Permeable Pile Groins

Abstract

Permeable pile groins have been built on the southern shores of the Baltic Sea in large numbers for a century and a half, yet no explanation has been found in the literature on how these function. It is shown that pile groins act as a hydraulic roughness on the longshore current and that their effect on waves is negligible. By reducing, but not blocking, the littoral current, the velocity differential between the velocity seaward and in the pile-groin fields is smaller than with impervious groins. Hence, the circulation in the groin field due to the fluid drag at the seaward boundary is weaker and is concentrated mainly in the velocity interface in the form of small eddies. The reduced littoral current velocity in the pile-groin fields leads to a reduction of turbulence produced at the bed by the wave-current interaction, to a reduced amount of sediment suspended, and to a thinner layer of suspended sediment. Hence, the amounts of sediment advected by superimposed currents are reduced. The reduction is most effective for erosive-wave conditions and enables trapping or holding of sediment until the continuity of littoral transport is reestablished and the cross-shore transports balance over time.