On August 17th 2004, after several hours of intense thundery rainfall in the drainage basin (202mm reported at Otterham) combined with a rising tide lead, a catastrophic flash flood ensued on the River Valancy, upstream of the village of Boscastle, N Cornwall, which, with the sheer volume of water (travelling at times up to 40 mph), broke its banks in the village. The force of the raging river swept over one hundred cars and three houses away, into the harbour and the open sea. A major rescue operation was launched by the RAF to airlift people to safety. A synoptic map of the situation (from UKMO) just after the event can be seen here and a satellite image here (courtesy Bernard Burton). Other areas locally such as the harbour village of Crackington Haven, north of Boscastle, were also flooded as well as parts of the River Ottery catchment area flowing east into the River Tamar.

Although the rain eased off in the evening, further rain fell in the W Cornwall area the following day (17th August) hampering the clean up efforts. Up to another 60mm fell in W Cornwall (more especially in the Camborne, Redruth and Helston areas) causing more local flooding with police advising motorists not to travel unless necessary.  

The technical background to the event  The synoptic set up that led to the devastating floods in the Boscastle region was a mixture of the general synoptic set up over the whole of the UK and also the local geography of this part of Cornwall. On the synoptic chart for 00Z on August 16th, a broad unstable S-SW airflow covered the entire SW Pensinsula. The winds aloft (700mb - one of the main steering levels) were strong from the SW. The airmass that covered Cornwall was unstable to modest temperatures. During the morning shower clouds developed inland as temperatures rose past the trigger temperature required to start convection. The 12Z Camborne ascent (see side panel) illustrated the convective potential; when convection had been initiated tops could easily reach around 38,000 ft (about 250mb). Once high tops were established the storm was able to "vent" quite well as strong winds aloft allowed air to move away from the storms centre whilst new growth could be initiated near the surface. Whilst temperatures were not that high in the area, they were just high enough to start a slight sea breeze along the north coast. With the prevailing surface wind from the S-SW this, along with the slightest sea breeze along the north coast, was enough to initiate a shower "train" convergence line or zone. (A shower train is a constant line of showers that continues to develop almost on the same spot whilst cells are advected along the train in the direction of the steering flow). The friction of the coastline in the location was also aligned to the 700mb flow and thus with the abundant moisture aloft, these shower cells quickly grew and released the large quantities of moisture stored aloft. Radar loops show that the shower cells initiated over the high ground in North Cornwall at around lunchtime on 16th; they grew and then an almost constant 'shower train' sat over the area affected for at least two hours. With rainfall readings being confirmed it seems likely that as much as 200mm fell in the space of 3 hours. This is in line with radar estimates from the afternoon and goes some way to explaining the events on the ground.

 

On the 16th of August 2004, 52 years to the day after the Lynmouth Floods in NorthDevon, a FLASH FLOOD hit the North Cornwall village of BOSCASTLE. The FLASH FLOOD damaged buildings and swept 50 cars away....

Remarkably, the floods of the 16th of August claimed no lives. A 3m wall of water forced its way through the village causing tremendous damage running into millions of pounds.

Weather report for day: "Scattered showers will begin to form during the morning, becoming heavy in places by the afternoon".

Catchment prone to flash flood was hit by a bad storm. Weather is becoming more extreme and intense.

Very thin soils.

Hard rock.

Steep catchment. Very steep slopes and the river closes in and gets narrower.

River Jordan, and River Valency are the 2 main rivers, which meet below the Wellington Hotel. Floor of room above the bar collapsed.

Water level was up to the top of the windows in Marine Terrace behind the hotel.

Several rivers meeting in narrow valley floor, and then on down into the harbour, which has a breakwater.

All 3 rivers in the basin went into flood.

Would have been overland flow in the fields higher up the catchment due to the intensity.

At least 100 cumecs during the flood - normally between 1 and 2 cumecs. 100 times the usual flow within 20 minutes. Very intense rainfall cell hit the catchment.

Layer of saturated air about half a kilometre deep. This was a particularly rare occasion called: CONVERGENCE. Large set of thundery showers moved in from ocean and met offshore winds. Forced the moist saturated air to rise and produce tremendous potential for rain.

River rose very quickly: rose about 8 inches in 2 hours, then 5 feet in around 20 minutes, meaning around a 6 foot rise within the hour.

Steep cliffs. Storm cells went spinning off the cliffs and generated uplift. Cliffs increased the rainfall.

Debris dams likely to be breaking in the upper half of the catchment - produced the wall of water which people saw coming down through the village. Blockages (trees etc.) hold up the water for a while, and a lake builds up behind, this then bursts and a surge of water comes down the valley - happened during the Lynmouth flood.

Top bridge parapet collapsed soon, and cars from the car park went over the bridge.

Graham King, local Coastguard observer was convinced that there were people in the cars that were being 'tossed around like cardboard boxes'. Hazard lights were on, and thought there were people in them. Cars were hitting the rooves of buildings.

Need to reduce VULNERABILITY in the future, which is why the rebuilding involves constructing a huge conduit. There is a much wider river channel built in LYNMOUTH.

Can we engineer the town in order to stop the event happening again ? Walls can be built, but they are ugly. Rivers which are diverted can revert to their original source. Need to protect people and property to reduce the risk: raise electrics, and use more flood resistant materials.

People could learn basic water rescue techniques. Create a community which can deal with such events.

People are positive about the future.

 

 Flash Flooding a 'freak event'

Flash floods such as those seen in north Cornwall are caused by a unique set of circumstances, weather experts say. While sustained rainfall is obviously key, it is the drainage and topography of the surrounding area which ultimately causes large scale flooding. Unfortunately, Boscastle, where 6cm (2ins) of rain fell in two hours on Monday causing a three-metre high wall of water to rush through the village, met the criteria. Phil Avery, of the BBC Weather Centre, said while the rainfall in this case was heavy it was certainly not exceptional. Slaughter Bridge near Camelford in Cornwall had the same rainfall in a two-hour period as Boscastle on Monday - as have other areas of the country in recent months. Mr Avery said: "If this had happened say in East Anglia where the land if flat you not have seen the flash floods. "What happened here is that the torrential rain was in an area where the conditions were such that the water was channelled through the village. "The topography of the area, the high ground, valleys and the fact there are only two rivers for the water to run into meant that we saw such devastation. "But the recent wet weather also played a role. The ground in the area was water logged, which meant the rain just sat on the surface." Met Office meteorologist Wayne Elliott agreed. "The crucial things in this case were the high ground and the area only having two rivers. Coupled with the already high water table, the conditions were just right," he said. The high ground acted as a trigger for the weather - forcing the air to rise and cool, creating rain, Mr Elliott added. And coastal winds converging on the area gave it extra "oomph". But despite a spate of flooding across the country in recent years, Mr Elliott said flooding was not becoming more common. "You cannot put it down to global warming. It really was a freak event. There is not much people can do to protect themselves. "We were forecasting heavy rain in our 4am bulletins but the key is when and where this is going to happen. If it had been 20 miles up the road, the situation would have been different."