Erosion patterns on a granular bed around a vertical cylinder

Abstract

We report on two different patterns that can be observed at the bed surface close to a vertical cylinder when submitted to a strong enough steady water flow. The classical scour pattern observed at the cylinder foot and due to the "horseshoe" vortex around occurs at a critical velocity Uc1 below the critical velocity Uc0 for erosion without any cylinder, thus under clear-water conditions. But we observe also another pattern downstream the cylinder which consists of two symmetrical ovoid holes that look like "bunny ears". This new scour pattern referred as BES can be observed at lower velocities that the horseshoe scour (HSS), above a critical velocity Uc2 < Uc1, with a timescale formation much higher that the one of HSS.