Environmental Engineering Reference
In-Depth Information
the movement of renourished sediment on to sea floor seagrass habitats, which
occurred during brief periods of storm wave action (Firman et al.
2011
).
A renourished beach berm should generally correspond to the level of the natu-
ral berm crest, determined from historical conditions at the site (USACE
2002
).
If this is not possible the natural berm height should be estimated from survey
data from berm levels on nearby beaches exposed to similar waves. Beach width
depends on the project objectives. For example, where a beach is being renour-
ished to improve amenity, the beach may need to maintain a minimum width,
while a minimum beach width following storms may be required to reduce dam-
age to adjacent property.
4.4.4 Beach Configuration and Orientation
There have been suggestions that once a beach attains a particular shape in plan
it will become stable. For example, beaches shaped by obliquely-arriving waves
alongside a headland or breakwater develop an asymmetrical curvature some-
times known as a crenulate, 'half-heart' or 'zeta-curve' configuration. The notion
that 'headland breakwaters' can be used to shape stable renourished beaches
within intervening compartments by attaining such a configuration, related to the
refraction of obliquely arriving waves, is based on the work of Silvester (
1976
),
who indicated that beach stability (in the sense of zero longshore drift) could be
attained when bays assumed this configuration. Headland breakwaters were con-
structed on the shore of East Coast Park in Singapore, where it was found that
longshore drift diminished as the intervening beaches attained a crenulate shape,
but erosion continued on the asymmetrical beaches formed in this way, so that the
problem of coastline instability remained.
Nevertheless, some beach outlines are more stable than others, and a renour-
ished beach will be more persistent if it is placed on an alignment that is com-
patible with incident wave regimes. Reference has been made to the importance
of aspect in relation to prevailing wave patterns in determining directions of
longshore drift (Fig.
4.21
), notably on the north-east coast of Port Phillip Bay,
Australia. The renourished beach at Wrightsville, North Carolina was initially
incorrectly aligned, but became more stable as the beach became realigned more
closely to the pattern of incoming waves. Breakwaters or terminal groynes can be
used to delimit a renourished beach in such a way that it would become correctly
aligned. Attention to dominant wave regimes and patterns of wave refraction
approaching a coastal sector can improve a beach renourishment project by select-
ing a suitable alignment for the project design. At Mission Bay and Kohimarama
in Auckland, New Zealand, renourished beaches contained between groynes were
built with an orientation related to the incident wave energy. They remained rela-
tively stable (Papps and Priestley
2005
). On the coast of Malta an artificial beach
was orientated to fit the pattern of incident waves (Firman et al.
2011
).
