Impact of Typhoon Chan-hom on the marine environment and sediment dynamics on the inner shelf of the East China Sea: In-situ seafloor observations

The suspended sediment transport in the inner shelf of the East China Sea (ECS) varies seasonally, and most of the suspended sediments are transported southward in winter. However, the influence of typhoons, which usually take place in summer, on the marine environments and sediment transport processes in the mud wedge area of the ECS inner shelf has not been elucidated. An in-situ monitoring platform moored approximately 440 km south of the Yangtze River mouth at a water depth of 40 m was resting on the seabed of the ECS inner shelf when Typhoon Chan-hom passed through the area. A 49-day data set of the temperature, suspended sediment concentration (SSC), chlorophyll a (chl a) concentration, and currents was recorded from this platform and analyzed in this study to characterize Typhoon Chan-hom and evaluate the associated sediment transport. The analysis of the data obtained shows that during the typhoon passage, the temperature decreased by approximately 0.8 °C, the maximum near-bottom current reached 152.0 cm/s, an average SSC of 919 mg/L was observed with peak hourly values exceeding 1785 mg/L, and the SSC and chl a concentrations reached approximately 50 and 10 times the corresponding values during the pre-typhoon period, respectively. During the typhoon passage, most of the suspended particles near the seabed were derived from the resuspension of bottom surface sediments. The chl a concentration near the seabed increased greatly because of surface sediment resuspension and the input of additional terrigenous nutrients transported by the typhoon passage. The net suspended sediment flux (SSF) associated with the passage of Typhoon Chan-hom was directed southwestward and landward. Sediment transport during Chan-hom accounted for approximately 89% of the total southwestward SSF and 44% of the total landward SSF during the whole observation period. Analysis of the wavelet power spectra reveals that in the upper water layers, the semidiurnal tidal currents prevailed before and after the typhoon passage; however, this dominant factor disappeared during the typhoon passage. Compared with winter storms in the ECS, summer typhoons such as Chan-hom mainly transport locally resuspended sediments, and sediment flux is landward in the across-shelf direction. Our study highlights the significant role of typhoons in the resuspension and redistribution of sediments and transport of suspended matter from north to south in the mud wedge area of the ECS.