Environmental Engineering Reference
In-Depth Information
19.1 INTRODUCTION:
THE HISTORICAL CONTEXT
North America, the vertical accretion rate of salt
marshes is directly related to the accumulation of
organic matter, rather than to inorganic matter
(Turner et al . 2000 ).
Global sea level rise after the last glacial period
caused a poorer drainage of the coastal hinterland and
a subsequent rise of the groundwater table in the adja-
cent low-lying inland zones which became marshy,
allowing peat formation over the underlying Pleis-
tocene subsurface. As a consequence of increased
marine infl uence, the freshwater marsh transformed
into areas of tidal salt marshes and intertidal mudfl ats,
or brackish lagoons. As a result, the basal peat layers
were covered by marine sediments, and with continu-
ous sea level rise the area became totally submerged.
This transgressive process continued until the mid-
Holocene, after which the coastline stabilized more or
less at its present position. As a result of the decline in
sea level rise, sedimentary processes became increas-
ingly dominant (Esselink 2000).
Coastal regions around the world are not only
affected by natural processes. In north-western Europe,
the coastal zone became increasingly shaped by human
activities undertaken to increase agricultural land
area, transport links and urbanization and coastal
defence. Most human activities in and exploitation of
intertidal fl ats were relatively unintrusive for a long
time, consisting primarily of small-scale fi shing and the
taking of shellfi sh by hand. With the advent of industri-
alization, however, over the twentieth century, and the
use of large nets and dredges, human exploitation pat-
terns of intertidal fl ats have come to infl uence the
natural processes a great deal indeed. It is not entirely
clear whether the same can be said for salt marshes,
where, embankments aside, grazing by domestic
animals has been the main human factor. Loss of extent
has had a signifi cant impact on salt marshes, resulting
in trunctation of the upper zone. Canalization and
dredging of estuaries resulted in widening and deepen-
ing of channels and loss of the pioneer zone. Both
resulted in loss of associated species. It is quite possible
that the grazing by domestic animals has replaced the
grazing that took place before human times by wild
large herbivores. The Baltic and southern European
coasts have been less affected by coastal defence works.
See Davy et al . (2009) for an overview on embankments
and land claim along the European coastline.
In Europe, the concepts of 'natural' and 'seminatu-
ral' salt marshes have been defi ned for the interna-
tional Wadden Sea (Esselink et al . 2009 ). Natural salt
This chapter deals with tidal salt marshes adjacent to
intertidal fl ats. Salt marshes and intertidal fl ats occur
along the edges of shallow seas with soft sediment
bottoms where the tidal range is considerable, at least
a meter or so (van de Kam et al . 2004), but also in the
absence of tides, as in the Baltic Sea. Low-lying inter-
tidal 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 mudfl ats in many tropical
areas. No intertidal deposits or salt marshes occur at
high latitudes (further 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. Hence, salt marshes
are mainly found in the temperate zone.
Salt marshes and intertidal fl ats are under complex
natural controls. The main external controls for the
tidal lands are the sea level and sediment supply
regimes. Upward sea level movements and auto-
compaction - that is, diminishing of the volume of the
sediment - combine to provide accomodation space
within which marshes build upwards. Mineralogenic
marshes consist of a vegetated platform dissected typi-
cally by extensive networks of blind-ended, branching
tidal creeks. The fl ow-resistant surface vegetation both
traps and binds tidally introduced mineral sediment,
but also contributes an organic component of indi-
genous origin to the deposit. When the sea level next
to mineralogenic marshes becomes stable or falls,
however, in response to century- or millennium-scale
fl uctuations, 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-western
Europe typically lack surface channels for intertidal
drainage (Allen 2000). At present very little peat
marsh occurs in Europe, except for the Baltic Sea
(Dijkema 1984). In contrast, the north-eastern coast
of North America features large coastal peat deposits
(Niering 1997). Along the south-eastern coast of
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