Environmental Engineering Reference
In-Depth Information
sources in 1983. This formed an artificial beach 1,200 m long and up to 200 m
wide in front of the cliffs and provided a beach for the seaside resort.
A similar project at Praia dos Tr￪s Castelos, to the west, failed when much of
the 50-70 m wide beach emplaced in 1983 disappeared within 5 years, evidently
because this sector was more exposed to wave scour than Praia da Rocha (Psuty
and Moreira
1990
).
Artificial beaches have been added on the seaward side of sea walls, as in the
Netherlands, and harbour breakwaters, as at Cullen Bay, near Darwin in northern
Australia. Here a beach of sand dredged from Darwin Harbour was placed in front
of a large boulder breakwater built to enclose a marina in a former bay in 1993.
This is a macrotidal coast, and the beach, emplaced around high tide level has
been combed down by wave action on ebbing tides to a wide concave profile.
At Ediz Hook, on the southern shores of the Strait of Juan de Fuca,
Washington, United States, a spit 5.6 km long and 27-275 m wide, shelters Port
Angeles Harbour. It had been supplied naturally with sand and gravel from the
west, partly from the Elhwa River and partly from the erosion of cliffs cut in
glacial drift, but the damming of that river and the building of sea walls to halt
erosion along the cliffed coast reduced the sediment supply, and beach erosion
became severe. In 1977-1978 rock revetments were built, and it was decided to
place gravelly material, quarried from glacial drift deposits west of Elhwa River
and brought by truck, on the outer shore of the spit. Supplemented in 1985 by fur-
ther such renourishment, the spit attained a relatively stable configuration (Galster
and Schwartz
1990
).
After the removal of dams on the Elwha River began in 2011 the supply of
fluvial sediment to the shore resumed, with accretion on beaches near the river
mouth.
Renourishment of beaches in south-east Queensland has been facilitated since
1988 by the use of the Port of Brisbane Authority dredge, modified to be able to
pump sand out over its bow (the rainbow method, (Sect.
4.3.1
, p. 57)) on to a beach.
It was first used to place 50,000 m
3
of sand on an eroded beach at Woorim, on Bribie
Island, north of Brisbane, and in 1992 to replenish Golden Beach at Caloundra, to
the north, with 70,000 m
3
of sand dredged from nearby Pumicestone Passage. The
rainbow technique has also been used to restore beaches in compartments between
the numerous groynes on the shore at Felixstowe, on the east coast of England.
4.3.2 Emplacement by Longshore Drift
Losses of beach sediment alongshore are a common cause of beach erosion.
Where longshore drift is unidirectional, or dominant in one direction, the losses
can be made good by injecting sediment updrift and allowing it to spread along the
eroded beach, as at Atlantic City, New Jersey, where sand deposited at the north-
eastern end has been distributed south-west along the city seafront by wave action
(Sect.
4.6
, p. 87).
