High Frequency Tail in Measured Wave Spectra of Steep to Breaking Sea States

Abstract

Abstract A recent study investigated whether it is physically feasible to generate the tail of a JONSWAP wave spectrum in a basin, and what the influence of wave breaking is on the energy content of this high frequency tail. It has been observed in basin measurements at MARIN that in steep sea states a dip starts to develop in the high frequency tail of the spectrum. This dip occurs due to non-linear wave interactions and/or wave breaking and a resulting energy shift and dissipation. This effect cannot be compensated by adding more energy at the wave maker at these frequencies, as the energy will continue to shift/dissipate as more non-linear interaction and wave breaking occurs. The objective of this research was to investigate whether a similar energy dip would occur in real environments or whether the effect is related to the fact that in lab environments the waves are generated at one location of the basin and not through wind blowing over the sea surface continuously. In cooperation with the University of Melbourne and MetOcean Research, several examples have been found in full scale measurements as well as non-linear wave simulations where a similar energy dip appears to occur. In this paper the physics involved in the reduced energy of the high frequency tail of very steep wave spectra are discussed and basin wave measurements are compared to nonlinear simulations and full scale measurements.