Case Studies, Boscastle and Barèges
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Case Studies, Boscastle and Barèges. Floods Case Study Boscastle Basic Facts and Figures Flood event began at 5.00pm on Monday 16th August 2004 185mm rain fell over a period of 8 hours over the 20 sq km (7.7sq miles) catchment area. Remnants of Hurricane Alex. Rain fell on ground already saturated reducing infiltration The Rivers Valency and Jordon meet in Boscastle and these rose overbank. Peak discharge of the river Valency 180 cumecs (6,300 cubic feet per second). Rate of flow 16kph. (10mph) Large trees, cars and buildings form part of the debris. 100 homes and businesses destroyed and 118 cars (some never found) , 5 caravans and several boats washed into the sea. Damage estimated at £50 million. Physical Causes Boscastle underlying geology that of impermeable shale and slate in some places leading to high runoff and little penetration. River valleys have flashy catchment because they are short and steep so water is accelerated down the valley. Event coincided with high tide that made matters worse Meteorological causes. In the two weeks prior to the flood there had been heavy antecedent rain 25% higher than normal. A slow moving area of low pressure further north was pulling in warm moisture laden air from over the Atlantic. The morning had been very hot with clear skies evaporating abundant unabsorbed surface water. A trough of low pressure developed over the area with extreme instability in the atmosphere. Line of thunderclouds over 10km high rose rapidly. How did flood defences cope? There weren’t any as the River Valcency and Jordan had never flooded before. Immediate effects Roads blocked by flood water making emergency access difficult except from the air. People trapped in buildings by the floodwater. Forced to seek refuge on the roofs Narrow valley meant only two helicopters at a time could operate. Property washed away. Property destroyed by debris. People left homeless and had to be accommodated. Day trippers had lost their cars and were stranded. Sewage main burst and made Boscastle inaccessible for health and safety reasons. Short Term Impacts. Floodwater damaged large number of properties. Possessions lost . Damage to ground floors of properties by sewerage. Repairs had to be made. Time consuming and some properties beyond repair. Big pay outs by insurance companies resulting in higher premiums for Boscastle residents. Damage to tourist industry with loss of attractions like witchcraft museum. Some tourists afraid to visit in case of a repeat. Environmental damage to wild life habitats and coastal pollution. Long Term Impacts New shops built with insurance money New visitor centre built. Visitor numbers actually increased with influx of people to view the damage. Flood defence responses New large culvert on the river Jordon to carry double the amount of water. Valency river widened and deepened. New higher bridges constructed. New car park built which is raised and set back from the river. New tree management scheme introduced to reduce chances of tree blockage. New sewage pumps and stations and new sewer pipe introduced. Case Study Barèges Village in the High Pyrenees 30km south of Lourdes. Basic Facts and Figures Barèges is a village situated in the high Pyrenees 30km (18 miles) south of Lourdes at a height of 1,250 metres (4,500 feet) Began on Tuesday 18th June worst floods since 1897. 110 to 180 rain fell over the 107 sq. km catchment. Followed cold spring which left copious amounts of snow still on the ground. Series of avalanches from February onwards blocked the river channel with snow rocks and tree debris the last being 31st May. River broke existing flood defences. Population and tourists evacuated via the Col du Tourmalet. 10 properties destroyed including the campsite and a hotel. Access road to the valley cut in 10 places. Estimated cost of repairing the roads 30 million euros. Physical Causes Underlying geology that of granite and schists, impermeable with high run off but fractured. Large amount of soft sediment on the south facing slope. Bareges occupies a narrow steep sided valley with short streams which soon reach bankful level at times of high rainfall. After avalanches rock and tree debris blocked the channel of the Bastan reducing capacity. Last avalanche occurred on May 31st. Snow bridges formed due to avalanches which reduced the channel capacity of the Bastan. Meteorological causes Exceptionally high snowfall in the winter metres on the Col du Tourmalet. Exceptionally cold spring. With low snow melt until late May early June. Three days of rain in mid June with high run off into river channels with reduced capacity. On Monday 17th June southerly wind known as La Balaguère produced high temperatures which accelerated the snow melt. (Fohn effect) Human Causes Some buildings restricting the river flow in certain localised places but this problem had been addressed in recent years. Extra bridges put in place to access some developments restricted river flow and trapped debris. How did flood defences and plans cope? The flood defences put in place in the 19th century largely held until day three when they were overwhelmed. Balancing tanks to take run off from car parks overwhelmed, Extensive planting of broad leafed species such as beech on the north side and south side ineffective because the cold weather arrested the development of leaves in spring and the trees were bare. Regulations for the non occupation of ground floors of buildings close to the river prevented building collapse. Large number of reservoirs controlling the headwaters of tributary streams on the south side could not cope and were purged adding to the amount of water in the River Bastan. People had already been moved from properties close to the river and lodged with families on higher ground. When essential services went down the population was evacuated. Immediate effects No water supply, electricity or sewage. Access road to the valley cut in 10 places. Entire village including residents, tourists and patients at the spa evacuated apart from 24 residents. Evacuation had to be carried out over the mountain via the Col du Tourmalet, which had to be cleared of snow. 400 people had to wait at the mountain restaurant at Superbareges at the foot of the Col until the convoy could move. Some homes and businesses washed away. All bridges except one washed away. One spa establishment severely damaged. River changed course and the campsite was overwhelmed. River channel deepened at the top of the village by 4m (13ft) in 6 hours. Short term impacts Floodwater damaged roads and infrastructure. Rocks and boulders strewn about the village. People used 4x4’s or feet and bicycles to move about. Water and bread delivered by helicopter to remaining residents and army. Army moved in requisitioning accommodation. Boulders and other debris had to be removed from the river channel sometimes by explosives. River had to be restored to original bed. Repairs had to be made to roads and bridges. Unsafe buildings had to be assessed and demolished if necessary. Income lost because event happened right at the beginning of the tourist season. Longer term impacts Loss of the campsite led to campervans and caravans being accommodated in the ski resort carpark. Access to the village via the Col du Tourmalet only. As soon as essential services restored some accommodation opened and welcomed hikers and cyclists and people taking the cure. Walking groups walked the footpaths and assessed damage. Loss of the abattoir meant that sheep could not be slaughtered in the area and the local lamb could not be sold under its protected status label. 60% of farmland lost in the surrounding area. 12 months on Debris removed. Bailey bridges installed. New style bridges installed. New road up from the valley installed. Nicknamed “the motorway” as it is much wider and has a new surface. Some areas beside the river left blank to allow the river to spread. Some elderly people subsequently died because of shock..