MEASUREMENTS IN THE STRAITS OF DOVER, AND THEIR RELATION TO THE NETHERLANDS CAOSTS. With the Rijkswaterstaat survey-vessel "Oceaan" in 1934-1935. INTRODUCTION. The Straits ofDover are in certain ways to be considered as the root ofthe Flemish­ Dutch coast as far as T exel and as the place where the most influential part of the tidal phenomena along this coast originates. Some uncertainty existed about the influence of a supposed enlargement ofthese Straits which was expressed in the following two questions, published by the "Bataafsch Genootschap der proefondervindelijke Wijsbegeerte" in September 1934. I. For the formation of our country the changes which have taken place in the Channel between England and France have been of great importance. The "Genootschap" desires an answer to the question in what measure the capacity of the Channel has been enlarged in bygone centuries by widening and scouring of its profile and perhaps by other causes. II. In past centuries the Channel has been enlarged by the widening and scouring ofits profile and perhaps by other causes. The "Genootschap" desires an answer to the question if - and in what measure - the enlargement of the capacity of the Channel has influenced, and still influences, the water movements and the highest water levels in the Dutch inlets. Also in the Dutch Parliament at the meeting of November 6th, 1935, the need of an investigation of these questions was stressed as follows: "Several members draw attention to the rising of the tides in the North Sea caused by a natural enlargement of the English Channel. The rate of this enlargement may be considered very slow, yet it causes larger tidal phenomena. These in their turn are influencing the tidal streams in Zeeland and South Holland and higher embankments might be necessary. It might be proved most unexpectedly by catastrophes that these embankments are no longer strong enough for the changed conditions and very expensive precautions may be required. These members think it desirable that the Government should investigate this matter and energetically effect a strengthening of the embank­ ments and a change in the draining works, if these are found to be necessary". According to existing theories our coastal sands were to be considered as forming a spit, grown in a north-easterly direction from the headland ofBlanc Nez. An amount of 10 to 12 million cubic metres ofsand and silt was believedto be moved annually to the North-East through the Straits of Dover and further on along the Flemish-Dutch­ Frisian coasts towards the mouth of the Elbe. This amount was calculated from the supposed rate of recession of the Channel coasts (BOUN1CEAU, LAMBLARD1E, DE MEY). Already in 1854 MARCHAL came to the conclusion that these Channel-materials are also mainly responsible for the accretions of the fertile "polders" in Zeeland, and he maintained that the marine portion of these accretions was 45 times larger than the fluvial portion. KELLER (1861), on the contrary, suggested that the sands in the Flemish banks were of fluvial origin and brought southward from the mouths of the rivers Rhine, Maas and ScheIdt. PLOCQ extended this theory by accepting two sand-drifts, one from 187 the south, the other from the north. Both might bring their sediments to the Flemish coasts. The question of the scouring out of the Straits of Dover had been subject to many speculations. Lord KELVIN wrote that the influence of a dam in these Straits would hardly affect the tide in the North Sea (northern part of the North Sea most likely!). Others concluded however that any change in the profile would cause a change in the tides along the Dutch coasts. An enlarged tidal action which actually exists at Flushing might be caused by a widened and deepened profile of the Straits of Dover. Other theories which caused some interest, were those held by the Dutch geologists that the Straits are ofcomparatively recent formation. They supposed that the Straits were formed about 3000 to 5000 years ago and an archeologist claimed in 1928 to have found definite proof that Caesar crossed the Channel mainly on horseback; an island, called Ictis, would have been situated between Gris Nez and Dover and wide coastal sand planes would have almost choked the Straits of Dover. After about 400 A.D. these Straits were supposed to have widened tremendously and enormous catastrophes along the Dutch coasts would have resulted (See § 25, 26). The above mentioned problems and some others were investigated with the "Oceaan" for two months in 1934 and some weeks in 1935. The stone-samples, which were taken during these investigations were examined by Dr. P. TESCH and Dr. TH. REINHOLD, both of the Government Geological Survey; the sands were classified by Prof. Dr. C. H. EDELMAN from Wageningen