During the afternoon of the 16 August 2004 an area of low pressure to the southwest of the UK helped a line of showers to develop just to the east of Boscastle, parallel to the north Cornwall coast from Newquay to Bude.
The showers aligned themselves with the wind and remained almost stationary, maintaining very heavy rain from around midday to 4 pm. In that time 200 mm of rain fell on the hills, fed into the River Valency and ultimately caused the floods in Boscastle.
The main triggers were convergence of the onshore southwesterly winds against the high ground of Bodmin Moor, which itself also helped to boost the intensity of the showers. The steepness of the valleys along the north coasts of Devon and Cornwall are particularly vulnerable to heavy rainfall as they efficiently collect water from the surrounding moors and rapidly transport it out to sea.
What were the impacts?
Two million tonnes of water flowed through Boscastle. The flash floods affected hundreds of homes and businesses, destroyed four properties in Boscastle and two at Crackington Haven, swept away about 115 vehicles and badly damaged roads, bridges, sewers and other infrastructure. Thankfully, due to a massive helicopter aided rescue operation by HM Coastguard and the military there were no fatalities.
What has changed in the last 10 years?
There have been three main changes since the Boscastle floods which have improved the way that severe weather and its impacts are managed:
The Environment Agency has made considerable investment into flood defences in Boscastle to help prevent a similar flood happening in the future. Working with professional partners, more than £10 million of improvements were carried out. This included widening and deepening the Valency River, and installing a flood culvert to improve flow in the Jordan River.
Andrew Houghton, of the local Environment Agency flood operations team, said: "Since their completion the new defences have worked successfully to protect communities across Boscastle and North Cornwall on several occasions, notably in 2010 and 2012.
"We have worked closely with people in Boscastle and other such rapid response catchments to raise awareness of the flood risk in these areas and develop community flood plans. Local residents and businesses in these catchments have been helped to achieve greater flood resilience and this work is ongoing."
The Environment Agency recently introduced a bespoke flood warning service for Boscastle which uses information from rain and river gauges in the area.
The Met Office and Environment Agency have formed the first of several working partnerships, the Flood Forecasting Centre. Combining expertise in weather forecasting and hydrology has helped to prepare communities for flooding during times of extreme weather.
At the time of the floods the operational forecast model had a resolution of 12 km, which was too large to be able to represent such a small scale collection of thunderstorms. Since 2004 the Boscastle case was re-run with a higher resolution research model which proved able to resolve the line of thunderstorms with much more accuracy and detail.
The 12 km model gave a weak signal of light rain across most of the Southwest Peninsula and provided little guidance. The higher resolution models showed the development of thunderstorms with much more representative intensity and location, which was very similar to the radar data from the actual time, shown below.
Humphrey Lean, Manager of the Met Office's Mesoscale Modelling Group, said: "This was a real step-change in the forecasting potential for small scale extreme weather and the model output demonstrated how well the Met Office's forecasting capability could work.
"Despite the Boscastle floods being a devastating sequence of events, it encouraged the development of higher resolution models which our meteorologists now use on a daily basis."
The latest forecast model, the UKV, operates with a 1.5 km resolution which is more likely to pick out small scale extreme weather with a longer lead time and allow the Met Office to provide advance warning and advice.
In 2011 the Met Office's National Severe Weather Warnings Service (NSWWS) changed from the previous threshold-based warnings to warnings centred around the likelihood of severe weather happening and the impact it might have. The Yellow, Amber and Red levels of warning help give the public and emergency responders more focused advice, helping them to be aware, prepared and ready to take action as necessary.
We have set up a network of Met Office Advisors around the UK who work with regional local planning groups and emergency responders to help them plan and prepare for severe weather. Working with our meteorologists these advisors ensure the local resilience teams are aware of how potentially severe weather situations are developing and what the impacts might be.
The rise of social media has become an increasingly useful way of making the public aware of upcoming severe weather. Through our social media channels we can rapidly reach our 218,000 Twitter, 60,000 Facebook and 115,000 Google+ followers with information about the latest forecasts, warnings and what to do in severe weather so they can plan, prepare and protect their property and families for the expected conditions. Social media also helps the public to tell us what is happening, for example, how much rain has fallen in their back garden through, amongst other channels, the Weather Observations Website (WOW).
Last updated: 8 June 2016