The small coastal town of Lynmouth became known throughout the world for the disaster, which struck in August 1952. On the night of the 15th, after continuous rain throughout the day, the rivers of the East and West Lyn rose suddenly and filled with the waters from their Exmoor catchments. Large boulders and rocks were carried in the flow towards the village, destroying houses, roads and bridges. Many lost their lives during that dark and terrifying night. The whole of Exmoor was affected and considerable damage was caused on the Barle, Exe, Heddon and Bray but the worst effects were at Lynmouth.
This is because the water draining from most of the northern side of Exmoor ends up in the East and West Lyn Rivers, which join at Lynmouth. Hundreds of thousands of years ago these rivers used to run to the sea much further to the west but during the Ice Age the side of their valley was eroded by the sea and, as a result, they fell to the sea along a much shorter and steeper course. As a result the waters descending on Lynmouth are particularly fast and erosive. The flood was one of the most spectacular and most studied in Britain.
Interest was shown in the small scale as well as the larger effects on the landscape. Green studied the effects on river courses, erosion and deposition and Gifford and Kidson studied land slipping and its causes in the upper reaches of the Exe. Whilst it is still possible to see landforms created by the flood and to calculate its flow from remaining flood channels, most of the evidence of the flood has now disappeared. At first it seemed that the flood confirmed the theory that most of the shaping of our landscape occurred during such violent events, which were perhaps hundreds of years apart.
However, work by Anderson and Calver on how the great scars and piles of boulders left by the flood have largely been removed by commonplace fluvial activity has changed our view of the shaping of landscape. Few now remember the disaster but its study has had far reaching effects upon our understanding of erosion and the way we deal with floods. Moorland Vegetation and Soils The catchment area of the Lyn rivers totals 39. 2 sq. miles, much of which is plateau drained by steep sided combes.
The plateau is covered in parts by moorland grasses growing from a wet, peaty ground and in others by heather and bracken on well-drained soils. During the 19th century the Knight family had tried to drain the northern plateau by ineffectually digging gutters to carry the water to the lowlands. Not until Frederic Knight used the steam plough to penetrate the hard pan, did thousands of acres of Exmoor become good pastureland. Yet he did not successfully drain the Chains, which, according to Burton, “remain to this day the wettest and wildest region of the moor”.
Despite the wetness of the Chains, the capacity of its peat to hold water has been reduced over the last century and a half by heavy grazing and burning. In addition there has been much reclamation of surrounding moor and heath in the 19th and early 20th centuries. Since 1947 there have been government grants for agricultural drainage and there is evidence that runoff is more rapid now than before that time. This rapid runoff has been blamed for the apparent increase in flash flooding.
Before examining the weather over Exmoor during 15th August 1952, it is relevant to assess the meteorological situation prior to the event. Drought had affected most of southern England during the second half of July of that year. Conditions broke down at the beginning of August to be followed by a period of changeable weather over the whole country. Maximum temperatures of 80 F were reported on only three days. Thunderstorms occurred daily in the country with the centre of activity changing from place to place. This led to an irregular distribution of the monthly rainfall.
During the evening of the 6th there were severe thunderstorms over London and the Home Counties. A record daily fall of 4. 83 inches was recorded at Boreham Wood, and rainfalls of “very rare” intensity were recorded in parts of North London. Further heavy falls and flooding were reported in Belfast, Cumberland and parts of Argyllshire on the 9th and 10th of August. Rain continued to fall in North Devon and West Somerset on all but two of the fourteen days prior; although there were no outstanding heavy falls, these were not small amounts (R. S. R, 1952).
A depression had formed at about 12. 00 G. M. T in the mid-Atlantic three days prior to the 15th, at about 47 N. , 34 W at a central pressure of 1016 mb and then subsequently moved east-southeast to 43 N. , 19 W at 00. 00 hours of the 14th at 1007 mb. Afterwards, it rounded an upper trough and then moved slowly northeast, parallel to the general thermal gradient and the 500 mb contours (Bleasdale and Douglas, 1952, 360). Although this depression had no frontal structure, warm air from France was drawn into the circulation as it approached Brittany.
Large moisture contents in the air around southern England combined with the warm thundery air from France indicated a high possibility of thunderstorms breaking out anywhere near the track of the depression, which in fact they did in most parts of south and south-east England. Continuous rain began at the Scilly Isles and at Culdrose in the early hours of the morning and spread to all parts of Devon, Cornwall and Somerset by midday. The nearest synoptic reporting station to Lynmouth, at Chivenor, reported incessant rain for almost 18 hours.