University and his staff (See Dr. J. A. BAAK's thesis: Regional Petrology in the Southern North Sea, Fe­ bruary 1936). Thanks are tendered to the English, French and Belgian authorities who assisted and facilitated the expedition. CHAPTER 1. § 2. The investigations with the ~~Oceaan" compared with some foreign ones. As the activities of the "Oceaan" are more or less different from other surveying ships a special chapter is devoted to explain its purpose and methods. Measurements of currents are taken by most countries around the North Sea. Mentioned are the British, German, Danish and Swedish measurements. Bottom­ samples of parts of this sea were described by VAN MIERLO (1899), BORLEY (1923) and PRATJE (1931). DANGEARD (1928) described the bottom of the Channel. Of the coastal researches the works of BRIQUET (1931) and the (American) COAST EROSION. BOARD (1933) are considered important. The activities with the "Oceaan" include stream-measurement, the taking of sea­ bed samples and coastal research. The main object is to know the influence of streams on the sea bed and the coasts. The instruments used on board are described in chapter VIII. A knowledge of the internal operation of tidal waters is necessary for planning engineering works in these waters, and may be considered essential for the maintenance of a country which hardly rises above sea-level and possesses relatively weak coasts. The investigation of the Dutch seas and inlets was started in 1930 with 3 ships ofabt. 30 Tons for inlet- and river work and the "Oceaan" for sea-work. Hydrography, history, geography and geology are touched by this research, but it is mainly hy­ draulical. The Russians seem to have started a similar investigation of their coasts and call 188 this a "hydrogeological" survey. Any experiment in a laboratory must be based upon a research "in natura". Laboratory-research alone is not enough; it even may be dangerous. § 3. Methods. An investigation as described above should include: A. Direct measurements with reliable instruments. Not everything can be measured with these instruments; about 10% of the tides (stormtides) cannot be measured that way, the lowest layer of about 10 cm cannot be reached, and the time is limited to a few tides at every point of measurement. B. Series taken with registering instruments. The combination of A and B gives the proceedings in the 4 dimensions: length, breadth, depth and time. C. Periodical soundings. These give the results ofthe currents and other forces, measured with the methods A and B. The difficulty that movement of the sand in the lowest layer of 10 cm thickness cannot be measured by the instruments, is largely eliminated by C, because more accurate total amounts of sand-displacements can be obtained by sounding. The research of the water- and sand movement in the lowest 10 cm is work for laboratories. In the Delft laboratory attention is given to this. D. The "film". By this is meant a series of old and recent charts, all on the same scale and referring to the same level (mean sea level = m.s.l. = the plane above which the areas ofthe tidal curves are as large as the areas below this plane). The"film" extends over a cycle of about one century, which is generally considered still too short. The slow changes of the bottom and shores, which escape attention when using the methods A, Band C, are registered here. § 4. Some practical points of view. Lack of knowledge is expensive and unsatisfactory, especially in works where harbours and channels are involved. A landing stage should not be built on the shore of a channel shifting its bed; a harbour entrance should not be made at silty parts of a stream, and brackish waters should be examined specially in a thorough way, because they are the most unreliable for works of any kind. Water charged with salt or silt is heavier than clean water and these differences in specific weight result in abnormal bed currents, which often are very important. Scouring capabilities of streams are a doubtful asset, because sand movements often mean deposits ofthis sand at places where it is not wanted. The best channels are those where no movement of material exists. For the rest no general rules should be given, because every region possesses its own regimen, its own waters, its own sands and silts and its own bed configurations. It is generally helpful to speak of "sand-streams". They often follow the rest­ streams or drifts of the water near the bottom. In the delta of the Rhine and Maas two kinds of sand are deposited; the river-sand coming down from Germany and Belgium and the sea-sand coming inland with the "underflood", which in a channel like the Rotterdam Waterway is very powerful. Formerly the river sand caused much trouble because it made the riverbeds grow higher and higher.