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Postcards from the edge

A picturesque Cornish village in the south west of England is getting an overhaul with a major new flood defence scheme that aims to blend in with the rural surroundings.

The flood that hit Boscastle on 16 August 2004 made headlines and the village on the north Cornwall coast became famous overnight for images of burst riverbanks, flooded houses and floating cars rather than for its reputation as a tourist hotspot. A £4.6M project is under way to try and protect it from succumbing to the elements, while allowing it to retain its charms.

The one-in-400-year event behind the well-documented damage caused trees and debris to block the River Valency (and the River Jordan that flows into it), and Halcrow Group engineers have come up with a flood defence design on behalf of its client the Environment Agency.

"Our remit is to create a design that will be able to cope with a one-in-75-year event happening," says Halcrow design manager Chris Green.
Green says the team ruled out traditional flood defence walls because of National Trust concerns about its visual impact on a village.

Instead, engineers came up with a design that includes widening and deepening the river channel in places along the Valency. Green says this should help cope with the fast approaching water as the rivers, which he says behave like alpine springs, gain speed while draining down the mountains.

The plans also involve replacing the Lower Bridge – an old packhorse bridge whose short span contributed to earlier flood problems. The new structure – a modern pre-cast concrete bridge with granite detailing – will be able to accommodate traffic coming in and out of the village more easily and will be double the original's width.

Contractor Carillion Regional Civil Engineering is carrying out the work on the project, part of which involves creating a new riverbank using an erosion control system.

Halcrow designed the system using Maccaferri products. Site workers have installed over 7000m2 wire mesh Reno Mattress units and MacMat R geo-matting to reinforce and protect an embankment between the Valency and the village car park.

The mesh units comprise flat wire cages 6m by 2m and 230mm, lined with Bidum filter fabric and filled with site won material, to create mattress-like lifts. This is protected by the three-dimensional geomat made of polypropylene monofilaments with a double twist wire mesh.

Site workers installed a continuous array of these units along a 200m long and 3.5m wide stretch to create a 30º from the horizontal embankment adjacent to the car park.

They complete the embankment with a cast in-situ concrete toe rail at its riverside foot and a barrier at the head. The barrier comprises a row of heavy section timber bollards set into a concrete strip. This creates a protection system to withstand an anticipated 5m/sec water flow and impact from any debris being washed down with the river.

A 150mm layer of topsoil completes the embankment and will encourage vegetative growth. In addition, extra holes cut at intervals in the geomat allow for future tree planting. The overall structure is a hard-engineered flood protection system, but is designed to blend in with the soft, rural look of the area.

Many of the vehicleswere washed from the car park in the 2004 flood event because of the burst riverbanks and high velocity water flow. However, the car park's level is being raised and is buffered from the river by the new embankment and a landscaped area. These effectively form a sacrificial flood plain, should another such event occur.

"This area is designed to flood during a one-in-five-year event," says Green. "It allows the water to spill on to the bank and landscaping because it's a case of putting the flood water in places where you don't mind it being."

Work began on the scheme in October 2006 and is being done in two autumn-to-spring phases, so as not to disrupt the summertime holiday seasons. The scheme is due to be finished by March.

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