Environmental Engineering Reference
In-Depth Information
Fig. 3.5
Groynes on the
Italian coast at Ravenna
have failed to retain beach
sediment, which has been
withdrawn seaward and lost
alongshore. © Geostudies
Fig. 3.6
Headland
breakwaters separating
beach compartments
fronting reclaimed land
on the south-east coast of
Singapore in the 1970s.
South-westerly wave action
formed asymmetrical beaches
between the breakwaters, but
these did not remain stable,
erosion continuing on the
asymmetrical alignments.
© Geostudies
compartment (Fig.
3.6
). Waves arriving at an angle to the coastline would shape
each beach into an asymmetrical 'half-heart' or zeta-curve configuration, which
would be relatively stable, with minimal losses alongshore. Unfortunately, this
procedure failed because erosion continued on the asymmetrical beaches between
the groynes.
Another approach to beach protection has been to build nearshore or offshore
breakwaters, detached structures parallel to the coastline, designed to interrupt and
reduce wave action and induce beach accretion by waves refracted round the ends
of the breakwaters, and also to shelter the accreted beach from erosive waves. At
Borth on the Ceredigion coastline of Wales, construction of two offshore breakwa-
ters and a submerged rock reef was completed in 2013 (Fig.
3.7
). The structures
accompanied beach renourishment. Incident wave patterns were modified so that
the adjacent beach was protected from strong wave action, and widened in the lee
of the breakwaters to form a cuspate spit. The standard of flood protection of the
adjacent village was thus increased.
Offshore breakwaters are most effective on tideless shores, as around the
Mediterranean Sea, where they do not have to face the problems of a regularly ris-
ing and falling sea. They are generally built parallel to the coastline, between 50 and
