Infragravity waves at a dissipative coast; evidence upon multi-resolution analysis

Simultaneous records of water level and wave-driven longshore current by spatially co-located devices in close shoreline proximity have been analyzed with a multi-resolution technique and singular spectrum analysis to provide convincing evidence that infragravity waves can actually exist at a dissipative coast with multiple bars. The records represent a 6-h evolution of wave climate and longshore current in the conditions of the beginning of storm recession. The sampling rate of 3 Hz generated a series of nearly 60,000 elements each allowing for a very detailed description of their spectral behavior throughout the whole frequency domain. The multi-resolution analysis with a suitably chosen wavelet function was performed to extract spectrally disjoint slow-varying components of water level and longshore current. After having been fine-tuned by singular spectrum analysis these components were cross-examined to see whether slow variations of water level are imprinted in the variability of longshore current. High amplitude and phase coincidence of one pair representing the period of 120 s yields convincing evidence that progressive infragravity waves are indeed encountered at dissipative shores. Another pair with the period of 30 s delivers much less firm evidence, so for the time being it cannot be regarded as an infragravity wave. The study also produced guidelines for further research aimed at in-situ detection of the length and cross-shore decay of infragravity waves in a dissipative environment.