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Preventative measures

Flood protection - Careful reshaping of the river owing through a National Trust village aims to prevent future ood disasters. Report and pictures by Adrian Greeman.

Civil engineering in Boscastle is helping to rescue the Cornish village from the aftereffects of 2004's devastating flood in more ways than one. As well as restructuring the channel of the river Valency and its crossings to avoid future catastrophes, the works are becoming a tourist attraction themselves, according to local hotel staff.

All this fits very nicely with the ethos of the Environment Agency, client for the ood protection works, and its contractor Carillion. Both plan to integrate as much as possible with the tiny community during the two-year project, explaining what is going on and trying to avoid disruption.

'It is one of the challenges on a scheme like this, ' says Ian Yelf, Carillion contract manager for the South West.

Enid Blyton's Famous Five would feel completely at home with Boscastle's 'secret cove' and the sharp S-bend at the harbour mouth which hides it behind the towering cliffs. But the picturesque narrow valley proved to be Boscastle's downfall in the summer of 2004.

A once in 400-year storm brought 200mm of rain in just five hours and the water flooded down to the village from a steep wooded catchment above.

The little harbour village was half destroyed as the deluge cascaded down the narrow valley, sweeping through houses and buildings. Five buildings were demolished and 47 others damaged, including the Hotel Wellington, which is built over a culverted tributary of the Valency (NCE, 2 September 2004).

According to the Environment Agency's Phil Barron, trees were part of the problem. Branches and trunks piled up behind the central road bridge along with cars swept from the car park.

Water was forced through houses and shops. Landowners, primarily the National Trust, have been asked to help manage the woods in future to remove debris.

The Environment Agency is spending £4M on reshaping the river and bridge construction.

Along the 750m length of the main village, the aim is to widen and deepen the river channel and remove potential blockages.

The lower brick arch bridge will be replaced with a wider crossing. At a couple of locations the river channel will be doubled in width with a perched berm separating the main channel from a secondary overspill channel and there will also be space for controlled ooding.

Revetment walls are being rebuilt: concrete backed in the village, and in original Cornish drystone in the National Trust harbour area. New car parks at the top of the valley will include an area for an overspill channel.

As well as extending the car park, a substantial muckshift is raising its height to eliminate the possibility of cars being washed away in any future ood. 'Most of the ll comes from the river bed, ' says Carillion site manager, Colin Mitchell. This material contains slate and quartz. It is processed by screening and washing, after which it is usable, though there is just enough silt content to make it hard going in wet conditions, 'acting like a sponge', says Mitchell.

The 000m 3 to be spread over the area will be in place by the end of March, when the tourist season begins. 'The new area will have a cellular geogrid surface with vegetation to maintain its meadow character, ' says Mitchell.

Meanwhile, the course of the river is being 'braided' to create a natural ow pattern and slow it down in the hope that any future ood debris will be deposited above the village. The Agency worked with the River Restoration Centre to achieve a natural ow and its consultant Halcrow has modelled the new scheme to assess spate ows.

Downstream, small pools have been created in the riverbed to add interest when the river is low, and to help sh populations.

These pools were carved out by an excavator working in the river bed. The 7.5t machine was tted with specialised buckets with ripperteeth, a hydraulic breaker and a roadheader-type cutter tool for harder parts.

Widening and deepening the lower rocky channel by around 1m has been quite difcult, says Mitchell, because the river bed consists of slate lying at an angle of about 45 o to the river line and at a vertical angle of 20 o. The machine operator also had to be aware of the ever present danger of flow in the river, which rises quickly from a trickle to flood after rain. 'Usually we were out about six hours before the warnings, ' explains Mitchell. The operator could not see what he was doing once water was 300mm-450mm deep, well before ood danger.

Work in the central village includes re-establishing some riverside platforms near properties and raising the backs of some buildings. As much of the original character of the buildings as possible is being retained in these works. Some minor work can be done in the summer, though after May the contractor has to rein back to allow another ood, this time of tourists, to pass through Boscastle. Full operations will resume in October.

Some of the most significant work is taking place downstream at the lower bridge, which is to be replaced next winter. After much local consultation and planning committee discussion, the new structure will be a modern bridge, designed to complement the surroundings rather than matching them.

'The river bed is widened and deepened at the harbour and a replacement arch bridge would have been much bigger - huge in the context, ' says Mitchell.

The widening also means rebuilding some of the harbour wall in the traditional north Cornwall drystone style. The National Trust insisted this must be done without concrete backing, which means a 100-year design life cannot be guaranteed. But the interlocking stone walling is actually very robust, claims Mitchell. Much local advice was received on which artisans were the best suited to carry out the work, he says.

A major task this winter has been the removal of a sewer crossing which runs across the river bed, connecting two local pipelines to a nearby sea outfall.

The pipeline forms a signicant obstacle in the river and a new tunnelled crossing replaces it, driven by an augur bore launched from a 7m deep shaft, to a second shaft near the Witchcraft Museum. Carillion excavated the shafts with miniexcavators and breakers, lining with precast concrete rings as the excavation progressed.

Subcontractor lan Watson has pushed the 27m long minitunnel beneath the river bed.

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