Tidal modulation of storm waves on a macrotidal flat in the Yellow Sea

On the open shores of the Yellow Sea along Korea’s Baeksu coast, a severe erosional phase in a tidal flat has been attributed to hydrodynamic processes in winter. To investigate the hydrodynamic behavior of winter storms on the intertidal flat, in situ measurements of currents and waves were obtained with an instrumented system deployed at a low tidal zone over a neap–spring tidal period in February 1999. The measurements included two storm events under different tidal conditions, i.e. the first storm occurred during a neap tide and the second weaker storm during a spring tide. Near-bottom velocities recorded during storm conditions at an elevation of 15 cm show that currents were dominantly directed alongshore with a maximum speed of 0.55 m/s and that the wave orbital velocity and significant wave height reached up to 1 m/s and 3m, respectively. In particular, it was observed that the magnitude of wave-related parameters was greater during the spring tide (the weaker storm) than during the neap tide, regardless of the intensity of the storms. Analysis of the relationship between wave height and water depth, and temporal variations in wave velocity spectra and skewness, show that the effect of storm conditions on waves was relevant to wave-breaking processes. The occurrence of harmonics in the broad-banded spectra of wave velocity demonstrates redistribution of wave energy due to the breaking of waves. Wave height (Hs) was linearly related to water depth (h) fluctuating with tides, yielding a correlation coefficient of 0.7 (¼Hs/h), which was close to the breaking limit. This tidal modulation of storm waves associated with wave-breaking processes is possibly responsible for the observed discrepancy between the intensity of storms and their hydrodynamic consequences on the intertidal flat.