The spatial and temporal behaviour of stratification in the Fehmarnbelt strait

Abstract

The amount of suspended sediment spill released during dredging activities in the Fehmarnbelt strait is monitored along the excavated trench to prevent negative effects on local ecology. Stratification due to the interaction of saline water from the North Sea and fresh water from the Baltic Sea forces bi-directional flow of the layer above and below the density gradient, causing bi-directional spread of the dredging plume perpendicular to the trench as well. During such an event, monitoring on both sides of the trench is required. By mapping out the spatial and temporal behaviour of stratification, a prediction can be given on where and when measuring on both sides of the trench will be needed to include the baroclinic effect. Figures created with monitoring data show that the density profile is influenced by the salinity, rather than the temperature. Therefore, stratification predominantly depends on the in and outflow of water with respect to the Baltic Sea. The figures also show a larger density gradient during out than inflow. It can also be observed that where the water depth is restricted to 10 meters, wind and bottom friction mix the entire water column. Therefore, stratification occurs predominantly during outflow in sections deeper than 10 meters, indicating the need for monitoring the bi-directional plume spread during such circumstances. Whether stratification occurs during inflow in sections deeper than 10 meters likely depends on the duration and strength
of the wind forcing and the initial strength of the density gradient prior to the inflow event. Further analysis should be done to confirm this. Signs of Ekman transport, return flows and the deflection of the currents towards deeper water can also be observed in the measurement figures. Since these processes affect the plume spread direction, additional research can be done on the behaviour of the current direction in the Fehmarnbelt.