2~
~ Stud) Arca
the basin of Spiekeroog only spans an area of 73.5 km
2 , whereas the basin of SyltR0m0-bight reaches 404 km' (Reise & Riethmuller 1998).
Waves
Backbarrier systems have considerably shallower waves than tidal areas without
barrier islands (for example Wurster Watt). The decrease of the waves is a result
of shelter through the chain of islands. Even with on-shore storms, the wave
propagation in the tidal channels decreases so strongly, that the wave height does
not exceed the height of wind waves (Niemeyer et al. 1994). The waves and their
significant wave heights were measured north (== seaward) of the ebb delta of the
Otzumer Balje, in the tidal inlet of the basin (Otzumer Balje), and near the sandtlat
of the Groninger Plate. North of the ebb delta the maximum values of the significant wave heights varied between 0.75 m and 2.75 m. In the tidal inlet, the height
varied between () m and 0.75 m. Similar values were measured on the Groninger
Plate. These levels clearly show that in the backbarrier tidal area of Spiekeroog a
strong attenuation of the waves takes place near the island. Inside the backbarrier
system the waves were not attenuated, sometimes even higher significant wave
heights were measured near the sandflats than in the tidal inlet Otzumer Balje. This
can be explained by the intluence of a superimposing effect and the morphology in
the Wadden Sea.
Drifting of tracers
To examine the drift of passive particles, so-called tracers, the hydrographic model
was coupled with a dispersion model to calculate the paths of the tracers using the
Monte-Carlo-Procedure (Maier-Reimer 1980). The results showed that the drift of
the tracers was intluenced by the following parameters: a) the site of start, b) the
time of start during the tidal cycle (high tide or low tide) c) and by the duration of
the observation.
After one tidal period the tracers were predominantly found near the starting
point. Starting at low tide, the tracers drifted until the turning point in the dominant
direction of the current and reached the vicinity of the starting site at the end of the
tidal period. Tracers which were started during high tide covered much longer
distances, but they returned to the starting position after one tide. With longer
duration of observation (more than one tidal period) this regularity was no longer
observed. Some tracers drifted into other water catchment areas (for example Accumer Ee) and only few were transported back into the backbarrier tidal flat of
Spiekeroog after a certain time. Tracers, which were started at high tide in the
Otzumer Balje, regularly left the observation area.
Sampling that takes place over a longer period of time (e.g. for more than 5
tidal cycles) does not offer the possibility of comparing the same water body. This
has to be taken in consideration when interpreting the data measured in the water
column.
Currents
In a backbarrier tidal flat current patterns are mainly characterized by the periodical falling and rising of the water. Areas of extreme currents are the deep channels
~ Stud) Arca
the basin of Spiekeroog only spans an area of 73.5 km
2 , whereas the basin of SyltR0m0-bight reaches 404 km' (Reise & Riethmuller 1998).
Waves
Backbarrier systems have considerably shallower waves than tidal areas without
barrier islands (for example Wurster Watt). The decrease of the waves is a result
of shelter through the chain of islands. Even with on-shore storms, the wave
propagation in the tidal channels decreases so strongly, that the wave height does
not exceed the height of wind waves (Niemeyer et al. 1994). The waves and their
significant wave heights were measured north (== seaward) of the ebb delta of the
Otzumer Balje, in the tidal inlet of the basin (Otzumer Balje), and near the sandtlat
of the Groninger Plate. North of the ebb delta the maximum values of the significant wave heights varied between 0.75 m and 2.75 m. In the tidal inlet, the height
varied between () m and 0.75 m. Similar values were measured on the Groninger
Plate. These levels clearly show that in the backbarrier tidal area of Spiekeroog a
strong attenuation of the waves takes place near the island. Inside the backbarrier
system the waves were not attenuated, sometimes even higher significant wave
heights were measured near the sandflats than in the tidal inlet Otzumer Balje. This
can be explained by the intluence of a superimposing effect and the morphology in
the Wadden Sea.
Drifting of tracers
To examine the drift of passive particles, so-called tracers, the hydrographic model
was coupled with a dispersion model to calculate the paths of the tracers using the
Monte-Carlo-Procedure (Maier-Reimer 1980). The results showed that the drift of
the tracers was intluenced by the following parameters: a) the site of start, b) the
time of start during the tidal cycle (high tide or low tide) c) and by the duration of
the observation.
After one tidal period the tracers were predominantly found near the starting
point. Starting at low tide, the tracers drifted until the turning point in the dominant
direction of the current and reached the vicinity of the starting site at the end of the
tidal period. Tracers which were started during high tide covered much longer
distances, but they returned to the starting position after one tide. With longer
duration of observation (more than one tidal period) this regularity was no longer
observed. Some tracers drifted into other water catchment areas (for example Accumer Ee) and only few were transported back into the backbarrier tidal flat of
Spiekeroog after a certain time. Tracers, which were started at high tide in the
Otzumer Balje, regularly left the observation area.
Sampling that takes place over a longer period of time (e.g. for more than 5
tidal cycles) does not offer the possibility of comparing the same water body. This
has to be taken in consideration when interpreting the data measured in the water
column.
Currents
In a backbarrier tidal flat current patterns are mainly characterized by the periodical falling and rising of the water. Areas of extreme currents are the deep channels