Erosion in the vicinity of structures

Erosion and transport of earth material is a leading threat for human activity and ecosystems. For instance, at the pier of a bridge, erosion can damage the entire structure leading to bridge failure. Achieving better hazard assessment requires coupling both the fluid dynamics around the structure (instabilities, wall effects) and the transport phenomena of solid materials. We investigate experimentally the local erosion of a granular bed near a fixed vertical cylinder that emerges from the bed. The onset of erosion arising at the base of the cylinder and usually ascribed to the wrapping horseshoe vortex (top image) is determined and rationalized by a flow contraction effect. We report another erosion pattern visible downstream of the cylinder that consists of two side-by-side elongated holes (bottom image). This pattern is observed for flow regimes close to the horseshoe scour onset, whose growth usually inhibits its spatiotemporal development..