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Flood alleviation: Hoe Valley, Surrey

This week marked an important milestone in the Hoe Valley Scheme near Woking, Surrey.

Community spirit

From Monday, nine community groups, including Girl Guides, army cadets and a local football club, began to migrate from a series of ageing huts into a shining new purpose-built community centre, complete with green roof, photovoltaic panels and a lush new football pitch grown from seed.

The move means the final stages of this wide-ranging regeneration and flood defence scheme by VolkerFitzpatrick and VolkerStevin can begin. The old community huts − located on a former landfill − will be demolished on 15 August, allowing the site to be remediated and left as a blank canvas for developers.

Hoe Valley

The impressive new community centre is a central trophy piece in this regeneration scheme. However, flood alleviation is the real heart of the project. Some 60 properties along the Hoe Stream − a tributary of the River Wey − were flooded in October 2000, prompting the Environment Agency to look more closely at the area and come up with a £3.7M scheme that would protect 83 properties against a 1 in 25-year flooding event.

However, Woking Borough Council had been developing its own plans to undertake regeneration and flood defence work in the area for 18 years. When this was made known, says Environment Agency south east flood risk management team leader Chris Savage, the two bodies agreed to work together to develop and fund the project.

“Working in partnership to get both schemes concurrent made sense”

Woking Borough Council strategic director Mark Rolt

As a result, the scheme was expanded to protect 198 properties against a 1 in 100-year flood through a combination of flood walls, earth bunds and compensatory storage ponds, while also achieving the council’s regeneration aims.

“Working in partnership to get both schemes concurrent made sense,” says Woking Borough Council strategic director Mark Rolt. The council formed a company called Thameswey Developments Limited (TDL) to deliver the Hoe Valley Scheme, and deemed it such a success that TDL will be retained for future projects.

Making the funding go further

Through TDL the council committed a substantial £40M to the project, some of which will be made back from the future sale of the remediated 25,000m² former landfill site.

A total of 80,000m³ of soil will be excavated and treated on site, before being re-used within the scheme to construct the flood embankments − an example of how the regeneration and flooding aspects of the project are interlinked, allowing the funding to go further.

The remediated soil was also used to raise land for the new football pitch, and will be used to build two large bunds at one end, which will do double duty as a match viewing platform.

It is easy to see how properties here flooded in 2000, as many of them stand very close to the water’s edge. This caused a headache at the design stage. “The challenge for us is building these soft defences in a residential area,” says VolkerFitzpatrick construction manager Deon Scholtz. “The average flood bund has a long footprint.”

Hard defence

While bunds were the favoured type of defence, the tight spacing between the river and the properties meant only 70% of the barriers could be soft, and the rest had to be hard defences − 250mm thick concrete walls with 205mm thick brick cladding, or sheet piles in particularly tight spots.

In places where the walls are not immediately overlooked by homes, the brick cladding is swapped for less attractive −but cheaper − exposed aggregate, 100mm thick.

The new bunds feature a geosynthetic clay liner on the river side, covered with 150mm of subsoil. Drainage is built into the structure to allow surface water to flow from the land side through valves to the river when necessary.

The construction programme was determined by the complex flood risk along the Hoe Stream − nothing could be constructed before its flood risk impact downstream was mitigated.

This resulted in complicated sequencing, and meant that flood walls have had to be constructed with large gaps to be closed up later.

Bridge replacement

Also contributing to the reduction in flood risk is the replacement of two road bridges − one Victorian and one built in the 1960s − whose small clearances restrict the river flow, and by raising a footbridge 2.24m. A third new road bridge will be built to provide access to the new community centre.

The Victorian structure, Elm Bridge, is by far the most challenging of the three bridges being worked on. A major thoroughfare, the bridge comprises two spans totaling 17.27m. The river passes under one span, and the other span is over an adjacent overflow channel for the river, which effectively creates an island when the river is swollen.

Development in the area has simply outstripped the bridge’s capacity. At just 5.1m wide the road is too narrow to allow vehicles to comfortably pass on the bridge itself; the pedestrian paths are also narrow, at 0.8m on one side and 1.5m on the other; and utility pipes fought for space beneath the deck, many of them ending up bolted onto the side of the bridge instead. “Every utility you can think of goes over this bridge,” says Scholtz.

Tight clearance

Crucially for flood mitigation, the bridge’s Victorian arches made clearance very tight for the river, with just 5.2m clearance width over the main channel, and an additional 3m clearance width for the overflow channel.

By contrast, the new bridge has a clearance width of 17.27m and height of 2.87m, and carries a 9m wide road with 3m pedestrian/cycle paths on either side. It is a single span reinforced concrete bridge with pre-stressed beams, clad in brick with brick parapets, echoing the appearance of the original Victorian structure.

The first stage in construction of this bridge was to build a cofferdam, then the many utility pipes were diverted onto a temporary bridge running alongside the existing one − a complicated operation that required the cooperation of all relevant utility companies in one single overnight operation in which utilities could be switched off for 30 minutes.

Every utility going over the bridge had to be kept live throughout the rest of the project.

The concrete box section of the new bridge was then constructed in situ, before work began to revert the utilities back across the permanent bridge. Finally, the cofferdam and temporary bridge will be removed, and the river will be realigned so that it runs between the existing river path and the overflow channel − and passes under the new box section.

Green corridor

Elm Bridge will re-open this autumn. The other replacement bridge, Park Access Bridge, will be open in November, while the new bridge giving access to the community centre has already opened. Ground remediation will finish at the end of September, and the flood defences − now around 60% complete − will be finished in mid-November.

With final landscaping work included, total project completion is set for February 2012. By that time the Hoe Valley will have gained not just new bridges, flood defences and community facilities, but a number of environmental benefits too.

The remediation of the landfill site − previously inaccessible to the public − will deliver a public space that forms the final jigsaw piece in a “green corridor” allowing residents to walk or cycle off-road from central Woking to the southern parts of the borough.

The scheme will also improve biodiversity and enhance previously “scrubby” parkland, says Savage. The Environment Agency itself has gained from the fact that the redevelopment will provide clear access to the river for maintenance, where access to the watercourse has previously been much more difficult.

Highways are benefitting too, with 432m of road widened, 385m of new road built, and four junctions upgraded.

Complex web

Between the utility companies, the community groups, the Environment Agency and other parties − including a school whose playground became a construction site, and the Royal Engineers who had to sign off a rifle range at the new community centre − the complex stakeholder interfacing on this project has been remarkable.

Even so, Scholtz says work is “well ahead” of schedule. “All parties are working together to see how we can reduce the time,” he says. It is an expeditious finish to a slow-growing project.

“The scheme has been mooted for 18 years and people said it would never happen,” says Rolt. Happily, those people have finally been proved wrong.

Project Team

Numerous stakeholders and a co-funding arrangement made this project an exemplar of collaboration.

“One of the things that made us choose Volker was that spread of knowledge,” says Woking Borough Council strategic director Mark Rolt. A joint venture between VolkerFitzpatrick and VolkerStevin - both owned by parent company VolkersWessels won the Hoe Valley Scheme in competitive tender against three other companies.

■ Main contractor VolkerFitzpatrick
■ Flood alleviation works VolkerStevin
■ Bridge joints and waterproofing VolkerLaser
■ Piling
VolkerSteel Foundations
■ Street lighting VolkerHighways
■ Main civils designer
Halcrow

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