CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Engineering

Volume 9 No. 3 September 2003

Special issue on wave studies in Tallinn Bay

Guest editorial; 155–156

Rein Raudsep

An experimental study of wind waves and ship wakes in Tallinn Bay; 157–184

Tarmo Soomere and Kalev Rannat

Abstract. The results of a field study of wind waves and ship wakes in the coastal area of Tallinn Bay in the years 2001–2002 are described. The main goal was to identify the typical and extreme parameters of ship wakes and to describe their energetic properties in different coastal areas. The basic features of wind wave fields in different wind conditions are estimated. The structure of the wake patterns of fast ferries, the maximum and typical values of wake heights and their spatial variability are analysed. Waves caused by fast ferries generally are higher than those excited by hydrofoils or conventional ferries. Their daily maximum height typically exceeds 1 m and may reach in extreme cases 1.7–2.3 m at a depth of 5 m at the eastern coast of the bay. The highest wave components frequently have periods of 10–15 s. Waves with such properties do not occur in natural conditions in the area in question. The spatial variation of the average energy density of the ship waves and its flux is relatively small in Tallinn Bay. The daily average frequency spectra of ship-generated waves show that different parts of the coastal zone are affected by waves with largely different properties.

Key words: wind waves, high-speed craft, wake wash, wave measurements, wave spectra, semi-enclosed seas.

Geological setting of areas endangered by waves generated by fast ferries in Tallinn Bay; 185–208

Jüri Kask, Aado Talpas, Andres Kask, and Klaus Schwarzer

Abstract. The paper considers the distribution of bottom deposits in Tallinn Bay in connection with the general geological setting. The shores subject to intense ship wash (western coast of the Viimsi Peninsula, SW coast of Aegna Island, and eastern coast of Naissaar Island) are discussed in greater detail. Distribution of bottom deposits is determined by the location of erosion areas and seafloor topography. The shores of the bay are mostly covered with coarse material washed out from glacial deposits, which reduces the influence of the waves. During the periods with low water level the waves generated by fast ferries influence deeper parts of the nearshore.

Key words: Tallinn Bay, fast ferries, shore, bottom deposits.

Possibilities of reconstruction of the wind regime over Tallinn Bay; 209–219

Sirje Keevallik

Abstract. An attempt has been made to reconstruct the wind regime over Tallinn Bay from coastal data. For this purpose, two ideal sites exist – Tallinn harbour and Naissaar Island. They describe the transformation of open sea conditions to the coastal ones. The average wind speed is 5.1 m/s at Naissaar decreasing to the value of 4.7 m/s near the harbour. The most probable wind direction is SW at the entrance of the bay and south for the inner part of it. Strong winds (11 m/s and more) blow mostly from SW at Naissaar and mostly from NW at Tallinn harbour. Such a difference is obviously caused by the topography of the bay as it is open to the NW; other directions are sheltered by islands and peninsulas. The secondary maximum of the strong wind directional distribution at NE seems to be similar for the whole bay. The SE and south winds are mostly weak. As these stations do not function any more, regressions were calculated to estimate the wind vector components on the bay from Harku and Kalbådagrund data. It has been shown that the zonal component of the sea winds in winter, spring, and autumn can be reconstructed from the Harku data. The same cannot be said about the meridional component. To restore the meridional component at Naissaar, Kalbådagrund data offer a better opportunity. The conclusion is that possibilities of reconstruction of the wind field properties based on Harku data are very limited.

Key words: wind regime, Tallinn Bay.

Fast ferries as a new key forcing factor in Tallinn Bay; 220–242

Tarmo Soomere, Jüri Elken, Jüri Kask, Sirje Keevallik, Tarmo Kõuts, Jaan Metsaveer, and Pearu Peterson

Abstract. The role of ship-generated waves in the total wave activity in Tallinn Bay, a semi-enclosed basin of the Gulf of Finland, is analysed. The heights of ship waves are moderate. Since the seabed in the vicinity of the shoreline is mostly covered with a pebble and cobble pavement which protects the shore from further erosion, the influence of the ship waves is negligible near the shoreline. Owing to the high density of fast ferry traffic, the annual mean energy of ship waves is 5–8% from the wind wave energy and the ship-induced wave energy flux (wave power) is 18–35% from the wind wave power. The highest components of ship wakes have the heights about 1 m and frequently periods of 10–15 s. Such waves do not occur in natural conditions in the area in question. They cause unusually high near-bottom velocities at the depths of 5–20 m. Thus, the fast ferry traffic 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. It may have significant influence on the aquatic biota.

Key words: fast ferries, ship wash, Gulf of Finland, wave measurements.