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Effects on the seabed by highspeed ferries
The aim of the project was to identify short-term effects on gravel seabeds very close to a high-speed ferry route.

The investigation focused on identifying immediate effects by passing high-speed ferries. This could either be ripped off algae vegetation from the substrate or reduced stability of the small stones. Reduced stability is expected to cause effects on the algae and fauna community over a longer time span than covered by this investigation.

The investigation was carried out north of the island Læsø at a water depth of approximately 10 m on 22/23 August 2001 under calm weather conditions (3 - 5 m/s from easterly and southerly directions).

The ferry company Stena Line AB participated in the project with the high-speed ferry "Stena Carisma" (88 m overall length, gross tonnage 480 t and service speed 38 knots).

Small stones with attached macrophytes were collected from a nearby stone reef at the same water depth, sorted in 3 sizes with an average diameter of 4 - 10 cm, 10 - 15 cm and 15 - 20 cm. Nine artificially mixed sediment seabeds were constructed, three for each stone size. The stones were gently pressed down in the sandy sediment imitating the mixed sediment from the nearby reef. A flat ion frame surrounded each stone bed.

Detailed current and wave measurements were made with an Inter-Ocean S4 Current Meter next to the artificial gravel beds at 1.28 cm above the seabed. One of the beds was video recorded during two passages.

A surface buoy marked the experimental area at the seabed and two buoys - one north and one south - marked the outer range of the two possible shipping lanes (Figure 1) Stena Carisma passed between the two buoys four out of five times with a speed of approx. 37 knots. The fourth passage was made with a near-critical speed of 18.9 knots.

The measurements show that the largest ship-generated waves and currents occurred with a near-critical speed. With this speed the wave height was approx. 1.5 m and the current speed was approx. 0.6 m/s (1.28 m above the seabed). These figures were measured approx. 41 m from the ship track. For normal operational conditions (i.e. at supercritical speed) the maximum wave height was 0.6 - 0.7 m and the associated current speed 0.35 - 0.5 m/s. It was also shown that linear wave theory yields an excellent estimate of the transient current speed above the seabed. This indicates that the wave impact on the seabed is caused by the overall wave system generated by the hull of the vessel and by the propulsion system.

Waves induced by the high-speed ferry were able to move gravel up to 625 g out of the frame. Sandy sediments were found on algae leaves after the first passage. No short-term effects as torn off algae individuals or leaves were registered.

The experiment showed that passing high-speed ferries might effect gravel communities in sheltered areas by destabilising the substrate. However, on this particular location natural induced waves and currents of the same size or larger occur so frequently and for longer periods than the 10 daily ship-generated waves, that effects on the benthic algal community are expected to be undetectable.

The project was financed by the Danish National Forest and Nature Agency and carried out by the National Environmental Research Institute and DHI Water and Environment.

Full report in pdf. format (671 KB)
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Helle Thomsen

01.11.2007


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