Wave-induced sediment resuspension in the Finnish Archipelago, Baltic Sea: Combining small-scale in situ measurements and large-scale numerical model simulations

Abstract. Sediment resuspension, driven by wind-wave-induced shear stresses, plays a crucial role in coastal water quality, biogeochemical cycles, and the dispersal of pollutants and organisms. If the shear stress from waves exceeds an erosion threshold, or critical shear stress, sediments are resuspended from the seabed. This critical shear stress is an essential parameter in sediment transport models, as it determines sediment erodibility. In this study, we implemented a high-resolution (20 m) spectral wave model to simulate wave-induced near-bottom velocities across the complex archipelago of southwestern Finland. Near-bottom shear stresses from the model and their respective critical values were estimated using seabed data, with results compared to critical shear stress values obtained through laboratory testing of in situ sediment samples. Model data suggested that the critical shear stress could be exceeded over 70 % of the time in certain areas. However, laboratory-determined critical shear stresses were 3–8 times higher than those derived from the model based on median grain size, with modelled shear stresses rarely exceeding the measured critical values. These discrepancies likely stem from unaccounted-for biological and biogeochemical properties of the sediments, which cannot be captured by a simple grain size-based model. We estimate that the accuracy of the wave model data used in this study are of secondary importance compared to the uncertainty of determining the critical shear stress.