The Elements of economic geology

Bibliographic data

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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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Engineering geology

Collection:: Economics Books

246 ECONOMIC GEOLOGY Shingle beaches similarly protect the shore. The shingle travels along the coast at a rate that can be measured by placing a load of bricks upon the shore and observing their drift month by month. If the shingle of a beach is renewed it affords a permanent defence. Coast protection in many places depends on prevention of the migration of shingle. The simplest method is the use of groynes, or ‘ horses,” which are barriers of timber, masonry, or cement, built across the beach. Well-placed groynes hold the beach material and protect the coast beside them ; but by retaining the shingle that would renew the beaches further along the coast they lead to more rapid erosion elsewhere. It was reported to the Coast Erosion Commission (Rep. iii, 1911, p. 110) that groyning the English coast would, in 1911, have cost £300 a mile, with the result that for every acre saved another would be lost elsewhere. Groynes are useful in protecting important positions: but they endanger adjacent land. The design and size of groynes should be adjusted to the local conditions. The system designed by E. Case for Dymchurch in Romney Marsh on the coast of Kent, is based on the principle that the main drift is between mean sea- level and low tide; hence low groynes rising two or at most 3 feet above the lower part of the beach are adequate, as they stop nearly all the drift and secure a slope at the natural angle of repose! Groynes are usually placed at right angles to the beach; but they may point leeward. If placed too close together groynes raise the water-level, and increase the backwash and thus hasten the transport of material down the beach. Harbour design is affected by some of the factors which control the efficiency of groynes. The most convenient site for a harbour is usually an estuary, where the river current or tide can be used to keep the channel clear. Some former British ports have been closed by silt. Chester was the chief port of the Western Midlands until the silting of the Dee estuary diverted the trade to Liverpool. London has maintained its position as a port. owing to the well-designed 1 Ed. Case, Rep. Brit. Assoc., 1899, p. 859; J. 8S. Owens and G. O Case, Coast Erosion and Foreshore Protection, 1008.

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