Discussion. Beach Replenishment: Sea Defences and a Review of the Role of Artificial Beach Replenishment.

Abstract

M r J. Duvivier, Lewis and DuvivierThe massive reclamation or replenishment of eroding beaches with sand dredged from the sea bed as an alternative to sea walls, breastworks or other conventional forms of sea defence is an attractive concept, and the Author is to be congratulated on bringing it forward for discussion. Whether it can be much cheaper and just as effective, as claimed, must surely depend upon the circumstances of each individual case: for example the depth, slope and composition of the sea bed and foreshore, the tidal range, degree of exposure to rough seas, the configuration of the coast line, littoral drift and last, but not by any means least, upon the administrative set-up and facilities for carrying out, maintaining and financing a major scheme of this type 60. To a generation of engineers and contractors who can build Europoort and the Dutch delta scheme and is prepared to carry out the Maplin project and enclose large areas of the Wash or Morecambe Bay (if it is allowed and paid to do so), there should be no special difficulty in dredging I , 2 or 3 million m3 of fine granular material from the sea bed and pumping it ashore. What is difficult to estimate to a consistent and acceptable degee of accuracy is how much it will cost to pump the necessary amount of sand ashore in the first place (I assume the Author's claims relate exclusively to sand as I can find no reference to replenishment with gravel from off-shore deposits), and, what is probably even more difficult, to predict what will subsequently happen to the reclamation under normal average weather conditions year in and year out.61. The Author accepts that on some coasts the provision of groynes may be helpful, even necessary (4 18), and the bulk of his criticism is directed against sea walls and in particular steep walls, which are said to cause loss of material from the beach (& 2 and 10) due to turbulence caused by wave reflexion from the wall. 62. I think, however, that it is unusual to build steep or vertical 'gravity' walls on beaches composed of soft, loose and erodible material. Such walls are normally found on coasts where the strata are relatively hard, e.g. Sunderland, Seaham, Sheringham, Cromer, Dover, Peacehaven, Brighton, Hove, Exmouth, etc., and provision is made by groyning the foreshore and, if necessary, by importing shingle from an outside source to protect the foundations and lower tidal levels of the walls against sea action. On soft, loose, erodible foreshores it is customary to spread the load over as wide an area as possible by means of sloping aprons or stepped foundations surmounted by relatively low curved superstructures backed by wide deckings and flood walls.63.