4.2 Impacts of the Sea on Coastal Fresh Water Systems
51
Fig. 4.2 Permeability of the upper soil layer along the Northwest German North Sea coast. High =
k f < 1 ∗ 10 –5 m/s, medium = 1 ∗ 10 –5 m/s ≤ k f < 1 ∗ 10 –4 m/s and low = k f > 1 ∗ 10 –4 m/s, spatially
variable = highly variable soil layer unable to allocate a specific permeability class. Source: adapted
from Ahlhorn et al. (2010)
4.2 Impacts of the Sea on Coastal Fresh Water Systems
The tidal oscillation is an important boundary condition influencing the water cycle of
coastal areas. Caused by the gravitational forces of the earth, the moon and the sun, the
sea water level periodically increases and decreases approximately shaped like a sine wave.
The gravitational attraction of moon and earth governs the rhythm of the oscillation due to
their small distance compared to the distance between sun and earth. Therefore, the lunar
orbit determines the phase and the frequency of the oscillation of the sea water level. Along
most of the coastlines, the periodic time is 12:24 hours. The amplitude of the oscillation
depends on the inference of the lunar tide with the solar tide (spring tide when both tidal
effects are adding up, versus neap tide when the solar tide is subtracted from the lunar tide).
The tidal forces have stronger impacts along coastlines compared to the high seas, and the
tidal range is strongly influenced by the shape of the coastline. The average amplitude
of the oscillation (the tidal range) ranges from several decimeters until more than 10 m
(Kelletat, 1989).
Figure 4.3 exemplifies a period of two days of a tidal gauge located in the North Sea
(tidal gauge Leyhörn). The tidal range is about 3 m, oscillating around mean sea level with
an oscillation period of around 12.5 hours. The consecutive tidal maximum (as well as the
minimum) reach approximately the same water level.
51
Fig. 4.2 Permeability of the upper soil layer along the Northwest German North Sea coast. High =
k f < 1 ∗ 10 –5 m/s, medium = 1 ∗ 10 –5 m/s ≤ k f < 1 ∗ 10 –4 m/s and low = k f > 1 ∗ 10 –4 m/s, spatially
variable = highly variable soil layer unable to allocate a specific permeability class. Source: adapted
from Ahlhorn et al. (2010)
4.2 Impacts of the Sea on Coastal Fresh Water Systems
The tidal oscillation is an important boundary condition influencing the water cycle of
coastal areas. Caused by the gravitational forces of the earth, the moon and the sun, the
sea water level periodically increases and decreases approximately shaped like a sine wave.
The gravitational attraction of moon and earth governs the rhythm of the oscillation due to
their small distance compared to the distance between sun and earth. Therefore, the lunar
orbit determines the phase and the frequency of the oscillation of the sea water level. Along
most of the coastlines, the periodic time is 12:24 hours. The amplitude of the oscillation
depends on the inference of the lunar tide with the solar tide (spring tide when both tidal
effects are adding up, versus neap tide when the solar tide is subtracted from the lunar tide).
The tidal forces have stronger impacts along coastlines compared to the high seas, and the
tidal range is strongly influenced by the shape of the coastline. The average amplitude
of the oscillation (the tidal range) ranges from several decimeters until more than 10 m
(Kelletat, 1989).
Figure 4.3 exemplifies a period of two days of a tidal gauge located in the North Sea
(tidal gauge Leyhörn). The tidal range is about 3 m, oscillating around mean sea level with
an oscillation period of around 12.5 hours. The consecutive tidal maximum (as well as the
minimum) reach approximately the same water level.