3.4 Morphology and Sedimentology
31
3.4
Morphology and Sedimentology of the Spiekeroog
Backbarrier System
Andreas Hild
3.4.1
Origin of the Wadden Sea
The origin of the intertidal areas of the southern North Sea is controlled by the
influence of tides, ongoing transgressive conditions and significant sediment advection. During the last Ice Age 15000-18000 years ago, the sea level was about
110-130 m lower than today (Jelgersma 1979). The melting of the glacial ice-caps
in the early phase of the Holocene transgression caused a relatively fast sea level
rise. Approximately 9000 years b. p. the sea level reached the northern Doggerbank and was about 45 m below the present sea level. From 8600 to 71 00 years
b. p. the sea level rose at a rate of approximately 2 cm a-i up to 15 m below mean
sea level. The base of the east Frisian islands, approx. 20 m below mean sea level,
was reached between 8000 and 7500 years b. p. (Flemming 1990). Afterwards, a
reduced rate of sea level rise occurred until 6500 years ago, up to 5-10 m below
the present sea level (Streif 1990).
3.4.2
History of the Coastal Evolution
The historical evolution of the east Frisian Wadden Sea is characterized by significant morphological and hydrographical changes (e.g. Ehlers 1988). These processes can be mainly traced back to human activities and have continued, with decreasing tendency, until the 20
lh
century. The analysis of historical maps shows the
reduction of the Harle Bay catchment area from 180 km
2
in the year 1362 to only
60 km 2 at present owing to land reclamation (Flemming 1990). This suggests a
larger tidal volume in the Wadden Sea before diking took place (Flemming 1990;
Flemming & Davis 1(94). As a result of the reduction of the catchment area, large
amounts of sand were relocated from the former ebb-delta leading to an eastward
growth of the island Spiekeroog. Due to the sea level rise, a slow shift of the islands occurs in south-easterly direction following the direction of the mean storm
fronts (Nummedal & Penland 1981). This was verified by the results of short drillcores from the islands that reached down to the Holocene and/or Pleistocene base
(Streif 1(90).
3.4.3
Morphology and Morphodynamics
The Wadden Sea ecosystem is characterized by the variability of morphology and
sediment grain size composition. The morphology of the backbarrier areas is
mainly formed by the influence of tidal currents. In the main inlet of the Spieker-
31
3.4
Morphology and Sedimentology of the Spiekeroog
Backbarrier System
Andreas Hild
3.4.1
Origin of the Wadden Sea
The origin of the intertidal areas of the southern North Sea is controlled by the
influence of tides, ongoing transgressive conditions and significant sediment advection. During the last Ice Age 15000-18000 years ago, the sea level was about
110-130 m lower than today (Jelgersma 1979). The melting of the glacial ice-caps
in the early phase of the Holocene transgression caused a relatively fast sea level
rise. Approximately 9000 years b. p. the sea level reached the northern Doggerbank and was about 45 m below the present sea level. From 8600 to 71 00 years
b. p. the sea level rose at a rate of approximately 2 cm a-i up to 15 m below mean
sea level. The base of the east Frisian islands, approx. 20 m below mean sea level,
was reached between 8000 and 7500 years b. p. (Flemming 1990). Afterwards, a
reduced rate of sea level rise occurred until 6500 years ago, up to 5-10 m below
the present sea level (Streif 1990).
3.4.2
History of the Coastal Evolution
The historical evolution of the east Frisian Wadden Sea is characterized by significant morphological and hydrographical changes (e.g. Ehlers 1988). These processes can be mainly traced back to human activities and have continued, with decreasing tendency, until the 20
lh
century. The analysis of historical maps shows the
reduction of the Harle Bay catchment area from 180 km
2
in the year 1362 to only
60 km 2 at present owing to land reclamation (Flemming 1990). This suggests a
larger tidal volume in the Wadden Sea before diking took place (Flemming 1990;
Flemming & Davis 1(94). As a result of the reduction of the catchment area, large
amounts of sand were relocated from the former ebb-delta leading to an eastward
growth of the island Spiekeroog. Due to the sea level rise, a slow shift of the islands occurs in south-easterly direction following the direction of the mean storm
fronts (Nummedal & Penland 1981). This was verified by the results of short drillcores from the islands that reached down to the Holocene and/or Pleistocene base
(Streif 1(90).
3.4.3
Morphology and Morphodynamics
The Wadden Sea ecosystem is characterized by the variability of morphology and
sediment grain size composition. The morphology of the backbarrier areas is
mainly formed by the influence of tidal currents. In the main inlet of the Spieker-