Weatherwatch: English summer storms

There used to be a saying that the English summer was three hot days followed by a thunderstorm. While that may be a pessimistic assessment it seems statistically correct that the most violent storms occur in the coming two months.

While many will remember the storm that devastated Boscastle in north Cornwall seven years ago on 16 August, it was the location of the storm and the terrain as well as the volume of rainfall that was the problem. Over four hours 200mm of rain fell in the rocky hills behind the village, causing streams to swell. The water rushed down the valley through the village, flooding 70 properties. Floating cars acted as battering rams destroying bridges.

Fifty years earlier a similar storm hit Martinstown in Dorset on 18 July, 1955. This would not be memorable at all except that it holds the official record for the largest one-day rainfall in the UK – 279mm (11ins). The storm came after a hot dry spell and there was some flooding, but not on the scale of Boscastle.

The reason was simple, the soil around Martinstown is chalk, not granite, and even with this volume of water a good percentage of the rain soaked directly into the soil. But meteorologists warn that with a warmer climate and more energy in the atmosphere violent storms could get worse. We should expect more large storms and where there is no safe escape for the water, dangerous flash floods.

Boscastle Floods 2004 – Lessons Learned - Veterus Consulting

This short video taken on the 6th anniversary of the Boscastle floods shows the incredible restoration work that has taken place since the flooding and offers a short glimpse at the beautiful village of Boscastle after the terrible floods that could have cost dozens of lives were it not for the skill, readiness and teamwork of the military and civilian agencies that responded on 16 Aug 2004.

On the 16th of August 2004 Boscastle in Cornwall suffered a catastrophic flash flooding event that nearly cost the lives of 100 people. A combination of unusual weather conditions led to incredibly high, and very localised rainfalls in the vicinity of the village as the remnants of Hurricane Alex combined with sea breezes to pile up 40,000ft high clouds that lingered and poured torrential rain onto the steep sloping valleys (“flashy catchments”) and roads leading into the Boscastle valley. In Boscastle itself 112mm of rain fell in 90 minutes!

A floodwatch was issued at 1239 after the Lesnewth rain gauge recorded 3mm of rain. At 1315 a further 15mm of rain fell in 15 minutes. After several more sudden downpours and lightning induced power cuts; the River Valency began to breach it’s banks at 1530. Half an hour later a 10ft high wall of floodwater rushed through the visitor carpark at an estimate 40mph and people began to be trapped in buildings by the torrential streams of water now flowing.

At 1622 rescue helicopters from RAF Chivenor and RNAS Culdrose were scrambled and an hour later they were already winching the first of 100 people to safety from the rooftops of rapidly disintegrating buildings as local emergency services and all available military and coastguard rescue helicopters rushed to the scene.

Studying this event we can see the importance of early warning and a fast, coordinated multi-agency response.

Looking at the recovery and restoration efforts since provides us with a best practice approach to flash flood mitigation – provided you have the budget for it.

The National Budget for Civil Protection in Pakistan is reportedly £500,000 and the area flooded is the size of England.

Boscastle Flood of 2004

Geography 360°

Floods in Action

Improvements in reservoir construction, operation and maintenance, proceedings of the 14th conference of the British Dam Society at the University of Durham from 6 to 9 September 2006

Flood Risk Assessment