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English Channel

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ENGLISH CHANNEL (commonly called "The Channel"; Fr. La Manche, "the sleeve"), the narrow sea separating England from France and lying entirely on the broad continental shelf which extends from Europe to Greenland. Its width is about I0o m. from Ushant to the Scilly isles and 22 m. at the Straits of Dover, and its length is about 35o m. The geological affinities are well marked on the two sides, as between the Devonian and granitic rocks of Cornwall and Brittany, the Jurassic of Portland and Calvados, and the Cretaceous of the Pays du Caux and the Isle of Wight and the Sussex coast as well as either shore of the Straits of Dover. The sea broke through the narrow neck at Dover in late Pleistocene times and the channel now covers what was once a wide valley. Coastal changes are still going on; Shakespeare cliff at Dover is said to have been cut back a mile during the Christian era, while Dungeness, Chesil Bank and the beach near Treguier are still growing. The area draining to the channel is about 8,000 sq.m. on the English side and 41,00o sq.m. on the French.

The bottom slopes fairly regularly from 20 to 3o fathoms in the Straits of Dover to 6o fathoms in the western entrance. A remarkable narrow depression, Hurd's Deep, runs for 7o m. parallel to the general direction of the channel to the north and north west of the Channel islands and approaches one of them, the Casquets, within 5 m.; 94 fathoms is found in it north of the latter point. A hole with Io5 fathoms occurs a few miles north west of Ushant. The Ioo fathom line, which here is also the edge of the continental shelf, approaches to within 120 M. south south-west of the Scilly isles ; it is generally marked by ripples and a change in the colour of the water. The bottom deposits are terrigenous:—white and yellow sand, often with black specks, fine broken shell, gravel and, less frequently, patches of larger stones or of mud. The channel is an important ground for both trawlers and drift net fishermen.

Salinity and Temperature.

Atlantic water with an aver age salinity of 35.4 per thousand enters the English channel through the southern part of the western entrance and flows east wards almost continuously at an average speed of 1.5 m. per day. On its way it is diluted with river water so that the salinity falls to less than 35.o along the coast eastward of Start point and to or less near the Isle of Wight and in the Baie de la Seine. The salinity in the centre of the Straits of Dover is about 35.1 and a tongue of water with a salinity not less than 35.o extends some distance into the narrows between England and the coasts of Belgium and Holland. Less water escapes eastwards or is removed by evaporation than enters from the Atlantic and from rivers, and the excess turns westwards and flows northwards be tween Lands End and the Scilly isles. Between the Bristol chan nel and Ireland it sets up a great anti-clockwise (cyclonic) cir culation which brings diluted Atlantic water southwards across the entrance to the channel, sometimes as far south as the Bay of Biscay; part of this is carried eastwards again. The variations in the channel are due in part to variations in the inflow from the west and in part to seasonal and meteorological changes.

The salinity of the Atlantic water in the Bay of Biscay varies with the seasons and from year to year. The maximum salinity generally reaches the British Isles in late summer or early autumn, and the maximum inflow into the channel from this source takes place between August and the end of February. The inflow from the Irish sea occurs generally in spring and summer. The salinity in the bay was high in 1902–o3 and in 1921 ; in 1922-23 it was low. There is no evidence that the flow up channel varies with the salinity outside, nor that high salinities in the channel are proof of high salinities in the bay. The variations in the channel are considerable, and since 1921 in particular high figures have been recorded in the sduthern part of the North sea and in the eastern part of the channel, due in all probability to water which has entered along the bottom. There is evidence for a two-year period in the salinity, and possibly for a period of 181 years.

The mean annual surface temperature increases from I I-11.5° C at the Straits of Dover to about 12° C at the western entrance. The annual range falls from I o–I I ° C in the eastern part to 5° C in the west. The mean maximum surface temperature, 16° C, occurs along the English coast at the beginning of September and a little later on the French side. In the western area the maxi mum is about 15°-16° and occurs in the first third of September. The mean minimum surface temperature is 5°-6° C in the Straits of Dover and 9° off the coast of Brittany. In the eastern part of the channel the water is well mixed by tidal action over the shallows, so that the salinity and temperature are generally uni form from surface to bottom at all times. In the western area, on the other hand, mixing is far less complete, especially in the summer, when the surface layer is so heated during the day that nightly radiation cannot cool the water enough to set up vertical convection currents extending to much deeper than 10-20 metres. The water becomes divided into two layers, separated by a very thin one in which the temperature falls rapidly and the salinity often increases ; this layer is known as the thermocline or dis continuity layer. Owing to this layering the average temperature in August in the western entrance is 5° C less on the bottom than at the surface. The upper layer generally vanishes in the autumn by mixing with the deeper water; if it has been unusually shal low, as generally is the case in a hot calm summer, the mean temperature in the autumn of the vertical column from surface to bottom will be lower than usual. The larger part of the seasonal variation of the temperature is due to local meteorological causes, but a sudden inflow of warm water from the west may cause a rise of temperature at a time when it is usually falling.

