Environmental Engineering Reference
In-Depth Information
features large coastal peat deposits (Niering 1997b).
Along the south-east coast of North America the
vertical accretion rate of salt marshes is directly
related to the accumulation of organic matter, but not
to inorganic matter (Turner et al. 2000).
Sea-level rise caused a deterioration of the drainage
of the hinterland and a subsequent rise of the
groundwater table in the adjacent low-lying inland zone
parallel to the coast. This zone became marshy, which
allowed peat formation on top of the underlying
Pleistocene subsurface. As a consequence of increased
marine influence, the fresh marsh transformed into an
area of tidal salt marshes and intertidal mudflats or
brackish lagoons. As a result the basal peat layer was
covered by marine sediments, before the area became
totally submerged. This transgressive process con-
tinued until the mid-Holocene, after which the coast-
line stabilized more or less at its present position. As
a result of the decline in sea-level rise, sedimentary
processes became more dominant (Esselink 2000). The
stratigraphic sequences that accumulated during the
Holocene beneath coastal marshes and high intertidal
flats typically present an alternation on a vertical scale
of silts (mineralogenic marshes, high intertidal flats)
and peats (organogenic highest intertidal/supratidal
marshes). Coastal barriers are represented by local accu-
mulation of sand and/or gravel. The silts and peats form
vertical alternations which are generally considered to
be related to fluctuations of the sea level around the
general upward trend (Allen 2000). The coastal land-
scape was not only affected by natural processes.
Gradually it became more and more shaped by human
activities, eventually resulting in artificial salt marshes
in front of seawalls along large parts of the coast.
The first seawalls were constructed against the
increased risk of flooding in the 10th century in the
northern Netherlands (Oost & de Boer 1994), and
during the 11th century in adjacent Germany (Behre
1995). The entire coastline was protected in the 13th
century. The first seawalls were constructed in the salt
marshes above the level of mean high tide, and hence
a strip of unprotected salt marsh remained generally
in front of the seawalls. The remaining salt marsh was
agriculturally exploited and was managed for coastal
protection. New marshes developed after the con-
struction of the first seawalls, especially in sheltered
bays (Oost & de Boer 1994). The new marshes, which
originated during the 12th to the 14th centuries, may
have evolved without human intervention. However,
human intervention was very likely from at least the
17th century onwards. Several techniques have been
applied to promote both vertical accretion and hor-
izontal expansion of salt marshes. At present the
majority of mainland salt marshes are artificial, such
as those currently found in Germany, the Netherlands
(Dijkema 1984) and the UK (Pye 2000). Back-barrier
marshes developed in the shelter of dunes, but also
as a result of the construction of artificial sand dikes
during the 20th century. These marshes always had a
natural drainage pattern.
13.2 Patterns and processes
13.2.1 Patterns and processes in
intertidal flats
The zonation of intertidal flats is largely a function
of the duration of submergence during high tide.
Parts of intertidal flats that are only submerged dur-
ing extreme spring high tides (and are not covered
by salt marshes or mangroves) are called supratidal
flats. Soft sediment shores in general, and intertidal
sand- and mudflats in particular, in contrast to rocky
shorelines, are in a state of dynamic flux (e.g.
Edelvang 1997). The nature of the sediments is deter-
mined by the sediment types available, the nature of
the currents, the tides and wind-generated waves, the
presence of ecosystem-engineering types of organism
(e.g. reef-building oysters, but also infaunal species
producing faecal pellets) and human activities such as
bottom-fishing and dredging. Good general introduc-
tions to the nature and occurrence of intertidal soft
sediment habitats can be found in Reise (1985),
Raffaelli and Hawkins (1996), Eisma et al. (1998) and,
especially, Little (2000).
Coarse-grained sediments are mostly found on
wave-exposed shores, but may even be found at
sheltered places if the currents are forceful enough.
Coarser sands can also be found in the surf-zone, form-
ing a beach with a steeper slope than the zone of fine-
grained sediments below (e.g. Rogers et al. 2003).
Fine-grained sediments accumulate in areas with some
shelter, with lower currents and less wave action. As
testified by the turbid colour of the surface waters above
sheltered muddy sediments, even here there is no stasis
 
Search WWH ::




Custom Search