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Shifting sediment A new approach to dredging could prompt a move to more sustanable port and harbour management.

A radical alternative to conventional dredging for the management and maintenance of shipping channels and harbours is making waves in the dredging industry - by smoothing waves on the seabed.

The Wing excavator uses propeller washing to agitate and locally redistribute sea and river bed sediments, removing sand- waves with remarkable accuracy.

The system is based on a fundamentally different approach to dredging. Usually dredging is carried out at widely spaced time intervals and tends to remove large quantities of sediment which are then disposed of at a considerable distance from the dredging site.

By contrast the wing does not transport the unwanted sediment out of the system. According to Bob Beaumont, managing director of its developer, East Sussex-based SILT (Seabed Impeller Levelling and Trenching): 'It challenges engineers, administrators, environmentalists and port authorities to re-examine their approach and philosophy towards dredging and move towards sustainable, long term environmentally responsible policies.'

While the effects of propellers on seabed sediments have been recognised since they were first fitted to ships in the 18th century, these effects have generally been negative. For example, constant shipping movements in shallow ports can cause considerable erosion to the seabed, particularly on the ebb tides. This in turn leads to sediment build-up on either side of the shipping channel.

Beaumont first saw the potential benefits of propeller washing in 1987 when working on a Victorian wreck which was cleared of sand and clay using a ship propeller.

He realised that if the propeller washing could be targeted and focused, there could be considerable engineering use, from port and harbour management to pipeline installation.

Key to developing this concept was the need for a stable, but manoeuvrable, platform from which the propellers could operate. This led to the Wing excavator design which is characterised by an inverted wing shape. This provides the stable platform, with the down force created giving dynamic stability against the tension from the suspension cables from the towing vessel.

Operational advantages of the Wing are that it can be operated from any vessel, such as a conventional tug boat or trawler, and does not need a dedicated dredger. It is easily transported

by road and can serve several areas easily with low mobilisation costs, and can cost effectively deal with very small dredging quantities.

Last spring SILT was invited by the Dutch Wilhelmshaven Waterway Authority (WSA) to carry out a demonstration trial in the Jade Channel in the North Sea. The aim was to assess the efficiency, stability and cost effectiveness of Wing tech- nology compared to established dred- ging methods. In particular WSA was interested in the ability to reduce the height of sand- waves to provide suf- ficient draft for incoming oil tankers.

The trial was assessed by Ove Arup. In its report Arup observed that when operated close to a seabed composed of sand, the Wing has the effect of scouring the sand and lifting it into suspension. Suspension is achieved by a combination of upward re- flection of the jet generated by the Wing's internal prop- ellers and turbulence within the jet

stream.

Under tidal current conditions, the differential flow of water over and under the Wing, which is shaped like an inverted aircraft wing, creates negative lift which serves to counteract the down- ward thrust from the impellers while in operation. The tidal flow also helps to carry away material lifted into suspension.

Depending on the strength of the tidal current and the grain size of the material - which determines the fall velocity - sediment will be redistributed at some distance down-current from the point of erosion.

The basic operational strategy proposed for the Jade demon- stration was to direct the Wing at the crests of sandwaves and allow tidal flow to carry the suspended material into the adjacent deeper water hollows.

Increases in water depth in excess of 2m were easily achieved at specific locations and over

the whole length of sandwave crests. Productivity averaged 1,700m3/h.

Arup's report concluded that despite teething problems and severe weather during the trial, 'the Wing presents a highly cost effective and technically advanced solution'.

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