23~
<1 Stahility Properties In the Wadden Sea
and position of the "Groninger Plate" have remained essentially constant during
the past 30 years; only its topographic height has decreased slightly (see 9.4.1).
A comparison of historical maps of the Wadden Sea (Chap. 3.4) reveals a slow
but continuous change of morphology because of the rising sea level and because
morphology has had to react to coastal engineering for several centuries. On the
other hand, notwithstanding these changes, the essential characteristics of the morphology seem to have remained unchanged for hundreds or even thousands of
years: extended tidal flats with tidal channels, sandflats, mudflats and muddy sands
as well as - before coastal engineering started - extended salt marshes.
A good demonstration of the resilience and high elasticity (short return time) of
the morphology was recorded at the "Swinnplate" after the ice winter in 1995/96
(Chap. 3.4): despite the partly extreme erosion during winter, the morphology
reorganized itself within just six months. The morphology of the backbarrier areas
is determined by the geometry of the overall tidal basin (see references cited in
Chap. 3.4) and is even resilient to changes induced by events like storms or ice
winters.
Sediment types in the backbarrier area (sandflat, mudflat, muddy sand) show a
large-scale zonation pattern which is determined by the landward gradient of the
decreasing kinetic energy of the water body. The closer to the coast, the more
chance the finer particles have of settling (Chap. 3.4). The majority of the tidal
flats in the Wadden Sea comprises sandflats or muddy sands, whereas mudflats are
restricted to a small strip adjacent to the coast (and to some protected bays and
biogenic mudbanks; Chaps. 3.4 and 3.5). Historical comparisons of the Wadden
Sea show that the proportion of mudflats is decreasing - a trend which will probably continue in the future (Reise et al. 1998).
Flemming and Nyandwi (1994) interpret the small strips of mud near the dikes
as natural mudflats which are cut off by the dikes. Without dikes, or if the dikes
lay behind their current position, the mudflats would be much larger. They conclude that the energy of the water near the dikes is too high to allow for the sedimentation of finer particles. This results in a net export of fine material from the
Wadden Sea, which in turn further decreases the mudflat areas.
The construction of dikes and the reclamation of land thus constrain the natural
processes of sedimentation and morphodynamics responsible for the diversity of
structure and habitat in the Wadden Sea. This means that from a sedimentological
point of view, there is no constancy or reference dynamics beyond a time frame of,
say, one or two decades. On the other hand, the loss of mudflats proceeds so
slowly that it has neither the dynamics nor the drama sufficient to arouse public
alarm and ultimately motivate decision-makers to develop and implement conservation plans. An additional problem in this context is that trends in the Wadden
Sea may easily be masked by disturbance events (storm tides, ice winters), making
it methodologically difficult to verify the trends.
Stability mechanisms
Hydrography and morphology mutually determine each other, and together they
determine the conditions for sedimentation. On a time scale of several decades,
there is a "morphodynamical equilibrium", i.e. morphology and, in turn, tidal currents change only marginally.
<1 Stahility Properties In the Wadden Sea
and position of the "Groninger Plate" have remained essentially constant during
the past 30 years; only its topographic height has decreased slightly (see 9.4.1).
A comparison of historical maps of the Wadden Sea (Chap. 3.4) reveals a slow
but continuous change of morphology because of the rising sea level and because
morphology has had to react to coastal engineering for several centuries. On the
other hand, notwithstanding these changes, the essential characteristics of the morphology seem to have remained unchanged for hundreds or even thousands of
years: extended tidal flats with tidal channels, sandflats, mudflats and muddy sands
as well as - before coastal engineering started - extended salt marshes.
A good demonstration of the resilience and high elasticity (short return time) of
the morphology was recorded at the "Swinnplate" after the ice winter in 1995/96
(Chap. 3.4): despite the partly extreme erosion during winter, the morphology
reorganized itself within just six months. The morphology of the backbarrier areas
is determined by the geometry of the overall tidal basin (see references cited in
Chap. 3.4) and is even resilient to changes induced by events like storms or ice
winters.
Sediment types in the backbarrier area (sandflat, mudflat, muddy sand) show a
large-scale zonation pattern which is determined by the landward gradient of the
decreasing kinetic energy of the water body. The closer to the coast, the more
chance the finer particles have of settling (Chap. 3.4). The majority of the tidal
flats in the Wadden Sea comprises sandflats or muddy sands, whereas mudflats are
restricted to a small strip adjacent to the coast (and to some protected bays and
biogenic mudbanks; Chaps. 3.4 and 3.5). Historical comparisons of the Wadden
Sea show that the proportion of mudflats is decreasing - a trend which will probably continue in the future (Reise et al. 1998).
Flemming and Nyandwi (1994) interpret the small strips of mud near the dikes
as natural mudflats which are cut off by the dikes. Without dikes, or if the dikes
lay behind their current position, the mudflats would be much larger. They conclude that the energy of the water near the dikes is too high to allow for the sedimentation of finer particles. This results in a net export of fine material from the
Wadden Sea, which in turn further decreases the mudflat areas.
The construction of dikes and the reclamation of land thus constrain the natural
processes of sedimentation and morphodynamics responsible for the diversity of
structure and habitat in the Wadden Sea. This means that from a sedimentological
point of view, there is no constancy or reference dynamics beyond a time frame of,
say, one or two decades. On the other hand, the loss of mudflats proceeds so
slowly that it has neither the dynamics nor the drama sufficient to arouse public
alarm and ultimately motivate decision-makers to develop and implement conservation plans. An additional problem in this context is that trends in the Wadden
Sea may easily be masked by disturbance events (storm tides, ice winters), making
it methodologically difficult to verify the trends.
Stability mechanisms
Hydrography and morphology mutually determine each other, and together they
determine the conditions for sedimentation. On a time scale of several decades,
there is a "morphodynamical equilibrium", i.e. morphology and, in turn, tidal currents change only marginally.