The experimental measurements of surface waves over a submerged rectangular obstacle upon a sandy bed reported by Tzang et al. (2008) have further been analyzed in detail by Hilbert-Huang transform (HHT). The harmonic components of regular waves at 5 sites in the vicinity of the obstacle are decomposed into the intrinsic mode functions (IMFs) by the ensemble empirical mode decomposition (EEMD) with postprocessing scheme using masking signal EMD (MSEMD). The results of Hilbert spectrum of waves derived by the HHT illustrate that considerable decay of harmonic amplitude occurs at all 5 sites when the sandy bed have been fluidized but is not observed in any of the unfluidized tests. Much more reductions of amplitude due to the fluidization response are found in the fundamental harmonic particularly during the building-up of excess pore pressure than in higher harmonics.

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