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WYE AGAIN?

A flood protection scheme is under construction in Hereford, but could it be too late? Report and photographs by Adrian Greeman.

Having an ironic sense of humour comes in useful on the site for a new flood protection scheme in the historic town of Hereford, near the Welsh border.

Work was initially held up by the floods of last summer when the River Wye burst its banks, and was suspended again for a week this January when NCE was on site – once again by more floodwater. Even the ground floor of the rented riverside site office was underwater.

"In between though, we have had relatively good conditions" acknowledges Andrew Wedlake, operation manager for contractor Morrison Construction which is carrying out the £5M scheme.

So when the weather has been favourable, earthworks, piling and installation of ground anchors have been able to continue.

The scheme creates a 2km long flood retention wall along a tight bend in the river which isolates an area of playing fields and parkland in the centre of the city. On the opposite bank is the historic cathedral with its medieval Mappa Mundi and upstream, there is an early 15th century stone bridge. From here, a public riverside walkway runs around the park perimeter.

The parkland is a natural flood plain and will remain so, but rising water often inundates around 200 houses and spills onto a road at the back of the park and a nearby car park.

The water even finds its way onto the central junction of the main road through the town, the A49, stopping traffic.

"The diversion during floods involves a 50 mile [80km] detour" says Suzanne Bland, project manager for client, the Environment Agency Wales. "As you can imagine that has a major economic impact."

An unusual variety of techniques has been needed for the scheme, from building up, reinforcing and anchoring an ancient river wall, to concreting, sheet piling and earthmoving.

Morrison began by raising the back of the park. A 2m high bund serves for much of this perimeter, and by careful design it has been blended into the parkland, using a very shallow shoulder slope of around 1:5.

A local supplier of recycled material was found for the 17,000t of cohesive material specified for the fill material, which had to be compacted to a density of 1.6t/m to create 164m of bund.

The final 100m section of flood protection where the park gives way to housing could not be built using bunds. Despite strong local support, building up embankments would have required too much land take and would have reduced the flood storage volume of the playing fields. A concrete wall up to 1.7m high serves to raise the boundary here instead. Pad foundations are 2.5m wide, creating a long cantilevering toe to resist overturning forces from flood water.

Further around the park there is a local swimming pool and amenity centre and the bund continues on the far side. Here the scheme had to cope with eight weeks of archaeological investigation before going ahead because a buried medieval fortification ditch lies across the park and is classed as a scheduled ancient monument.

An excavation down to about 3m along a 30m length was completed just before the main works began, confirming historical knowledge of the ditch but not revealing much more.

The final 140m of the boundary, taking it to the river bank, has required sheet piling to deal with the limited space available. The big footings [for a conventional wall] needed too much space and excavations would have cut into the root envelope of nearby lime trees," says Bland. An arboriculturalist advised that the trees could be permanently damaged or killed if foundations encroached on the root network.

"The piles cut into gravel underlying top soil and cut off some groundwater flow during floods," says Wedlake. "It has already cut basement flows".

These 7m long piles are faced with stone and brickwork for the 2m above ground on the park side and with timber cladding on the other where the "wall" faces nearby housing.

But installing deep piles requires clear headroom, which in some locations was interrupted by the overhang of the trees.

Morrison explored various options including using pre-auguring to loosen the ground, although did not help much. It also tried using a Giken push pile system, where a hydraulic unit clamps onto three existing piles and uses them as a reaction pile to force in the next one.

But the gravel's weak frictional resistance meant that installed piles were pulled out instead of the new one going in.

Finally a Müller hammer "resonance free" piling system was used. This comes with a secondary hammer sitting along the pile row which is synchronised with the main hammer to deliver a counter-pulse, the effect being to cancel out vibrations.

Unfortunately, running this system meant supporting the 5t of hammers from a crane, which meant some unavoidable damage to the trees at one end of the row.

At the river side a road ramp and up-and-over steps have been built for the footpath. Beyond that is a 100m length of the river wall, close to housing, which has involved some of the most complex works. The stone wall is being raised with a brick addition sitting on new concrete foundations. Piers along the wall extend the height and will allow timber infills to be installed during floods to give an additional 600mm height.

"All of that sits on a concrete foundation over the original stone wall," explains Wedlake. The concrete is anchored vertically into Raglan mudstone bedrock about 6m down with about 30 anchors. Another two rows of 30 raking anchors have also been installed into the stone wall itself and into the bedrock.

Drilling work for this, by subcontractor System Geotechnique was made difficult by ecological constraints. The Wye is a Site of Special Scientific Interest, home to the protected Shad fish, which spawns in the river and requires undisturbed passage in spring and late summer.

"We had to stop all river work for two periods of two weeks each" says Wedlake. More importantly a platform in the river for drilling operation was ruled out and anchors had to be installed from above on the riverside walkway.

"We have also had only one access point from this end onto the walkway," says Wedlake "which has kept us to a tight linear sequencing for the work."

Still to do is the brickwork for the wall and below the water level, an infill for an erosion cavity in the old stone wall. For this last task the contractor is using a Fabriform grout bag with a 200m3 capacity.

Divers will install the bespoke bag, into the cavity which has been restrained by mesh formwork. Grout is then pumped into the bag to fill the void and provide underpinning support for the bank and houses above.

A final 500m length of the protection has meanwhile been carried out upstream by supermarket giant Asda – a planning requirement in order to build a large store on the other side of the main road. Asda also contributed £2M to the overall funding of the whole scheme which should finish this April, unless further rain hinders construction.

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