Environmental Engineering Reference
In-Depth Information
attack the neighbouring sector, causing beach erosion there. If the sea wall is then
extended along the coast to counter this beach erosion, a 'domino sequence' may
ensue, with beach erosion beyond each limit of the extended sea wall. This hap-
pened at Point Lonsdale in Victoria, Australia, where each new sector of sea wall
has been built on a set-back alignment on the eroded shore (Fig.
2.12
).
2.9 Interception of Longshore Drift by Breakwaters
Breakwaters have been built to stabilise river mouths or lagoon entrances in order
to improve their navigation, or create boat harbours. Where beach sediment is
drifting alongshore there is interception on the updrift side of the breakwaters,
and beach erosion on the downdrift side as the sediment supply is cut off. The
downdrift erosion caused by breakwaters may spread for several tens of kilome-
tres (Bruun
1995
) and can prove irreversible (El-Asmar and White
2002
). On the
east coast of Florida breakwaters have been built to stabilise several tidal entrances
through sand barriers, and in each case southward longshore drift has been inter-
cepted to prograde the beach on the northern side, and beach erosion has ensued
on the southern side, deprived of a longshore sand supply. The erosion at Upham
on the west coast of Florida is caused by a significant deficit in the southward
longshore sediment transport, due to the structures at the Blind Pass inlet (Elko
and Davis
2006
, Elko et al.
2005
). Practically, no sand bypasses Blind Pass to
reach Upham Beach (Roberts and Wang
2012
). Finkl and Esteves (
1998
) estimated
that structures blocking littoral drift accounted for 72 % of Florida's beach erosion.
The seaport at Chennai (formerly Madras) on the south east coast of India
underwent massive expansion programmes in the last two decades, which resulted
in substantial changes in the geomorphology of the down-drift side of the port
(Ramana Murthy et al.
2008
). Nearly 400 ha of beach was lost as a result of ero-
sion, which resulted in the construction of a seawall.
Until about a century ago there was a shingle beach, maintained by eastward
drift, beneath the Chalk cliffs between Dover and Deal in the United Kingdom
but this has almost disappeared as the result of the interception of shingle drifting
alongshore by the breakwaters at Dover Harbour, west of which the beach has pro-
graded. In Lyme Bay on the south coast of England the dominant eastward long-
shore drift of shingle has been intercepted by landslide lobes and rock falls, each
of which act as breakwaters, causing beach erosion downdrift.
2.10 Increased Losses of Beach Sediment to the Backshore
Sand is swept from the beach to the backshore by strong onshore winds
(Fig.
2.13
), or when storms wash beach sediment beyond the back of the beach,
or over into lagoons, swales or swamps, or the mouths of rivers. If losses from the
