Boscastle August 2004Case Study of a flooding incident in the U.K.

What are we learning today Dino?

• “The location of Boscastle.• The cause of the flood• The effects of the flooding• How can it be stopped from

happening again – Boss”.

Where is Boscastle?

A small town on the north coast of Cornwall. 30 kms north west of Plymouth.

CAUSE 1: VERY HEAVY RAIN

Most of the rain fell in a five hour periodPeak intensities were in excess of 30mm/hr (0.5mm per minute)

A month’s rain fell in just 2 hours

Studies of extreme rainfall patterns have concluded that freak floods are more likely to occur in June, July and August than at any other month of the year.This is when atmospheric conditions, such as a warm ground surface, lead to the uplift of air masses which subsequently cool, producing cloud and rainfall formations.

At midday, on the 16th August 2004, heavy, thundery showers had developed across the South West, these were the remnants of Hurricane Alex which had crossed the Atlantic.

Bands of showers aligned themselves with winds that had converged along the coastal high ground around Boscastle, creating Cumulonimbus clouds 12192m (40,000ft) high and kept them stationary for many hours.

CAUSE 2: THE STEEP SIDED VALLEYS

It has been estimated that the Boscastle valley’s catchment area exceeds 23sq kms spanning inland to Bodmin Moor where many small rivers spring.

The steep sided valleys that converge down to the sea, known in the trade as “flashy catchments”, act as huge funnels and can produce true flash floods after a sudden cloudburst or prolonged heavy rainfall.

River ValencyRiver Jordan Bodmin Moor

CAUSE 3 – SLATEY IMPERMEABLE ROCKS with CLAY SOILS

The cause was the combination of

• intense convectional rain,

• local topography and

• the geology

= resulting in a flash flood no one could have predicted.

Cause 4 The site of Boscastle?

The harbour area is on low ground beside the sea

and on the flood plain of two rivers.

12.15 Rain gauge at nearby Lesnewth some 4km (2½ miles) up the valley, shows no rainfalland it is dry in Boscastle’s harbour area, yet there are torrential showers at Camelfordand at the top of Boscastle.

What happened?

15.30 River Valency begins to break its banks

Cars swept awayCars were swept out to sea, bridges were washed away and people clung to rooftops and trees for safety as torrential rain hit the area. Emergency workers mounted a huge operation to rescue residents and holidaymakers along a 32-km (20-mile) stretch of the north Cornwall coast around Boscastle.

16.00 a 3 metre wall of water runs through Boscastle car park at 40 mph.

17.00 Rescue Helicopters.

In an operation lasting from mid-afternoon until 2:30 AM, a fleet of seven helicopters rescued about 150 people clinging to trees and the roofs of buildings and cars. Amazingly, no major injuries or loss of life were reported.

55 residents were airlifted out by the Royal Air Force after the flooding, 35 BBC staff were flown in by other means.

Aftermath – short term rescue and clear up

Medium Term : Effect on tourismAbout 90% of Boscastle’s economy is dependent on tourism. After the flood, more than 20 accommodation providers were forced to shut, many of them individually owned bed and breakfasts. As about two thirds of the business is done during the six week school holiday, the effects were even more devastating with half the three weeks remaining.

The timing of disasters such as that in Boscastle is important. As this happened during the day people were awake and could be rescued by the emergency services. If it had happened during the night most people would have been asleep and there would have been a higher likelihood of injuries or even deaths.

What can be done – to stop it happening again?

Freak event – only once in 400 years but:

Control developments on flood plain

Bigger culverts on both rivers

What was done in Lynmouth up the coast? Boscastle was 52 years to the day after Lynmouth – where over 50 live were lost!