Natural and anthropogenic wave forcing in the Tallinn Bay, Baltic Sea

摘要

The Tallinn Bay is a 10x20-km semi-enclosed area of the Gulf of Finland. Since tidal currents are negligible in this area, wind waves and local currents that both cause moderate near-bottom velocities off shoreline govern the beach processes. In recent years, high-speed ship traffic has increased considerably and amounts to 70 traverse per day. The ships frequently sail with speeds close to the critical velocity. Comparison of a long-term reconstruction of wind wave climate with the direct ship wave measurement shows that, in the coastal zone of the bay the mean energy of ship waves is 7-10% from the bulk wave energy and wave-induced power as high as 40% from the bulk wave power. The annual maximum of significant wind wave heights is frequently <1.5 m (periods 5-6 s) whereas fast ferries generate waves with the heights about 1 m. The highest component of ship wake has frequently periods 10-15 s and causes unusually high near-bottom velocities at the depths of 5-20 m. Thus, the wake of fast ferries is a new forcing component of vital impact on the local ecosystem that may cause considerable intensification of beach processes as well as enhanced vertical mixing in the water body and have significant influence on the aquatic wildlife.