Flash Flood

August 2004

Flash floods in Cornwall devastated the tourist village of Boscastle during August. The settlement was badly damaged leading to an extensive clear-up and on-going repair work, after heavy, intense rainfall caused local rivers to burst their banks.

These were the heaviest rains in living memory for the people of the village - 185mm fell in just five hours. Given that the total annual rainfall for much of southern England is around 1,000mm, this is a lot of water to have arrived in such a very short time! Boscastle is also situated near the mouth of a very narrow river valley, which made matters worse. Flood risk is heightened whenever storm waters are denied a wide flood plain to spread into: in a narrow valley, whatever land exists either side of the channel will become rapidly submerged, once the river has burst its banks.

Happily, no one died on this occasion, despite the great hydraulic force of the water flowing through the town’s streets. In contrast, property damage was high. At least thirty cars were washed straight into the harbour and many more were left upturned and badly damaged. A three-metre high wave of water was reported to have crashed through one street at 40 miles per hour. Fridge-freezers were picked up and swept out of kitchens as water entered properties. Six properties collapsed entirely, as a result of traction and abrasional damage.

The first signs of danger came at 2.00pm on 16 August, when cast iron manhole covers in the town’s streets began to shudder, indicating that forceful storm waters were gathering in the sewers beneath. By mid-afternoon, the Valency and Jordan had over-topped their banks. At 5.00pm, the Clovelly Clothing shop was dragged 20 feet away from its site by floodwater flowing down the main street and subsequently collapsed (The Guardian, 18 August 2004). As evening fell, soaked locals and holidaymakers huddled together in the village sports hall while helicopters winches lifted stranded home-owners off the roofs of their own houses.

Sections in this article include:

• What were the physical and human causes of this hazard?

• What were the consequences of the Boscastle flood?

• Is global warming to blame?

• What are the comparisons with Lynmouth (1952)?

• UK: A turbulent summer?

• What were the causes of the hazard?

A hazard is a negative interaction between physical and human systems. The causes of a hazard therefore encompass (1) the physical factors responsible (in this case, heavy rainfall and catchment characteristics that promote rapid surface run-off) and (2) the human factors that have brought people, knowingly or not, into an environment where they are now at risk.

What were the physical factors?

• Three rivers – the Valency, Jordan and Paradise - converge on the village of Boscastle. The majority of the damage was attributable to the Valency.

• Heavy rain was caused by extreme frontal activity. In total, an input of three million tonnes of water was added to a tiny drainage basin, whose size is just 40 square kilometres.

• Attention must be paid not just to the total volume of rain but also the intensity with which it fell. 185mm arrived in just five hours, the majority falling in the first two hours. Under such conditions, infiltration-excess overland flow is inevitable, with the rate of input of rainwater greatly exceeding the infiltration capacity of any soil type.

• The soils were already saturated from previous rainfall earlier in the week, encouraging overland flow to begin even sooner.

• The three river valleys are very steep and narrow. A broader floodplain would have helped to soak up excess water and to dissipate energy more effectively (through an increased hydraulic radius). 

• The steep valley sides mean that soils are thin, as a result of mass movement, with limited storage capacity.

• The parent material is old, hard sandstone with limited permeability. The rivers here are naturally flashy.

• Surrounding vegetation includes agricultural land with limited interception storage, although there is some forestry along the riverbanks. (So conditions would have been even worse, without these patches of woodland!)

• The rain coincided with high tide in the bay. This restricted the rate of exit of floodwater into the harbour.

What were the human factors?

• The settlement of Boscastle has been allowed to develop on a narrow flood plain on the west coast of England, where rainfall is often high. Early settlers would have been attracted by site factors such as the presence of a fresh water supply and access to a natural harbour. There are now around 1,000 permanent residents, most of who rely upon the situation of the settlement to provide tourist revenues (it is close to the South West Coast Path).

• The rainfall hit at the worst time of year when the settlement population doubles to 2,000 as tourists arrive, many of who are following the South West Coast Path. Much higher levels of motor vehicle damage were also experienced, as a result of this influx. In addition, shops were carrying greater levels of stock than at other times of the year.

• Although new flood defences were set to be built in October, work had not yet started.

• Overall, excellent emergency services and Environment Agency response meant no lives lost. However, due to the transient nature of the tourist population, it took a long time to clearly establish that there had been no fatalities.

An Environment Agency commissioned report into the event to inform risk management for the future will be published in January 2005. Was the event rare? Should the river channel be adapted to cope with similar flows in the future? Is that practical? Should properties be re-build in the harbour area? For latest news see BBC website.

