Sedimentological parameters and erosion behaviour of submarine coastal sediments in the south-western Baltic Sea

The aim of this study was to evaluate the erodibility of submarine coastal sediments for the purpose of modelling sediment dynamics in Mecklenburg Bay, south-western Baltic Sea. Erosion thresholds derived from experiments with a device microcosm on cores of fine sand (n=5, mean grain size=132 mu m) and mud (n=5, medium silt size, mean=21 mu m), collected at different times of the year, were compared to theoretical critical shear stress velocities based on grain-size measurements. For this purpose, a sedimentological map of natural surface sediments was constructed for the study area. Calculated values for critical shear stress velocities (u* cr-Hjulstrom) are 1.2 cm s -1 for fine sand, and 3.75 cm s -1 for cohesive mud. At the mud station, erosion experiments showed an initial transport of the fluffy surface layer (u* cr-initial) at a mean critical shear stress velocity of 0.39 cm s -1. Initial rolling transport at the fine sand station for single sand grains was recorded at values of 0.5 cm s -1. At higher shear stress velocities, the two sediment types showed diverging erosion behaviour. Measurable erosion (e>5.0 times 10 -6 kg m -2 s -1) of fine sand starts at a mean critical shear stress velocity (u* cr-erosion) of 1.15 cm s -1 whereas fluffy surface material on mud cores was eroded at mean u* cr-erosion of 0.62 cm s -1. This indicates that measured erosion thresholds at the fine sand site fit well to calculated critical shear stress velocities whereas calculated erosion thresholds for cohesive mud are roughly 6 times higher than measured values. As erosion behaviour at the mud station was dominated by fluffy surface material, the comparability of measured and calculated threshold values may be reduced. The underlying silt-sized sediment itself was stable due to cohesive effects. This behaviour has to be taken into consideration by using sediment types instead of mean grain sizes for mapping and modelling sediment dynamics. A comparison of the near-bottom hydrodynamic conditions in the study area and experimentally derived critical shear stress velocities suggests that particle transport is controlled by storm events whereas under calm conditions shear stress velocities do not exceed the critical values. [PUBLICATION ABSTRACT]