Environmental Engineering Reference
In-Depth Information
protected by seawalls in the 13th century. Initially
arable crops were grown on the levees, but ditching
and the construction of embankments, dated from the
1st century
BC
to the 2nd century
AD
, allowed crops
to be grown on the salt marshes. The majority of the
marshes were exploited for livestock grazing from
the early settlements onwards, and haymaking from
the 1st-3rd centuries
AD
onwards. The high frequency
of subfossils of
Juncus gerardi
and the low frequency
of
Elymus
spp. in artificial mounds led to the conclu-
sion that pristine salt marshes were scarce during most
of the occupation period (Esselink 2000). Nowadays
salt marshes that have never been grazed by livestock
are only found at the eastern point of the Wadden
Sea islands. The oldest dates back to 1930 and is found
on the island of Terschelling, the Netherlands.
Mature marshes were embanked (see section 13.1).
The incentives for embankments have gradually
changed during the 20th century from land claims for
agriculture to coastal protection. During the early
20th century large-scale accretion works have been
started to create artificial salt marshes that were to
be reclaimed for agriculture, but some decades later
this was no longer economically feasible due to
changes in both socio-economic conditions and agri-
cultural policies. From about the 1970s there has
been a growing recognition that the remaining salt
marshes, though largely developed as a result of
human intervention, have an important nature-
conservation interest (Esselink 2000). During the 1980s
a no-net-loss policy was developed in the Netherlands
for the existing mainland marshes. The accretion
works protect the existing marshes from erosion. Hence
this policy has become the main reason to maintain
accretion works in the Netherlands during the 1980s
and 1990s. At the same time, increasing areas of arti-
ficial salt marshes were designated as nature reserves
in Denmark, Germany and the Netherlands, and were
included in national parks. Livestock grazing decreased,
especially in Germany. Nowadays about 40% of the
salt marshes of the Wadden Sea support livestock
grazing. The cessation of the creation of artificial
marshes, including zonation, resulted in a loss of
pioneer communities. The existing marshes undergo
a process of maturation, and pioneer and young
marshes become lost as a result. This process is still
enhanced when livestock grazing on the marshes
ceases.
North American salt marshes have a different his-
tory. Along the east coast, tidal marshes formed within
the last 3 - 4000 years as sea-level rise slowed to
about 1 mm yr
−1
, favouring the establishment of the
initial colonizer
Spartina alterniflora
, a 1-2 m tall grass
(Niering 1997b). The accumulation of organic matter
controls the accumulation of inorganic matter, not the
reverse. Below-ground plant material is very import-
ant in maintaining salt marshes once they are estab-
lished. During the Colonial period (17-19th centuries)
salt marshes were mown, grazed, ditched and em-
banked in order to make them more suitable for agri-
cultural exploitation. In conjunction with these early
activities, some ditching and diking were done to
regulate tidal flooding. However, these impacts were
minor compared to those that followed the Industrial
Revolution (1850s) when, with increased mechaniza-
tion, marshes were dredged for marinas, filled for
development, ditched for mosquito control, filled with
dredge spills and tidal-gated in order to prevent upland
flooding. The subsequent invasion of
Ph. australis
in
disturbed coastal wetlands resulted in outcompeting
of the native plant species and desiccation as a result
of strong transpiration out of its habitat (Bertness 1999).
Wetland protection laws since the 1970s, and no-net-
loss policies have led to restoration efforts. Moreover,
with the past decade Open Marsh Water Management
has been widely practised using biological control which
favours small fish to control mosquitos and simultan-
eously promotes restoration (Niering 1997b).
13.4.2 Erosion, reclamation and
embankment
Salt marshes have been eroding rapidly in south-west
England during the past 150 years, and particularly
in the past few decades. The mechanisms of erosion
include landward recession of the marsh edge, wave
erosion of the marsh surface, internal dissection due
to enlargement and coalescence of tidal creeks and mud
basins, and direct removal due to human activities.
Increased wind and wave energy is supposed to con-
tribute most strongly to erosion. Increased mean sea
level and tidal range are underlying factors leading
to coastal 'squeezing' of salt marshes between the sea
and seawalls (Pye 2000, Cooper
et al.
2001). Estab-
lishing seawalls on the intertidal flats makes them
