Elements of a Large Eddy Simulation (LES) method are examined emphasizing application to practical real-scale open-channel flows. Though LES methods have been validated by developers as specific methods and models are proposed or applied, most of these validations are not intended for hydraulic applications. We validate a LES method for analyzing open-channel flows in real applications such as design of hydraulic structures. It is shown that proper treatment of wall boundary condition is necessary and a simple but practical wall stress model is sufficient to obtain accurate mean-flow and the instantaneous large-scale turbulence structure with moderate grid resolution. It is suggested that the turbulent stresses are determined more by shear-stress carrying large motions than by energy-dissipating small scale motions. Specific examples include very high Reynolds number flows in smooth and rough surface channels with three-dimensional mean flow and large changes with separation and recirculation.