Wave-induced sediment resuspension potential in the Finnish Archipelago, Baltic Sea: integrating field measurements with large-scale numerical model simulations

Sediment resuspension, driven by wind-wave-induced shear stress, is a key process influencing coastal water quality, biogeochemical cycles, and the transport of pollutants and organisms. The critical shear stress, τcr, is a central parameter in sediment transport models, since initiation of motion can occur when wave-induced shear stress exceeds the critical value. In this study, we implemented a high-resolution (20 m) spectral wave model to simulate near-bottom orbital velocities across the complex archipelago of southwestern Finland. We then used laboratory measurements from in situ sediment samples to determine a model for the critical shear stress that accounts for physical properties using the median grain size and the dry bulk density, and the time-varying biological variation using chlorophyll a. Our proposed model, τcrd50,ρB,Chla(t), explained 66 % of the variation of the measured critical shear stress for our data collected from three different sediment types (Mud, Sand and Mixed sediments). The modelled mean critical shear stress differed between sediment classes, with values of 0.49 N m−2 for Mud, 1.56 N m−2 for Sand, and 1.02 N m−2 for Mixed sediments. The variability in the critical shear stress around the mean values driven by a non-constant biological contribution was approximately 30 % for Mud and Sand, and approximately 50 % for Mixed sediments. Finally, we used a class-level map of the sea floor and the in situ grain size data to translate the wave model orbital velocities to near-bottom shear stresses. Based on the numerical model data, the critical shear stresses from the newly proposed model, τcr(d50,ρB,Chla(t)), were rarely exceeded based on only wave-induced motions in most of the model grid, but could, nonetheless, be exceeded to up around 10 % of the times in smaller areas. This study highlights the importance of incorporating both physical and biological factors – and their temporal dynamics – into sediment transport models to achieve reliable predictions of critical shear stresses and resuspension potential.