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 wet, peaty ground and in others by heather and bracken on well drained soils. These soils do not extend much further than 4 feet deep (Wolf 1953). S.H Burton, author of 'Exmoor', wrote in the journal 'Weather' in 1952:

   "The Chains is the name given to the north-west plateau of Exmoor. This plateau lies above the 1,500 foot contour, its higher point such as Chapman Barrows, Wood Barrow and Chains Barrow being nearly up to the 1,600 foot line. It is the watershed between the rivers flowing south and west - the Barle and the Exe and their tributaries, and the rivers flowing to the north - Farley Water and Hoaroak Water, the main tributaries of the East Lyn, and the West Lyn with its numerous feeders.

   The soil of Exmoor is of two kinds. The dry land soil is loamy on top and clayed below and drains rapidly. It is to be found mainly in the valleys of Exmoor and, though it is deficient in lime, heavy dressing makes it fertile. The wet land soil is peaty. Just below the surface a thin ironstone pan occurs which prevents drainage until the pan is cracked by subsoil ploughing" (Burton, 1952. 334).

During the 19th century the Knight family had tried to drain the northern plateau by ineffectually digging gutters to carry the water to the rivers. Not until Frederic Knight used the steam plough to penetrate the hard pan did thousands of acres of Exmoor become good pasture land. Yet he did not successfully drain the Chains, which, according to Burton, "remain to this day the wettest and wildest region of the moor". S.H Burton continues with:

   "To reach the Chains from Lynmouth involves a climb of fifteen hundred feet in four miles, and that is not the worse part of the journey...Even in a good summer the route is wet, and for yards at a time the walker must stride or jump from tussock to tussock to avoid the bogs where black water oozes sluggishly over sphagnum moss and under the white heads of cotton grass" (Burton, 1952. 335).

Despite the wetness of the Chains, the amount of peat and its capacity to hold water has been reduced over the last century and a half by peat cutting, 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.

The Weather

Before examining the weather over Exmoor during 15th August 1952, it is relevant to assess the situation prior to the event. Drought had effected 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-south-east 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 north-east, 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 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, ( Marshall 1952) but, as Marshall explains:

   "This was not merely a case of orthodox rainfall distribution around a frontal depression on high ground. It was complicated by thunderstorms all over the area of heavy rain, and one set of storms seems to have come up from Brittany in the south south-east wind which was observed at 700 mb over Brest at 300 GMT on the 15th"( Marshall 1952 , 341).

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 9.00 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.

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 matter 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.

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 river bed 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 that 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).

The Landscape

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 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 head waters 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 sandstones break up 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).

It is rare that both the East and West Lyn peak at the same point, for the West Lyn rises first if the rains come in from the south-west. The East Lyn rises first with a easterly or south-easterly wind direction (Keene and Elsom, 1990). On the night of the 15th both the rivers peaked together, causing competition for the limited space at the confluence of the two rivers.