3.3 Hydrographic ConditIOns
27
counterclockwise, sweeping tides along the coast, i.e. from west to east in Lower
Saxony, with a semidiurnal tide of 12 hours and 25 minutes. Near the coast the
tidal amplitude and the direction of the tide are increasingly influenced by the
interaction of the tides with the morphology of the area.
Tidal range
The mean tidal range is calculated as the difference between the heights of the
mean high tide and the mean low tide. With a tidal range of 2.7 m at the western
end of Spiekeroog island and of 2.9 m at the mainland the conditions in this area
are classified as mesotidal.
Because of the slack tide and the reflection of the tide, the tidal range is higher
near the coast than on the seaward margin of the area. An increasing tidal range
can also be observed at single sites in the backbarrier system of Spiekeroog. At the
water gauge of Spiekeroog the tidal range lies between 2.64 m and 2.88 m, while
at the gauge of the Groninger Plate the tidal range varied between 2.76 m and
3.cl2 m. These values were taken from measurement campaigns in spring and
autumn and include neap as well as spring tides.
Sea level
The sea level in the backbarrier system of Spiekeroog is influenced by the impulse
of the North Sea, which in turn is the result of the tidal forces and the meteorological conditions. Southerly and easterly winds cause a decrease of the sea level,
whereas north-west winds raise the sea level at the coast (Reineck 1987). During
the winter months of the study period, maximal differences of the sea level were
measured with ± 60 em on average. In the remaining months the deviation from
the mean was lower, so that the time was characterized by mean sea levels. The
highest sea level during the study period was recorded on the 28 'h of January 1994
with +910 cm on the tide gauge (equals +410 cm above mean sea level). This was
the highest record at this tide gauge since it was installed in 1987. A comparison
with the tides of earlier storm floods of the years 1906, 1962 and 1976 was not
possible.
Water volumes
The transport of the water masses into the catchment area occurs to 70 % over the
tidal inlet of the Otzumer Balje. The remaining water masses enter the area over
the water sheds. Modelling the conditions during a neap/spring tidal cycle in summer 1994 showed that the water volume during low tide reached values between
30 and 90 million m', and during high tide between 240 and 330 million m 3 The
difference between the water volume at mean tidal level (equals mean sea level)
(110 million m') and the maximal volume during high tide showed that the tidal
volume can reach 220 million m' water. This means that at maximum extension
the water volume had trebled.
A comparison of the tidal basin of Spiekeroog with that of the Sylt-R0m0-bight
shows that with a mean water volume of 570 million m' the basin of Sylt contains
substantially more water and that its tidal volume of 559 million m' is much higher
than that of the backbarrier system of Spiekeroog. But it must be considered that
27
counterclockwise, sweeping tides along the coast, i.e. from west to east in Lower
Saxony, with a semidiurnal tide of 12 hours and 25 minutes. Near the coast the
tidal amplitude and the direction of the tide are increasingly influenced by the
interaction of the tides with the morphology of the area.
Tidal range
The mean tidal range is calculated as the difference between the heights of the
mean high tide and the mean low tide. With a tidal range of 2.7 m at the western
end of Spiekeroog island and of 2.9 m at the mainland the conditions in this area
are classified as mesotidal.
Because of the slack tide and the reflection of the tide, the tidal range is higher
near the coast than on the seaward margin of the area. An increasing tidal range
can also be observed at single sites in the backbarrier system of Spiekeroog. At the
water gauge of Spiekeroog the tidal range lies between 2.64 m and 2.88 m, while
at the gauge of the Groninger Plate the tidal range varied between 2.76 m and
3.cl2 m. These values were taken from measurement campaigns in spring and
autumn and include neap as well as spring tides.
Sea level
The sea level in the backbarrier system of Spiekeroog is influenced by the impulse
of the North Sea, which in turn is the result of the tidal forces and the meteorological conditions. Southerly and easterly winds cause a decrease of the sea level,
whereas north-west winds raise the sea level at the coast (Reineck 1987). During
the winter months of the study period, maximal differences of the sea level were
measured with ± 60 em on average. In the remaining months the deviation from
the mean was lower, so that the time was characterized by mean sea levels. The
highest sea level during the study period was recorded on the 28 'h of January 1994
with +910 cm on the tide gauge (equals +410 cm above mean sea level). This was
the highest record at this tide gauge since it was installed in 1987. A comparison
with the tides of earlier storm floods of the years 1906, 1962 and 1976 was not
possible.
Water volumes
The transport of the water masses into the catchment area occurs to 70 % over the
tidal inlet of the Otzumer Balje. The remaining water masses enter the area over
the water sheds. Modelling the conditions during a neap/spring tidal cycle in summer 1994 showed that the water volume during low tide reached values between
30 and 90 million m', and during high tide between 240 and 330 million m 3 The
difference between the water volume at mean tidal level (equals mean sea level)
(110 million m') and the maximal volume during high tide showed that the tidal
volume can reach 220 million m' water. This means that at maximum extension
the water volume had trebled.
A comparison of the tidal basin of Spiekeroog with that of the Sylt-R0m0-bight
shows that with a mean water volume of 570 million m' the basin of Sylt contains
substantially more water and that its tidal volume of 559 million m' is much higher
than that of the backbarrier system of Spiekeroog. But it must be considered that