Sediment movement in the German Bight
Halfway through the BAW research project “Model-based analysis of long-term morphodynamic processes in the German Bight”, the first results are becoming evident.
Transport processes in the course of time
How will the tidal flats and forelands of the German North Sea coast adapt if sea level rises as a result of climate change? Answering this question is important not only for the safety of the coastal dykes, but also for the approach channels to the marine ports. On the one hand, the shallow water in the estuary zone has a significant influence on the tidal and sediment regime in the tidal rivers and thus influences the future maintenance of the fairways to the seaports. On the other, experience has shown that in an observation period of many decades, the small-scale transport processes in the German Bight and the outer zones of the estuaries are also influenced by the transport processes which occur in the North Sea as a whole.
The dimensions of these large-scale transport processes can be discerned clearly in the satellite photograph of the surface distribution of sediment plumes from the estuary mouths (Fig. 1). However, this phenomenon has not yet undergone extensive scientific study because little is known about the actual transport processes in the North Sea, in particular in the German Bight: for example, solid, comprehensive knowledge is lacking on the in situ material on the seabed, the structure of the bed or the relevant forces which initiate the transport, such as wind and sea swell. And, finally, we do not yet have tools suitable for calculating these complex transport processes.
The BAW has the lead management of this research project
In the context of a research priority of the Coastal Engineering Research Council (KFKI) as put out to tender in a competition, the BAW ’s application for a research project on the topic Auf von integrierten Modellsystemen zur Analyse der langfristigen Morphodynamik in der deutschen Bucht or (AUFMOD) [Model-based analysis of long-term morphodynamic processes in the German Bight] was successful. Nine other partners are cooperating in the project, which is being led by the BAW. Started at the end of 2009, the sponsorship runs initially until 2012 (see: www.kfki.de/prj-aufmod/en).
Integral approach in the work strategy
Cooperation between specialists from various disciplines – from oceanography through geology to the engineering sciences – characterises the integral approach in the work strategy of this joint research project. In a multi-model approach for the German Bight, the experts involved use several simulation models in five sub-projects to calculate the transport processes and their impact on the bathymetry and the diversity of bedforms. The basis of this modelling is a bed model for describing the time variations in the bathymetry of the sea bed, the forms of the bed and the composition of the sediments.
The BAW is acting as the coordinator for the joint project and is working principally on Sub-project 3. Here the specialists involved are preparing a scale-bridging, process-based 3-D modelling of the overall system of the North Sea, the German Bight, the estuaries and the Wadden Sea with a high resolution. In a further phase, these models will then be validated for time periods spanning several years, on the basis of consistent bathymetric data. And in order to demonstrate the range of possible results of morphodynamic calculations, the BAW uses several modelling methods simultaneously: UnTRIM with SediMorph and K-model, Delft3D and SWAN and MARINA.
Interaction between the sub-projects
The excellent cooperation between the various sub-projects is critical to the success of the project as a whole. Not only the data-based models, but also the numerical simulation models interact in a variety of ways:
The basis of the simulation models is a comprehensive database of the bathymetry, the sedimentology (fractional sediment covering of the bed) and the form of the bed (bed friction); this database is acquired from Sub-projects 1 and 2 and flows into Sub-projects 3, 4 and 5. Simultaneously, the alterable composition of the bed sediments, and parameters of the bed forms, supplies information for the validation and calibration of the morphodynamic modelling.
In cooperation of Sub-projects 1, 2 and 3, the model grid and relevant allocation of sediment parameters of the bed are produced for the German Bight model and the models of the sub-areas and are utilised by Sub-projects 3, 4 and 5. Furthermore, the simulation results from Sub-project 3 form the basis for the model control and comparative analyses of the long-term simulations in the sub-projects 4 and 5. Finally, the analysis of the sedimentological time series and the production of a bed classification system (Sub-project 1) as well as the knowledge of the dynamics of bed forms (Sub-project 2) are important components for the assessment and improvement of the morphodynamic models in Sub-projects 3, 4 and 5.
Promising project results
In order to achieve a comprehensive overview over the processes of sediment transport in the German Bight, it was important to first of all analyse the geomorphological changes taking place on a large scale and in the long-term. No easy task, as in order to analyse the processes that cause these changes against the background of a physically plausible chain of effects, it was necessary to collate all available data for the marine and coastal bathymetry (including locally occurring bed forms) and, above all, for the sedimentology and to format this in a way suited to the project. In addition, it was necessary – for the support of the morphodynamic analyses – to acquire consistent data on the bed morphology, for various earlier periods of time which have still to be defined, as required, in the course of the project.
For this reason, the decision was taken very early in the project to generate a comprehensive software-supported bed model, which organises jointly the data on bathymetry and sedimentology, and could process this data functionally, that is, according to specific stipulations, and make it available to the users in a manner suited to their requirements. The decision for this course of action necessitated doing this extremely comprehensive, basic development work at the beginning of the project. It will, however, prove worthwhile later in the project, all the more so as the functional bed model can be equipped with methods for data-based analyses. Furthermore, the scientists and engineers did not forget to choose a priori methods in this joint project which may lead to the development of functional products for the government research institutes which are participating in the project.
North Sea littoral states also provide data
The sediment processes in the German Bight must, on the one hand, be considered in the context of the North Sea as a whole. To do this, it was necessary to acquire comprehensive data from the states with North Sea coastlines and to include these in the functional bed model. On the other hand, processes that are limited by locality, and partly by time, can also influence sediment movement over larger areas and longer periods of time. For this reason the focus throughout the project is on specific, locally limited areas. These include regions on the shelf, that is, the coastal zone, in the foreshore (littoral) areas, in areas around islands and in the Wadden Sea. The marine waterways are of particular interest, primarily the estuary mouths, as well as certain conspicuous sedimentological features, such as the formation of the muddy area south of Helgoland, where the sediments are being laid down in a rotational current in a flat hill formation. To complement the data already acquired at various places before the project began, additional data will be gathered with measuring methods designed specifically for the project; in this way, in an analysis oriented towards measurement results, the understanding of the process that is acquired with models can be improved on a longer-term basis. The interpretation of the current measurement trips which is presently being carried out for the project, is time-consuming and the results serve primarily for direct interpretation. In the final year of the project it will be essential to examine in how far this data can be included in the simulation models.
In the context of the use of deterministic process models, the researchers are pursuing a consistent multi-model approach in the project using simulation models with differing process resolution in order to estimate the spread of the results. These process models are divided into process-based and process-oriented simulation models on the basis of their basic approach. For the kinds of model, which are based on unstructured modelling methods, mostly identical grids and boundary values are used. As far as possible, uniform post-processing will be carried out to enable a comparison of the model results with uniform methods. These agreements and regulations agreed for the entire project are intended to ensure that the phrase “integrated model system” does not remain merely empty words.
The structuring of the integrated model system has now been successfully carried out and will be completed during the year 2011. The continuation of the simulation projects for the intended investigation periods (year, decade, century) will remain a challenge in the second half of the project. In the third project year, the researchers want to achieve a conclusive synthesis of all the relevant results from the sub-projects. After all, the chief purpose is to achieve a cross-discipline, joint evaluation in order to fulfil the research objectives defined by the KFKI: to obtain a comprehensive overview of sediment movement in the German Bight.