Flow structure and bottom friction of the nonlinear turbulent boundary layer under stormy waves

Wave environment equivalent to the full-scale nearshore storm was generated in a large wave flume, and the flow within wave boundary layer was measured using an acoustic Vectrino profiler. Analysis were made on various factors including the velocity profile, phase lead, boundary layer thickness, steady streaming, turbulence as well as bottom shear stress, and comparisons were made with existing studies using oscillatory flow tunnels. Results show that the phase lead in the boundary layer is less significant compared to previous tunnel experiments, generally smaller than 20 degrees. Free surface and vertical velocity lead to prominent wave-induced Reynolds stress at the top of the boundary layer, resulting in a thinner boundary layer. Both wave-induced Reynolds stress and wave shape asymmetry contribute to the steady streaming, making its direction either onshore or offshore. Relationship of friction factor versus relative roughness generally agrees with the exponential expression in the literatures for rough turbulent boundary layer, however the value of friction factor is larger. It is stressed that the friction factor would be several times higher if the free stream velocity is calculated using linear wave theory, which is questionable as the measured waves in the large wave flume were shown to be highly nonlinear. To compensate the nonlinear effects which significantly contribute to the maximum bed shear stress, a velocity skewness factor based on second-order Stokes wave theory was introduced. For predicting the intra-wave bed shear stress, a time-varying friction factor was also constructed, which was proved to have acceptable precision compared with experimental data. These formulations have simple, explicit expressions that can be used in practical engineering applications, especially for the nonlinear waves under nearshore storm conditions.