Frequency Downshifting in Wave Spectra in Coastal Zone and Its Influence on Mudbank Formation

Abstract

The evolution of wind waves in coastal zones leads to changes in the shape of the wave spectrum. Along the coast of Kerala, due to the presence of mudbanks during the southwest monsoon, we could observe downshifting of the peak frequency in the wave spectral data. The present study aims at proving the mechanism of frequency downshifting and possible influence of the downshifting process on mudbank formation. The results of SWASH (Simulating WAves till SHore) modeling and bispectral analysis shows that frequency downshifting occurs due to the difference nonlinear triad interactions of the main frequency peak of the wave spectrum with frequencies of the infragravity range independent of the viscosity of the medium. The increase in wave dissipation accelerates frequency downshifting additionally, decreasing the wave energy in the main peak frequency. It is shown that frequency downshifting can be one of the possible wave mechanisms of mudbank formation due to essentially different wave attenuation coefficients at the beginning and end of this process. For muddy cohesive sediments, it will lead to formation with an erosive profile at first and then an accumulative profile, i.e., mudbank formation.

Wave Climate Variability and Occurrence of Mudbanks Along the Southwest Coast of India

“…Shynu et al (2017) carried out seasonal time series measurements of suspended particulate matter in the MBs region and postulated that the dissipated wave energy might have probably eroded the bottom sediment and formed the near-bed fluid mud. As was recently shown in Saprykina et al (2020), wave dissipation leads to frequency downshifting process in wave spectra due to difference of wave attenuation coefficients at the beginning and the end of this process will lead to formation first erosive and then accumulative profile, i.e., MB. Though different hypotheses were proposed for explaining the formation of MB, none of them is conclusive due to lack of convincing scientific/field supports holistically.…”

“…Because the appearance of MBs depends on the heights of the waves and, as the analysis of Figure 9 shows, the wave height threshold for their formation is 2 m. As shown in Saprykina et al (2020), the mechanism of MBs formation due to the wave transformation depends on the rate of dissipation of their energy above the bottom profile. In this case, it is important that must be two characteristic regions: first with a strong dissipation of the wave energy and then with a weak one, which will lead to the formation of an erosive (weighing of particles) and then an accumulative bottom profile due to their transfer, i.e., the formation of an MB.…”

Method of Studying Modulation Effects of Wind and Swell Waves on Tidal and Seiche Oscillations

“…What is interesting is question of the evolution of the wind wave spectrum, which has many mechanisms. For example, [23,24] describes the mechanism of change in the wave spectrum, while waves are going out to a shallow; in this case, due to nonlinear interactions with the bottom, the frequency of the wave process decreases. In [25], numerical and experimental studies of the process of nonlinear energy transfer between two main spectral maxima conditioned by the mechanism of dispersive focusing indicate that nonlinear energy transfer plays a greater role than linear superposition.…”