Environmental Engineering Reference
In-Depth Information
preferable to abandon the groyne field and provide a supply of sand at the updrift
end sufficient to maintain a protective beach along the coast by longshore drift.
Groynes are of little use where sediment from the renourished beach between
them is being withdrawn to the sea floor, as at Virginia Beach on the Atlantic coast
of the United States, or the German North Sea island of Nordeney (Kunz
1990
).
In such conditions it may be possible to retain a renourished beach by building
nearshore underwater breakwaters to prevent seaward losses, as at Niigata in Japan
(Chill et al.
1989
).
Artificial structures such as marinas may act as traps for seasonally drift-
ing sediment, as at Sandringham Harbour on the NE coast of Port Phillip Bay,
Australia. When the first survey was carried out in 1861 this beach compartment
had a low receding cliff in soft Red Bluff Sand bordered by a sandy beach that
drifted to and fro seasonally. Cliff recession prompted the building of a sea wall in
the late 1930s, when the cliffs were graded back to an artificial slope and planted
with vegetation. As a sequel, the beach between Green Point and Hampton was
depleted, because it was no longer maintained by the supply of sand washed out
of the receding cliff by runoff and wave scour. This beach was further reduced fol-
lowing the building of a boat harbour (Fig.
4.27
a), with construction and elabora-
tion of a large boulder breakwater at Picnic Point, to the south (Fig.
4.27
b). As on
the other beaches on the north-east coast of Port Phillip Bay, there is northward
drift of beach sand between November and April, and southward drift from May
Fig. 4.27
Changes on the coast at Sandringham, Port Phillip Bay Australia.
a
The natural con-
figuration in 1861,
b
seasonal alternations of longshore drift on the beach resulted in its depletion
after a breakwater was built, and southward-drifting sand accumulated in Sandringham harbour,
c
renourished beaches are now maintained between groynes. © Geostudies
