We analyze the extra strain rates that characterize a curved, thin mixing layer induced at an unsteady spilling breaker. We focus on the flow curvature, which induces some extra rates of strain that should be accounted for in algebraic-type turbulence closures. The comparison between the analytical formulation proposed by Brocchini and co-workers for a single-phase turbulent thin layer of fluid and the data, obtained from a Particle Image Velocimetry (PIV) dedicated experimental program, reveals that the order of magnitude of the extra rates of strain induced by the streamline curvature, is comparable with that of the simple shear. This differs from what observed for the geometric curvature terms and from what occurs at hydraulic jumps, typically used to model steady breakers.