Boscastle Floods
CAUSE 1: VERY HEAVY RAIN
* Most of the rain fell over five hours.
* Peak 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.
CAUSE 3 - SLATEY IMPERMEABLE ROCKS with CLAY SOILS
The cause was the combination of
• intense convectional rain,
• local topography and
CAUSE 1: VERY HEAVY RAIN
* Most of the rain fell over five hours.
* Peak 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.
CAUSE 3 - SLATEY IMPERMEABLE ROCKS with CLAY SOILS
The cause was the combination of
• intense convectional rain,
• local topography and
