Geography Reference
In-Depth Information
compensates for tidal wetlands lost elsewhere. At
the present time, therefore, managed retreat trials
are being implemented in several locations,
including the UK and the Californian and Pacific
northwest coasts of the USA. In the UK, the
county of Essex in eastern England is protected
by 430 km of seawalls, a defence system
strengthened in the aftermath of the disastrous
floods of 1953 but one now reaching the end of
its design life. Some 60 per cent of these seawalls
are protected by fronting salt marsh, but erosion
of these marshes has accelerated since the 1970s.
Thus several managed retreat trials, including
monitoring studies on water, sediment and
nutrient exchanges; soil physical and chemical
changes; vegetation re-establishment; and
sedimentation/accretion processes, are being
undertaken in this region. One such experiment
is at Tollesbury Fleet on the Blackwater estuary, a
21 ha site re-flooded in August 1995 after a
controlled breach of the old sea wall (Plate 8.2). A
former drainage channel to a now defunct sluice
on the eastern side of the site divides the site into
a lower, northern area previously sown with clover
from a higher, sloping southern area formerly
under cereal crops (Figure 8.5).
There are considerable unknowns as to the
long-term performance of such schemes. One
crucial set of questions revolves around the
observation that usually new tidal exchange
reactivates former salt marsh surfaces converted to
agricultural use on enclosure from the sea.
Dewatering, compaction and soil chemistry
changes on isolation from the marine
environment mean that re-flooded surfaces are
likely to be much altered from natural marsh
surfaces (Portnoy and Giblin 1997). Furthermore,
neighbouring marshes outside reclaimed areas will
have continued to accrete vertically, and thus there
are likely to be height differences— perhaps of the
order of 1.0-1.5 m in systems dominated by
inorganic sediments (Pethick and Burd 1995)—
between higher natural and lower reactivated
marsh surfaces. One of the challenges for managed
retreat site design, therefore, is to ensure that these
height differences are eliminated by rapid
sedimentation. This can only be achieved through
a proper understanding of salt marsh development
processes and how they can be manipulated for
management purposes. At Tollesbury, natural rates
of elevation change, measured on naturally
vegetated salt marsh surfaces outside the managed
retreat site, have averaged
c
. 5 mm a
-1
since
monitoring began in 1995 (sites 1 and 2, Figure
8.5; Table 8.1), a figure consistent with the surfaces
keeping pace with local estimated sea level rise
Plate 8.2
Managed
realignment of an estuarine
shoreline: Tollesbury Fleet,
Blackwater estuary, Essex,
England. The view is bisected
by the now breached old
seawall defence line. To the
left foreground, natural salt
marsh is just being flooded by
the rising tide. To the right in
the distance, small boats
have sailed through the
breach into the lower, already
well-flooded managed retreat
site
(photograph: T.Spencer)
.
