Estimating wave-induced bottom shear stresses in the Gulf of Bothnia

Sea floor erosion can be induced by waves, bottom currents and ice. Although the Gulf of Bothnia in the Baltic Sea is a relatively small basin, the record value of significant wave height is 8.1 m with highest individual wave of 15 m. In the present climate the seasonal ice cover limits the wave growth during winter time, but in the future climate it is estimated that ice extent will reduce which can lead to more severe wave climate. Thus, the effect of waves on the bottom sediment erosion is expected to increase. We used wave model WAVEWATCH III to do a 30-year high-resolution hindcast for the Gulf of Bothnia. The hindcast wave parameters were validated against wave buoy and altimeter wave measurements to ensure good quality of the wave hindcast. The hindcast near-bottom orbital velocities and amplitudes were used to estimate wave-induced bottom shear stress. These calculations are based on the wave spectra, taking into account the effect of different wave heights and wave lengths. The results were used to evaluate the extent of areas that experience significant wave-induced bottom stress under the present climate. Furthermore, the results show how often and for how long periods the wave-induced stress exceeds the critical values for sediment resuspension to take place. The estimates of the critical values for resuspension are calculated utilising the seabed sediment data available for the Gulf of Bothnia. The adequacy of the results is evaluated by comparing the known erosional seafloor areas to the ones estmated based on the hindcast values. This study is part of the SmartSea project of the Strategic Research Council of the Academy of Finland (grant no. 292 985).