Boscastle flood of 2004

Boscastle flood of 2004

The old Cornish Stores shop
Date 16 August 2004
Location Boscastle; Crackington Haven
Deaths 0
Property damage Extensive
Looking upstream from the bridge after the flood

The Boscastle flood of 2004 (Cornish: An Lanwes Kastel Boterel 2004) occurred on Monday, 16 August 2004 in the two villages of Boscastle and Crackington Haven[1] in Cornwall, England, United Kingdom. The villages suffered extensive damage after flash floods caused by an exceptional amount of rain that fell over eight hours that afternoon. The flood in Boscastle was filmed and extensively reported but the floods in Crackington Haven and Rocky Valley were not mentioned beyond the local news. The floods were the worst in local memory. A study commissioned by the Environment Agency from hydraulics consulting firm HR Wallingford concluded that it was among the most extreme ever experienced in Britain. The peak flow was about 140 m³/s, between 5:00pm and 6:00pm BST. The annual chance of this (or a greater) flood in any one year is about 1 in 400. The probability each year of the heaviest three-hour rainfall is about 1 in 1300 (although rainfall probability is not the same as flood probability). At midday on 16 August 2004, heavy thundery showers had developed across the South West due to a weak disturbance to the northeast of the United Kingdom.[2]

The last time Boscastle had suffered notable flooding was in 1996 as a result of Hurricane Lili, but floods are recorded in 1847, 1957, 3 June 1958 (one man drowned) and 1963. On 16 August 1952 the small town of Lynmouth, 50 miles (80 km) north-east along the north coast in Devon near Exmoor, suffered extensive damage in a catastrophic flood, in which 34 people lost their lives. Coincidentally, this was 52 years to the day before Boscastle's 2004 flood.

Pre-Flood Characteristics

The Boscastle flood was caused by both anthropogenic and physical characteristics. Boscastle was a high flood risk area before the storm event due to its unique flood prone physical characteristics. Physical characteristics included are impermeable bedrock, thin soil, lack of vegetation, very steep converging topography, many fluvial systems joining in a confined area, a temperate wet climate and land use. With the combination of high precipitation (orographic and frontall type rainfall) from the storm event, the timing of the high tide and the debris forming a temporary dam a serve large flood event was generated.

Major Geology Bedrock Formations

The area around Boscastle has a combination of igneous, metamorphic, siliclastic sedimentary and carbonate sedimentary rocks with influence of faults making the area hydrogeological complex.

Boscastle Formation

Exposures are located in the Northern part of Boscastle village itself and makes up the moderate relief to the North. Bedrock has a thickness of hundreds of metres which varies due to uplift and downlift of faulting. Lithological characteristics of this formation are dark grey to black pyritous mudstones with thin laminated siltstones which are punctuate by areas of thick to thin bedded grey coarse to fine grained quartzose sandstones. Adjacent to intertonguing contacts with the Buckator Formation, the sandstones are calcareous, shelly and accompanied by thick beds of bioclastic limestone (Softgrounds) and nodules of silicified limestone (Hardgrounds). This means that there may be several small scale water ways within the lithology due to the permeability differences. The mudstones are locally burrowed. Rare thin beds of tuffaceous mudstone, and tuffs and lavas also occur which may be permeable due to cooling cracks. Overall the bedrock is relatively impermeable with a low infiltration rate which is natural prone to flooding.[3][4][5][6]

Yeolmbridge Formation

Exposures are located in the Southern Part of Boscastle village itself and make up the moderate topography at the centre of the map. Lithological characteristics of this formation are green and black mudstones with variably abundance of siltstone laminae with scattered medium to thin bedded sandstones, limestone nodules and units of nodular cephalopod limestone. Thin tuff beds and lava are sporadic. Overall the bedrock is relatively semi-permeable which means it is natural prone to flooding.[3][4][5][6]

Tredorn Slate Formation

Exposures are located on the vary South Western outskirts of Boscastle Village itself and make up the Southern moderate topography. The B3266 which is a major road and is essential for transport, evacuation and trade is built upon this formation. Lithological characteristics of this formation are greenish grey quartz with chlorite or mica slate. Locally there are interbedded thinly bedded lenticular bioclastic limestone and dolomitic beds which are 0.15m thick with siltstone, sandstone and rare tuff beds. Overall the bedrock is relatively impermeable which means it has a very low infiltration rate so the area is natural prone to flooding.[3][4][5][6]

Superficial/Drift Geology

Drift Geology is the unconsolidated sediment on top of the bedrock. Alluvial and river terrace deposits are the only types of drift in the Boscastle area. Exposures only exist in small abundance within the River Jordan and River Valency. This means that the majority of the area is influenced by impermeable igneous, metamorphic and carbonate rocks. These rocks type have thin soils which are highly susceptible to over saturation which would cause flooding.[3][4][5][6]

Structural Geology

Structural geology is the study of the mechanics and stresses of rocks. There are many faults within Boscastle village itself and in the surrounding higher topographic areas. The faults are highly important for understanding flooding because they can provide channel ways for water. This means that water will become concentrated and channelized into a relatively small area. This is especially essential to understand in fault dominating and impermeable bedrock areas like Boscastle.

