Geography Reference
In-Depth Information
the earlier expansion of the Nile delta has been
replaced by rapid recession with the removal of
sediment supply following the completion and
closure of the Aswan High Dam in 1964 (Stanley
and Warne 1993). Finally, at the present time,
tropical deforestation is creating the latest
geographical focus in the developing tropics for
sediment pulses and coastal progradation. It seems
likely that the familiar pattern of shoreline
problems will follow: initial difficulties with
navigation and siltation as river sediments disrupt
coastal settlements and infrastructure; the
construction of sediment control measures; and
the erosion that follows the combination of the
cessation of accelerated sediment inputs and the
interference from structures in the natural
processes of sediment redistribution.
beach replenishment efforts has been mixed
(Davidson
et al
. 1992) and not all these differences
can be explained by large-scale regional
differences in shoreline type or sea level change
(Leonard
et al
. 1990). Thus, of 110 such schemes in
the Gulf of Mexico, 23 per cent were found to
have persisted for over five years, 54 per cent for
one to five years and 23 per cent for less than one
year (Dixon and Pilkey 1989).
It is instructive to look at failed beach
nourishment projects; many identify a lack of
knowledge of coastal geomorphology as a key
component in failure. The great majority of beach
nourishment schemes are undertaken because of
the loss of the existing beach; thus it is essential to
know why the beach is being eroded and where
the sediment sink is located, whether to landward,
to seaward or alongshore. Thus any individual
beach must be set in its regional context, in terms
of both shoreline morphology (both natural and
artificial) and wind, wave and tidal regimes
(including the role, if any, of extreme events). Such
pre-planning should help to determine the nature,
volume and location of sediment emplacement. A
second lesson from scheme failure is the need to
monitor the performance of beach recharge after
emplacement, as the following case study
illustrates.
CASE STUDIES
Coarse sediments
It is clear from the preceding discussion that a
substantial beach is an excellent form of coastal
defence and that there are considerable
management benefits from maintaining or
improving a beach frontage through beach
nourishment/ replenishment. The advantages of
this approach are three-fold. First, such natural
defences are morphodynamically active and hence
can adjust to changing wind and wave conditions
in a way that is not possible for a fixed defence
line. Second, reduced wave run-up and flooding
behind defence lines, as a result of wave energy
reflection or dissipation, is achieved without the
need to emplace sediment-retaining structures,
which then create down-coast sediment starvation
and erosion problems. Third, beach nourishment
often provides the additional benefit of recreating
a valuable recreational and amenity asset. The
attractiveness of this form of shoreline
improvement to operating authorities can be seen
from the rapid increase in the adoption of
nourishment schemes, first introduced in the USA
in the 1920s, from the 1970s onwards (Pilkey
1995). However, the performance of individual
Bournemouth beach, southern England
Bournemouth beach forms the central section of
the 30 km long broad embayment of Poole Bay
on the coast of southern England (Figure 8.4A).
The predominant wave direction is from the
southwest, coinciding with the maximum fetch
from the Atlantic Ocean. The bay is backed by low,
erodible cliffs stabilised to varying degrees; the
characteristic beach sediment is fine sand to the
west, but it coarsens to gravels towards
Hengistbury Head in the east (May 1990). The
town of Bournemouth relies upon its sandy
beaches to help to sustain an economically
important tourist industry and has traditionally
used the standard coastal protection measures of
seawalls and groyne fields to prevent urban
flooding and retain beach sediments, respectively.