The largest falls of rain were located on the left-hand side of the low track, as expected. Cold, moist and unstable air ascending up the northward facing slopes of Exmoor introduced more moisture into the already heavy raining area. This may well have been decisive in producing the excessive rainfall in the Lyn catchments (Marshall 1952). In a rain gauge on Longstone Barrow on the ridge running from west to east at approximately five miles south of the coast, a voluntary observer, Mr C. H. Archer of Wootton Courtenay, measured 9. 00 inches of rainfall for the 24-hour period beginning at 9. G. M. T on the 15th.
Two other measurements were made with standard gauges; these were 7. 58 inches at Challacombe, and 7. 35 inches Honeymead near Simonsbath. Both of these were on lower ground within a few miles of Longstone Barrow. Unfortunately, there were no rain gauges in the heart of Exmoor: only those of Wootton Courtenay in the east and Chivenor at some 14 miles west south west of Longstone Barrow. These were carefully analysed using intervals of 6 minutes along the time scale, and the results are shown below (Bleasdale and Douglas, 1952).
Despite the distance between the two stations there is agreement in the timing of the most intense peaks of the day, where the difference is only a mater of a few minutes. A composite picture of the storm incorporating the information from both Wootton Courtenay and Chivenor would divulge that the rain started some time during the morning at approximately 11. 00 to 12. 00 G. M. T. Heavy periods occurred during the afternoon with brighter intervals. The first exceptional downpour occurred after a darkening of the sky and peculiar colour effects between 15. 30 and 17. 30 G. M. T and thereafter easing.
Although reports vary, torrential rain occurred during 18. 30 and 22. 30 G. M. T, easing off to rain of little importance in most places at approximately 02. 00 G. M. T on the 16th. There were some reports of rain continuing throughout the night and into the next morning (Bleasdale and Douglas, 1952). At the time, the rainfall over Exmoor produced one of the three heaviest falls in 24 hours ever recorded in the British Isles from records dating back to 1862. These were at Bruton on the 28th of June 1917 of 9. 56 inches, and Cannington on the 18th August 1924 of 9. 40 inches.
As Bleasdale and Douglas point out, it is rather strange that these cases both occurred in Somerset, and now third highest to be added to the list is in Devon some half a mile from the Somerset border. The Passage of the Waters As a great deal of rain had fallen on the catchment during the previous two weeks and the evaporation during the day of the flood was negligible, the permeability of the surface of the area was minimal. The storage capacity of the vegetation, peat or soil and rocks was taken up by the rain which fell prior to the 15th, leaving no opportunity for infiltration to occur.
On the smooth, convex hills there were few storage dips and ponds, and this capacity would have quickly filled after the first rains (Dobbie and Wolf, 1953). When the soil profile becomes completely suffused, saturation excess develops into overland flow or surface runoff (Burt, 1986). On the high moors this would have been consistent with the movement of a sheet of water only hindered by the closely grown thick-stemmed plants and the artificial channels cut during the nineteenth century.
These being inadequate during normal times could carry only a small portion of this exceptional rainfall. While the average velocity of flow may have only been a few inches per second over the summit, by the time it had travelled hundreds and, in some cases, thousands of feet before reaching a stream or the edge of the moor it would now be travelling at 10 to 20 feet per second and at a depth of several inches. This may well have taken as long as two hours. As the water passed over the valley edge and down the sides the velocity increased to several feet per second.
Reaching the streams, this then increased again to 5 to 10 feet per second. At the height of the flood the time of flow from the headwaters of the West Lyn down to the sea was less than one hour (Dobbie and Wolf, 1953). As the valleys were narrow and deeply incised there was little room for flood storage, the result being that the waters were confined to the narrow river channel (Harris, 1992). The Devonian shales and sandstones break up easily into boulders of all sizes, including smaller fragments and sand.
Transportation is minimal during normal flow conditions, yet during flood conditions much material can be moved and the valley sides are loosened up allowing this material to be carried away by the following floods (Dobbie and Wolf, 1953). Additionally, long, low intensity rainstorms can wet slopes to the point of failure but cannot produce flood peaks to damage the actual channel. Storms of high intensity that produce rapid slope runoff for a short period of time, are capable of devastating small order channels and transporting large quantities of material to the lowland reaches.