Geomorphology

Geomorphology is the study of topographic land forms and surface processes. Boscastle is situated on low-lying flat topography which is surrounded by areas of highly steep converging topography. Southern topography has higher relief ranging from 30 to 258 meters with very steep slopes around 40–70 degrees. Northern topographic has lower relief ranging from 20 to 200 meters with very steep slopes around 55 to 75 degrees. This is highly significant because water runoff will be very fast and will become concentrated in the converging topographic areas creating a large sudden output of water.

Hydrogeology

Hydrogeology is the specialized area of geology concerned with the study of water underground, subsurface and on the surface of the Earth. There are over 10 natural water springs within 1400m radius of Boscastle village. The springs form on converging topography where sudden topographic change always the water to escape the impermeable underlying bedrock. This is highly significant because there is already a constant flow of water in the converging topographic areas without the influence of precipitation so a flood event during high precipitation events is highly probable.[3][4][5][6]

Hydrology

Hydrology is the specialised science concerned with the study of the earth's water in the atmosphere and its movement in relation to topography. There are 3 major river junctions near Boscastle. Junction A (SX 09915 91246) is between the River Jordan and River Valency. Junction B (SX 10464 91131) is between the River Valency and spring originated river. Junction C (SX 10910 90987) is between the River Valency and Tregrylls plantation river. The river sizes are realtivley small which means they capacity to hold excess volumes of water is slow which in turn contributes to flooding events. There are other minor fluvial junctions between streams and springs outlets. This is highly significant because these junctions allow two bodies of water to intermix in a short period of time causing the daughter body of water to have an increased volume. Due to the high abundance and narrow complex shapes of fluvial systems in the Boscastle area it is highly prone to flooding from a hydrological viewpoint.[3][4][5][6]

Environmental Analysis

Environmental Science/Geology is an applied science concerned with the practical application of the principles of both geology and environmental in the solving of environmental problems influenced by physical and anthropogenic factors. Vegetation and land use are major factors of flooding in Boscastle. The natural vegetation is grassland/scrubland due the thin soil influenced by the parent rock. This means that the areas natural capabilities to uptake and infiltrate water is poor compared to more highly vegetated areas in Britain like Pembrokeshire woodland. The anthropogenic land use has made the area even more susceptible to flooding. Around 70% of original vegetation or natural deciduous sparse trees have gone since the 1800s. The main modern land uses in Boscastle are farming, fish farming and plantations. Farmland becomes easily saturated due to the compression of soil by cattle and machinery. Also the crops' ability to infiltrate water compared to natural vegetation is relatively poor. Further upstream the River Valency around 3000 metres from its mouth is a dense area of fish farming with 9 separate ponds densely packed in a small area. The River Valency Meanders have been made large which means the residual time of the water is longer so the water will stay in the fluvial system for longer. This also means a large volume of water is constantly trapped at the surface. Tregrylls Plantation is a large area consisting of densely packed spruce pine trees which are all the same age. The infiltration capabilities compared to a natural forest are poor due to the lack of intercepting layers in the area. The densely packed nature of the spruce pine stops sunlight reaching the plantation floor which in turn causes a further lack of vegetation to infiltrate and slow down the runoff of water. Also the settlement itself has impermeable layers like tarmac, concrete and a Victorian drainage system which is not suitable for a modern major flooding event.[3][4][5][6]

Meteorological

Boscastle has a temperate climate. There are continuous levels of precipitation all year round which means the ground and soil is constantly saturated to an extent. This means that the soil infiltration rate is greatly reduced which leads to more runoff which contributes to floods.[3][4][5][6]

Causes of the flood

The main factors that caused the flood are meteorological and anthropogenic. Both factors are both highly variable.

Meteorological

A combination of mixing airs from Atlantic storm and prevailing winds are the main factors. The rain was caused due to warm air absorbing moisture due to residual heat from the Atlantic Ocean which traveled towards the South West Cornish coast within the prevailing winds. Upon contact with the topographically high vertical coast the winds experienced a strong up drafting force thus causing internal moisture to reach the atmosphere and consequently cool as a string of storm cumunonumbous clouds. With convergence and coalescence the enhanced moisture levels resulted in heavy precipitation on the afternoon of 16 August 2004. The precipitation type was orographic/relief rainfall and frontall rainfall.[7]

Anthropogenic

Second large flash flood was caused by cars, waste and trees blocking the stone bridge creating a temporary dam. With increasing volumes of water pressure increased causing the dam to breach which created a sudden outlet of water to accumulate. [8]

Tidal

High tide occurred at 13.00pm which magnified the severity of River flooding. Tides around August are very large and are almost the size of spring tides. Also tides are very warm at August but reach they peak temperature around October which means evaporation allowed more moisture to be regenerated into storm clouds. [9]

