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Defending the unlucky Severn

Flooding & coastal Shrewsbury flood barrier

Shrewsbury residents were initially reluctant to have a flood barrier, but inundation in 2000 changed their minds, discovers Andrew Mylius.

Work on building flood defences for Shrewsbury, Gloucestershire, got off to an inauspicious start. Floods engulfed low lying areas of the town and lapped at the site offices for the whole of February 2002, just after the scheme began on site, putting the project a month behind schedule.

A year on, though, the first 700m of a new barrier system is almost in place and ready to hold back the surging River Severn.

Plans for a barrier defending Shrewsbury were first put forward in 1993, but were dropped in the face of public opposition.

The permanent masonry structure proposed was deemed too ugly, and the threat of inundation too small. But after 400 properties were swamped in the autumn of 2000 opinion was ready to change.

Sloshing through the town in waders, Tony Blair and John Prescott promised Shrewsbury protection within 12 months.

The Environment Agency appointed consultant Black & Veatch to have a new look at defence options, and in January last year construction of a £4.2M, 3m tall barrier began.

Shrewsbury has been divided into 20 defensible 'cells'. Black & Veatch has set out to protect the first of these, containing some of the town's oldest and most vulnerable buildings, from a 1:100 year or greater event.

However, locals want easy access to and views of the river for the periods it is not in flood.

On the section of river being tackled by Black & Veatch, and contractor for the scheme Dean & Dyball, the river is bordered by a section of towpath, behind which lies an industrial estate.

There is also a car park, backed by a rag-tag collection of gardens. The line of defence runs along the towpath and the back of the car park, and here it has been possible to install permanent barriers, says project manager Simon Lewins - they are brick and masonry-clad reinforced concrete walls that have simply replaced existing fences.

The river is flanked over short stretches by buildings, and here walls have been reinforced and waterproofed.

Where it crosses public space or keeping views of the river is essential, though, 1.5m high reinforced earth embankments and a demountable barrier system supplied by German firm IBS are being used. Embankments are tall enough to withstand a 1:20 year flood. Putting an extra 1.5m demountable fence on top brings it up to full height. There are also sections using full 3m high demountable panels.

The demountable barrier system adopted at Shrewsbury has been used at Bewdley, also on the Severn. If necessary the towns will be able to swap components, Lewins says. Panels are slotted between demountable posts that fix to stanchions set into the ground. Posts are braced from behind by back-stays to resist bending moments exerted by water pressure.

To prevent water forcing its way under the barrier and through the made ground, sand and silts found along the barrier's line, it has been necessary to install a 14m deep sheet piled footing, toed into impermeable alluvium.

The piled footing is capped with a reinforced concrete ground beam on which reinforced concrete walls or demountable barrier fixings are constructed. To resist surcharge as river levels rise, and to cope with wind loading and impact by heavy flood-borne objects, 20m long raked piles have also been installed. These are tied into the ground beam with a specially designed steel flange which spreads loads evenly.

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