Tidal turbines operate in a highly unsteady environment, which causes largeamplitude load fluctuations to the rotor. This can result in dynamic and fatigue failures. Hence, it is critical that the unsteady loads are accurately predicted. A rotor's blade can experience stall delay, load hysteresis and dynamic stall. Yet, the significance of these effects for a full-scale axial-flow turbine are unclear. To investigate, we develop a simple model for the unsteady hydrodynamics of the rotor and consider field measurements of the onset flow. We find that when

Tidal turbines encounter a range of unsteady flow conditions, some of which may induce severe load fluctuations. Rotor blades can experience stall delay, load hysteresis and dynamic stall. Yet, the range of flow conditions which cause these effects for a full-scale axial-flow turbine are unclear. In this work we carry out a parameter study across a range of flow conditions by modelling root bending moment responses. We show how unsteadiness manifests along the span of the blade, the unsteady phenomena occurring and the conditions which induce the most significant load fluctuations. We find that waves and turbulence are the main sources of unsteadiness, and that extreme waves dominate over extreme turbulence. A yaw misalignment increases the load fluctuations but reduces the maximum peak. Large yaw angles, low tip-speed ratios, and very large waves lead to dynamic stall increasing the mean loads. Conversely, added mass effects mostly attenuate the loadings.