Geography Reference
In-Depth Information
Table 8.1 Average rates of surface elevation change,
1995-7, outside and inside the managed retreat site at
Tollesbury Fleet, Blackwater estuary, Essex, UK.
difficulty with which the remoter parts of the site
receive sediment inputs (see Figure 8.5, Table 8.1).
Continued monitoring over the next few years
should provide interesting information on
changing temporal and spatial patterns in
sedimentation and on plant establishment and
subsequent vegetation dynamics.
Finally, and while the above discussion has
concentrated upon the need to better understand
the internal dynamics of managed retreat sites, it
should be recognised that the wider uptake of such
schemes, and the establishment of progressively
larger managed retreat areas, will generate a need
to understand how such schemes will impact on
broader-scale, whole-estuary hydrodynamics and
sedimentation.
Notes: *See Figure 8.5 for station locations.
**Negative numbers indicate surface lowering. Work forms
part of an ongoing collaborative project with the US
Geological Survey (Dr D.Cahoon), Louisiana Universities'
Marine Consortium (Dr D.J.Reed) and University College,
London (Dr J.R.French).
accumulation of organic material is the prime
input), studies of long-term marsh development
show rapid vertical height gains, which slow with
time (often after c . 100 years) as progressively
higher marsh surfaces are flooded by fewer and
fewer tidal incursions (French 1993). Within this
behaviour, however, recent studies at single-tide
to tidal-monthly timescales have been able to
resolve the smaller-scale detail of sedimentation
dynamics (e.g. French et al. 1995b; Leonard 1997),
showing that proximity to tidal feeder channels,
or 'creeks', is a prime control on patterns of
sediment deposition. Thus one way to increase
sediment delivery to, and within, a managed
retreat site may be to consider either the re-
excavation of old tidal creeks or to construct a
new network of artificial creeks, taking design
rules from fluvial geomorphology. As salt marsh
channels also aid plant establishment through
substrate drainage and dewatering, then such
approaches should be seriously considered (French
1995). However, such work does need to be
undertaken with care: poorly designed networks
may promote erosion rather than sedimentation
(e.g. Haltiner et al . 1997), and channel design may
also have implications for withinchannel
hydrodynamics and nutrient exchanges (e.g.
Emmerson et al . 1997). At Tollesbury, it appears
that one of the problems of site design is the
CONCLUSIONS
It should be clear from this review that there are
fundamental discrepancies between the way in
which many coastlines have been modified and
subsequently maintained and the way in which
natural processes operate; these discrepancies often
manifest themselves in severe coastal erosion
problems. These difficulties are bad enough
without the additional problems introduced by
administrative frameworks that tend to fragment
coastal management into small units and support
local interests above regional and larger-scale
concerns. Countries, and indeed regional
economic groupings, should therefore in the first
instance work towards larger-scale, integrated
coastal management plans. Clearly with limited
resources, some form of prioritisation will be
required. This must, for any extensive stretch of
coastline, decide between one of three strategies:
1
'Hold the line' by providing robust and
reliable defences;
2
'Accommodate' shoreline change by allowing
continued occupancy but with adaptive
measures, including adjustments to periodic
flooding by modifying buildings and access
routes or by the acceptance of periodic
inundation; and
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