Environmental Engineering Reference
In-Depth Information
2.8 A Change in the Angle of Incidence of Waves
Beaches tend to become adjusted to the prevailing wave regime, and then to respond
to short-term changes in the direction of incident waves and the angle at which they
arrive at the coast. A persistent change in the angle of incidence of waves can result
in alteration or intensification of longshore drift, leading to beach erosion. Such a
change followed the construction of the Portland Harbour breakwater in Victoria,
Australia in 1957, when the onset of beach erosion on the adjacent coast at Dutton
Way resulted from intensified longshore drift in response to the change in wave
approach. Beach erosion then spread eastward along the coast of Portland Bay.
Oblique waves generated by passing ships (boat swash) can modify the inci-
dent wave regime. This has led to erosion on Point King Beach at Sorrento on
Port Phillip Bay, Australia, where the beach was re-shaped as intensified longshore
drift depleted the eastern end and led to accretion at the western end (Bird
2011
).
Refraction of waves over the ebb shoal at the entrance to John's Pass, West
coast of Florida causes a local reversal in longshore sediment transport contribut-
ing to severe erosion on Sunshine Beach (Wang et al.
2011
).
Wave attack on a beach sector may intensify as a result of the lowering of
the beach profile on the neighbouring sector, allowing stronger waves to arrive
obliquely, and thus accelerate longshore drift. A beach profile may be lowered
as the result of sea wall construction and scour by reflected storm waves on one
sector, so that larger oblique waves can then move through the deepened water to
Fig. 2.12
In response to
coastal erosion a sea wall
was built at Point Lonsdale
in 1900. This resulted in
beach depletion by reflected
waves (cf Fig.
2.15
), and also
the lowering of the beach
to the north, followed by
accelerated erosion there.
The sea wall was extended
northward in the 1930s, with
a similar result, and further
set-back extensions were
made in 1947, 1966 and
1977. © Geostudies
