| The old Cornish Stores shop | |
| Duration: | 1 day |
| Fatalities: | 0 |
| Damages: | Extensive |
| Areas affected: | Boscastle; Crackington Haven |
The Boscastle flood of 2004 occurred on Monday, 16 August 2004 in the two villages of Boscastle and Crackington Haven in Cornwall, England, United Kingdom. The villages suffered extensive damage after flash floods caused by an exceptional amount of rain that fell over eight hours that afternoon. The flood in Boscastle was filmed and extensively reported but the floods in Crackington Haven and Rocky Valley were not mentioned beyond the local news. The floods were the worst in local memory. A study commissioned by the Environment Agency from hydraulics consulting firm HR Wallingford concluded that it was among the most extreme ever experienced in Britain. The peak flow was about 140 m³/s, between 5:00pm and 6:00pm BST. The annual chance of this (or a greater) flood in any one year is about 1 in 400. The probability each year of the heaviest three-hour rainfall is about 1 in 1300 (although rainfall probability is not the same as flood probability). At midday on the 16th August 2004, heavy thundery showers had developed across the South West; these were the remnants of Hurricane Alex (2004) which had crossed the Atlantic. The last time Boscastle had suffered notable flooding was in 1996 as a result of Hurricane Lili, but floods are recorded in 1847, 1957, 3 June 1958 (one man drowned) and 1963. On 16 August 1952 the small town of Lynmouth, 50 miles (80 km) north-east along the north coast in Devon near Exmoor, suffered extensive damage in a catastrophic flood, in which 34 people lost their lives. Coincidentally, this was 52 years to the day before Boscastle's 2004 flood.
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On the 16th, warm air picking up moisture - due to residual heat from the Atlantic sea - travelled towards the South West Cornish coast as prevailing winds. Upon contact with the topographically vertical coast, these winds experienced a strong up-drafting force thus causing internal moisture to reach the atmosphere, and consequently cool as a string of storm clouds. With convergence and coalescence, enhanced moisture levels resulted in heavy rainfall on the afternoon of 16 August 2004. 185 mm (7 inches) of rain fell over the high ground just inland of Boscastle. At the peak of the downpour, at about 15:40 GMT, 24.1mm of rain (almost one inch) was recorded as falling in just 15 minutes at Lesnewth, 2.5 miles (4 km) up the valley from Boscastle. In Boscastle, 89 mm (3.5 inches) of rain was recorded in 60 minutes. The rain was very localised: four of the nearest 10 rain gauges, all within a few miles of Boscastle, showed less than 3 mm of rain that day. The cause of the very heavy localised rain is thought to be an extreme example of what has become known as the Brown Willy effect.
The torrential rain led to a 2 m (7 ft) rise in river levels in one hour. A 3 m (10 ft) wave, believed to have been triggered by water pooling behind debris caught under a bridge and then being suddenly released as the bridge collapsed, surged down the main road. Water speed was over 4 m/s (10 mph), more than enough to cause structural damage. It is estimated that 20,000,000 cubic metres (5.3×109 US gal) of water flowed through Boscastle that day alone. The steep valley sides, and the saturated surface ensured a high amount of surface run-off.
Changes in farming practice in the area also possibly contributed, sewage could have been a cause as well, with a reduction of trees and hedges higher up the valley causing water to flow through more quickly than would have been the case in the past. Fortunately, no one died in the flood.
However, an episode of Discovery Channel's Perfect Disaster states that the floods might have been caused by a phenomenon called a "blocking high". A blocking high is a large area of static high pressure. It can happen anywhere in the world, and the effect is deadly because the high pressure can stall other weather systems around it.
75 cars, 5 caravans, 6 buildings and several boats were washed into the sea; approximately 100 homes and businesses were destroyed; trees were uprooted and debris were scattered over a large area. In an operation lasting from mid-afternoon until 2:30 AM, a fleet of 7 helicopters rescued about 150 people clinging to trees and the roofs of buildings and cars. No major injuries or loss of life were reported.
Most work takes place in the winter season (October–May), during the off-season. The carpark is reduced to half the space (120 spaces) in winter, for works to take place, and then back to 240 spaces in summer.
The main structure of the former lower bridge survived the flood, however the stone walls did not, and were washed away. On 1 May 2005, the official reopening of the village, wooden fences were used on the bridge to temporarily replace the stone walls. The bridge used to have a concrete plaque on it saying "This bridge is the private property of the lord of the manor, August 1887". This was lost during the flood, but then recovered from the harbour in good condition. This bridge has now been replaced with a new one.
During the flood of 2004, 14 cars became lodged beneath it, this had caused a huge backlog of flood water and debris, adding to the damage caused in the surrounding area.
The original proposal was for a stone bridge, this was rejected. The second proposal is for a modern concrete bridge, with steel railings. Public consultations were held and villagers were asked to select their preference from four designs; most did not choose the one which has been proposed. This plan was rejected.
The new bridge is a few metres further down the river than the old bridge. The bridge was installed on 18 December 2007, and was made by Cornish Concrete, a company based near Truro. The main arch is made from concrete, with metal railings.
The old stone bridge, which was over 100 years old, was demolished in early April 2008.
Boscastle flooded again, although not nearly as badly as the 2004 floods. After an afternoon of intense localised rainfall, and a week of steady rain everyday, a small flood happened on 21 June 2007. 30 mm fell on the area in one hour. Roads had become flooded in the area and in the village, most water came from the saturated fields around the village. Many drains had become blocked. Roads around the Tintagel, Camelford and Davidstow area were closed off to stop people visiting the village. River levels were alarmingly high, but the banks contained the water. However the many culverts of the River Jordan had overflowed onto the villages roads, adding to the amount of water on the roads. The new storm culvert joining the River Jordan to the River Valency was at full capacity, but did not flood. Services and organisations called in included:
The Environment Agency's flood defences installed after the 2004 flood worked and kept the water in the river channel. A few properties in the village were flooded by 3 ft (0.91 m) from water flowing down the streets, rather than from the river flooding, although the damage was not nearly as bad as the 2004 flood. The two main roads (B3266, B3263) were blocked with flood waste and debris, although this was cleared and the roads reopened the following morning. The Environment Agency looked at the culverts around the village and will change them all to modern drainage
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