Boscastle flood of 2004

Boscastle flood of 2004

The old Cornish Stores shop
Date	16 August 2004
Location	Boscastle; Crackington Haven
Deaths	0
Property damage	Extensive

The Boscastle flood of 2004 (Cornish: An Lanwes Kastel Boterel 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 16 August 2004, heavy thundery showers had developed across the South West due to a weak disturbance to the northeast of the United Kingdom.

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.

On the 16th, warm air picking up moisture – due to residual heat from the Atlantic Ocean – 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: Otterham and Lesnewth, both a few miles inland from Boscastle, recorded 24 hour totals of 200 mm and 185 mm respectively on the day, whereas four of the nearest 10 rain gauges showed less than 3 mm of rain in the same period. 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.