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The rheological behaviors of suspended sediments are crucial for investigating near‐bed particle dynamics; for these investigations, the viscosity of a highly concentrated suspension is primarily focused on quantifying its resistance to deformation. By using a rotational viscometer, the relationships between relative viscosities (ηr) for different sediment types and their volumetric concentrations (ϕ) are studied for low‐to high‐concentration slurries. The results show that the conventional Einstein formula for diluted sand severely underestimates ηr. In the case of pure silts and non‐cohesive quartz, ηr demonstrates a gradual linear increase, reaching values around order of 101 as ϕ increases within the range of 0.2–0.4. Meanwhile, the ηr values of clays exhibit an exponential rise before leveling off at a plateau. Specifically, the ηr values of kaolinite, montmorillonite, and bentonite rise with ϕ lower than 0.3 and reach to plateau of values of thousands, which rise more rapidly than chlorite and illite. The modified viscosity model based on Costa (2005, https://doi.org/10.1029/2005GL024303) agrees reasonably well with observations and produces similar ηr ∼ ϕ relationships. In addition, the model is coupled with a hydrodynamic model to simulate the deposition of a thickened tailings slurry and a one‐dimensional dam break. The proposed model performs well in predicting the quasi‐equilibrium profiles of the non‐Newtonian fluid, which are validated by available analytical solutions. The results suggest that future models should be focused on the effects of flow properties, especially for non‐Newtonian fluids, and on large‐scale modeling applications, which can increase the accuracy of predictions of the transport characteristics of sediments and pollutants in rivers, lakes, and coastal areas. Plain Language Summary: Viscosity describes the resistance of a fluid, and it is defined as the ratio of shear stress to shear rate. Flow that is laden with sediment particles increases relative viscosity (ηr). Einstein (1906, https://doi.org/10.3929/ethz‐a‐000565688) conducted pioneering work to relate ηr with solid volumetric concentration (ϕ); this work is valid for dilute suspensions with coarse, non‐cohesive particles. Nevertheless, the near‐bed layer in muddy coastal waters is usually abundant with hyper‐concentrated cohesive sediment flocs, which may result in deviations using conventional equations for the viscosity. In this study, 79 experiments with eight different sediments (i.e., five clays, two silts, and quartz) are conducted to investigate the ηr ∼ ϕ relationship. We propose a modified viscosity model to further illustrate the impacts of sediment concentration, composition, and size on relative viscosity. For this model, only one parameter can be adjusted based on our experimental results with different minerals. In future research, our model could be expanded to incorporate the influence of mud/sand fractions, thereby capturing the rheological characteristics of mixed sediments more comprehensively. Key Points: The relative viscosity was measured for clays, silts, and quartz under different concentrations or flow environments by using a viscometerThe conventional Einstein formula severely underestimates the relative viscosity for fluid containing high concentrated cohesive sedimentsAn improved viscosity model was proposed to predict the ηr ∼ Φ relationship for different sediment suspensions [ABSTRACT FROM AUTHOR]