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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. • The surface SSC-discharge curve follows an anticlockwise hysteresis pattern. • The surface SSCs after the extreme floods are lower than before. • The turbidity maximum zone location is logarithmically related to river discharge. [ABSTRACT FROM AUTHOR]

Among all marine hazards, storm surges cause the most severe damage, and East Asia is one of the regions in the world most affected by tropical cyclones (TCs). With the increase in global mean temperature, the future changes of tropical cyclone (TC) activities are of great concern in East Asian countries. However, due to the inconsistency among different TC datasets, there is no uniform understanding of the spatial and temporal distribution and trends of TC activities in the eastern sea area of China (ESAC). In this study, all TC wind fields affecting ESAC during 1949–2019 were calculated using the Fujita-Takahashi formula, and a new TC dataset with a spatial resolution of 0.1° × 0.1° was thus obtained. Based on this dataset, the spatial-temporal variations of TC activity were analyzed. Three stages (1949–1967, 1968–1993, 1994–2019) of TC activities in ESAC can be identified using Mann Kendall Trend Test, which is closely related to El Niño-Southern Oscillation, sunspot activity, and Pacific Decadal Oscillation. A comparison of the accumulative number and duration of TCs in the three stages shows increasing trend toward the coasts and an northward. To address the issue of shelf storm deposition records on ESAC, we compared the published storm depositional records retrieved from several cores with typhoon impact data from the same sites based on the dataset of this study. We found that the preservation potential of the storm deposition record on ESAC ranged from 8% to 33%, with a high preservation potential in shallow water. These findings reveal the distribution and variability of TC impacts, which can be used to provide a scientific basis for disaster prevention and mitigation planning and adjustment of key coastal protection zones, as well as to evaluate the value of marine sedimentary records better. We have established a new and more comprehensive dataset (0.1° × 0.1°) to analyze the spatial-temporal distribution of tropical cyclone(TC) activity on the seas. Also due to the latitudinal and longitudinal effects of TC effect, the coastal cities in northern China may suffer from more severe storm surge disasters in the future. We also included sedimentary records to help us understand more ancient TC activity. [Display omitted] • We provide a new high-resolution (0.1° x 0.1°) and more detailed dataset of tropical cyclones.. • Coastal cities in northern China are likely to suffer from more severe storm surge disasters in the future. • The tropical cyclone dataset coupled with the sedimentary record can provide a better understanding of paleostorm activity. [ABSTRACT FROM AUTHOR]

Reclamation in estuaries can greatly change the channel geometry and hydrodynamic conditions and these changes may have significant impacts on spatial and temporal distribution of the turbidity maximum zone. This study focuses on the effects of a large area of reclamation built in 2007–2018 and the behavior of the turbidity maximum zone along the North Channel of the Yangtze Estuary. Data were collected of bathymetry in the North Channel, tidal elevations at Sheshan Station, river discharge at Datong Station and turbidity, retrieved from six Landsat remote sensing images in the dry season from 2006 to 2019. In-situ measured data on flow velocity and suspended sediment concentration were obtained in the dry season of 2003 and 2018. Analysis of the data revealed that reclamations, which led to narrowing (0.86–2.74 km) and fixing of the channel, caused erosion of 0.19–3.72 m in the deep channel and deposition on the tidal flats. Furthermore, it was found that the length of the turbidity maximum zone decreased: its landward boundary shifted 5 km seaward during spring tide and 17 km seaward during neap tide in the dry season. The position of the seaward boundary wandered within a range of 3 km, being further downstream during neap tide than that during spring tide. A conceptual model of changes in the borders of the turbidity maximum zone in response to reclamation is proposed. After the reclamation works, the deeper and narrower channel intensified ebb-dominance of the flow velocity. The coarsening of bed sediment weakened resuspension and decreased the bottom tidally averaged suspended sediment concentration. These changes led to a significant decline in the depth-mean of tidally averaged suspended sediment concentration and caused the seaward movement of the landward boundary of the turbidity maximum zone. • The landward boundary of the turbidity maximum zone has moved seawards. • Ebb dominance is enhanced in the deep channel via channel narrowing and deepening. • Coarsening of surface sediments has weakened resuspension. [ABSTRACT FROM AUTHOR]

Climate change and human activity have exerted significant influences on the sediment load and channel morphology of the Changjiang River system, China. However, our knowledge of their influence on flood regime on the centennial to millennial timescales remains limited, and this is mainly because of the difficulty in directly determining the long-term hydrological variability of the Changjiang River over the period before hydrological gauges were established. Based on multiproxy analysis that combines chronological, sedimentological, and geochemical analysis of a 4.8-m-long sediment core retrieved from the subaqueous delta of the Changjiang River, this study establishes a flood history for the Changjiang River during the late Holocene. Our palaeoflood reconstruction revealed 14 multi-decadal periods of extreme floods (19–66 BCE, 25–80, 255–350, 415–475, 550–710, 740–835, 970–990, 1080–1130, 1170–1235, 1275–1390, 1440–1500, 1560–1730, 1810–1830, and 1950–2011 CE). These flood units match well with the observed large floods, documentary records of floods, and sedimentary flood deposits, confirming that the coarse units in the sequence are flood-derived and are regionally representative. Comparing with regional and global palaeoclimate records indicates that major flood events of the Changjiang are strongly modulated by the Asian summer monsoon and the El Niño–Southern Oscillation (ENSO), but that anthropogenic impacts (e.g. artificial channelization and land clearance) have also greatly amplified the flooding frequency over the past 600 yr. • We report a history of extreme flood events on the Chanjiang River over the past 2000 yr. • Periods of increased extreme flood activity in the Changjiang River typically coincide with El Niño periods and weak EASM phases. • Human modifications have greatly amplified flood activities in the Changjiang River over the past 600 yr. [ABSTRACT FROM AUTHOR]