Sediment dynamics on a tidal flat in macro-tidal Hangzhou Bay during Typhoon Mitag

Abstract

The hydrodynamics and sediment characteristics of tidal flats during typhoons are of great significance for coastal geomorphology and ecosystems. In this study, field records of waves, currents, and suspended sediment concentration (SSC) in the southern tidal flat (Andong) in the macro-tidal turbid Hangzhou Bay, China, were collected during Typhoon Mitag. The field data showed that there was little variance in the magnitude of the tidal currents during the typhoon. The directions were disordered with the peak ebbing currents changing from the southeast to the east, presumably due to the winds and northerly currents from the tidal creeks during the precipitation period. The significant wave heights (Hs) and turbulence kinetic energy (TKE) significantly increased during Typhoon Mitag. The SSC obtained by the optical backscatter sensor (OBS) near the bottom level rose rapidly to 6–7 kg/m3 during the initial stage of tidal cycles during calm conditions but decreased to 4–6 kg/m3 during Typhoon Mitag, probably due of the enhanced upward sediment transport by turbulent mixing. During normal weather conditions, the bed shear stresses induced by currents (τc) were mostly larger than those induced by waves (τw), while the τw exceeded the τc during the storm. The instantaneous bed shear stresses induced by currents and waves were mostly lower than the critical shear stress. Extracting turbulence from waves effectively diminishes the overestimation of turbulence. In calm conditions, the small-scale features (>0.5 Hz) of the scalograms exhibited some degree of disorder owing to the random nature of turbulence. Small-scale characteristics were embedded in relatively large-scale motions during extreme conditions, and a small quantity of plume-like streaks appeared between the frequency bands of 1/16 and 1 Hz. The temporal distributions of the high wavelet power regions of U'W' and c'W' coincided with each other, indicating the critical role of intermittent turbulence in sediment dynamics. The results of this study shed light on the study of coastal morphology and marine disasters.