In some parts of the world people are used to the kind of flood that seems to come out of nowhere. And although this is rare in the UK it is no less dangerous. 'Flash floods can involve a very sudden danger and the strong risk of loss of life, ' says UK Environment Agency flood defence manager John Curtin.
'Unlike a normal flood, where there could be an 18 hour period before the waters rise, this can happen extremely quickly. And the water levels can be fast moving and engulfing, rather than lapping around your feet and gradually rising.' People can lose their lives in normal floods, he says, but here the danger is exceptional.
'You have to evacuate the area immediately.' The Agency had no remit for warning or defence against flash flooding before the torrent that hit Boscastle in north Cornwall last August (NCE 19 August).
The only previous notable, similar event was the 1951 Lynmouth disaster.
Investigations into the Boscastle flood by the Agency - with consultants including HR Wallingford and the Met Office - suggest that such events will remain extremely rare. Judging from historical records, weather data and local knowledge, there is just a 1:400 chance of a repeat event.
'That is important for the local people and for planning purposes, ' says Tim Woods, the Agency officer leading the Boscastle investigation. 'Unlike a normally flood prone area, we can advise the planning authority that it is OK to rebuild structures.' Nonetheless, Woods as advised against placing anyone or anything in the path of potential future flash fl ood flows. 'You would not locate a camping site or car park on exposed slopes or river bank meadows for example.' But as part of its flood warning role, the Environment Agency has now put fl ash flooding on its risk map, despite the rarity of the events.
'You might never be able to issue advance warnings of the kind possible in normal flood prone areas, ' says Curtin, 'but you could give planning authorities notice of the risks and also set up some kind of alert system.' A national team is now working on a set of criteria for potential flash fl ood areas.
These can then be matched against the detailed GIS (geographic information system) databases that are increasingly held by government, regional and local authorities, to pick out vulnerable catchments.
Working out precisely what criteria to use will take some time, says Curtin. 'The problem is that you do not want to set the remit too wide - you could end up including thousands of small areas with a very low risk.' But Boscastle showed that geographic and geological features are very important.
Boscastle is hemmed in by exceptionally steep slopes on a small valley, with a high number of tributaries adding to flow surges. Impermeable slatey rock making up the river bed prevent flows soaking away.
Vegetation can absorb rainfall and then slow down fl ows, and to compensate for changing land use patterns around Boscastle, which have reduced ground cover, future action on flood risk reduction may involve strategic planting of trees and bushes.
The settlement pattern of a valley is important, as there is no point in warning an area with just a few pastures in it.
And the location and geography of the village is key. 'The west coast is generally wetter than the east, north and west, ' says a Met Offi ce spokesman.
Coastal locations subject to sea breezes are particularly vulnerable, he adds.
At Boscastle the tight valley and steep slopes pushed air upwards, adding sea breeze convergence to existing warm air convection currents. Combined with very wet, energy-laden air coming in from a summer ocean the result was a selfperpetuating area of rainstorm known as a 'super-cell'. This is a type of giant cumulo-nimbus cloud 'which in turn can spawn daughter cells'.
Super cells typically remain stationary, which is one reason why the Boscastle storm was so localised.