Effects of Wave Coherence on Longshore Variability of Nearshore Wave Processes

Coherent waves are pairs of waves having identical frequency and waveform with constant phase difference leading to a stationary wave interference in the wavefield of phase-resolving wave models. The resulting stationary interference due to the presence of coherent water waves results in the longshore variation of the wave height in longshore uniform topography. Consequently, this variation affects the radiation stress throughout the domain and leads to the longshore variation in mean water level, mean current field, shear wave amplitude, wave setup, and wave runup. Here, we have developed a new method of wave energy spectrum discretization for the phase-resolving wave models like FUNWAVE-TVD, which either eliminates the coherent waves at the offshore boundary condition or limits them to a controlled number. Then, effects of the coherent waves on some nearshore wave processes are probed using both a flat bottom case as well as a longshore uniform sloping beach. The results shed light on the effects of wave coherence on the nearshore wave processes and demonstrate the usefulness of the newly developed wavemaker as a tool for the scientific community to further assess the hydrodynamics in nearshore environments. Plain Language Summary It is well known that two wave trains having identical wave frequency and different wave angles result in alongshore non-uniformity of the wave field. These waves are called coherent waves that can affect nearshore wave processes, leading to alongshore variations in wave height, mean water level, and surfzone currents on an alongshore uniform beach. However, quantitative understanding of such effects under field conditions is rather limited. In this study, we develop a new method to generate directional random wave fields with various degrees of wave coherence in a phase-resolving Boussinesq wave model, which either eliminates the coherent waves at the offshore boundary condition or limits them to a controlled number. Then, effects of wave interference on nearshore wave processes are probed using numerical experiments on a flat bottom and an alongshore uniform barred beach. The results shed light on the role coherent waves play in alongshore variations of wave setup, wave runup, longshore currents, and rip currents. The new method can be implemented into other phase-resolving wave models for nearshore processes to quantify the effects of wave coherence on surfzone hydrodynamics and morphodynamics.

Automated summary

Coherent waves are pairs of waves with the same frequency and waveform, but with constant phase difference, leading to stationary wave interference in the wave field. The presence of coherent water waves can cause longshore variations in wave parameters, affecting radiation stress and other wave characteristics.