The Lynmouth flood was remarkable for being both a slope flood and channel flood, in all terms a “rarity” (Newson, 1989). River Maintenance During the 1930’s economic recession, there was little maintenance carried out to the valley sides as these were held in private ownership. As a result large trees of sycamore, birch and ash were growing in the riverbed and among shoals and shattered rock at the side (Harris, 1992). Since the flood this situation has changed as various government agencies have taken responsibility for flood prevention and river maintenance.
As the waters cascaded into the narrow steep sided valleys of the East and West Lyn, the rushing waters became torrents bearing uprooted trees, boulders and other debris which acted as battering rams dislodging other material and blocking culverts and bridges in a remorseless flood towards the sea (Binding, 1997; Dobbie and Wolf, 1953). According to Einstein, a velocity of 15 feet per second will move a rock measuring 3 feet cubed, although super-critical velocities of 20-30 feet per second are common and may account for a boulder weighing 7. 5 tons that was found in the basement of a Lynmouth hotel (Dobbie and Wolf, 1953).
One reason why the flood damage was so high stands out very clearly when examining this 1830 print. This shows the East Lyn confined between the abutments of a bridge, yet wide and unconfined elsewhere. Photograph 6 shows how further development had encroached upon the river course, narrowing the channel. Note the buttresses on the left wall. After the Rains had Stopped Once the flood had subsided the full extent of damage both erosional and depositional could be assessed. Trees stripped of their bark were intermingled with enormous piles of boulders and occasionally interspersed was the wreckage of human habitation and property.
River banks had been ripped out exposing dangling roots; walls and hedges were left with gaps torn through; potholes had been gauged out of the ground leaving bewildered trout swimming in them (Green, 1955). The river course had been altered, shortening the length by cutting through meanders. In places floodwater was forced out of the constricted riverbed and cut back from the banks were it re-entered the channel until the blocked channel was completely bypassed. Following the constrictions where the valley opened out and the gradient slackened, boulder deposits were found.
Yet on most of the rivers the gradient was so high and the flood plains so narrow that minimal storage could be achieved. The size of the individual boulders shifted depended upon the availability of boulder material and the water velocity to shift them. The largest moved was of some 350 cu. ft. found in the West Lyn, while the East Lyn produced smaller and more rounded material. Peat was eroded from the headwaters of the valley as the waters undercut the already saturated ground, causing land slips of peat up to 40 ft by 18 ft by 5 ft thick to be carried floating or rolling several hundreds of yards downstream.
Gouging of the ground downstream from breaches in stonewalls resulted in pits of between 2 and 8 feet deep. Similar gouging caused the undermining of bridges on their upstream side, as waters were forced down and underneath the arches removing the drift material supporting the bridge piers. Potholes caused by trenching measured to depths of 12 ft as surface water moved downwards in sheets, entrenching itself in ruts and then rapidly downcutting. Slipping occurred in the drift deposits overlying the Devonian rock, as water ran over the impermeable layer, sweeping away up to two foot of the saturated drift and further inducing slumping.
The two small reservoirs at Woolhanger and North Furzehill, both on West Lyn tributaries, burst. Suggestions that these may have been an important contribution to the flood disaster have been discredited as all the debris from the burst was deposited some 200 yards downstream with little damage to the valley below (Green, 1955). During the period that followed the flood disaster it became noticeable that rainfall of unusual intensity caused a very rapid rise in the river levels.
This immediate increase in run-off level prior to the flood was originally attributed to the effects of exhaustion of water retention within the soils. Subsequently it became evident that this situation was more permanent in that there had been an improvement in the discharge pattern of the whole catchments basin with scouring of new tributaries and widening of the channel (Dobbie and Wolf, 1953). The flood’s devastating effects upon buildings and bridges occurred in two ways. Firstly, the sheer weight of the floodwaters and its load of trees and boulders pounding against the artificial structures accomplished direct battery.
The main street of Lynmouth was damaged in this way, as was the bridge at Glebe House, Malmsmead, where both its piers were based on the solid Devonian rock and could stand the weight of debris and water behind. There the bridge broke at its weakest point, the junction of the span and the sides. The second way of property destruction was the undermining of structures that were built on easily erodable drift deposits. Most of the ruination of houses and bridges was due to this undermining action (Green, 1955).