Shipping firms want deeper dredging for a new generation of mega- ships. Environmentalists want to see wildlife in coastal and estuary habitats left to flourish undisturbed. Adrian Greeman reports from the UK's largest container port, Felixstowe, where a balance has been struck.
The bar-tailed godwit is one of Europe's more interesting migratory wader birds. Along with several other species it is also an interesting civil engineering problem.
Other adjectives might spring to the minds of port engineers at Harwich and Felixstowe on the east coast. Making sure they do not disturb these birds during a £31M dredging operation has been one of the environmental issues taken on in 18 months of delicate negotiation.
Both ports lie at the Orwell river mouth, though Felixstowe on the north side is by far the more significant. It is Britain's biggest container port and has grown rapidly during the 1990s. Felixstowe has one of the longest quays in Europe, including a new 630m length with 14m deep water alongside.
Deep sea is close by, however, it is not quite close enough for the newest generation of 'ultra post-Panamax' container ships, which draw more than 14m. These giants pass, just, at high tide, along a 9km long sea channel which was dredged to 13m lowest water in 1994. They only make it partially loaded.
Felixstowe, privately owned and run by Hong Kong port operator Hutchinson Port Holdings since 1992, wants to expand, and recently signed a deal with international shipping giant Maersk. According to civil engineer George Steele, the port's head of engineering, the shipper was promised better access. That meant asking the Harwich Haven Authority, which controls navigation, for dredging.
The channel must be deepened to 14.5m and extended to 18km. One of Dutch- owned Westminster Dredging's biggest vessels is now working in the outer 6km. But a lot had to happen to get it there.
Shallow mud has formed in flats all the way up the Orwell and Stour rivers. Both are critical natural areas and protected zones, designated Sites of Special Scientific Interest, Special Protection Areas - 'the lot', as assistant harbour engineer John Brien puts it in his office across the water in Harwich. The sea coast to north and south, though not assigned SSSI and SPA status, is jealously monitored by organisations like the Royal Society for the Protection of Birds and English Nature. Essex and Suffolk county councils also have concerns.
After dredging, mud flats could be changed all along the river by the deeper tidal regime, it was concluded, following compulsory environmental impact studies. Models by Hydraulic Research, working with Posford Duvivier, suggested a 15mm average drop of the harbour water level and changes to silt deposition.
'The worst case would be loss of 4ha of mud flats immediately and progressive erosion of 2.5ha annually over five years,' says Steele, although he is not convinced.
A deal has been struck for mitigation which has been 'a kind of test case', explains HHA's harbour engineer Richard Allen. 'It is one I do not mind publicising. We have been guinea pigs and have set a precedent for other harbours.'
HHA won permission to go ahead with dredging after arguing its commercial benefits would be overridingly in the public interest. It will compensate for lost natural environment by providing an equivalent land area to the lost flats. And since it does not own any inter-tidal mud flats of scientific interest, it is going to create some.
'There will be a managed retreat from some reclaimed land,' says Allen. 'A bund will be built around a 16.5ha area and then the sea wall will be breached.'
The area chosen is riverside, upstream from Felixstowe, on farmland leased from Trinity College, Cambridge, for 67 years and next to a nature reserve previously created by the container port.
Agreeing environmental compensation cost HHA and Felixstowe months. Negotiating prices took time too, says Steele ruefully.
There are other measures. Harwich will deliberately spill fine material during routine dredging to wash upstream. It could cost them a premium since it will reduce dredging efficiency.
Meanwhile, Britain's biggest capital dredge proceeds. It may be, as Brien describes it, simply a giant muckshift of some 17M.m3 of material, but it is taking place underwater on mixed ground.
'We lie on the boundary of the ice sheets from the last Ice Age,' says Brien, 'And have varied stiff clays, sand, silts, gravels and in places, large boulders. Deeper down there is London Clay.'
Investigation is difficult since 'you can't walk it and kick it'. But previous works had left a fair amount of information and that was supplemented by a 150 trial pit survey, using a backhoe mounted on a spudded pontoon.
Westminster also repeatedly surveys the channel using a new 20m 'wide swathe' echo sounder which processes 'gigabytes of data' and can give a detailed picture of the channel bed. Hard spots and high spots can be identified as work proceeds. Where the rippers on the suction head of the trailer suction hopper boat cannot penetrate, Brien says they will bring in a 'big backhoe on a barge' to carve out the hard spots.
The same survey aids deposition at the Roughs Tower, a sand bank licensed for disposal. 'We don't have a lot of room. There is 22M.m3 of space to the -9m mark and we have 15M.m3 to drop. Every shipload is precisely positioned,' he says. Westminster's boats use the latest real time comparative GPS during dredging and dumping.
As part of the agreement some 2M.m3 of sand or gravel, uncontaminated with silt or clay, must go to shore deposition. Fifteen different coastal improvement and erosion control schemes are benefiting as Volvo A30 articulated trucks and Caterpillar D9s spread material pumped to shore. 'English Nature helps to fund some of this work,' adds Brien.
Work on the main £28M contract began finally in November last year and is expected to take 14 months. Total cost is between £31M and £32M, the difference going on consultancy and mitigation. HHA pays back a 15-year loan from the Prudential using the £10,000 conservancy fee paid by each incoming vessel. Felixstowe underwrites the loan.