Cardiff Canoe Slalom
Publication: Contract Journal, 7 October 2009
Journalist: Paul Thompson
In south Wales Dean & Dyball Civil Engineering is steering a course to deliver the UK’s first olympic-standard pumped water canoe slalom flume. Paul Thompson reports.
For those who haven’t visited Cardiff for a decade or so, take some friendly advice and do. From the tired old port town it had become has emerged a capital city that bursts with vibrancy. Even in the depths of one of the worst economic downturns of recent history, Cardiff seems to have a positivity about it that is almost infectious.
Much of this renaissance can be attributed to the positive effect the redevelopment of the derelict docklands around Cardiff and Penarth has had on the rest of the city. The Cardiff Bay project has seen huge amounts of investment flow into the area since Cardiff’s tidal barrage was completed at the turn of the millennium.
This dam has helped turn uninspiring tidal mudflats into a 500-acre freshwater lake and eight miles of waterfront. It is in the western corner of the lake that the latest bid to attract investment into the region is being built.
Dean & Dyball Civil Engineering is masterminding the construction of a 250m olympic-standard canoe slalom course and white water rafting centre plus offices, changing facilities and a café as part of the International Sports Village – Cardiff’s bid to become a worldwide sporting centre of excellence and destination resort.
Project manager, Neil Chambers, is the man charged with negotiating the twists and turns of a turbulent project and delivering the course – the first of its kind in the UK.
“There is no other pumped flow facility in the UK, and although it obviously has no link with the 2012 games in London, it will offer an important training facility in the run-up to them,” he says.
World-class course
Wedged in a tight site alongside the River Ely just before it empties into the bay, the white water canoe course boomerangs around a 180-degree bend halfway along its 254m length. It will surge world-class athletes from the top to the bottom, with a few obstacles thrown in along the way, in just two-and-a-half minutes.
The canoeists will emerge at the bottom of the main channel in the retained water pool (RWP) ready to paddle onto a conveyor belt which will lift them and their boat back up to the top of the flume ready to go again. And although this pool affords worn out white water rafters the opportunity of getting their breath back in more tranquil waters, it is actually the main 14,000 cubic metre water storage tank for the scheme.
It is from here that four 6m high ITT Flygt pumps, manufactured in Sweden and weighing around five tonnes each, will suck up the water, forcing it through the pumping station and into the main channel at a rate of 16 cubic metres per second.
But driving the steel tube and sheet pile combi wall that separates the River Ely and the busy Penarth Marina from the RWP proved as difficult an obstacle to overcome as an upstream slalom gate in a kayak.
The series of 22m long tubes ranging from 1,067mm to 1,619mm in diameter are driven some 3m into the rockhead. The sizes and arrangement differ according to the fluctuating bed levels and pressure they are resisting. Between each of the rubes two or three standard sheet piles are driven to the rockhead, depending on the tube placement. Eventually the clutches of the steel sheets will be welded tight.
It is a delicate operation, with a tolerance of just 20mm when driving the tubes linearly. Waver from this and infill sections would have to be cut from steel sheets to patch up the final section. However, this wasn’t the worst of Chambers’ headaches.
“The Environment Agency only allowed us a working window from the beginning of December to the end of March to install the RWP wall because of the migratory patterns of fish. Our major concern was being able to complete everything by that date,” he says.
Change of plan
A jack up floating pontoon and a 180-tonne crawler crane were used to handle the steel tubes. The initial plan was to install them sequentially from the southern tie in to the northern. However, one of the first tubes to go in refused alongside part of an old timber dockside structure. Despite the best efforts of a team of divers and cranes, it could not be budged even after a week of pulling and pushing.
Fortunately, because the refused tube was close to the existing dockside, the team was able to take the decision to continue with the installation of the combi wall and leave a gap, before coming back and digging it out by placing a rock ramp and using a long-reach excavator. Even then the RWP wall completion was a tight run thing.
“I think we finally removed the obstruction on 28th March, got the last tube in on 29th and finally plugged the gap on the last day of the working window,” says Chambers.
Fish rescue
A rescue of all the fish that had become trapped during the construction of the RWP saw a surprising 3,500 returned to the river, including one large salmon – a tribute to the increasing cleanliness of the waters that empty into Cardiff Bay.
Desilting resulted in 5,000 cubic metres of material being removed from the RWP, dried on site and sent off for recycling, with particular care taken during excavation around the large, timber coal wharf – the last remnant of the dockside’s former glory – which enjoys ancient monument status. This wharf sits in the centre of the RWP and will, once renovated, become part of the viewing gallery for spectators viewing the boiling water of the rapids.
For now though, the focus is on placing stone fill within the RWP to bring it up to level and getting the huge pumps installed in the pumping station before the whole pool can be flooded and tested for leakage.
It will be daunting when completed, says Chambers, not least because he will doubtless be one of the first guinea pigs to be sent down. It is a sport of which his experience is somewhat limited. Now might be a good time to practice.
White Water Wonder
Designed by French specialist engineer HydroStadium, the 13m wide course channel is of relatively simple construction – a 270mm thick reinforced concrete base slab with 200mm thick reinforced concrete walls at right angles to it.
Over its 254m length the course falls by 4.5m – just enough to ensure it keeps the required 16 cubic metres per second water velocity, which is delivered by the four pumps. The clever bit that ensures the difficulty of the course lies in the location of obstacles along its bed. These will create the eddies and whirlpools so intrinsic to the sport.
Sections of the cast slab have been left out along the length of the course, and it is here that multiple plastic fixing brackets will be cast. These will sit proud of the slab and be benched in around using hand-applied concrete. Where the bracket sections do not sit next to the wall of the course, Chambers is looking at the possibility of infilling uses of old car tyres, anchored down and shotcreted to help imitate undulating river banks.
But the main feature of the brackets is slots where individual plastic barrels called omniflots are fixed to help create turbulence. The height, number and exact location of these barrels can be tweaked to alter the conditions in the channel at any time.