What were the consequences of the flood?

The consequences can be categorised into immediate and long-term impacts upon the local society, economy and built environment, in addition, there have been physical changes to the drainage basin.

(1) What were the immediate losses?

Heavy property damage 
Six properties were destroyed outright. Most others will require between £15,000 and £30,000 for repairs. Shared amenities such as the village green are now covered with silt and up-ended cars. These are serious costs for a small community with a seasonal employment problem, due to its over-reliance on summer tourism. Insurance companies will reimburse most people. However, some home and car owners will not receive compensation if (a) they lack insurance cover or (b) they find that they are not entitled to payment because insurers regard this unusual event as an ‘Act of God’.

Infrastructure disruption 
Both bridges in the village have been destroyed and sections of road have been swept away. Telephone, water, electricity and gas supplies were all immediately interrupted.

Irreplaceable loss of historical artefacts 
The ‘Witch Museum’ – which is 50 years old and receives 50,000 visitors a year – has seen some of its unique contents damaged (BBC R4, Today, 18 August 2004).

Physical trauma 
No-one died, but at least one resident suffered a heart attack.

(2) What is the long-term impact?

Economic losses 
Much of rural Cornwall is classified as a deprived region. It is one of the UK’s poorest rural counties, with EU Objective One status, meaning that incomes are below 75% of the EU average (Financial Times, 18 August 2004). A victim of early deindustrialisation, the region’s mining industries are now long-gone leaving it over-dependent on tourism. Luckily, a strong association with Arthurian legends and with the writer Thomas Hardy has helped foster high visitor-numbers for Boscastle (Hardy met his wife there and helped build St Juliot’s church). However, most shops will now stay shut for the rest of the season and the bad publicity is likely to reduce tourist numbers during future years, resulting in a negative multiplier effect for the entire local community.

Wider regional impact 
In addition, the effect may spread beyond Boscastle if other river-line settlements are perceived to be at risk by tourists. Boscastle businesses can claim compensation from their insurance companies (claims for ‘disruption to trading’ in Boscastle could amount to £15m). However, others businesses elsewhere in Cornwall cannot, even though they too may suffer reduced trade next year. This is a cause for concern, with tourism accounting for 30% of Cornwall’s GDP. The population doubles during July and August each year, with tourists spending up to £1 billion throughout the county.

House price falls 
People will find the value of their homes permanently reduced, now that Boscastle is associated with a serious flood risk. It has been suggested that values have halved (The Guardian, 19 August 2004).

Mental trauma 
Many residents will suffer stress and anxiety during the year ahead, with insurance loss adjusters advising that it may be six months before properties are sufficiently repaired for homeowners to permanently return (The Guardian, 21 August 2004). Environment Agency post-event surveys have repeatedly shown that this is one of the worst problems that flood victims face: they cannot physically return to their homes even when the floodwaters have receded. North Cornwall District Council banned Boscastle residents from returning to even look at their homes for the first 10 days, while structural engineers inspected properties. Health and Safety legislation also required that 76 up-ended cars, masses of uprooted trees and sewage-contaminated silts needed to be moved from the village streets before they could be re-opened to the public. Only now can actual interior repairs such as re-plastering begin, and this will take many more months. In some cases, the historic character of the houses in Boscastle is likely to cause extra problems. Many are Grade II Listed buildings, which means that repairs will take even longer, as restoration will require specialist attention.

(3) What are the physical changes?

Sediment accretion 
Much of the village’s open land, including gardens and the village green, has been covered with fresh beds of alluvial silt. As the floodwaters receded and the three rivers lost energy, widespread deposition of sediments resulted (most houses now contain a two-foot deep deposit of mud).

River channel changes 
The bed of the River Jordan is an amazing 10 feet higher than before the storm, as a result of sediment transport and deposition (The Independent, 18 August 2004). It has also changed its course. The Environment Agency has delivered sandbags to Boscastle and is currently helping to re-route it.

Is global warming to blame?

‘The Met Office said that heavy, 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.

The historical record shows that Boscastle has always been a hazardous environment and prone to steep rises in discharge after heavy rain. However, recent events are more severe than any other hydrograph event on record. Could global warming be a culprit? Increasingly, many meteorologists and hydrologists believe so. Flash floods have, of course, always been a feature of the UK’s climate, located as we are along a “battle zone” where hot tropical air routinely makes aggressive contact with colder polar air. Whenever these two air masses converge, storms can occur as the warmer, less dense tropical air rises above the colder air and rapidly condenses. However, global warming makes the potential for extreme storm damage greater. If tropical air masses are warming, in response to climate change, then their potential to hold moisture is increasing (water vapour content is determined by the amount of heat energy available). As they over-ride the cold air over the UK, the volume and intensity of frontal rainfall that is released may now be increasing.

