Spreading and sedimentation of spill from dredging for the Øresund fixed link - Comparison of modelling results with geochemical identification of spilled sediments

During the period 1995-1998, in total 14·10⁶ tonnes of seabed materials (limestone and clay till) were dredged as part of the construction of a fixed link between Sweden and Denmark. Of this amount, approximately 0.6·10⁶ tonnes (4.2%) were spilled in the water. The purpose of the present investigation is to validate the modelling results with respect to accumulation of spilled sediments, using geochemical identification of spilled sediments. During construction, the sediment spill was measured on a continuous scale. Using the sediment spill data and a 2 dimensional numerical model, developed by DHI, the sediment spreading and the sedimentation to the seabed were modelled. Sediment samples were collected from 31 positions in Øresund in June 1999, and the amount of spilled sediments accumulated at the seabed was measured, using geochemical identification. The results were compared with the modelling results. The hindcast modelling shows that the majority of the spilled sediments (approximately 73%) had accumulated in the deepest parts of the Øresund strait, north and south of the alignment. The geochemical identification indicates a higher accumulation of spilled sediments in the shallow-water areas close to the link than indicated in the modelling. The reason is believed to be filtering of the water column and bioturbation in the seabed, caused by mussels and other benthic fauna. In order to model the resuspension dynamics of the deposited spilled sediments correctly it is necessary to take into account the effect of the benthic animals.