Why does more sand get removed at the place where we stand on the shore when the waves recede?
Erosion of sand around our feet when we stand on a beach happens due to the formation of `horseshoe vortices‘. To understand what a horseshoe vortex is, imagine the following situation: A vertical solid cylinder is fixed to a large horizontal plate to form an inverted T-shaped assembly.
A viscous fluid (maybe air or water) flows horizontally over the cylinder side of the plate. As the flow proceeds over the plate and around the solid cylinder, several horseshoe shaped vortices are formed near the `neck of the T.’
In this case the `legs’ of the horseshoe vortex are oriented in the downstream flow direction, and the core of the vortex forms a low-pressure zone. When we stand on the beach, our legs and feet play the role of the cylinder, and the sandy beach is like the plate mentioned before.
As the core of the horseshoe vortices is a low-pressure zone, sand particles are sucked into these vortex cores. The sand particles are then washed away by the retreating seawater in the downstream direction, revealing depressions in the coast around our feet once the wave recedes.
Horseshoe vortices are a civil engineer’s nightmare. Soil can get eroded due to this effect from around the foot of the pillars or piers supporting bridges, leading to structural collapse.