Morphodynamics of the Wadden Sea and its barrier island system
Highlights
► We review on research to morphodynamics of Wadden Sea and its barrier islands system. ► We analyse the knowledge requirements from a coastal zone management point of view. ► We identify the knowledge gaps to inspire and stimulate research.
Introduction
The Wadden Sea is a coastal wetland of exceptional size, great beauty and richness in unique natural assets (Fig. 1). It contains the largest coherent tidal flat area in the world. It has developed a unique geomorphology with its specific combination of physical factors and their interaction with the regional biota. In 2009 the Dutch and large part of the German Wadden Sea became a UNESCO World Heritage Site.
The Wadden Sea, the tidal inlets and the North Sea coasts of the Wadden Islands exhibit a dynamic behaviour. The dynamics concerns the flow of water and air, and transport, erosion and sedimentation of sand and mud. These processes result in an ever-changing morphology (topography/bathymetry) of the islands, channels and tidal flats. This dynamic development – of shape and nature of the Wadden Sea area – form, together with the biotic systems, the Wadden system.
Although the dynamic development of the morphology is a very characteristic and natural aspect of the Wadden system, there are concerns about developments due to changing environmental conditions and to human interferences. Climate change and sea-level rise – including changes in tidal range such as observed in the German Bight – are major challenges for guarantying the safety against flooding, for conserving the unique character of the morphological and ecological system and for managing the human activities in harmony with nature. Examples of human interferences in the Wadden Sea are the closures of the Zuiderzee and the Lauwerszee, land reclamation, mining of gas and salt leading to land subsidence, dredging and dumping activities including maintenance of navigation channels and sand nourishment for the maintenance of the North Sea coast. Coastal projects outside the Wadden Sea may be of influence as well. Examples of these projects include the land reclamation and harbour development of Maasvlakte 2 and all planned/proposed major works along the Dutch coast such as the construction of artificial islands or large-scale nourishments. Knowledge on the morphodynamics of the Wadden Sea is therefore essential to protect, conserve and wisely manage the Wadden Sea.
Morphodynamic research of the Wadden Sea system has to deliver the essential knowledge and tools (such as models) to assess the qualitative and quantitative coastal developments that will occur in response to changing environmental conditions and human measures.
Morphodynamical development is the result of the interaction between water movement, sediment transport and bottom changes (Fig. 2). Likewise, morphodynamic research includes studies of hydrodynamics, sediment transport, morphology and interaction with biological processes. Biological processes play an important role in (de)mobilizing sediment and in generating eco-morphological landscape units such as mussel beds and salt marshes. Hydrodynamic studies have to include the effects of tide-, wind- and density-driven flow patterns as well as the impact of waves and wave-driven currents. Sediment transport concerns sand, mud and sand-mud mixtures. Therefore morphological studies should not only concern changes of geometry and bathymetry, but also changes in substrate or bottom composition ought to be considered. The bottom composition is of great importance for the ecological system. Mud is a carrier for nutrients and contaminants and the mud content in the bottom has a substantial impact on flora and fauna. Conversely, flora and fauna may also affect water movement, sediment transport and morphological changes. Biogeomorphology is a recently developed discipline focusing on the interaction between morphological and biological processes and their impact on coastal geomorphology.
The objective of this paper is to identify gaps in our knowledge on coastal morphodynamics of the Wadden Sea area. For that purpose we present an overview of the state of the art on morphodynamic research and confront this knowledge with major questions that have to be addressed by coastal zone managers and policymakers who share a responsibility for the complex Wadden Sea ecosystem.
Although biological processes are very important for the Wadden Sea (Fig. 2) we will focus on the physical processes, i.e. the interaction between water movement, sediment transport and changes of morphology and bottom composition. First a review is given on the state of the art on the existing knowledge in Chapter 2. In Chapter 3 the knowledge requirement is evaluated from the integrated coastal zone management perspective. The knowledge gaps are identified in Chapter 4 by combining Chapter 2 and Chapter 3.
Section snippets
Large-scale sediment budget
By large-scale sediment budget we mean sediment volume changes due to sedimentation and erosion of large areas at the scale of an entire tidal basin in the Wadden Sea and the corresponding North Sea coastal area. These changes give insights into the import–export of sediment through the tidal inlets. Understanding of the large-scale sediment budget is important for the coastal maintenance as well as for the sustainable management of the Wadden Sea.
In 1990 the concept of Basal Coastline (“Basis
Knowledge requirements
Knowledge of the morphological development is primarily needed for policy and management, not only for current affairs, but also for the long-term. Furthermore, knowledge requirements also arise from scientific research and the curiosity-driven wish to understand the behaviour of a system. Therefore, it appears to be difficult to clearly distinguish the need for knowledge based on these two different categories. The same understanding can be used to understand the behaviour of a system and to
Knowledge gaps
The knowledge gaps are identified by combining the previous two chapters. They are summarized below using the same outline as is used in Chapter 2.
Ethical statement
The work described in or manuscript has not been published previously. It is not under consideration for publication elsewhere. Its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere including electronically in the same form, in English or in any other language, without the written consent of the copyright-holder.
Acknowledgements
This paper is based on a previous document or position paper (in Dutch) initiated by the Wadden Academy (part of the Royal Dutch Academy of Arts and Sciences) to discuss relevant geoscientific issues of the Wadden Sea (see Speelman et al., 2009; Wadden Academy, 2009). This position paper addresses three major subjects: the geological development and structure of the region (including the presence and development of mineral/natural resources), the Holocene coastal evolution and “present-day”
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