This paper revises several types of dredging: for mineral recovery from beaches or offshore, navigation, and beach nourishment. Worldwide examples are cited. Dredging usually has both hydraulic and ecological repercussions. Significant hydraulic problems arise when dredging radically alters the coastal sediment budget, sediment supply to the coast, and wave refraction patterns near the coast. serious damage to the marine ecosystem my_ occur. These matters require father study; dredging programs should be monitored.
It is barely necessary to emphasize the dynamic character of the coastal environment, and (certainly in high-energy situations) the very considerable measure of sediment transport that may occur. Coastal environments, many with dynamic-equilibrium regimes, are easily, and sometimes irreversibly, modified by man intervention. It may seem somewhat ironic that in countries such as Britain, man has, for many hundreds of years, endeavored to clear river mouths and harbor mouths of the sediments tending to choke them in the interests of safe navigation, and yet has had to cling desperately to the sediments resident between rivers and harbors to secure adequate protection against coastal flooding and erosion. More recently, in response to growing recreational and amenity demands, coasts have assumed enormous importance; population pressures upon the more accessible beaches have increased, and man is actively engaged in conserving, improving, and even creating new beaches. Socio-economic developments are making increasing demands on raw materials of all kinds, many of which can be recovered economically from the marine environment.
Fortunately, improving technology makes it possible to redistribute sediments in a fairly controlled manner to the over-all advantage of coastal users; for instance, in beach nourishment programs, in the transference of material from the accreting to the eroding flank of a harbor entrance, and in the maintenance dredging of a harbor-approach channel. In these cases, there may be little alteration as far as the total quantity of sediment in the coastal sediment circulation system is concerned.
In contrast, there are many instances in which sediments are deliberately removed from coastal systems, and are not necessarily being replaced; for example, in the removal of beach near shore material or material normally resident in the deeper water offshore, for industrial an other purposes.
Obviously, all dredging and dumping practices have some environmental consequences. If carried out without due regard for hydraulic and ecological consequences, serious repercussions may result. Coastal dredging generally falls into the following categories:
beach dredging for industrial purposes,
beach offshore dredging for beach nourishment,
near shore-offshore dredging for navigational purposes, and
offshore dredging for industrial purposes.
Mero15 describes how natural processes lead to the concentration of certain valuable minerals within dunes, inertial areas, submerged beaches, and submerged relic stream channels. Submerged beach deposits, some of those in the Black Sea, for example, may be significantly larger than most exposed beaches. Littoral drift and beach-near shore sediment exchange processes have tended to concentrate mineral grains into zones, usually at or close to the base of the sand layer, frequently in contact with a basal surface such as a wave-cut bench, and especially in the shoreward part of the backshore zone. 17 Seasonal gales with their cut-and-fill tend to replenish the stocks of heavy miner