer is pulled out at a given velocity and inclination (bottom: experimental set-up).

We built a laboratory-scale experiment in which a sediment layer, initially created under water, is pulled out of a bath of water (from the perspective of the layer, the water flows off). Depending on velocity and inclination, a multitude of patterns can be observed.

The advantage of a laboratory-scale experiment is the precise control over the parameters: water level decrease velocity, inclination, thickness of the erodable layer, size of the grains making up the sediment, … This allowed us to calculate the forces and to show that the Shield number (ratio of shear and weight) – well kown to geologists studying river bed dynamics – determines the threshold at which patterns appear. Furthermore, we were able to show that a multitude of patterns could be observed just by varying two control parameters, and map the corresponding regions in the velocity-inclination space.

For more details, please refer to the publication on that subject.

Last modified: 18 Jul 2020