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BOSCASTLE FLOODS: CASE STUDY SUMMARY

 

An example from an MEDC

 

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, forming thunder clouds 10 km high over Bodmin Moor – as air rose, it cooled, water vapour condensed into water droplets and huge amounts of intense - very heavy - rain fell.

 

Causes: other:

o     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.

o     Impermeable rocks & thin soils increased surface run-off.

o     Steep slopes – rapid runoff

o     Narrow, steep sided valley – nowhere for water to overflow

o     Confluence of Rivers Valency & Jordan is just above the village, with a third tributary joining lower down – so water from all 3 poured through the village

o     A very high – ‘spring’ tide – made it difficult for water to flow out to sea

o     The short lag time until peak discharge and the rapid rise in river levels resulted in a flash flood with 100 x

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