A suction dredger is being used to remove nutrient-enriched sediment from part of the water bed of the Norfolk Broads because surface algae is thriving on the phosphates and nitrates it contains. This, in turn, is affecting the balance of the Broads' delicate ecosystem and the passage of tourist boats.
Algae deny oxygen to aquatic plant life as they grow and silt up at the bottom when they die, impeding navigational routes.
The Water Group is using a 9.2m long auger suction dredger to remove a 0.3m layer of sediment from Barton Broad and pump it along a 1km discharge pipe to specially constructed inland lagoons to dry out. About 300,000m3 of this eutrophied (over-enriched) material, caused by sewage effluent and leaching from agricultural farmland, will be removed over a period of five years as part of the Broads Authority's Clear Water 2000 project.
A horizontal auger cutter, 230mm in diameter, is connected to an 8m long polypropylene suction pipe trailing from the vessel. The auger consists of two helix screws, with differing threads, feeding material to the centre of the cutter before being pumped up the suction pipe and along the discharge pipe. This method of dredging allows about 9,000m3 of sludge to be removed each week.
The Water Group's John Bingham emphasises the degree of care needed. 'Dead algae form most of the sediment which is very fine, so it has to be sucked up with the minimum amount of disturbance to the water. If not, the eutrophied material would spread further into the Broad,' he says.
A hydraulically adjustable, 360mm diameter mud shield totally covers the working equipment on all but the cutting face. This avoids aggravating the sediment.
The suction dredger, secured to a cable held in place between two mobile anchors, works across the Broad in 200m stages. An operator winches the dredger backwards and forwards along the wire to manoeuvre the auger cutter into place.
John Ash, manager for the Clear Water 2000 project, says obstacles beneath the surface can unwittingly be retrieved due to the amount of material being dredged. 'The suction pipe sometimes becomes blocked with debris from the Broad's floor, such as metal objects and wood - even wreckage from a crashed aircraft was found. A couple of hours are usually lost a week due to the subsequent unblocking of our equipment,' he says.
Before the dredging process began, a preliminary analysis of the Broad was undertaken by the National Rivers Authority (whose role has been embraced by the Environment Agency) for any evidence of contamination in the water or the eutrophied sediment.
It was decided after consultation with engineering consultant Anthony D Bates Partnership that inland lagoons would be suitable to hold the dredged material, as there was no contamination risk to humans or animals.
Former arable land nearby is consequently being refortified with material- rich nutrients up to a depth of 0.75m. It will be covered with a layer of top soil to provide a high-yield crop growth area.
Samples of soil are regularly taken from the lagoons by the Agricultural Development Advisory Service, and the clean water running back into the Broads is monitored by the Environment Agency. 'At the very minimum, the material exposed must do no harm to the land or any living creature. There is a presumption that it will actually do some good,' said senior partner Anthony Bates.
The removal of the material from the Broad not only removes chemicals for the short term, but encourages other plants and aquatic animals to develop. Wildlife had totally disappeared from some areas of the Broad as oxygen was denied to water plants, restricting suitable environmental habitats. Bittern and otters are now starting to make a comeback.
This article has been produced for NCE by Barrett, Byrd Associates.