:1.3 Hydrographic Conditions
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and the water sheds. In the channel of Otzumer Balje the direction of the dominant
current is directed by the topography. With the incoming tide the water tlows
south-east and with low tide the water streams north-west. At mean wind conditions the velocity of the currents in the deep channel are roughly similar during
high tide and low tide. They are highest at half tide when they reach maximal
values of 0.7-1.3 m S'. At the watersheds the direction of the rising and falling
waters can not be distinguished that clearly. The direction of the current varies, no
clear maximum can be distinguished and the velocities of the current are low with
values between 0-0.2 m S'.
The velocities of the currents are not only influenced by the morphology but
also by the spring and neap tide and by the wind. From neap to spring tide the
velocity increases. With wind coming from north and west, the velocities intensify.
Easterly and southerly winds caused no intensification of the current speed but a
reduction depending on the wind strength.
The two studied sandtlats, Groninger Plate and Swinnplate showed a great variability in velocity and direction of the current. Comparing both sandflats, the velocity of the current was higher at the Swinnplate than at the Groninger Plate. At
the Swinnplate a decrease in velocity was found from the edge to the centre. Particularly low values were found in the east of the flat, because this area is sheltered
by the higher plateau of the tlat. The directions of the ebb and flood tide showed
only low variations during a tidal period as well as during neap/spring tidal cycles.
In general, the rising water flowed east and the decreasing water flowed off in
westerly direction. Because of the higher current velocity during falling water the
Swinnplate was designated as ebb tide dominated.
At the Groninger Plate the current was considerably more variable than at the
Swinnplate. The directions of the ebb and tlood tide varied in the different areas of
the tlat. In the central and western areas the incoming waters flowed in southeasterly direction on to the tlat and off the tlat to the west. In the northern part of
the tlat the water tlowed east-south-east during tlood and westward during ebb
tide. The higher situated easterly area is tlooded from south to east and the ebb
water moves in all directions. Because of the higher current velocity during tlood,
this tlat was designated to be tlood tide dominated.
Size of the intertidal areas
Intertidal areas (tidal flats) are periodically submerged at high tides. According to
the model calculation, the size of the areas emerging at low tide varies, because of
the intluence of the neap/spring tidal cycle and meteorological conditions. A model
calculation for the period from I st to 10'" of August 1994 showed fluctuations in
the size of the intertidal areas. Maximum extensions of tidal tlats during a
neap/spring tidal cycle were calculated to reach 40 and 59 km 2 (48-70 % of the
total area) and a minimal size was calculated with values between 10 and 17 km 2
(11-20 % of the total area).
Areas, which fell dry for more than six hours were generally located near the
mainland or near the island. Inside the backbarrier tidal flat of Spiekeroog only
parts of the Janssand, the Martensplate and the Eversplate emerged for more than
six hours. Groninger Plate and Swinnplate are also intertidal areas, their exposure
times varied between one to four hours, depending on neap/spring tide and the
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and the water sheds. In the channel of Otzumer Balje the direction of the dominant
current is directed by the topography. With the incoming tide the water tlows
south-east and with low tide the water streams north-west. At mean wind conditions the velocity of the currents in the deep channel are roughly similar during
high tide and low tide. They are highest at half tide when they reach maximal
values of 0.7-1.3 m S'. At the watersheds the direction of the rising and falling
waters can not be distinguished that clearly. The direction of the current varies, no
clear maximum can be distinguished and the velocities of the current are low with
values between 0-0.2 m S'.
The velocities of the currents are not only influenced by the morphology but
also by the spring and neap tide and by the wind. From neap to spring tide the
velocity increases. With wind coming from north and west, the velocities intensify.
Easterly and southerly winds caused no intensification of the current speed but a
reduction depending on the wind strength.
The two studied sandtlats, Groninger Plate and Swinnplate showed a great variability in velocity and direction of the current. Comparing both sandflats, the velocity of the current was higher at the Swinnplate than at the Groninger Plate. At
the Swinnplate a decrease in velocity was found from the edge to the centre. Particularly low values were found in the east of the flat, because this area is sheltered
by the higher plateau of the tlat. The directions of the ebb and flood tide showed
only low variations during a tidal period as well as during neap/spring tidal cycles.
In general, the rising water flowed east and the decreasing water flowed off in
westerly direction. Because of the higher current velocity during falling water the
Swinnplate was designated as ebb tide dominated.
At the Groninger Plate the current was considerably more variable than at the
Swinnplate. The directions of the ebb and tlood tide varied in the different areas of
the tlat. In the central and western areas the incoming waters flowed in southeasterly direction on to the tlat and off the tlat to the west. In the northern part of
the tlat the water tlowed east-south-east during tlood and westward during ebb
tide. The higher situated easterly area is tlooded from south to east and the ebb
water moves in all directions. Because of the higher current velocity during tlood,
this tlat was designated to be tlood tide dominated.
Size of the intertidal areas
Intertidal areas (tidal flats) are periodically submerged at high tides. According to
the model calculation, the size of the areas emerging at low tide varies, because of
the intluence of the neap/spring tidal cycle and meteorological conditions. A model
calculation for the period from I st to 10'" of August 1994 showed fluctuations in
the size of the intertidal areas. Maximum extensions of tidal tlats during a
neap/spring tidal cycle were calculated to reach 40 and 59 km 2 (48-70 % of the
total area) and a minimal size was calculated with values between 10 and 17 km 2
(11-20 % of the total area).
Areas, which fell dry for more than six hours were generally located near the
mainland or near the island. Inside the backbarrier tidal flat of Spiekeroog only
parts of the Janssand, the Martensplate and the Eversplate emerged for more than
six hours. Groninger Plate and Swinnplate are also intertidal areas, their exposure
times varied between one to four hours, depending on neap/spring tide and the