LABORATORY EXPERIMENTS OF BICHROMATIC WAVE GROUPS PROPAGATION ON A GENTLE SLOPE BEACH PROFILE AND ENERGY TRANSFER TO LOW AND HIGH FREQUENCY COMPONENTS

Abstract

This work presents a first analysis of experimental data studying the influence of the frequency bandwidth on the propagation of bichromatic wave groups over a constant 1:100 beach slope. The use of a large spatial cross-shore resolution and Bi-Spectral analysis techniques allows the identification of nonlinear energy transfers along the propagation of wave groups. During wave-group shoaling, nonlinear coupling between the primary wave frequencies results in a larger growth of superharmonics for narrow-banded wave conditions, increasing the skewness of the wave and leading to eventual instabilities and earlier high frequency (hf) wave breaking compared to the broad-banded wave condition. Regarding the growth of low frequency (lf) component, the data analysis has shown a larger growth of the incident bound long wave (IBLW) for broad-banded wave conditions. It is generally assumed that the transferred energy from the primary wave components to subharmonics does not affect the short wave energy budget. Here, the opposite is hypothesised, and a larger growth of the IBLW for broad-banded wave conditions is accompanied of a larger reduction of the primary wave components, a reduced growth of hf components and, consequently, a reduction in the growth of hf wave asymmetry during wave group shoaling. Conversely for narrow-banded wave conditions, a reduced IBLW growth is associated with a larger growth of hf wave asymmetry. After hf wave breaking, within the low frequency domain (lf), the IBLW decays slightly for narrow-banded conditions, consistent with a reduction in radiation stress forcing. This involves a nonlinear energy transfer from the wave group frequency back to hf components. The remaining lf energy, Outgoing Free Long Wave (OFLW), reflects back at the shoreline. However, for broad-banded wave conditions, strong dissipation and minimal reflection of lf components occurs close to the shoreline, which might be caused by lf wave breaking.