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Heads out of the sand

The performance of the time-trusted sandbag has been heavily challenged since the mega floods of summer 2007. Mark Hansford reports on how technology from the highways sector could soon offer a better solution.

The slipformed concrete safety barrier is becoming a common sight on the central reservations of the UK’s motorways. It could soon also become a familiar sight for flood defence works, due to its robustness and long-term low-maintenance needs.

That is because transport infrastructure group Britpave, encouraged by the success of the concrete safety barrier in the highways industry, has been examining potential alternative applications for the slipformed barrier. And given the increased threat and occurrence of flooding in the UK, due to extreme weather patterns and the unrelenting pressure to build on flood risk areas, the need for effective, and cost effective, flood defence systems is great.

Current systems barely stand up to scrutiny. In his review of the summer 2007 floods, Sir Michael Pitt was sceptical on the performance of the UK’s approach to flood defence and in particular its reliance on sandbags. "Strategic sandbagging can be successfully used alongside roads or adjacent to important buildings to prevent them from flooding, but the work needs to be done properly by experts," he said in his final report, the conclusions from which have now been adopted by government.

"The review was unable to obtain any significant evidence that sandbags were particularly effective during the 2007 summer floods in providing protection to individual households. "The general provision of sandbags should be phased out in favour of better products such as kite-marked flood boards, air bricks and other forms of temporary defence," Pitt’s report concludes. Pitt goes on to recommend that government develops a single set of guidance for local authorities and the public on the use and usefulness of sandbags and other alternatives, rather than leaving the matter wholly to local discretion.

So now is clearly a good time to be offering up a better product. "The slipformed concrete barrier, for a number of reasons, is a highly cost-effective and efficient solution," claims Britpave director David Jones. Britpave is, of course, not alone in racing to get a system to market. But its barrier is well progressed and the argument in favour well honed. "In addition to the long-term robustness and minimum maintenance, the approach was found to be particularly well suited for flood barriers," he says, adding that following presentations from Britpave, the Environment Agency is interested in the potential of concrete flood barriers.

Click here for a cross section of a typical Slipformed flood defence

The key to any flood barrier system is for it to remain standing when subjected to hydrostatic loading. In this respect, the barrier is a mini-dam. The mass of a slipformed concrete barrier is similar to that of a concrete dam with the only real difference being that the water pressure is applied for a short duration. To ensure that it can withstand the water pressure, the stability of the barrier needs to be able to resist overturning and sliding. Sufficient concrete mass is required to counteract overturning forces. The trapezoidal profile shape of the concrete barrier provides further stability.

In addition to the mass, incorporating a cut-off wall and drainage in significant horizontal joints can reduce overturning forces. This has the advantage of reducing the size of the barrier required. Resistance to sliding relies on the provision of a suitable foundation width and depth to give sufficient friction/adhesion and passive resistance to immobilise the positive sliding forces.

Different levels of flooding will require different foundations and mass. These would have to be taken account of during the design process. Bearing in mind the relatively short time for which a flood barrier is expected to function, Jones does not consider it essential for the barrier system to be watertight. "There is no need for special waterstops at each joint due to the temporary nature of flooding," he says. Accepting a minor volume of leakage would allow this water to be routed to a sump location from where it could be easily pumped back to the watercourse.

Suitable joint details and limiting the magnitude of shrinkage of the concrete mix would ensure minimum leakage. Construction of the slipformed barrier, due to its simple and uniformed shape, is simple and fast, claims Jones: "Depending upon the cross section of the barrier being formed, it is possible to construct up to 200m per eight hour shift per machine. "The barrier is also cost effective. Based upon experience of construction of concrete road safety barriers, the cost of a 900mm high flood barrier is expected to be very competitive. The 50 year design life is impressive as well and it is possible for the barrier to be serviceable for over 100 years as there is no risk of concrete deterioration resulting from the corrosion of the reinforcement," says Jones.

There are also environmental benefits. Land take is minimal as only a limited width is required – a typical barrier will need an 800mm wide foundation – and the barriers do not require high performance concrete. Both fly ash and ground granulated blast furnace can be used as cement replacement. In addition recycled aggregates and concrete from demolished structures can be used.

The barriers can also be made to be more aesthetically pleasing with the use of imprinting to mimic stone or brick walls and the use of coloured concrete or exposed aggregate finishes. "The slipform concrete barrier has tremendous potential for providing flood barrier systems that are fast and cost effective to construct and offer long-term minimum maintenance," says Jones. "Working closely with the Environment Agency, Britpave is now preparing technical guidance on the design, construction and maintenance of typical concrete flood barrier systems."

Currently only a handful of temporary barriers are available on the market. These products are highly engineered and as such are extremely expensive, from £400 to over £1,000 per linear metre.

In the right scenarios they work well, but it is hard to see them ever replacing the ubiquitous sandbag. Most require bolting to the ground, tend to be slow to assemble, demand skilled personnel for assembly, are bulky and inflexible and bespoke to each scenario. So claims Simon Phelps, founder of Fluvial Innovations and inventor of the Floodstop barrier – another flood barrier innovation taken straight from the highways sector.

Phelps’ barrier has been under development since the summer of 2004, where the budding inventor chose to spend his final year in academia at Bournemouth University developing a rapidly deployable flood barrier that could truly supersede sandbags in cost and function. Four and a half years later the Floodstop barrier has been sold to local authorities, government bodies and private households throughout the UK.

The barrier is a modular plastic system, usually seen on highway road works. No bolting to the ground is required and the modular units are simply connected using slide-in keys, creating a flexible watertight barrier. Units also nest together to minimise storage and haulage requirements.

One council has been won over: "Hampshire County Council has recently ordered a significant quantity of the Floodstop barrier as we believe this product will be an improvement on sandbags in dealing with flood water," says the council’s former chairman of the emergency planning society Ian Hoult.

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