Location: Boscastle, N. Cornwall; Rivers Valency & Jordan
Date: 16 August 2004
Causes: immediate: a flash flood following fall of 50mm rain in one hour, 125mm in a few hours; this resulted from warm moist air moving onshore meeting air moving overland from the SW ~ air was forced to rise rapidly,– as air rose, and it rained heavily.
A very wet August (2 times average rain) meant that the ground was already saturated; water couldn’t be absorbed so rapidly entered the rivers as surface run-off.
Impermeable rocks & thin soils increased surface run-off.
Steep slopes – rapid runoff
Narrow, steep sided valley – nowhere for water to overflow
The short lag time until peak discharge and the rapid rise in river levels resulted in a flash flood with 100 x the normal river discharge flowing through Boscastle 20 mins after the storm began.
Effects: immediate: a wall of water swept through the village destroying everything in its path
50+ cars, and caravans were swept out to sea
6 buildings were swept away eg Wellington Hotel, others were badly damaged eg Witchcraft Museum
Many other houses, shops etc were flooded, with mud + sewage as well as water; possessions also ruined.
Bridges swept away
Roads under 2.75m of water
Damage to sewers/water/electricity supply
Boulders & trees swept through streets
No deaths, few serious injuires
Effects: longer term: cost of building & infrastructure repairs, clean up, new river defence works cost to insurance companies - & premiums probably rose
Emergency services – 6