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Abstract: The influence of three different species of tubicolous worms (Pygospio elegans, Polydora ciliata and Lagis koreni) on the hydrodynamic bottom roughness length (z 0) was analysed in this study. Flume experiments and geospatial methods were combined to determine the potential interactions between worm tubes and the near-bed flow regime and the resulting effects on sediment transport in the south-western Baltic Sea. The three selected species are common in the area of interest (3539 km2). Their species-specific population densities were taken from existing macrozoobenthos datasets and transferred into a Geographic Information System (GIS). In analogy to the sediment roughness length, the hydrodynamic roughness lengths generated by the tubicolous worms were calculated and corresponding sediment transport values, derived from flume experiments with artificial tube lawns, were geospatially analysed using GIS. In order to show the direct influence of worm tubes on the surrounding sediment surface flume experiments were conducted at two given current velocities of 20 cm s−1 for sediment displacement effects and 5 cm s−1 for deposition effects. The roughness length was shown to increase by a factor of 2 to 30 in the presence of biogenic structures such as the worm tubes. The near-bed hydrodynamic conditions are significantly influenced at low roughness densities through independent or isolated flow conditions at 0.7 to 1.9% and at high roughness densities between 4.2 and 7.5%, resulting in unaffected sediment surfaces through “skimming flow”, as well. The GIS analysis revealed that this effect occurs over 4% (137 km2) of the area of investigation, whereas sediment displacement at roughness densities between 0.7 and 1.9% due to increased turbulence is the predominant effect over 33% (present on 1172 km2) of the area of investigation. These findings reveal the important influence of species-generated microtopography on sediment transport processes. [Copyright &y& Elsevier]

Abstract: The development of the bed bathymetry of an experimental dumping area was followed over three-and-a-half years by means of multibeam echosounder techniques. Two types of material were discharged in the bight of Mecklenburg in the Baltic Sea in approximately 20 m of water depth. One set of the discharges was 2900 m3 of glacial till and the other set was a 2400 m3 mixture of glacial till, sand and minor amounts of cohesive matter. Only approximately 2500 m3 (86%) of the glacial till and 1500 m3 (63%) of the mixed soil materials were deposited on the seabed. This means that already during the dumping process a considerable part of the sediment material drifted away. The glacial till formed crater-like rings of 30 m diameter with peaks up to 1.4 m above seabed, whereas the spatial structure of the mixed soil material was somewhat more diffuse, but with similar magnitudes in the peaks and troughs. The morphological changes were small and their quantification required a high measuring precision in the order of few cm in the vertical. The dominant processes of surface deformation was flattening of peaks and filling of troughs. The speed of this process decreased with horizontal scale: structures of less than 4 m horizontal extension had a trend to disappear within less than five years, whereas structures of larger than 8 m extension showed little change and are estimated to remain detectable for many decades. In contrast to the reworking of the matter inside the dumping structures, no net transport of material out of the dumping area could be detected. Extrapolating the observed morphological changes into the future it is estimated that without significant decrease in internal shear strength of the disposed till the structures will persist for at least 70 years. This can be attributed to the high internal stability of the dumped glacial till and the low hydrodynamic forces present at the seabed in this region. [Copyright &y& Elsevier]

To simulate transport of clastic material in the Baltic a sediment transport module is linked to a Baltic Sea Model that is based on the Modular Ocean Model-MOM3. In order to describe the properties of the seabed sediment parameters as mean grain size, critical shear velocity and bed roughness length must be provided as input data to the numerical model system. To obtain maps of these quantities for the Baltic Sea area the proxy-target concept is applied. As proxy-variable the mean grain size of the sediment types is used. For different sediment samples the critical shear velocity was measured and serves as the target variable. Using the relation between the sediment classifications based on the mean grain size (proxy) and the measured critical shear velocity (target) a map of the critical shear velocity in the Baltic is derived. In January 1993 several extreme strong storm events occurred in the Baltic. Using this period for a model calculation maximum values of current and wave induced bottom shear velocities were obtained. Comparing these model results with the critical shear velocity distribution provided by the proxy-target concept we identify potential erosion areas. Further we show the transport path of material initially deposited in the Mecklenburgian Bight. [ABSTRACT FROM AUTHOR]