Two constants dominate the creation of the just completed Cardiff barrage in Wales - tide and silt. It is the reason why the dam was built and it is the reason why construction of the barrage was sometimes complex and difficult. Silt has infiltrated works, cut divers' visibility to zero.
Silt has even dogged recent commissioning trials, blocking water level sensors and forcing re-evaluation of controls for the giant sluices that now regulate water flows in and out of the impounded bay lake. Now more of an irritation than a problem, silt has delayed the change to fresh water in the new lake until next year.
Millions of tonnes of silt particles are carried in the nearly world- record level Atlantic tides which surge over a 13.8m range in the Severn estuary, past the Welsh coast. The particles flocculate and gather together before dropping endlessly on to the shore, forming thick shifting mudbanks and flats in rivers, estuaries and bays, including Cardiff's.
Mud, appearing as an unsightly rolling 'wasteland' in the Cardiff bay for the 14 hours a day of low tide, is what prompted calls for a barrage in the first place in the late 1980s. By sealing the bay between the Penarth headland and the old docks area (see box) it was hoped to create a pleasant watery parkland area, good for the city in general and a means to stimulate regeneration of the old dock.
'As you can see, even on a sunny day this is not exactly a blue Mediterranean water region,' comments one of the project's engineers looking out on the murky sea. But inland, where once the mud glistened, new still water in Cardiff Bay is clear and with a fair wind even looks blue.
The recently impounded 200ha enclosed area behind the 1,100m length of the barrage, is already producing a transformation in the dock. New buildings are reflected in the clearer water and at weekends the sails of smaller yachts and dinghies from the local marina have begun to dot the bay.
This is exactly the picturesque scene that Cardiff Bay Development Corporation (CBDC) had in mind when it put forwards the scheme in 1989, says engineering manager David Crompton. 'All the economic tests showed that we could stimulate more interest, inward investment and job creation with a barrage.'
'We wanted water in the bay. People like to develop next to water,' says Crompton. Since the bay encompasses two river estuaries it will be possible to make this fresh water, once the barrage controls are fine tuned.
Even allowing for an overall £200M investment, the barrage is justified in terms of development. That figure covers the likely £118M outcome price for the construction itself as well as environmental mitigation works, relocation of sewage outlets in the rivers and bay, groundwater protection and initial legislative work, and design and project management fees. The increase on the orginal £94M cost is due to inflation and contingency increases for unforeseen problems, says Crompton. Project manager Bechtel adds that the sum is within the original outturn estimate.
The CBDC's job was similar to that of the London Dockland Development Corporation, to regenerate an old and now disused docklands area. In Cardiff's case this is a 100ha sprawl of land around the bay from south Cardiff through to the Queen Alexandra docks, with over 12km of waterfront.
The port had been one of the Britain's most prosperous in the steamship era, exporting the hard black Welsh valley anthracite all over the world. It was admittedly a little rough.
The bay was created when the docks were built, some 100 years ago, on reclaimed land. It was then regularly dredged. Silt inundation is relatively recent as trade throughput dropped from 13Mt annually to just 2Mt. Only the outer Wrack channel gets regular maintenance dredging to accept 38 000t vessels.
Heavy industry, steel, engineering and mining have almost gone too. 'We got the remit to create 30,000 jobs, 5M sq ft of offices and industrial space plus 6,000 new homes,' says Crompton, spending some £450M over 13 years. 'We wanted to attract £2,000M of investment.'
He says results so far are good, if not as fast as he would have liked. 'We have £1,000M worth of investment at present, including a £400M Nippon glassworks, a major retail park, large office developments and the St David's & Spa waterfront hotel, Cardiff's first five-star facility. We have built or have under construction 3,500 houses, triggering a house price boom, just as we thought.'
'But we always knew this was a 15 year perspective and there is further to go. But I can see where the other £1,000M is coming from.'
CBDC decided early on that were three key items of infrastructure; a motorway link to the M4 and London, a city centre to harbour avenue and the barrage.
The roads are there but the step from idea to reality for the barrage was not so easy. Even though a team of maritime engineers, dam specialists and others from consultant Gibb began the complex studies for the work in 1987, it would be years before the construction got under way. To build the barrage it meant a Parliamentary Act was needed - and not everybody was in favour.
In particular there were major environmental objections because the mud flats are feeding grounds for various birds including redshank and dunlin. Bird protection and environmental groups were opposed to the scheme, saying that it would disrupt feeding. They were also concerned that this was the first time development was being allowed on a designated Site of Special Scientific Interest.
CBDC argued that the site was a small proportion of the SSSI which stretches along the Severn estuary and that alternative feeding grounds for the birds would be available, something which post-impoundment studies seem to bear out. Tagged birds are appearing in nearby estuaries.
Provision of a £9.3M wetlands area created from old fly ash tipping lagoons along the coast has helped mollify some, though not all, the opposition. Agreement too to include a £7M fish pass structure in the barrage has countered other concerns, especially about the trout and salmon which used the Taff and Ely rivers flowing into the bay. Though during the industrial epoch these streams did not sustain many fish, recently the rivers have become much cleaner.
'A second line of opposition came from Cardiff property owners who were concerned about the potential for groundwater rise in the city,' says Crompton. Once the lake is impounded at 4.5m above mean sea level, groundwater could rise.
A major survey has been undertaken of around 25,000 properties to assess current condition and this will be repeated in two years to inspect possible changes and assess any due compensation.
Dampness rather than structural damage is the more likely problem if any occur.
'That protection goes on for 20 years,' says Crompton. 'And we have installed dewatering systems in a number of possibly critical areas,' he adds.
Royal Assent for the scheme was given in 1993. But that was only after a first attempt with a Private Members Bill had foundered on the main vote.
'It was talked out in 1991 and I have to say the vote at that time was one of the great disappointments I have felt.' The second bill was backed by the then Conservative Government as a hybrid.
There were positive arguments for the barrage too, he says. Apart from the dam's development purpose, it has an important environmental function. Cardiff suffers high tide flooding and the barrage will remove the danger of overtopping even on the highest tides, which can be up to 7.9m. 'And it will not increase the danger of river flash flooding; in fact it slightly diminishes it,' he adds.
The new basin will accommodate up to six hours of the highest anticipated flows in the two rivers during the high tide period, for release through the sluices as the tide drops again. Low tide can fall to -6.3m.
Ensuring that the control systems can accommodate this water volume and release it correctly is currently the sub-ject of commissioning tests.Cardiff City Council is due to take over that operating function on 31 March this year, though discussion on facilities management in the short term were still under way as this supplement was prepared.
Meanwhile David Crompton, who retires at the same time, with an OBE to his account for the work he has done, is pleased to see the general mood now largely accepting of the barrage, though there are 'still some opposition voices'.
'But I think that is changing as people see the end result,' he says.