Environmental Engineering Reference
In-Depth Information
19.3
THREATS AND DISTURBANCES
of artifi cial sand dikes during the twentieth century
(Dijkema 1987). These marshes always had a natural
drainage pattern with creeks.
Embankment is the most defi nitive threat for the
occurence of salt marshes, and occurs worldwide.
Other threats are erosion, reduction of the tidal ampli-
tude and decrease in salinity of inundating water.
Intensive livestock grazing is considered a threat in
many parts of the world. Overgrazing results in short
unifom swards without tall vegetation for plant and
animal species to survive. Also, the potential for sedi-
ment capture and wave attenuation has reduced. More-
over, evaporation and salinization take place. However,
in Europe most salt marshes have always been exploited
for either grazing or cutting. As most salt marshes are
agriculturally exploited, the cessation of exploitation is
a sudden change in long-term management and thus
causes disturbance. After abandonment , a single
plant species can become dominant with subsequent
losses of characteristic halobiontic species (i.e. groups
or organisms tolerating saline conditions). However,
some exceptions wil be discussed below.
19.3.2 Erosion, reduction of salinity and
tidal amplitude
Salt marshes have been eroding rapidly in south-
western England during the past 150 years, and par-
ticularly 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). Establishing seawalls on the inter-
tidal fl ats makes them more vulnerable to erosion. In
seminatural marshes with sedimentation fi elds, the
abandonment of accretion works results in retreating
of the marsh edge (Esselink 2000).
As one result of future rising sea levels caused by
global climate warming, widespread salt marsh erosion
is predicted. A long-term experiment was carried out
on this subject, at the Wadden Sea island of Ameland,
the Netherlands, in a seminatural marsh system with
drainage assured by natural creeks. As a result of gas
extraction, soil subsidence of 10 cm was observed over
a 15-year period, affecting both the low- and high-salt
marsh (Dijkema 1997). The net elevation of low-
marsh plots did not change, indicating that sedimen-
tation kept pace with subsidence. In contrast, net
elevation of the high-marsh plots decreased by 10 cm,
indicating that no extra sedimentation took place.
Neither in the lower nor in the upper plots of the salt
marsh did vegetation change (Dijkema 1997).
Another important cause of losses of salt marshes is
coastal protection by shortening the coastline of estua-
rine coasts. Desalinization causes the transformation
of salt marsh communities into communities adapted
to freshwater conditions. Continued grazing by live-
stock retards the losses of halophytic plant species
(Westhoff & Sykora 1979). Moreover, grazing benefi ts
short turfs that are favoured by winter-staging geese,
as in the former salt marshes. Undisturbed succession
results in scrub and forest with characteristic bird
species (van Wieren 1998).
19.3.1
Embankments
In Europe, the fi rst seawalls were constructed against
the increased risk of fl ooding in the tenth century in
the northern Netherlands (Oost & de Boer 1994), and
during the eleventh century in adjacent Germany
(Behre 1995). The entire North Sea coastline of the
Netherlands and Germany was protected in the thir-
teenth century. The fi rst seawalls were constructed in
the salt marshes above the level of mean high tide, and
hence, in most places, a strip of unprotected salt marsh
remained in front of the seawalls. New marshes devel-
oped after the construction of the fi rst seawalls, espe-
cially in sheltered bays (Oost & de Boer 1994). The
new marshes, which originated during the twelfth-
fourteenth centuries, may have evolved without
human intervention. However, human intervention
was very likely from at least the seventeenth century
onwards. Several techniques have been applied to
promote both vertical accretion and horizontal expan-
sion of salt marshes. At present, the majority of main-
land salt marshes are seminatural, resulting from
sedimentation fi elds with intensively engineered ditch-
ing for drainage, such as those currently found in
Denmark, Germany, the Netherlands (Dijkema 1984)
and the United States (Pye 2000). Back barrier marshes
developed in the shelter of dunes, but also in the shelter
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