The effects of extreme flood events on the turbidity maximum zone in the Yangtze (Changjiang) Estuary, China

Extreme flood events can significantly disturb hydro-sediment dynamics and affect turbidity maximum zone (TMZ) distributions, which are crucial to estuary evolution. This study used field data and eight Sentinel-2A/B MSI, GF-1/6 WFV, and Landsat-8 OLI satellite images to examine the behavior of the TMZ in the North Passage (NP) and South Passage (SP) of the Yangtze Estuary under the influence of the 2020 extreme flood event. Images covered average annual, usual flood, and extreme flood (river discharge above 70,000 m3/s) conditions between 2018 and 2020. The surface suspended sediment concentration (SSSC) of the Yangtze Estuary was retrieved and verified by the in-situ measured turbidity. Relative SSSC (RSSSC), the ratio of retrieved SSSC to region-averaged SSSC, was then calculated to determine the TMZ (RSSSC>1). The results show that the relation between the SSSC measured in the TMZ core of the SP and river discharge at Datong Gauging Station demonstrated an anticlockwise hysteresis, which can be divided into three stages. In the second stage, the observed SSSCs during the extreme flood were 0.18- 0.68 kg/m3, 0.10- 0.17 kg/m3 higher than before. SSSCs after the extreme flood were lower than those with similar discharge before it in this stage. The TMZ boundary location is correlated logarithmically with river discharge, and the upper boundary moves more seaward than the lower boundary. They moved 6- 20 and - 4- 11 km seaward in the NP and migrated 4- 13 and - 7- 9 km seaward in the SP during the extreme flood, respectively, compared with average annual and usual flood conditions. These findings further clarify TMZ characteristics in extreme floods and can be used as a reference for ecological and waterway projects considering the impacts of extreme flood events.