COASTAL DEFENCE 629
structures, flow-induced vibrations of structures such as piles may also occur,
threatening the structural integrity (see Sections 14.12 and 14.13).
Global warming causes rise of sea water temperatures and melting of
the polar ice both of which result in rising sea levels making the efficacy of
some of the existing structures as measures against flooding questionable
in the future. Engineers are therefore beginning to introduce future water
levels due to global warming into design calculations.
15.2 Coastal defence
15.2.1 GeneralCoastal defence is used to describe natural features or man-made meas-
ures that prevent coastal flooding and prevent the existing shoreline from
erosion. Natural coastal defence is provided by shorelines in the form of
cliffs, sand dunes and beaches. Cliffs can be either erodible or non-
erodible. The former is a good source of beach material and the latter pro-
tects the shoreline from the severity of the sea possibly causing depletion
of coastal sediments in their vicinity. Wind action is responsible for trans-
porting and depositing sand to form sand dunes along the coast. They are
very effective as sea defence against flooding during extreme stormy con-
ditions. Vegetation, particularly grass, protects sand dunes from eroding
due to wind and traps sand transported by wind. Coastal processes affect-
ing the sediment motion and physical and geological features of the coast-
line help form a healthy natural beach which is the best form of coastal
defence.
In planning engineering works, the factors that must be taken into
account are the causes of erosion of the existing coastline, the direction
and the magnitude of the waves and surges, the tidal range, the transport
of sediments, the effect of the planned structure on the coastal régime and,
of course, economic, social, and environmental factors. Before proceeding
with a coastal defence scheme, cost–benefit and environmental impact
studies are thus essential.
Coastline protection may involve schemes such as seawalls, break-
waters parallel to the shoreline, groynes and provision of beach nourish-
ment. The interaction of waves with these structures is clearly very
important in their design.
Beaches adjust themselves to absorb the energy of the waves; if the
wave climate exhibits large variability, the beach profiles become complex.
When the waves break, the flow is intensely turbulent and large quantities
of sediment may be transported in suspension. The subsequent onrush of
water can carry even coarse sediments towards the beach. The downrush is
slow and can transport relatively small sediments offshore. If the energy of