Medem Channel profiles based on stand-alone model simulation and corrections with nudging

State-of-the-art process-based models have shown to be applicable to the simulation and prediction of coastal morphodynamics. On annual to decadal temporal scales, these models may show limitations in reproducing complex natural morphological evolution patterns, such as the movement of bars and tidal channels, e.g. the observed decadal migration of the Medem Channel in the Elbe Estuary, German Bight. Here a morphodynamic model is shown to simulate the hydrodynamics and sediment budgets of the domain to some extent, but fails to adequately reproduce the pronounced channel migration, due to the insufficient implementation of bank erosion processes. In order to allow for long-term simulations of the domain, a nudging method has been introduced to update the model-predicted bathymetries with observations. The model-predicted bathymetry is nudged towards true states in annual time steps. Sensitivity analysis of a user-defined correlation length scale, for the definition of the background error covariance matrix during the nudging procedure, suggests that the optimal error correlation length is similar to the grid cell size, here 80-90 m. Additionally, spatially heterogeneous correlation lengths produce more realistic channel depths than do spatially homogeneous correlation lengths. Consecutive application of the nudging method compensates for the (stand-alone) model prediction errors and corrects the channel migration pattern, with a Brier skill score of 0.78. The proposed nudging method in this study serves as an analytical approach to update model predictions towards a predefined 'true' state for the spatiotemporal interpolation of incomplete morphological data in long-term simulations.

Clay mineral assemblages in the western Baltic Sea

Surface samples and nine cores from the western Baltic Sea and marginal water bodies were investigated for clay mineral composition. The clay mineral assemblages of recent sediments are rather homogeneous. Variations result mainly from the erosion of different glacial source deposits. High percentages of illite and low kaolinite/chlorite and quartz/feldspar ratios are characteristic for this glacial source. Advection of kaolinite-rich suspensions from the North Sea is believed to account for higher kaolinite/chlorite ratios in the Mecklenburg Bight. A contribution of the rivers Trave and Oder to the western Baltic Sea is indicated by increased smectite values in marginal water bodies. They correspond to increased kaolinite/chlorite and quartz/feldspar ratios. In the main basins the river signal is diluted beyond recognition. Cores from the Arkona, Bornholm and Gotland Basins penetrate through post-Littorina muds and sediments of the Ancylus Lake/Yoldia Sea into Late Glacial sediments of the Baltic Ice Lake. Clay mineral assemblages are characterized by an increase in kaolinite/chlorite ratios from Late Glacial to Holocene sediments, with a distinct shift at each facies change. This allows the distinction and core to core correlation of main lithological units with kaolinite/chlorite ratios. Kaolinite enrichment of Holocene muds corresponds to a brackish-marine facies and may reflect influx of kaolinite-rich suspensions from the North Sea. Cores from the lagoon of the Oderhaff show fluctuations in the contributions of the two main sediment sources: river suspension and glacial deposits during the Late Glacial and Postglacial sequence. Lacustrine sediments, which were deposited prior to 5500 years B.P. are characterized by smectite, kaolinite and quartz from the drainage area of the Oder river. Erosion of coastal and offshore glacial boulder clays with the Littorina transgression supplied a marine component rich in illite, chlorite and feldspars to the brackish muds of the Oderhaff.