36 Die Küste, 74 ICCE (2008), 31-44 
wave energy, fine-grained particles have been eliminated from the sheltered backbarrier areas 
behind the islands (e.g. van Straaten and Kuenen, 1957; Flemming and BartholomA, 1997). 
The deposits in the channels cover a wide range of different sediment types. It can reach 
from very coarse lag sediments of Pleistocene till (Ahrendt, 1992) over strongly cohesive 
Holocene layers (Berner et al., 1986; Ricklefs and Asp, 2006) to outcrops of peat. Most com 
mon, however, are recent sandy to partly muddy deposits (e. g. Poerbandono and Mayerle, 
2005). On pure sandy channel beds, mega-ripples and submarine dunes are very common 
(Ulrich, 1979; Mayerle et al., 2005). Especially in the main tidal channels of North Frisia, 
where coarser sandy sediments prevail, the seabed is partly characterized by sand waves of 
up to several meters height (Ulrich and Pasenau, 1973; Hennings et al., 2004). 
The system of the tidal flats has always undergone an intensive internal rearrangement 
of deposited sediments. According to Zeiler et al. (2004), bed-load transport rates of 
400 m 3 nr 1 a -1 in the North Frisian Wadden Sea are around five times higher than in the ad 
jacent open water zone of the German Bight. Especially under the influence of an increasing 
sea level and/or an augmented mean tidal range, a deepening of inlets and tidal channels is 
expected (Hofstede, 2002) and can be partly observed (e.g. Higelke, 1998). Another evi 
dence of the adaptation of the depositional environment to a changing hydrodynamic forcing 
is the landward migration of characteristic morphological elements. This holds for the sandy 
barriers with a migration rate between 15 and 27 m a -1 (Hofstede, 1997) along the meso-tidal 
coasts of East and North Frisia as well as for the small supra-tidal sands (Ehlers, 1988; 
Runte, 1994) and exposed sand banks of the open tidal flats in the low macro-tidal innermost 
German Bight (Ricklefs et al., 2005). 
Input from the English Channel, erosion of the East Anglian coast as well as discharge 
from the rivers can be considered as the main sources of fine-grained material (silt and clay) 
in the North Sea. Based on clay mineral analyses, Pache et al. (2008) could demonstrate that 
the Helgoland mud area is a main deposition centre for suspended particulate matter dis 
charged by the River Elbe. The distribution of fine-grained material (especially clay) is cou 
pled to hydrodynamic transport patterns in the German Bight. According to Eisma and Irion 
(1988) most of the fine-grained sediment deposition takes place in the tidal flats (Wadden 
Sea). While 3-7 X 10 6 tons of suspended particulate matter (SPM) is accumulated in the Ger 
man Bight, 12-19 X 10 6 tons per year are transported into the Norwegian Channel (Dyer 
and Moffat, 1998). 
2.3 Estuaries 
In the Elbe and Weser Estuary, the seabed is dominated by distinct morphological fea 
tures such as channels and sand spits, sand waves in channels or longitudinal sand bars (Rei- 
neck and Singh, 1973) dependent on the strong tidal influence, grain size distribution and 
sediment availability. 
Pronounced migrating bed-forms occur on the sandy river bed of the Elbe and Weser 
estuary. Wever and Stendal (2000) measured a migration speed of a minimum of 45 cm d~' 
in the Elbe River. For the Weser/Jade estuary, minimum migration speed is higher by a factor 
of 2 (Schrottke et al., 2006). The migration of these bed-forms shows a cyclic pattern which 
correlates with the tidal cycle.