Environmental Engineering Reference
In-Depth Information
and added to the next beach. Eventually the losses of sand depleted the renour-
ished beach until it no longer prograded sufficiently in summer to leak past Green
Point, and by 1994 it had become fairly stable, apart from continuing seasonal
alternations within the beach compartment. The beach has become narrow at the
southern end.
Two more groynes were added south of New Street in 1996, and intervening
beaches then renourished. These beaches have persisted, and show that a renour-
ished beach can be maintained when a beach compartment is divided into seg-
ments by building groynes.
Offshore breakwaters have been used to create a pattern of refracted waves that
will concentrate sand deposition and prograde the beach in the lee of the breakwa-
ter. This has been illustrated at Santa Monica, California.
At Port Hueneme in California, a breakwater was built parallel to the coast on
the updrift side of the harbour entrance in 1940, and the sandy cusp that formed
on the beach landward of it (Fig.
4.29
) has been excavated at the rate of about
400,000 m
3
/year by a floating dredge to produce sand which is used to replenish
wasting beaches on the downdrift shore (Johnson
1959
).
It has been suggested that a floating breakwater, anchored off successive sectors
of the shore, could induce local accretion of sand and gravel by shoreward drift of
sediment to renourish a beach in stages along the coast. Reference has been made
to the use of submarine breakwaters to diminish wave scour and protect a renour-
ished beach at Niigata, Japan. At Marina di Cecina, on the Tuscan coast, Italy,
beach renourishment was accompanied by the building of retentive groynes with
undersea extensions. These have been successful, but two of the submerged break-
waters caused current scour leading to a sediment deficit and some beach erosion,
and these are being allowed to disintegrate (Ciprani et al.
1992
).
Fig. 4.29
Sand by-passing at
Port Hueneme, California.
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