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starts to colonize. Mid-marsh vegetation will
colonize when inundation frequencies fall below
c.
230 times a year, and mature marsh below 100
(Adam 1990). Ultimately, in the upper reaches
of the higher marsh, where tidal inundation is
rare, freshwater vegetation will appear.
A further significant aspect of deltas and
estuaries is a more negative one. The formation
of extensive areas of vegetation has, historic-
ally, seen this land being regarded as prime for
development. This development has taken many
forms. Initially large areas of salt marshes were
drained and converted to farmland; such areas
in the UK include the Fenland around The Wash
(47,000 ha), and bordering major estuaries, such
as the Severn (
c
. 8000 ha). Both of these areas
have an almost continuous history of land claim
from Romano-British times. In other examples,
c
. 2000 ha of marshes have been claimed since
the nineteenth century in the Ribble estuary; and
in the Dee estuary, around 6000 ha have been
claimed since the eighteenth century (Davidson
et al. 1991). More recently, with reduced demand
for agricultural land, marshes around major
estuaries and deltas have become prime sites on
which to develop ports and marinas. Similarly,
industrial growth has regarded such land as cheap
and highly desirable. The Thaw estuary in South
Wales, for example, was completely claimed in
1850 for construction of a power station, large
areas of the Orwell estuary in Suffolk were lost
in the nineteenth and twentieth centuries to
construct Felixstow docks, and successive piece-
meal land claim since the mid-sixteenth century
for naval and commercial port facilities has
significantly reduced the salt marsh resource of
Portsmouth harbour (Davidson et al. 1991) (see
also section 7.5, Fig. 7.14).
Land claim does not only serve to reduce
the areas of vegetation in estuaries and deltas,
but also has an impact on morphology and func-
tioning. Fundamentally, land claim constrains
an estuary, in that it reduces space for water to
occupy. The volume that the flood tide occupies
is known as the tidal prism. Clearly, as more
marsh is claimed, the space available for the
flood-tide water to inundate is reduced, and
so the tidal prism becomes constrained into a
smaller space. The amount of water entering
the estuary remains the same, however, with the
result that the only way of accommodating it
is for the height of the water surface to increase.
This has some potentially serious implications.
First, it increases the risk of defence overtopping;
and second, it means that the remaining marsh
will be covered more frequently, for longer
periods, and to greater depths, potentially lead-
ing to vegetation loss. This situation is, in fact,
analogous to that which occurs during sea-
level rise (see section 7.3). Significantly, how-
ever, some examples of historic land claim are
now proving to be short sighted. When claimed,
salt marsh soils dewater and contract, falling in
elevation relative to any marsh remaining out-
side the line of defence. Hence, anything built on
this land is soon below high water mark. In a
world of increasing sea-level rise, this is now a
major problem.
From the above, it can be seen that estuaries
are areas of intense human impact and influence,
yet they are also areas where natural processes
can be particularly dynamic. This diverse range
of interests and demands placed on estuaries and
deltas represents a key dichotomy. On the one
hand, these sedimentary environments are seen
as highly dynamic and ecologically important,
yet on the other, they are pollutant and sediment
sinks and subject to a range of human pressures,
such as land claim and port development. As a
result, many of the large deltas and estuaries
of the world have had to be protected through
designation of major conservation status.
7.1.3 The classification of deltas and estuaries
Variations in grain size, sediment supply, fresh-
water discharge, tidal range and wave activity
will lead to considerable variation between
estuaries and deltas. In estuaries, differences
between the amount of sea water entering dur-
ing each tidal cycle are critical in classification.
At the simplest level, estuaries may be classified
purely on the basis of tidal range (the amount
by which the water level rises between low and
high tide). Estuaries with a tidal range less than
2 m are known as microtidal, those with a range
