Geoscience Reference
In-Depth Information
With respect to deltas, continued sea-level rise
may have an impact on the seaward edge, in that
the sea will start to dominate over the processes
of river discharge and fluvial sediment input. It
is even argued that in many situations, the pro-
gradation of deltas will cease (Bird 1993) and
that the low-lying lands of the delta will be more
prone to flooding as a result of storm-wave over-
topping and increased brackish water penetra-
tion up channels. Bird (1993) argues that a rise
in sea-level of 1 m would cause the submergence
of most of the seaward parts of the Nile delta,
with the coastline moving several kilometres
inland. Similarly, Milliman et al. (1989) suggest
that much of the delta region of Bangladesh
would be lost. Clearly, such change is dramatic
and difficult to prevent. Defence provision is one
response, but to cope with a sea-level rise of 1 m,
and given that storm-wave height would also
increase correspondingly with respect to current
levels, the potential cost of the hard engineering
necessary would be extreme.
On a smaller, but no less significant level,
the survival of vegetation communities faced
with sea-level rise is linked to two factors. First,
the issue of coastal squeeze can cause marshes
to experience lateral retreat and loss of higher
marsh communities (see Fig. 7.13). Second is
the threat of drowning. For a marsh surface to
survive in the face of sea-level rise, it needs to
grow vertically and keep pace with the deepen-
ing sea-level. Allen (1997, 2000) developed a
simple model for quantifying this process. Given
that the vegetation surface receives sediment in
two forms, minerogenic and organic (see Allen
(2000) for detailed review), then the total amount
of material received has to exceed the amount of
sea-level rise. Hence
of sea-level rise is greater than vertical marsh
growth, and the marsh is in danger of drowning.
Conversely, if
E
is positive, then the vertical
growth is out-pacing sea-level rise. Although
such rates can be measured for a contemporary
system, prediction of the likelihood of future
marsh survival is difficult. This is because there
are many unknowns with respect to estuarine
response to progressive sea-level rise, particu-
larly in respect of the sediment budget and
hydrodynamics of the system.
Another significant aspect of sea-level rise is
that the associated increase in marsh erosion
and increased height and salinity of the water
table will accelerate the reworking of a range of
environmental pollutants. Case Study 7.3 detailed
this idea from an erosion point of view, but
chemical stability also becomes a factor when
ground saturation and/or salinity increase. The
key question to ask is where will this material
go? Increased sediment loads due to marsh and
mudflat erosion may be washed out to sea, or
may remain in the estuary, depending on tidal
symmetry. Either way, this may cause future
management problems.
Δ
7.6.2 Impacts of increased anthropogenic
disturbance
Coupled with sea-level rise, humans still see
estuaries as areas of cheap land for development.
Although with the advent of estuary manage-
ment plans, development has become much more
restricted, there are still future issues that per-
tain to this area of estuarine and delta impact.
The growing leisure industry is imposing greater
demands for marinas and sailing centres. As a
1993 English Nature report stated, there was not
one major estuarine system in England that did
not either have, or have plans for, a major marina
development (Pye & French 1993). Similarly,
the growth in ship size means that capital dredg-
ing needs to be carried out in many ports and
harbours in order to develop berthing capacity.
Again, this represents further capacity to change
tidal and current activity. Furthermore, as popu-
lation grows, so the demand for fresh water will
see more dams being constructed in catchments.
S
min
S
org
Δ
E
=Δ
+Δ
−Δ
M
−Δ
P
where
E
is the net change in marsh surface
elevation,
Δ
S
min
is the thickness of minerogenic
sediment added to the marsh surface,
Δ
S
org
is
the thickness of organic sediment added to the
marsh surface,
Δ
Δ
M
is the change in relative
sea-level and
P
is the height change resulting
from compaction. If
Δ
Δ
E
is negative, then the rate