The Tides

of the English channel, that is, the rise and fall of the water, are due to an undulation from the Atlantic which reaches Ushant on the days of new and full moon at about 3 h. 4o m. and the Scilly isles at about 4 h. 20 m. As it passes up the channel it is retarded along the shores so that the crest presumably becomes convex eastwards. The range from trough to crest is always greater on the French than on the English side, 19 to 23 ft. between Ushant and Ile de Bas but only 15 to 16 ft. between Lands End and Start point.

The time of high water is progressively later as far eastward as Portland, but from this place to Selsea Bill and near Havre on the French coast there are great irregularities, which take the form of double tides or of prolonged stands at one height. They are due to the increase in range and displacement in time, rela tively to the semidiurnal tidal constituents, of the quarter-diurnal constituents, presumably by the bottom configuration (see TIDES).

Between the Isle of Wight and the mainland double tides do not occur east of Cowes ; at Cowes there is occasionally a double high water but generally only a prolonged stand at high water. West of Cowes and in Southampton Water, as far as Hurst, double high waters occur with an interval between them of about 2 hours. This interval increases progressively westward and is about 21 hours at Christchurch, 3-1 hours at Poole and 4 hours at Swanage. West of Swanage the double tide coincides more nearly with low water and at Weymouth and Portland double low waters occur, with an interval of about 3 hours between them, locally known as the Gulder. Double tides do not occur on the French coast but there is a prolonged stand at high water at Havre. In the basin eastward of the narrows high tide occurs at nearly the same time everywhere and at Dover it is at 11 h. 24 m. at new and full moon.

Tidal Streams.

When the free tidal wave of the Atlantic ocean strikes the edge of the continental shelf in about Too fathoms it is converted into a wave of translation which gives rise to the tidal streams. In the English channel the flow is on the whole parallel to the axis but there is a set into bays, in which the stream turns earlier than in the offing, sometimes as much as two hours earlier. The streams are described most easily by imagining the whole area to be divided into definite portions.

The Channel stream lies between the lines joining Start point to the Casquets in the Channel islands and Beachy Head to Pointe d'Ailly near Dieppe, the Intermediate stream occupies the Straits of Dover between the last line and another drawn from the North Foreland to Dunquerque, and the Southern North Sea stream lies between this and a line from the Leman and Ower lightship off the coast of Norfolk to the Hook of Holland. The reference time is the time of high water at Dover. The Channel stream and the North Sea stream set towards one another and the straits when the water is rising at Dover and in the opposite direction as it falls, meeting and separating on a moving line within the limits of the Intermediate stream. When the water begins to fall at Dover the line is off Beachy Head and during the fall it moves eastward and reaches the eastern limit of the Intermediate stream on the North Foreland-Dunquerque line at low water. By this time the two main streams have ceased to run but the Intermedi ate stream is still setting westwards. When the water begins to rise at Dover both main streams set towards the straits ; the North Sea stream runs with the Intermediate and the line of junction is again off Beachy Head and moves eastwards to reach the North Foreland at high water, by which time the main streams have slacked and the Intermediate stream is running eastward alone. During the next hour the main streams begin to ebb, and Intermediate stream runs with the North Sea stream and the line of separation is again off Beachy Head.

The maximum speed in the Channel stream, apart from races off headlands, is about 31 knots in the narrows south of the Isle of Wight. In the Straits of Dover it is not as a rule over 3 knots but rises to 5 knots at places near the Goodwin Sands. The flow into the North sea is greater than that in the opposite direction by an amount which on the average would carry a drifting body 24 m. in a lunar day, but this flow varies greatly with the wind and is sometimes reversed. At the other entrance to the channel, west ward of a line drawn from the Scilly isles to Ushant, the stream continually changes its direction through north and east and makes a complete circuit of the compass in a tidal period, setting north and east when the water is falling at Dover and south and west when it is rising there. The maximum rate is 1 i knots and there is a slight predominating flow into the channel. In the area between this line and another drawn from Start point to the Casquets in the Channel islands there is a mixed stream partaking of the character of the streams on each side of it and the maxi mum rate in the open is 21 knots. There is a strong set into the Gulf of St. Malo as the water falls at Dover and in the race of Alderney between the island and the French coast the speed reaches 7 to 8 knots. As the tide rises at Dover there is a strong set round Cape Barfleur into the Baie de la Seine.

