Modeling of sediment resuspension in Neva Bay during strong wind events

Abstract

Wind waves play the major role in the bottom sediments resuspension in coastal areas. However, the available results of field observations imply that only the consideration of the combined effect of currents and wind waves can give correct model estimates of resuspension intensity. In present study a three-dimensional circulation model of Neva Bay (Gulf of Finland, Baltic Sea) was used to simulate the bottom sediments resuspension due to currents and wind waves. The characteristics of wind waves are calculated from the wave model SWAN. They are then used to calculate the bottom shear stress resulting from the nonlinear wave-current interaction. The resuspension model takes into account the difference in physical characteristics of two main bottom sediments in Neva Bay: sand and silt, with a possibility to also allow for the presence of clay fraction. The variable sinking velocity of suspended particles is used, account being taken of the effects of settling velocity reduction due to high concentrations called hindered settling, and flocculation. The model also takes into account cohesion and packing effects of the bottom sediments. Satellite images with total suspended matter (TSM) data were used to calibrate the resuspension model. Also the results of model runs aimed at estimating the intensity and frequency of resuspension events during extremely strong winds in 2008 over the Neva Bay are presented. The model runs carried out for the ice-free period from May 1 until November 30 have shown that the significant resuspension events in Neva Bay occurred during autumn from the end of September until the end of November and were caused mainly by strong western winds.