Geography Reference
In-Depth Information
However, in the 1960s this system began to fail,
with falling sea levels and increasing damage to
seawall structures. A pilot beach nourishment
scheme (Beach Improvement Scheme (BIS) 1) was
implemented in 1970, with 84.5×10
3
m
3
of
dredged sand emplaced along a 1.8 km frontage.
This was followed by a large-scale scheme (BIS 2)
in 1974-75, when 654×10
3
m
3
of sand was
pumped directly onto beaches over an 8.5 km
frontage, with an estimated additional 749× 10
3
m
3
of material, as 'leakage' from the pumping
process, being deposited in the shallow nearshore
zone. A third emplacement (BIS 3) was
undertaken between 1988 and 1990, adding 999×
10
3
m
3
of fill dredged from the entrance to Poole
Harbour (Harlow and Cooper 1996). The reasons
for this third phase of beach replenishment are
clear from the synthesis of data on beach dynamics
gathered from almost forty beach profile lines on
some fifty occasions since 1974 (Figure 8.4B). The
total beach volume increased from a minimum of
6×10
6
m
3
in 1975 to a peak of 7.7×10
6
m
3
in 1979
as a result of both the direct emplacement of beach
material and the onshore migration of 'leaked'
material. Thereafter, beach volume decreased to
6.9×10
6
m
3
in 1988, the accompanying narrowing
of the beaches and the onshore migration of the
mean high water mark necessitating the BIS 3
scheme. This restored beach volume to over 8×10
6
m
3
in 1990. By late 1993, beach volume had
decreased to 7.9×10
6
m
3
, suggesting the necessity
for the next phase of beach replenishment, with a
factor included for potential near-future sea level
rise, before 2003 (Figure 8.4B). The important
conclusion from this study is the need for not only
sensible preemplacement planning of nourishment
but also the value of ongoing monitoring once
emplacement has taken place. Monitoring must be
regular and sustained to establish trends in beach
volume change from the high degree of variability
between successive individual surveys, and extend
far enough offshore to encompass shallow subtidal
as well as intertidal changes. It is only through an
ongoing management commitment of this kind
that the technique of beach nourishment can be
'fine-tuned' for optimal performance, thus
avoiding the traditional, and costly, crude design
practice of 'over-filling' (often by 40 per cent;
Verhagen 1992) nourished beaches to allow for
unmonitored losses.
Fine sediments
It is not only beaches that provide important
natural coastal buffer zones. Intertidal mudflats and
vegetated mangrove swamps and salt marshes also
perform similar energy-dissipating functions
(Pethick 1992).
At the coastal scale, salt marshes within estuaries
reduce tidal range and flooding potentials through
frictional drag on water sufaces and by allowing
the high tide stage of water on marsh surfaces
(Burd 1995). Recent field measurements have
shown that marsh surfaces can dissipate between
47 and 99 per cent of incident wave energy over
distances of 200 m or less (Moeller
et al
. 1996),
thus reducing wave run-up and overtopping risk
on marsh-fronted sea defences. However, on many
coasts land reclamation has at worst removed and
at best reduced the width of such fronting marshes.
Furthermore, there is evidence in many localities
that remaining marshes have been subjected to
accelerated loss in recent decades, perhaps as a
result of sea level rise. On natural coastlines,
marshes will migrate landwards as sea level rises to
maintain their position in the tidal frame.
However, on protected coastlines, this migration
is prevented by a landward defence line, and marsh
volumes cannot be preserved. The loss of fronting
salt marsh both results in increased wave action
against the defence and removes mechanical
support from its toe; the likelihood of
undermining and collapse thus generates a
demand for costly re-engineering of the defence
line.
However, an alternative approach is that
provided by 'managed retreat' (also known as
'coastal setback' or 'shoreline realignment')
whereby the defence line is repositioned in a more
landward position. Such schemes are attractive for
physical reasons, as they create an immediate
energy-dissipating zone in front of the new
defence line, and for ecological/conservation
reasons, as replacement intertidal habitat
