The Boscastle flood of 16 August 2004: Characteristics, causes and consequences

On 16 August 2004, exceptionally intense and prolonged storm conditions centred over the North Cornwall coast and the headwaters of Bodmin Moor generated extreme flooding in and above the coastal villages of Boscastle and Crackington Haven. The event – which wreaked havoc and damage, but led to no direct loss of life – captured considerable media attention. “What happened? Why there? How? How unusual was it? What’s the risk of it happening again, there and elsewhere? What’s being done? [and yes] Is climate change to blame?” style questions peppered the press, and occupied the minds of many, at the time and after. This paper addresses the “what happened?” question. It reports on a flood forensics study carried out for the Environment Agency by HR Wallingford, with support from Halcrow, Royal Haskoning, The Met Office, CEH Wallingford and others. The event, from storm to runoff to flood to impact, has been reconstructed using best available evidence and applying best possible analyses
(meteorological, hydrological and hydraulic). Propagation mechanisms – and features like the reported “walls of water” observed during the event - are investigated with a numerical model calibrated against evidence of peak water levels. Estimates are provided of the peak flow and peak water levels experienced, and their probabilities; these place the event amongst the most extreme ever to have occurred in the region and in the UK.