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16 August 2004


Flooding in north Cornwall


Synoptic situation

Heavy, thundery downpours developed by midday across south-west England on 16 August 2004. These showers formed bands which aligned themselves with the wind helping to maintain the heavy rain across certain areas of north Cornwall for several hours. The trigger mechanisms for these storms appeared to be convergence of winds along the coast and the high ground in the local area which also helped to generate showers. It would appear that the serious nature of these floods was exacerbated by the local topography around Boscastle.

Synoptic situation below for 1800 GMT on Monday 16 August 2004.

Analysis chart

South-west England 1971-2000 rainfall averages for August

More information on climate in south-west England

 

Other similar serious flooding events that have occurred in the past include:

  • July 1955 Martinstown Dorset (279.4 mm in 24 hours)
  • June 1917 Bruton, Somerset (242.8 mm in 24 hours)
  • August 1924 Cannington, Somerset (238.8 mm in 24 hours)
  • August 1952 Lynmouth, Devon (229.5 mm in 24 hours)
  • June 1957 Camelford, Cornwall (203.2 mm in 24 hours)

 

Rainfall information

The following data was obtained from the Environment Agency on 19 August 2004. No quality control has yet been undertaken on the data.

Map of locations

24-hour rainfall totals ending 0900 GMT 17 August 2004
  NGR 24 hour total (mm) Main event total (mm)
Otterham SX 169 916 200.4 -
Trevalec, Lesnewth SX 134 900 184.9 181.0 (11:30 to 16:30 GMT)
Trevalec, Lesnewth TBR SX 134 900 155.8 152.8 (12:15 to 16:19 GMT)
Creddacott  SX 231 956 123.0 -
Slaughterbridge TBR SX 109 857 76.5 -
Bude SS 208 063 46.7 -

TBR = Tipping Bucket Rain gauge

The 24-hour value from Otterham would give a return period in excess of 200 years (using the Flood Estimation Handbook method).

Lesnewth rainfall graph

Graph below showing 15-minute rainfall totals for Lesnewth TBR (Tipping Bucket Rain gauge).

Lesnewth rainfall graph

Map showing radar rainfall accumulations

Radar rainfall accumulations

Radar imagery: Click for full size

The animation below uses 15-minute data between 1030 GMT and 1930 GMT. The images are composites, based on a mixture of the best resolutions available.

Radar 16 August

Please note: The empirical relationship between radar reflectivity and rainfall rate is fixed whereas in reality this is highly dependent on precipitation type and is very different for rain and hail.