Architecture: The price of turning mud into gold: Cardiff's new seaside plans are spectacular, but do they really improve on nature? asks Nicholas Schoon

THIS IS Britain's most ambitious and expensive attempt to improve on nature. The planting of the New Forest and the landscaping of the great country house gardens were modest experiments in comparison to the Cardiff Bay Barrage. What is to be improved here in South Wales is the hundreds of acres of grey-brown mud exposed in the bay for most of the day, year in, year out. The Government and its servant, the Cardiff Bay Development Corporation, believe the sight of all this silt is ugly and claim that, unless hidden, it will forever deter the developers that Cardiff needs to boost its economy.

But build a pounds 150m dam across the mouth of the bay and - hey, presto] - the mud disappears forever beneath a freshwater lake covering a square mile. White sails skim across blue water. Office blocks and 'prestige' flats spring up along eight miles of once-scruffy, largely derelict waterfront. A hundred shops, restaurants and street markets bloom. A site (but not the funds) has even been earmarked for a 2,000-seat Sydney Harbour-style opera house. All this sounds dramatic, and it is. Whatever the merits of the scheme, the architecture and design of the Cardiff barrage will make it one of the most spectacular coastal developments in Britain.

'People will discover a sort of Covent Garden by the sea,' says Michael Boyce, the corporation's chief executive. But it will not be sea any more. It will be a sanitised freshwater marina. Will this really be an improvement on the bird-pecked mudflats? Horrified nature lovers want to know what is so ugly about mudflats. The bay is but one of hundreds of muddy estuaries that indent the British Isles, providing a feeding ground for millions of wading birds, ducks and geese. Any ugliness, they argue, is in the eye of the beholder.

Environmentalists and bird lovers have fought the barrage since it was proposed six years ago, through Parliament, through the local papers, all the way to Brussels with a still unresolved complaint that the European Commission's birds directive will be breached. But they have lost. The corporation, backed by the Government, has never budged from the line that only with a freshwater lake could the full potential of Cardiff's run-down southern quarter - with its vestigial port, abandoned heavy industry and poor housing - be realised.

Next month the Bill legislating for the barrage is expected to receive its royal assent. The main construction contract should be awarded in October and by 1998 the structure should be complete. Hard luck for 5,000 dunlin and redshank that each winter fly to the bay, a government-designated Site of Special Scientific Interest.

The corporation is already celebrating. Next Tuesday in London the Princess of Wales will be the guest of honour at a gala presentation. Guests will hear the orchestra and chorus of the Welsh National Opera give a premiere performance of Gareth Wood's Cardiff Bay Overture. Anthony Hopkins, Shirley Bassey, perhaps even Tom Jones, will pay remote tribute using videotape.

The mudflats and wading birds are doomed. Now the Welsh capital has to worry about what kind of development it attracts, what sort of lake and what kind of barrage it wants. Stretching from Queen Alexandra Dock to the Victorian suburb of Penarth, it will be, first and foremost, a heavy- duty engineering structure. It will have to sit on soft sediments and cope with strengthening currents as the bay is gradually closed off. The barrage will also have to stand up to one of the highest tidal ranges in the world - nearly 50ft - keeping the fresh water inside at the mean high tide mark whatever the level outside.

The consulting engineers, Sir Alexander Gibb and Partners, have decided on an embankment with a rock core sitting on the gravel that lies some 15ft below the muddy bay bottom. To seaward will be a steeply sloping rock face armoured with limestone boulders several feet across. To landward will be sloping sand that will waterproof the structure. It will be as wide as 100 yards at its base. This embankment will occupy most of the barrage's two-thirds-of-a-mile length.

At the western, Penarth, end will be five big sluice gates to let the waters of the Taff and Ely flow to sea and a fish pass for the few salmon brave enough to venture up these grubby (but improving) rivers. There will also be three small locks for fishing vessels and pleasure craft, with harbour walls outside to shelter boats waiting to sail in. The ship-sized lock serving Cardiff docks will be to seaward of the barrage.

The corporation and the engineers realised that the barrage ought to look beautiful - or at least not ugly. So it called in the architects Alsop & Stormer, the inspired avant-garde practice whose recent designs include Tottenham Hale station in north London and the Museum of the Word in Swansea, as well as radical transport projects in Hamburg and Marseilles, to devise ways in which a hard-edged engineering structure could be transformed into something that residents might view with affection and tourists would come to admire.

The architects are no strangers to the bay. They designed the corporation's award-winning visitor centre, which has become a docklands landmark. They were taken to the Netherlands to look at some of the huge concrete barrage structures with which the Dutch have tamed the North Sea. And they came away thinking: 'It mustn't look like this.'

Little can be done to change the brute visual power of the barrage, the architects say; the trick is in making it interesting and appealing close up.

'The first thing we did was to try to get a good geometry,' says Peter Clash, project architect. Gareth Jones, the Welsh-born, New York-based sculptor, was brought in and joined with Will Alsop and Mr Clash in trying out some shapes. They settled on most of an S - three gentle curves flowing from the straight, reinforced concrete part of the barrage with its sluices and locks into the headland at Queen Alexandra Dock.

This makes the structure more interesting to walk along as curves are rounded and new stretches revealed. Walkers will be welcome; the main idea is to make the barrage into a linear park with car parks - and hopefully bus stops - at each end. Alsop & Stormer has proposed three 'experiences'; a rugged walk along a path on the seaward side looking south across the Severn estuary to distant Somerset, a crest walk along the ridgetop with views to land and sea, and a 'softer, more eventful' lakeside walk looking north towards the city.

Mr Clash says the architects want to provide some shelter from wind and rain, points of interest, even some surprises. There will be three piers for fishermen stretching out from the steeply sloping seaward side. The architects have toyed with the idea of creating artificial rock pools. The grassed and gently shelving landward side will have bumps and hollows and a snaking water margin.

Just offshore will be a small, densely planted island. Groynes will break up the view and create sheltered spots where shrubs can grow and people can picnic. The plan is to house the inevitable cafe and visitor shop inside a circular control tower from which the locks and sluices will be governed.

The architects would like a viewing deck at the top, a vantage point from which to savour the curves. 'This is a bare minimum,' Mr Clash says. 'I think more can be done.' How much farther they can go in pleasing locals and visitors and how much will actually be built remains to be seen.

Although it would not complain, it seems that Alsop & Stormer has had a slightly bruising encounter with Bechtel, the US engineering corporation that has been appointed project managers by the corporation. Bechtel carried out a 'value engineering' exercise, a sort of brainstorming session to come up with ideas for saving money. After a lot of thought these were whittled down to five proposals, saving pounds 6m. The biggest is to steepen the seaward face of the barrage and alter the curves of the embankment. 'It's not a huge S any more, but it's still fairly organic,' says Mr Clash.

There can be little doubt that Will Alsop and his team of artists and architects are doing their best to make the Cardiff Bay Barrage the most attractive and adventurous of its type yet seen. But the big question remains: what is wrong with mudflats? They have their own beauty, their own distinctive ecology. Cardiff's failure lies not in its choice of architects and engineers for the barrage project, but in not realising that it could have made a virtue of a natural resource that most of us would only be pleased to have on our doorsteps.

(Photographs omitted)