Swell-driven sediment resuspension in the Yangtze Estuary during tropical cyclone events

Abstract

Tropical-cyclone-induced waves in coastal waters are considered to greatly influence sediment resuspension. Much attention has been paid to the effect of mixed waves (wind waves and swells) during tropical cyclone events. However, little is known about the difference between the effects of swells (driven by offshore tropical cyclones) and wind waves (generated by local winds) on coastal sediment resuspension. In this study, field observations including waves, currents, and suspended sediment concentrations (SSCs) were obtained at an offshore station (mean water depth 20 m) in the Yangtze Estuary, China, during tropical cyclones Meranti and Malakas in 2016. These two tropical cyclones caused the bed shear stress derived from swells (τs) to increase by 5.5–9.5 times relative to the values in calm weather (average τs increased from 0.011 – 0.053 N/s2 to 0.077–0.372 N/s2). The impact of swells on sediment resuspension was much greater than that of wind waves during these two tropical cyclones. The tide-averaged τs (0.011–0.372 N/s2) was 3–16 times greater than the shear stress derived from wind waves (0.003–0.056 N/s2) during the two tropical cyclones. The SSCs was found to have increased by ∼7.5 times (from 0.056 to 0.473 kg/m3) even when tropical cyclones with wind speeds of >41 m/s near their centers occurred 500–1500 km away from the observation station. We estimate that there are on average at least eight tropical cyclones each year that may influence the subaqueous Yangtze Delta by means of swells, considering the impact of remote tropical cyclones. These findings indicate that the swells induced by tropical cyclones play an important role in sediment resuspension in open China coasts, which contributes to our understanding of the influence of swells on coastal sediment resuspension during tropical cyclones.