What are the comparisons with Lynmouth?

The recent event is very similar to the case of the River Lyn and the village of Lynmouth in 1952 – a favourite GCSE and AS Level case study for many schools. Spookily enough, both floods occurred at the same time of year – between August 15-16! If you are using Lynmouth as a case study at your school, why not draw comparisons with Boscastle? You might look for both similarities and differences in the type of flood event, the level of hazard risk and the human and physical consequences of the flooding.
The river Lyn flows through a small, narrow drainage basin over impermeable rocks on the northern edge of Exmoor in North Devon, only 50 miles north-east of Boscastle. In 1952, 230mm of rain fell in just 14 hours. Soils were already saturated from previous rains, further increasing rates of overland flow. The river broke its banks to flow down the main street of the settlement of Lynmouth. 34 lives were lost, while 90 houses and 130 cars were damaged or destroyed.

However, unlike the more recent flood in Boscastle, a large portion of the blame for the Lynmouth floods could be placed with the construction of a road bridge over the Lyn with a very narrow arch. This became blocked with boulders and vegetation in the early stages of the storm. They had been produced by small landslides in tributary valleys and transferred down the valley in a series of ‘pulses’ (The Guardian, 20 August 2004). This was a major contributing factor to the river bursting its banks close to the bridge in such a devastating manner.

Actively drawing lessons from Lynmouth, geologists from the Royal Geological Survey have been flying over the valleys of Boscastle with radar devices, thermal cameras and other detectors in search of potential landslides (The Guardian, 20 August 2004). Should one occur now, and be followed by heavy rain, then Boscastle could face even greater flooding before the end of autumn, if any of the rivers become blocked!

Making comparisons:

• Levels of property damage in Lynmouth ’52 and Boscastle ’04 are very similar, notably the number of properties damaged.

• Loss of life is very different. No lives were lost in 2004, partly reflecting improvements in flood warning techniques, levels of public education and emergency service preparation for such extreme hazard events. Seven Navy and coastguard helicopters as well as 17 fire engines were quickly scrambled to help stranded residents escape from the roofs of their houses. The Environment Agency web site now provides 24-hour flood warning information, which may have alerted people of the impending danger.

• The same physical factor - extreme frontal rainfall - was the primary cause of the hazard in both cases. 185mm in five hours in Boscastle, compared with 230mm over 14 hours in Lynmouth. Both events also occurred in the same part of the world under near-identical conditions of geology and relief. However, the hazard impact was greater at Lynmouth, claiming 34 lives, due to one key additional factor: the blocking of the river Lyn’s narrow-arched road-bridge.

A turbulent summer for Britain?

This was not the only meteorological disaster to hit the UK during a turbulent summer:

• In early August, heavy rainfall in London overwhelmed the Victorian sewer system, leading to a massive discharge of 600,000 tonnes of untreated sewage into the Thames that killed 10,000 fish (The Guardian, 05 August 2004). A child died during the same storm, after being struck by lightning.

• 57 people had to be airlifted to safety in Scotland on 18 August by RAF Air King helicopters, after landslips blocked the A85 road in the Lochearnhead area. The mass movement was attributable to heavy rains that fell over a two-hour period. One truck was even swept off the road by the mudslide (The Guardian, 19 August 2004).

• The farming industry has been left in crisis, after widespread crop failures attributable to the wettest August on record in many parts of the country. In particular, oil seed rape yields have been drastically reduced, while half of the UK wheat crop has been lost. Typically, losses of between £50/acre and £100/acre are expected, which could bring the total bill to around £1 billion (BBC R4, PM programme, 26 August 2004. Writing in The Guardian (28 August 2004), one farmer compared conditions with the 1845 potato famine in Ireland – “high humidity alongside a lot of rain”. As a result, many farmers may now abandon cereal crops and grow grass instead, taking advantage of a new EU subsidy for grassland that commences next year.

• Overall, this has been the wettest summer since 1956. More than double the normal amount of rain fell in England and Wales during June, July and August. In many parts of southern and eastern England, the 274mm that has fallen is around one third of the expected annual total. Wetter years have been known in the past, notably in 1912. However, this is notably more rain than in recent wet years such as 1999 and 1980 (The Guardian, 30 August 2004). Given that last summer was the hottest on record and that this summer is the wettest for 50 years, perhaps the UK is indeed moving towards a more unpredictable and extreme climatic regime, as some meteorologist are currently suggesting.

