23.07.2010

World Heritage Site Wadden Sea: Geology

The Wadden Sea has only developed over the past 8,000 years and thus is a very young ecosystem in terms of geomorphology and evolution. It shows the post-ice age development of a sand barrier coast in a moderate climate with the conditions of a rising sea level.
Highsand Kachelot - a symbol for the dynamic development of the Wadden Sea

Outstanding examples representing major stages of the Earth’s history from the Lower Saxon Wadden Sea (criterion No. VIII of the application guidelines for world heritage)

The Wadden Sea ecosystem is a mud flat and barrier island system with extensive salt marshes, which is unique in size and variety.

Typical features of this sand and mud flat system include the high dynamics and the continuous change of Wadden, troughs and tideways.

In the Lower Saxon part of the Wadden Sea, the gigantic mud flat tideway systems of the Hohe Weg Watt (located in the Weser estuary), Am Knechtsand (Wursten coast) or on the far sides of the East Frisian Islands impressively demonstrate the dynamic interplay between sea and land in all its varieties.

Featuring considerable differences between their landward and their seaward sides, the salt marshes and dunes, the barrier islands from Borkum to Wangerooge, located in the west between the Ems estuary and the Jade river, protect the tidal zone between the islands and the mainland with its differentiation into sand, mixed and mud flats from the North Sea. In the east, between Jade river and Elbe estuary, where the higher tidal range primarily favours sandbanks but not barrier islands, a highly dynamic system of constantly changing sandbanks, troughs and tideways extends across a large area. Sand river islands on high ground such as Knechtsand may assume the character of a real island over the years, which, however, may also disappear again. The geology of the Lower Saxon Wadden Sea continues to be characterised by the Ems, Jade, Weser and Elbe estuaries as well as by the large Ley and Jade bays with their extensive salt marshes. These natural features dominate the landscape and the sea panorama and are intensified by the constant struggle of mankind with the area, which has been going on for over a thousand years; the main dike line provides impressive proof of that.

The Lower Saxon Wadden Sea contains many examples of post-ice age coastal morphology and the dynamic interplay of physical and biological processes. Particularly illustrative parts of the world natural heritage include the Geest cliff near Cuxhaven, where the Wadden Sea still directly adjoins the mainland and no measures of coast protection have been taken as yet, the Dollart, Ley and Jade bays as testimony to the latest transgressions, and the worldwide unique Schwimmende Moor ("swimming bog”) near Sehestedt as a remnant of the formerly huge areas of raised bog.

Despite human interference, the development and rejuvenation of landforms including the entire range of habitats are ensured due to the permanent dynamic natural processes, and thus the functions of the ecosystem are preserved. The appearance and disappearance of the Kachelotplate, a huge sandbank west of the island of Juist showing first signs of the generation of an island demonstrates that the world natural heritage area still allows enough room for the geomorphologic processes to take place without any disturbance.

The morphological development of the Wadden Sea ecosystem is mostly dominated by the tides; however, wind loads and waves play an important role, too. The shifting of the sand that everybody can experience at high winds on the beaches of the East Frisian Islands and which eventually causes the formation of dunes and their dynamics or the changes to entire mud flat and tideway systems after a severe storm tide demonstrate the impacts of these forces impressively.

The geomorphologic developments of dune islands, salt marshes and mud flats are inseparably linked. The basis is formed by the sediments within the system that are transported by water and wind and sorted and distributed in accordance with the physical conditions. Erosion and sedimentation ensure a constant redistribution. This way, dunes, salt marshes or sand, mixed or mud flats develop in certain areas. Protected by dunes, large salt marshes emerge on the islands. The chain of dune islands and offshore sandbanks protects the mud flat behind it and triggers the development of the large currents between the islands with the associated tideway systems. The waves and currents in turn influence the conditions of sedimentation. Close to the islands, sand flats dominate which are followed by extensive transitional areas of mixed flats, while mud flats are predominant along the mainland coastline and mostly in the bays such as the Jade bay where the current is less intense. In contrast to similar systems in other parts of the world, eelgrass or cordgrass grow only occasionally on the flats here since the great dynamics and the mobility of the sediments prevent a full coverage with erect vegetation in the Wadden Sea. Due to this, a coastal landscape of unique character has emerged, characterised by sand and flat areas without vegetation divided by a complicated pattern of tideways and currents.

In the different stages of the dunes on the islands, the tideways, mud flats and salt marshes, a broad spectrum of bio-geomorphologic processes can be found. What is special about it is that the changes of the geomorphologic features of the landscape do not take thousands of years but may already occur within a man’s lifespan. Only 50 years ago, the island of Spiekeroog, for example, was only about half of its present size. Sand areas formerly free of vegetation have turned into extensive dune and salt marsh habitats.

These morphodynamic adaptations are possible because the wadden sea system can still naturally respond to changes and largely develop without disturbance. The development of Spiekeroog is a good example of the strong hydraulic and aeolian dynamics, which forms the eye-catching morphological changes that vary enormously in terms of spatial and temporal extension: from the shifting of entire islands over centuries to the generation of tiny dunes protected by a shell for only a few hours.

Worldwide, there are only very few areas that permit such a direct observation of the dynamic adaptation to bio-geomorphologic processes within one generation as the "dynamic hot spots” of the world natural heritage area: the east ends of the East Frisian Islands, the processes of alluvial deposit and erosion in the salt marshes or flat areas that become dry during low tide.