Environmental Engineering Reference
In-Depth Information
Table 13.4
Outline of the different landscape types from natural to artificial landscape features and examples
for land use.
Drainage type
Erosion protection
Type of landscape
Land use
Creeks
No groynes
Natural
No
Creeks
No groynes
Semi-natural
Grazing/cutting/no
Ditches
Groynes
Semi-natural
Grazing/cutting/no
up to several metres deep seems to result in a nat-
ural drainage pattern during the fast sedimentation
process. Salt marshes have been grazed since their
emergence. Livestock grazing is beneficial for winter-
staging birds, but not for many breeding birds.
Authorities in charge of restoration should be aware
that striving for natural salt marshes is often a con-
tradiction. Moreover, targets can ask for contradictory
measurements. Different approaches are currently
practised and/or proposed to counteract the afore-
mentioned changes in salt marshes. We discuss
approaches including the interaction of sedimentation
and vegetation. Hence, mitigation projects such as on
dredged spoil are not taken into account (Zedler &
Callaway 1999, Zedler & Lindig-Cisneros 2000).
When we adopt the aim to develop a diversity of
salt-marsh vegetation, reflecting the geomorpho-
logical conditions of the habitat, the following
definitions may be useful.
Natural landscapes
feature
geomorphological conditions that are not affected by
humans. They show a natural drainage system with
meandering creeks and levees with higher elevation
than the adjacent depressions. Natural landscapes
occur in sandy back-barrier conditions or in parts of
former Wadden Sea bays along the mainland coast.
In contrast,
semi-natural landscapes
have a wide-
stretched creek system but are affected in their geo-
morphological conditions by artificial drainage and/or
by measures to enhance livestock grazing or cutting.
Semi-natural landscapes are found on islands, in
foreland clay marshes and in marshes with sedimen-
tation fields and an artificial drainage system, i.e.
ditches. They also include de-embanked summer-
polders having an artificial drainage system without
groynes. Clay pits or the establishment and mainten-
ance of sedimentation fields without an artificial
drainage can result in a natural drainage system
with meandering creeks and a naturally developing
elevation structure.
Characteristic salt-marsh plant species can be
present in all landscape types. However, their
abundance in typical salt-marsh vegetation types and
their spatial arrangement in the vegetation structure
can be affected by land use. Geomorphological
conditions may change in the long term - decades -
whereas the effects of changes in land use can occur
within a few years. This hierarchical arrangement
implies that changes in land use cannot result in, for
example, a transition from an artificial marsh into a
natural or semi-natural marsh (Table 13.4).
13.6.2 De-embankments
Embankments have not only caused an interruption
in salinity but also in sedimentation. The unem-
banked marsh in front of the newly created polder
featured a continuous rise in net surface elevation.
Differences in soil level in front of and behind the
seawall or summer-dike will be greater when the
polder is intensively drained for agricultural pur-
poses. This will also hold for coastal systems with
accumulation of peat where great shrinkage can take
place (Roman
et al.
1995). For the sake of coastal
protection and the costs of seawall maintenance it is
assumed that a well-inundated tidal marsh with a good
rate of sedimentation in front of the seawall or
summer-dike is better than a low-lying polder with-
out sedimentation. Polders established at the edge of
the salt marsh or even at the intertidal flats suffer