In the wake of a hazardous August, the newspapers have been brimming over with statistics relating to disasters and their costs. Here are just a few of them:

• The average number of climate-related disasters in Europe has doubled in the last 10 years.

• 20,000 deaths may be attributable to the European heat-wave of 2003.

• 600,000 people were affected by the European floods of 2002, with economic losses topping £10 billion and causing over 100 deaths.

• The most dramatic flooding in UK history was in 1829, when the River Findhorn, in Scotland, rose in height by 15 metres.

• The heaviest rain recorded in 24 hours in the UK was 279.4mm in Martinstown, Dorset, in 1955.

• The worst flash flood in the US occurred in 1972, when 237 were killed along Rapid Creek in South Dakota. 140 campers also died in the Big Thompson canyon near Denver, Colorado.

• 1,442 people died in storms in the US between 1994 and 2000. Each year, damages amount to around $1 billion.

A Level exam tips

Make sure you focus upon the keywords when answering hydrological questions. Here are two questions that can both be answered using the Boscastle flood as a case study. Look carefully at how the wording varies between the two examples and at the way in which the answers are focused on the command words.

1. With reference to a named drainage basin, examine the factors that cause flooding

Student’s answer: The cause of a flood – when a river overflows its banks – is often a result of overland flow bringing water to a river more quickly than it can cope with, causing discharge to rise rapidly. In Boscastle (Cornwall), in 2004, the River Valency and Jordan burst their banks after 185mm of rain fell in just five hours. This was the main cause – such intense rain will always result in overland flow. Other factors also contributed. Soils were saturated from previous rainfall, while steep valley sides (water has no time to infiltrate the soil before running down-slope) and a lack of trees on agricultural land (so limited interception stores) also contributed to overland flow and flooding occurring.

Examiner’s comment: ‘This answer would score well, as it not only clearly focuses upon the factors but evaluates their relative importance. This is one aspect of the command word ‘examine’ that I might expect an A-grade student to be thinking about, without prompting.’

2. With reference to a named drainage basin, describe and explain the flows of the hydrological cycle

Student’s answer: The River Valency, in Cornwall, is a ‘flashy’ river. It is capable of very quick changes in its rate of flow and discharge. Overland flow can often occur in this basin, where soils are thin, old red sandstone rocks are not very permeable, valley sides are steep and interception stores are small because much of the land is agricultural and not wooded. Large inputs of rainfall come from Atlantic weather fronts, such as the 185mm of rain that fell in five hours on 16 August 2004. All of this results in rapid inputs and flows of water through the basin hydrological cycle.

Examiner’s comment: ‘Although this student is using a case study that relates to flooding, he/she has been careful to discuss only the parts of the story that are relevant to the question set – namely, the workings of the water cycle. Details of how many cars were destroyed or houses were flooded at Boscastle are not required here. In fact, there is no reference to floods here at all, nor does there need to be.’

For extended essay writing on hazards, make sure you can distinguish between short-term and long-term impacts, as the article suggests. The comparisons with Lynmouth may also be useful, especially for an essay such as:

Why do the impacts of hazards vary from place-to-place and from time-to-time?

By comparing Lynmouth and Boscastle we can make comparisons based upon the different local conditions (such as the importance of the design of the bridge over the River Lyn). We can also make a longitudinal (over time) survey, emphasising how much better the UK has got at dealing with emergencies since the 1950s, with the advent of new technologies, and with the work of the Environment Agency.

11-16 Questions

(1) Put yourself in the position of a homeowner in Boscastle who cannot return home yet and also worked for a living in one of the businesses that has now been destroyed. Using the information provided in the ‘Geography in the News’ report, write a short account outlining the challenges that you would face over the next year.

• Who might help you and give assistance?

• How long might it take for help to come?

• What particular problems will winter and the December holiday season bring?

(2) Hazards and resources are both defined as ‘relationships’ between people and the environment. They are different sides of the same coin. Hazards can be seen as a bad, or negative, relationship, whereas resources are a good, or positive, relationship. Many things found in nature are a bit of both, like rivers.

Draw a table that shows how the following things can function, at different times, as both a hazard and as a resource.

• Rivers

• Storm winds

• Arid weather (hot and very dry)

• Volcanic activity

• Oil

Try to think of some other additions for your table.

This report was written by Dr Simon Oakes. Simon teaches at Mander Portman Woodward, London and also works for the Flood Hazard Research Centre, Middlesex University. He is a senior examiner for Edexcel.