Implications of fast-ferry wakes for semi-sheltered beaches: a case study at Aegna Island, Baltic Sea

Abstract

The almost-tideless Tallinn Bay, the Baltic Sea. is one of the few places in the world where high-speed ferries frequently operate close to the shoreline and where wake-waves may have a significant effect on the morphology and the sediment dynamics on medium-energy beaches, in particular, because of the difference of the wake propagation direction from that of dominant wind waves. The properties of ship waves were measured continuously during four weeks in summer 2008 offshore from a semi-sheltered beach located ~2700 m from the sailing line. Beach profiles were measured up to several times a day for more than 20 days. An adjacent jetty restricts sediment transport from the east. Overnight and during high-energy wave conditions. wind generated waves build the beach adjacent to the jetty. During calm periods the beach is not replenished and significant loss of sediment across the beach profile is evident due to ship wakes. The beach therefore, never reaches an equilibrium shape, as might normally be expected on the up-drift side of a groin. Instead, the area offshore adjacent to the jetty serves as a sink for the beach sediments.