Far-field vessel wakes in Tallinn Bay

Abstract

The properties of wave fields induced by high-speed ferries and recently introduced conventional ferries with increased cruise speeds are analysed for a site in Tallinn Bay, the Gulf of Finland, the Baltic Sea, located about 3 km from the sailing line and up to 8 km from the wave production area. The analysis is based on high-resolution profiling of the water surface for about 650 wakes from fast ferries, measured during 4 weeks in June-July 2008. The new large conventional ferries with cruise speeds of 25-30 knots (~45-55 km/h) sail at near-critical speeds along extensive sections of eastern Tallinn Bay, and excite wakes equivalent to those of high-speed ferries. The peak periods of these wakes are between 10 and 13 s. The typical daily highest ship wave is approximately 1.2 m, measured prior to wake breaking. The largest recorded ship wave in calm conditions had a height of 1.5 m and in the presence of some wind wave background 1.7 m. The cumulative impact of ship wakes results in a gradual increase in the suspended matter concentration in near-bottom water over the course of a day. The largest and longest ship waves produce considerable wave runup at the coast and prevent several coastal sections from achieving an equilibrium state. The largest ship waves have an asymmetric shape both in terms of the water surface elevation above and below the mean level and in terms of the shape of the wave front and back. The overall intensity of anthropogenic waves has remained at the same level as it was in the year 2002, although the ships that produced the highest waves in the past are no longer in service

Variability in spatial patterns of long nonlinear waves from fast ferries in Tallinn Bay

“…The seabed in deeper waters (down to about 15 m) comprises an almost continuous sheet of mixed finer sediments (sand and gravel) with some quite large boulders (∼1 m above seabed) and clusters of boulders (Kask et al, 2003;Parnell et al, 2008). Below 15 m depth the seabed is generally smooth and covered with finer sediments.…”

“…The measurement program was carried out in June-July 2008 on the SW coast of Aegna, immediately west of a jetty with low wave-reflecting properties Parnell et al (2008). An ultrasonic echosounder (LOG aLevel ® ) mounted on a tripod approximately 100 m from the coast, was used to record wave properties offshore, outside the main shoaling and surf zone.…”

“…This is particularly true for a large part of the coast of Tallinn Bay with a belt of boulders near the waterline, down to the depths of about 2 m (Kask et al, 2003;Parnell et al, 2008). On the other hand, considerable parts of the coast have gently sloping beaches, where high runup from ship waves are hazards both in terms of potential intensification of coastal processes and with respect to the safety of people and property (Parnell and Kofoed-Hansen, 2001).…”

“…Parnell et al (2008) analyzed wakes recorded at a distance of 2.5-3 km from the ship track. As the crests of the highest ship wakes make quite a large angle with respect to the sailing line, the waves recorded at Aegna had travelled a considerably longer distance from their point of generation.…”

Nonlinear long-wave deformation and runup in a basin of varying depth

“…Unusual and short-lived freak or rogue waves occur on the coast and result in the loss of lives every year (Kharif and Pelinovsky, 2003;Didenkulova et al, 2006a;Kharif et al, 2009). High-speed ferries operating in the coastal zone induce large waves, which can lead to a beach erosion and seabed change (Soomere, 2007;Parnell et al, 2008).…”