On the 16th, warm air picking up moisture – due to residual heat from the Atlantic Ocean – travelled towards the South West Cornish coast as prevailing winds. Upon contact with the topographically vertical coast, these winds experienced a strong up-drafting force thus causing internal moisture to reach the atmosphere, and consequently cool as a string of storm clouds. With convergence and coalescence, enhanced moisture levels resulted in heavy rainfall on the afternoon of 16 August 2004. 185 mm (7 inches) of rain fell over the high ground just inland of Boscastle. At the peak of the downpour, at about 15:40 GMT, 24.1mm of rain (almost one inch) was recorded as falling in just 15 minutes at Lesnewth, 2.5 miles (4 km) up the valley from Boscastle. In Boscastle, 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 over 4 m/s (10 mph), more than enough to cause structural damage. It is estimated that 20,000,000 cubic metres (5.3×109 US gal) of water flowed through Boscastle that day alone. The steep valley sides, and the saturated surface ensured a high amount of surface run-off.

Changes in farming practice in the area also possibly contributed, sewage could have been a cause as well, with a reduction of trees and hedges higher up the valley causing water to flow through more quickly than would have been the case in the past. Fortunately, no one died in the flood.

However, an episode of Discovery Channel's Perfect Disaster states 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.

Impact of the flood

75 cars, 5 caravans, 6 buildings and several boats were washed into the sea; approximately 100 homes and businesses were destroyed; trees were uprooted and debris were scattered over a large area. In an operation lasting from mid-afternoon until 2:30 AM, a fleet of 7 Westland Sea King helicopters rescued about 150 people clinging to trees and the roofs of buildings and cars. No major injuries or loss of life were reported.

Work since the flood

The rebuilt Harbour Light in October 2006, now used as tea rooms. The shop is over the river.

Most work takes place in the winter season (October–May), during the off-season. The carpark is reduced to half the space (120 spaces) in winter, for works to take place, and then back to 240 spaces in summer.

2004

2005

2006

2007

2008

The lower bridge

The New Lower Bridge, taken on 20 December 2007.

The main structure of the former lower bridge survived the flood, however the stone walls did not, and were washed away. On 1 May 2005, the official reopening of the village, wooden fences were used on the bridge to temporarily replace the stone walls. The bridge used to have a concrete plaque on it saying "This bridge is the private property of the lord of the manor, August 1887". This was lost during the flood, but then recovered from the harbour in good condition. This bridge has now been replaced with a new one.

During the flood of 2004, 14 cars became lodged beneath it, this had caused a huge backlog of flood water and debris, adding to the damage caused in the surrounding area.

The original proposal was for a stone bridge, this was rejected. The second proposal is for a modern concrete bridge, with steel railings. Public consultations were held and villagers were asked to select their preference from four designs; most did not choose the one which has been proposed. This plan was rejected.

The new bridge is a few metres further down the river than the old bridge. The bridge was installed on 18 December 2007, and was made by Cornish Concrete, a company based near Truro. The main arch is made from reinforced concrete, with metal railings.

The old stone bridge, which was over 100 years old, was demolished in early April 2008.

Mini flood – 21 June 2007

Boscastle flooded again, although not nearly as badly as the 2004 floods. After an afternoon of intense localised rainfall, and a week of steady rain everyday, a small flood happened on 21 June 2007. 30 mm fell on the area in one hour. Roads had become flooded in the area and in the village, most water came from the saturated fields around the village. Many drains had become blocked. Roads around the Tintagel, Camelford and Davidstow area were closed off to stop people visiting the village. River levels were alarmingly high, but the banks contained the water. However the many culverts of the River Jordan had overflowed onto the villages roads, adding to the amount of water on the roads. The new storm culvert joining the River Jordan to the River Valency was at full capacity, but did not flood. Services and organisations called in included:

The Environment Agency's flood defences installed after the 2004 flood worked and kept the water in the river channel. A few properties in the village were flooded by 3 ft (0.91 m) from water flowing down the streets, rather than from the river flooding, although the damage was not nearly as bad as the 2004 flood. The two main roads (B3266, B3263) were blocked with flood waste and debris, although this was cleared and the roads reopened the following morning. The Environment Agency looked at the culverts around the village and will change them all to modern drainage

See also

References

  1. ↑ "Crackington Haven". cornwall365.co.uk. Cornwall 365. Retrieved 22 January 2015.
  2. ↑ "17 August 2004 surface analysis". Institute of Meteorology. Free University of Berlin. Retrieved 4 April 2012.
  3. 1 2 3 4 5 6 7 8 British Geological Society Regional Map, 2012
  4. 1 2 3 4 5 6 7 8 Ordnance Survey Regional Map, 2012
  5. 1 2 3 4 5 6 7 8 Soil Mechanics, R.F Craig, 1997, Book
  6. 1 2 3 4 5 6 7 8 Tectonics of Sedimentary Basins, Cathy Busby and Antonio Azor, 2012, Book
  7. ↑
  8. ↑
  9. ↑
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