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Boscastle: the clean up begins

News analysis Boscastle

Flash flooding at Boscastle left a muddy trail of destruction. Ruby Kitching met the engineers working to the village get back on its feet.

Men, women and children trudged back to Boscastle last Friday to begin picking out possessions from the decomposing vegetation and mud caking the ground floors of their homes.

Dressed in Wellington boots, contamination suits and hard hats, it is difficult to tell them apart from the contractors workers alongside them to shore up their homes and muck out their streets. The centre of Boscastle looks somewhere between a demolition site and a construction site.

Over 75mm of rain fell in the area around Boscastle in just two hours on Monday 16 August (NCE 19/26 August) which caused the Valency and Jordan rivers to burst their banks.

Residents speak of a 3m high 'tidal wave' charging through streets sweeping up trees and cars in its path.

Engineers believe that walls and hedges acted as a succession of bottlenecks in each river catchment, allowing the staggering quantity of rain water to build up. As each obstacle was overwhelmed a surge of water was released.

The bottleneck effect was heightened by constrictions in the Valency and Jordan allowing water to deluge Boscastle.

The Environment Agency is now working with consultant Halcrow and water research company HR Wallingford to figure out how this bottleneck effect can be reduced or eliminated (see News this week).

One bottleneck is a narrow stretch of the Jordan which passes along a 600mm wide by 1,100mm deep channel behind the Marine Terrace cottages in south Boscastle. During the floods, the river deposited mud and rock from upstream, raising the bed level by 2m.

Flowing at this elevated level, the swollen Jordan was able to blast through the cottage windows.

The Environment Agency was aware of this bottleneck and had planned to upgrade the channel this month. The project is now being reviewed.

Restoring the channel to its full section has been one of the first tasks undertaken by the Agency to improve river flow.

'We had to dig down in sections while continually stabilising the excavation before getting down to the original bed level, ' says the Agency's area flood defence manager, Gordon Trapmore. Excavated material has also been used to build up the banks either side of the Jordan.

Beyond Marine Terrace, the Jordan cascades down to a wider but shallower culvert beneath the Wellington Hotel before joining the River Valency.

During the flood, boulders carried by the Jordan blocked the culvert beneath the hotel, forcing water to back up and burst through the ground floor of the building.

Meanwhile, water flowing behind the Marine Terrace cottages thundered through the second floor window of the hotel, forcing its way out of the building through the ground floor window.

Trapmore says inspection of the culvert has since revealed that the corrugated steel sides of the culvert collapsed during the flood, making the blockage more severe.

'At the moment we're trying to remove the obstructions beneath the Wellington using jetting pumps, but if that doesn't work we'll have to start mining under the hotel, ' he warns.

At Marine Terrace, the channel will probably have to be widened.

Trapmore says the Jordan is a very steep fast-flowing river, 'in excess of a 1:10 slope', with a very small catchment. Torrents of rain water swept up huge quantities of earth and vegetation.

Flows into the Valency were similar, and when it burst its banks it took to the village carpark, located in its floodplain. Water forced its way under the tarmac surface, breaking it up and carrying cars with it.

Four buildings were also destroyed by passing debris and flooding and a further two have required propping where key loadbearing elements have been wiped out (see structures box).

'It's a miracle that there was no loss of life - that's down to fast thinking locals and a well co-ordinated emergency response teams, ' says Horrocks.

'We're now working towards restoring Boscastle at least to its pre-flood state, ' he adds.

Buildings

'A lot of the buildings were so full of silt and tree trunks, we couldn't actually assess them at first, ' says North Cornwall District Council senior building control surveyor Darren French. He and his team have been inspecting structures in Boscastle since the flood receded.

'But the biggest problem has been stopping people returning to their homes to collect things.'

The Wellington Hotel and Marine Terrace cottages are still out of bounds as Environment Agency engineers decide how to repair the flood damage (see main story).

The flood meted out severe damage to weak old masonry buildings and modern cavity wall structures alike. Boscastle visitor centre, located in the car park, was entirely demolished; built in 1994, it was constructed using blockwork cavity walls with a stone outer leaf.

The older buildings have generally fared slightly better.

'All the blockwork walls which met the floodwaters head on fell through, but the old stone walls which are at least 600mm thick in places remained intact, ' says structural engineer John Davis.

'The lime mortar used in the old stone walls is also slightly more flexible than the brittle sand and cement mortar used between the blockwork, ' he adds.

He is confident that most foundations are intact. 'The rock is very close to the surface around here and most structures are keyed into it in some way, ' he says.

The Riverside Hotel and a bakery have required internal propping to support upper floor beams where walls have collapsed.

Roads

Flood damage to roads varies from drainage media being washed away and drains blocked to whole sections of road being destroyed. In many locations in Boscastle and for some 16km around, water found its way beneath the bituminous surface and broke it up.

Cornwall County Council estimates the cost of repairing roads and footpaths will be about £1M.

Most road bridges remained intact as cars and debris soon became jammed under them preventing further damage, although cars swept up by the Valency crashed through the parapet of the main reinforced concrete road bridge in Boscastle.

Temporary concrete barriers have been placed along the bridge allowing heavy machinery to pass over it safely.

Power

Western Power Delivery 'quickly isolated the electricity coming into the harbour area so emergency services could go in and do their work', says team leader Dave Gilbert. 'By the following day we started assessing the damage and restoring supply by bringing in two 100kV generators and fuel tanks. These were air lifted in.'

The firm is now constructing 250m of trenches to relay cables to houses.

Water supply and sewerage

South West Water was one of the first to set up camp in the village to begin the operation of restoring the water supply and sewerage network.

'So far we've focused on installing a new mains water connection above ground and reconnecting the main sewer, ' says South West Water programme leader Kim Vanstone.

The main sewer runs alongside the River Valency, whose banks were washed away during the flood, allowing debris to puncture the pipe.

Other work has involved locating blockages by sending cameras down pipes, flushing debris through sewers, and clearing out manholes filled with silt.

Constant updates

The Coastguard was scrambled by the owner of Boscastle's Witchcraft Museum when his property was inundated. The alarm was then relayed to the fire brigade which set off Cornwall County Council's 'multi-agency flood response plan' - first issued to all relevant agencies just six months ago.

'The first three hours following the floods were crucial to the success of the recovery of Boscastle, ' says the council's network manager Mark Stephenson.

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