While it is true that fluid dynamics typically assumes a no-slip condition at the surface, where there is no relative motion between the fluid and the solid surface, the polishing of rocks into pebbles by water flow is not solely dependent on fluid friction.
The process by which rocks are polished into pebbles by water flow is known as abrasion or erosion. It involves the mechanical action of sediment particles, including rocks and other solid materials, carried by the fluid as they impact and interact with the surface of the rocks.
When water flows over rocks, it often contains suspended sediment particles, such as sand, silt, and small rocks. As the water moves, these particles are transported and collide with the rocks present in the stream or riverbed. The impact and movement of these particles against the rocks cause abrasion or erosion.
The abrasive action of the sediment particles can be further enhanced by factors like turbulence, hydraulic forces, and the presence of other rock fragments. Over time, repeated collisions and rubbing of these particles against the rocks lead to the gradual removal of small fragments and the smoothing and rounding of their surfaces.
While the fluid dynamics assumption of no-slip condition helps describe the flow of fluids over surfaces, it does not prevent the mechanical action of sediment particles and the resultant abrasion. The presence of sediment particles in the flowing water is key to the process of rock polishing and the formation of pebbles.