The prevailing winds are westerly and greatly preponderate over easterly winds. Gales are generally due to the passage of a de pression eastwards and are commonest from October to March inclusive. Calms are rare. Fogs are frequent at all places and are commonest when anticyclonic conditions prevail, especially in the winter. The winds 'lave considerable influence on the tidal streams, especially in the Straits of Dover.

An account of the tides of the English channel according to the dynamical theory is given in the article TIDES.

The Channel Tunnel Scheme.

Unbroken railway communi cation between England and France would avoid the delay and expense incurred in the transhipment of goods and many schemes have been proposed to this end. Train ferries were proposed during the first half of the 19th century and were actually used for a time during the World War. On account of a small difference between the sizes of the English and French stock, English rolling stock could be used on the French lines but it would not be possible to use French stock on English lines. A tunnel through the chalk was proposed at the beginning of the 19th century but the scheme was first put into practical form by Thome, who exhibited a model of his tunnel at the Great Exhibition in 1867. Later he joined forces with William Lowe, an English engineer, who also had been preparing plans. Their scheme was adopted by an international committee which asked govern ment aid in driving an experimental drift. A commission appointed by the French ministry of Public Works reported favourably on the plan but the outbreak of the Franco-Prussian war put an end to further progress. After the conclusion of peace negotia tions were resumed between the British and French Governments and an agreement was reached on such points as jurisdiction, the right of blocking the tunnel, etc. In 1875 the Channel Tunnel company obtained an act authorizing it to undertake preliminary work near St. Margarets bay and the French Submarine rail way obtained permission to begin work at Sangatte. The English company failed to raise the necessary money and was bought up in 1886 by the Submarine Continental Railway company which had been f ormed in 1881. In 1881 the South Eastern Railway company obtained an act giving powers for experimental borings and carried out some preliminary works which were taken over in 1882 by the Submarine Continental Railway company in return for cash and shares. In 1883 a joint committee of both Houses of Parliament decided that the building of the tunnel was inex pedient and brought the scheme to an end for a time. In 1886 the Submarine company absorbed the Channel Tunnel company, and in 1887 it changed its name to "The Channel Tunnel Com pany, Limited" with the sanction of the Board of Trade. It promoted an unsuccessful bill in 1906. After the World War the question was again raised, but in 1924 the Committee of Imperial Defence reported against the scheme. Early in 1929 however the matter was again brought up by the Tunnel Parliamentary Com mittee, and both Houses were circularized for an opinion. Con siderable interest was aroused and suggestions were made for a bridge across the straits of Dover similar to those of MM. Schneider and Hersent in 1889, but this is not regarded favour ably by experts.

The present scheme provides for a trial gallery, which would be enlarged into two tunnels, each carrying a single railway track. Geologically the conditions are good, since the boring would be made through Chalk Marl which is impervious to water, and between Dover and Calais the conformation is uni form. The total length of the tunnel would be 34 m., 24 m. of which would be under the sea, its greatest depth being 95 ft. below the flannel bed (26o ft. below sea-level) .

BIBLIOGRAPHY.-For

conformation and tides: Channel Pilot; Tides Bibliography.-For conformation and tides: Channel Pilot; Tides and Tidal Streams of the British Islands; Chart 1,598 and others on larger scales; Atlas of Tidal Streams of the British Isles; all published by the Admiralty. For temperature and salinity: H. W. Harvey, Rap. et procis-verbaux, vol. vi., xxxvii. ; Bull. Suppl. (1908) ; all published by the Cons. Perm. Internat. pour l'Exploration de la Mer (Copen hagen) . For Tunnel: Blue Books "Correspondence . . .," Commercial No. 6 (1875) and C. 3,358 (1882) ; Rep. Joint. Sel. Comm. . . . Houses of Lords and Commons . . . (1883) ; Brit. Assoc. Reports (1882), P. 404, Hawkshaw, and (1899) , p. 75o, Boyd Dawkins ; Rodakowski, The Channel Ferry (London, 2905) ; Parl. Debates, vol. 175, 1782 (July 1924) ; memoranda by Channel Tunnel Company. (D. J. M.)

water, dover, stream, north, salinity, tunnel and french