One recurring question is: why does the seabed have to be resurveyed again and again? To answer this question, some facts about the changeability of the seabed will be explained in the following. Especially in the coastal waters of the German North Sea, the tides and other currents are continually changing the morphology of the seabed. The following example shows an area in the Elbe estuary, near Gelbsand.
In the time from 1994 to 1998, the surveying frequency was once a year. The graph shows a 1994 section (blue) and a 1998 section (red) for comparison.
Where in 1994 one still sees a fairway with a depth of over 8m, a tidal flats area has formed just five years later. The maximum difference (not visible in the graph) is almost 10 metres!
An areal comparison of this region in 1997 and 2000 shows similar changes.
This example shows that many areas, especially shallow waters, are continuously changing and require repeat surveys every one to three years. As changes in the seabed are not normally visible, surveys have to be performed systematically (single beam or sweep surveys) in order to obtain reliable images of the seabed topography.
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