Environmental Engineering Reference
In-Depth Information
13
Restoration of intertidal flats and
tidal salt marshes
Jan P. Bakker and Theunis Piersma
13.1 Introduction: the historical context
the taking of shellfish by hand. With the advent of
motorized power over the past century and the use of
large nets and dredges, however, human exploitation
patterns of intertidal flats have come to influence the
natural processes a great deal. It is not entirely clear
whether the same can be said for salt marshes, where,
reclamations aside, grazing has been the main human
factor. It is quite possible that the grazing by domestic
animals has replaced the grazing that took place
before human times by large herbivores. Neverthe-
less, the main external controls for the tidal lands are
the sea-level and sediment-supply regimes. Upward
sea-level movements and autocompaction - that is,
diminishing of the volume of the sediment - combine
to provide accomodation space within which marshes
build upward. Mineralogenic marshes consist of a
vegetated platform dissected typically by extensive
networks of blind-ended, branching tidal creeks.
The flow-resistant surface vegetation both traps and
binds tidally introduced mineral sediment, but also con-
tributes an organic component of indigenous origin
to the deposit. When the sea level becomes stable or
falls, however, in response to century- or millennium-
scale fluctuations, the organic sediment component
becomes dominant and mineralogenic marshes are
transformed into organogenic ones. Because peat is
such a porous and permeable sediment, and there is
little or no tidal inundation, organogenic marshes in
north-west Europe typically lack surface channels
for intertidal drainage (Allen 2000). At present very
little peat marsh occurs in Europe (Dijkema 1984).
In contrast, the north-east coast of North America
This chapter deals with intertidal flats and the adjac-
ent tidal salt marshes. Tidal flats and salt marshes occur
along the edges of shallow seas with soft sediment
bottoms where the tidal range is considerable, at least
a metre or so (Eisma
et al.
1998, van de Kam
et al.
2004). The low-lying
intertidal
areas are largely
barren except for the occurrence of
Zostera
(sea- or
eelgrass) or
Spartina
meadows and reefs formed by
shellfish or tubeworms. Intertidal areas are inundated
at least once a day, and make a place for more
irregularly inundated areas of salt marsh higher up.
In tropical areas, and even some benign temperate areas
such as northernmost New Zealand, the upper parts
of intertidal areas may be covered by mangrove
forests rather than salt marsh. Such mangroves have
the tendency also to cover the regularly inundated parts
of intertidal soft sediments, thus reducing the extent
of mudflats in many tropical areas. No intertidal
deposits or salt marshes occur at high latitudes (fur-
ther north than 70 -73°N). Here coastlines are either
ice-covered for most of the year or disturbed by
moving ice too frequently for soft sediment deposits
or vegetation to build up.
Intertidal flats and salt marshes are under complex
natural controls. In most parts of the world, and
certainly in Europe, intertidal flats and salt marshes
experienced human exploitation from the mid-
Holocene period onwards. Most of the human exploi-
tation of intertidal flats was relatively unintrusive for
a long time, as it consisted of small-scale fishing and
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