Hydrological impact of an extreme precipitation event the Boscastle August 2004 flood

The rainfall that led to the flood in the villages of Boscastle and Crackington Haven on the north Cornwall coast on the 16 August 2004 was an example of localized, intense rainfall, falling over an already wet catchment area. It is similar, in this conjunction of very intense rainfalls and wet antecedent conditions, to the flood generating rainfall that occurred in Lynmouth, Devon, 80 km further north-east along the coast, also on 16 August in 1952. In the Lynmouth flood, a maximum of 229.5 mm of rainfall were recorded in 24 h, but the flood that was generated occurred at night and 37 people were killed. Fortunately in Boscastle nobody was killed, but 104 people trapped by the flood were lifted to safety by helicopter from the roofs of buildings and vehicles. Fifty-eight properties were damaged and four had to be demolished. The estimate of total damages was put at £2 m. It is not the first time that Boscastle has been damaged by a floods; events are also recorded in 1827, 1847, 1957, 1958, 1963 and 1996. Even since 2004 there has been another 'mini-flood' in Boscastle in 2007.

The weather conditions on 16 August 2004 show a low-pressure area centred to the west of Ireland. Winds on the Cornish coast were from the south-west with convective cells being triggered along squall lines aligned with the wind. The intensity of these cells was increased by the topography as they crossed the coast, leading to very local high precipitation. Of the ten nearest raing auges to Boscastle, four showed less than 3 mm of rain that day. In Boscastle itself, 89 mm was recorded in 60min, and at Lesnewth 4 km up the valley, 24 mm in 15 min and 184.9 mm in the day (Fig. 9.3).1 The highest

Lesnewth 5 min rainfall intensity

16 August 2004

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Fig. 9.3 Accumulated radar rainfall totals (after Golding et al., 2005, with kind permission of John Wiley & Sons) and 5-min tipping bucket rainfall record at Lesnewth, Boscastle flood event, 16 August 2004 (after Burt, 2005, with kind permission of John Wiley & Sons).

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Time UTC

Fig. 9.3 Accumulated radar rainfall totals (after Golding et al., 2005, with kind permission of John Wiley & Sons) and 5-min tipping bucket rainfall record at Lesnewth, Boscastle flood event, 16 August 2004 (after Burt, 2005, with kind permission of John Wiley & Sons).

daily rainfall observed in the area was 200.4 mm at Otterham. A similar event, in Maidenhead, Somerset, in July 1901, provided the largest recorded 60-min rainfall in the UK (see Table 9.2), while similar or more extreme daily falls have been recorded in July 1955 at Martinstown, Dorset (279.4 mm; Table 9.2); in June 1917 at Bruton, Somerset (242.8 mm); in August 1924 at Cannington, Somerset (238.8 mm); and in June 1957 at Camelford, Cornwall (203.2 mm in 24 h).

This type of event is very difficult to predict, both in location and in intensity of the rainfall. Post-event analyses using numerical weather prediction (NWP) models, run at a high-resolution 1-km grid scale, have been able to reproduce the recorded patterns of rainfall in the event reasonably well (Golding et al., 2005), but such models are not yet run at such fine resolution operationally. It is also difficult to predict the runoff produced by such an event because this will depend strongly on the antecedent wetness of the catchment area involved (see Chapter 12). In the Boscastle event, the damages caused seem to have been made worse because of small bridges becoming blocked with debris (including in one case a car), with consequent failure leading to a steep flood wave, up to 3 m high, flowing down the valley at velocities of the order of 4ms-1.

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