Flooding at Boscastle

• According to the Environment Agency – The devastating flash flood in Boscastle was among the most extreme ever recorded in Britain.

Possible causes of flooding

• Snow melting on higher ground releasing water held in store

• Increased run off in tributary valleys increasing the flow of a river

• Impermeable rock preventing water from soaking into the ground

• The water table may be high because the ground is already saturated from previous periods of heavy rain.

• Steep slopes may cause increased surface flow on the sides of the valley

Weather report for day:

• "Scattered showers will begin to form during the morning, becoming heavy in places by the afternoon".

• Synoptic situationHeavy, 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.

Theory: Flash Floods

• A flash flood is a sudden flood event caused by a hydrologic response of the drainage basin. Flash floods are normally strongly localized and associated with extreme showers or thunderstorm activity, when high rates of precipitation occur in a short period of time.

• Flash floods usually arise when rain falls over a small catchment basin. The terrain may channel the extreme run-off to produce a flood peak that reaches its maximum in just a few minutes or hours. Flash floods are increasingly observed in urban areas where the surface is unable to absorb large amounts of water in a short period. Often the impetus and velocity of flood water are much more important than the associated water levels and duration in terms of potential impact.

Huge funnels!

• Whilst torrential rain is key to the onset of flash flooding, the drainage and topography of the surrounding area determines the scale and impact of the event. When there is torrential rain that cannot be soaked up or drained away, this leads to ‘run-off’ – water running over the soil rather than sinking into it.

• This run-off can cause localised but severe flooding. In places such as Boscastle, steep-sided valleys can act as huge funnels for this water, channelling it very quickly down to the sea, and sometimes with devastating results for local communities. This is what happened in Lynmouth on exactly the same day 52 years ago

Causes• The rainfall on the afternoon of 16 August 2004, was very heavy. 200.4 mm (8

inches) of rain fell over the high ground just inland from the village. At the peak of the downpour, at about 15:45 GMT, 24 mm of rain (almost one inch) was recorded as falling in just 15 minutes at Lesnewth, 2.5 miles (4 km) up the valley from Penzance. In Boscastle itself, 89 mm (3.5 inches) of rain was recorded in 60 minutes. The rain was very localised: four of the nearest 10 rain gauges, all within a few miles of Boscastle, showed less than 3 mm of rain that day. The cause of the very heavy localised rain is thought to be an extreme example of what has become known as the Brown Willy effect.

• The torrential rain led to a 2 m (7 ft) rise in river levels in one hour. A 3 m (10 ft) wave — believed to have been triggered by water pooling behind debris caught under a bridge, and then being suddenly released as the bridge collapsed — surged down the main road. Water speed was in excess of 4 m/s (10 mph), more than sufficient to cause structural damage. It is estimated that 2 million tonnes (440 million gallons) of water flowed through Boscastle alone that day.

• Other people believe that the floods might have been caused by a phenomenon called a 'blocking high'. A 'blocking high' is a large area of static high pressure. It can happen anywhere in the world, and the effect is deadly because the high pressure can stall other weather systems around it. In this case, the high pressure blocked a small thunderstorm which, because the blocking high kept it stationary, dumped all of its water over the Boscastle area. (source –Wikepedia)

When and Where? (16.8.04)

• Boscastle is the only natural harbour for 20 miles along the North Cornwall Coast. For a hundred years the village was a thriving port, but the coming of the railways, whilst ending the isolation of North Cornwall, soon saw its decline as a trading port. Since then however, Boscastle has developed as a tourist destination with thousands of people visiting the village every Summer

http://www.bbc.co.uk/cornwall/uncovered/stories/august2004/boscastle_floods.shtml

Impact• Approximately 75 cars and 5 caravans, six buildings and several

boats were washed into the sea; approximately 100 homes and businesses were destroyed; trees were uprooted and debris was scattered over a large area. In an operation lasting from mid-afternoon until 2:30 AM, a fleet of seven helicopters rescued about 150 people clinging to trees and the roofs of buildings and cars. No major injuries or loss of life were reported.

• Most of the tourist attractions and shops are in the oldest parts of the town, in the areas most affected by the flood at the bottom of the river valleys. The visitor centre was partly demolished, and the Museum of Witchcraft was also severely damaged. The ground floors of many buildings were covered with many inches of mud washed in by the flood waters. Following the rescues on 16 August 2004, emergency services cleared debris that had built up beneath and over the bridge at the centre of the village, and waters receded. Several buildings were demolished as a result of damage caused by the floods.