The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Engineering geology

Collection:: Economics Books

COASTAL WORKS 239 and are about 50 feet high. In the North Atlantic the maxi- mum is about 40 feet. Vaughan Cornish (Waves of the Sea, 1910, p. 53), during a strong gale in December, 1900, measured many waves 29 feet high, and some of 43 feet. The height depends upon the * fetch,” i.e. the width of open water to windward ; if the fetch is more than 39 miles the height of the waves (H) in feet is one and a half times the square root of the fetch (D), i.e. H = 1-54/D; if the fetch is less than 39 miles H = 1-5 (2:5 — 4¢/D). The heights of waves, according to the formulas, are as follows :(— a Fetch. Wave Height. Fetch. Wave Height. A Fetch. Wave Height. L, — = I mile = 3 ft. 10 miles = 5 ft, 6 in. 20 ,, =2"%ft. 1in. 30 miles = 8 ft. 4 in. go ,, = oft 5in. 50 ., ==I0ft.6 in. 100 miles = 15 ft. 200 °,, =2I ft.5in. 200 ,, = 26 ft. The waves in Lake Geneva are 8 feet high where they have a fetch of 40 miles, those on Lake Superior 20-25 feet high with a fetch of over 300 miles. The depth of disturbance of a wave is equal to its length ; the maximum length of ordinary waves in the Atlantic is 600 feet, and they disturb fine sediment to the depth of about 600 feet or 100 fathoms. The action diminishes rapidly with depth. The displacement of water particles at a depth equal to the length of the wave is only zly and at double that depth is only sggyy of that of the surface. At special points waves and currents move material far below the ordinary limit of wave action.. Lobster pots in the English Channel are sometimes filled with coarse shingle at the depth of 180-200 feet. Seaweeds which live not less than 200 feet deep are washed ashore with stones attached to their roots, and must have been torn from the sea-floor by waves. The cover of telegraph cables is cut by drifting sand at the depth of 600 feet, and silt is moved at greater depths. The transport of beach material depends on the angle at which waves strike the shore. A wave which rushes obliquely up the beach returns by the shorter steeper course at right angles to the shore; it carries material along a zigzag course. The backwash may be concentrated and strike a more power- ful blow than the oncoming wave: thus at Dunbar a wave

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