For some people, the widespread floods of summer 2007 were a foretaste of the kind of natural disaster that results from man-made climate change. For many in the water industry, it was also a stark reminder of how urgently we need to upgrade our water infrastructure.
One of those areas worst hit by the 2007 floods was Wakefield, West Yorkshire, a city with a long history of flooding due to river modifications associated with its industrial development. Serious flooding has hit Wakefield five times since 1940, with a further three floods within just the past few years.
Following the 2007 floods, more than £12 million has been spent on projects to reduce the flood risk from the River Calder and Wash Dike. And now Wakefield Council, in partnership with the Environment Agency, is nearing completion of a £1.3 million project to contain floodwater from the Oakenshaw Beck in the Agbrigg district of the city.
Although normally a small and unremarkable stream, Oakenshaw Beck is notorious for flooding and in 2007 more than 400 properties were inundated when the beck overflowed.
Main contractor CA Blackwell (Contracts) Ltd from Wakefield, won the contract to construct a system of storage ponds and flood embankments near Agbrigg to allow floodwater to be diverted away from the natural watercourse during flood conditions. The system then discharges the stored water back into the stream when it returns to normal levels.
Besides excavating a series of ponds and swales to accept the floodwater, Blackwell’s had to install two concrete structures, plus associated concrete pipes, to transfer water to and from the storage ponds.
The upstream control structure is a 12m long, 6.5m wide chamber containing a penstock and valve system which intercepts high water flows and feeds the surplus into the storage ponds.
Downstream, a substantial concrete pumping station and downstream control structure have been installed to enable stored water to be pumped back into the stream after the risk of flooding has passed.
The original scheme proposed traditional in-situ concrete techniques to build these two structures. This would have involved assembling temporary formwork within the excavations, fixing steel reinforcement and then pouring the concrete and leaving it to cure until it had achieved enough strength to allow the formwork to be struck and removed.
But this is a lengthy and labour-intensive process, and CA Blackwell was keen to explore opportunities to reduce on-site activities, exposure to bad weather and speed up construction times.
“Waterfront, the company which supplied the penstocks and valves, suggested off-site construction methods as a more efficient alternative,” explains Blackwell Project Manager Phil Holden.
“By manufacturing the concrete elements off-site we could reduce the risk of disruption from adverse weather conditions and also reduce the amount of time spent working in a live natural watercourse. There were significant environmental benefits.”
Via Waterfront, Blackwell contacted Kijlstra, one of Europe’s leading suppliers of specialised drainage systems. Headquartered in The Netherlands – where much of the country is built on low-lying reclaimed land and flood conditions are a constant threat – Kijlstra has pioneered the design and production of pre-cast concrete components for drainage applications.
But although now commonplace in continental Europe, the use of pre-cast in the UK water industry is still relatively unknown. On the Oakenshaw Beck project, Kijlstra’s solution was an entirely new concept for most of the construction team.
“We had used small pre-cast components on previous projects but this was the first time we had considered using pre-cast for an entire structure,” says Mr Holden.
Convincing JBA Design Consultants, which had produced Blackwell’s winning design, to approve the use of in-situ concrete was the first major challenge, admits Wieger Faber, sales engineer for Kijlstra: “They were a little unsure I think, at first.”
However, once the technical suitability of the method had been demonstrated to everybody’s satisfaction, the practical, environmental and time-saving advantages of pre-cast were enough to tip the balance in its favour, adds Mr Faber.
The upstream control structure comprises pre-cast Kijlstra wall units grouted onto an in-situ concrete base slab with Kijlstra headwalls located at either end. “The headwalls are located back-to-back and a penstock is installed at one end to control the water flow,” explains Wieger Faber.
The penstock can be opened and closed manually but electronic remote control also allows the valve to be operated remotely at the flick of a switch.
The downstream structure and pumping station also comprises pre-cast wall sections sitting on an in-situ base with Kijlstra top slabs and two headwalls.
Adopting the Kijlstra solution meant that Blackwell’s site team was on a sharp learning curve. “The guys on site had never used the system before and there were a couple of issues with alignment of the precast units and with grouting the steel dowel-bars,” admits Mr Holden.
“But Kijlstra were magnificent. They provided us with first-class technical backup so that, even though this was the first time we used the system, we quickly got used to it and produced a good result,” he adds.
According to Wieger Faber, building this type of structure with pre-cast units can save huge amounts of time. “We installed an entire chamber in one day. With in-situ concrete it could have taken four or five weeks to complete,” he says.
On this project, the time-savings were less dramatic, mainly due to the construction team’s unfamiliarity with the system. “Taking into account the total contract time and the changes we had to make, we estimate that the Kijlstra units still cut the amount of time spent on site by about a week,” says Phil Holden.
CA Blackwell is now an enthusiastic convert to the off-site technique and Mr Holden believes that it offers considerable scope for environmental and time-saving benefits: “In fact, we are now actively looking for ways of incorporating this type of pre-cast concrete in future projects of this type,” he says.
A timelapse video of the successful Kijlstra installation is available to view at www.blackwellgroup.co.uk