The Changjiang Estuary and Hangzhou Bay system has experienced river damming and estuarine engineering in the last decades. However, few studies focused on the shifts in its sediment dynamics due to such human activities. In this study multi-decadal development of sediment dynamics in the transitional zone of the two large estuaries was analyzed, based on the synchronous hydrographic data in the winter of 2023, 2014 and 1983. The results revealed significant changes in regional hydrodynamics and suspended sediment transport, despite the continuous good correlations between the current velocity, suspended sediment concentration (SSC), water/sediment fluxes and tidal range. Specifically, the current velocity has been decreased by 8 - 21% after 2014, mainly due to the land reclamation (implemented around 2016) with several groins stretching into deep water and altering alongshore hydrodynamics. The SSC has decreased further by 29 - 38% in addition to the significant decrease during 1983 - 2014. The SSC changes are related to the combination of river damming which induced sediment load reduction and land reclamation which enclosed a large amount of sediment. Furthermore, the sediment transport from Changjiang Estuary to Hangzhou Bay decreased by 36% - 53%, explaining the observed bed erosion in the northern bay mouth in recent years. The findings are also relevant for studies on sediment dynamics in other large estuaries worldwide.@en

Understanding tidal dynamics in estuaries is essential for tidal predictions and assessments of sediment transport and associated morphological changes. Most studies on river-tide interaction ignored the influences of morphological evolutions under natural conditions such as the seasonal and interannual variations of river discharge. This study analyzes the multiple-timescale tidal dynamics in the Qiantang Estuary, a macro-tidal estuary in China with an extremely active morphological evolution. A large dataset including water levels at representative stations, river discharges and bathymetries since 1980 has been collected. The results of the analysis show that within a spring-neap cycle, the tidal amplification in the upper estuary is stronger during spring tide than during neap tide. This unexpected behavior is due to the high sediment concentration and the unique longitudinal profile of the estuary. On the seasonal and interannual timescales, the low water levels in the upper estuary depend on the local bathymetrical conditions. Tidal ranges in the upper estuary are larger in the high flow season and years, than in the low flow season and years, due to the erosion at high flow, in contrast to estuaries with less active morphological changes. During low flow season and years, the bed is gradually recovered, the low waters are elevated, and the tidal ranges decrease accordingly. A good relationship exists between the tidal ranges and the depth of the upper estuary. In the lower estuary, the flood dominance increases continuously due to embankment. In the upper estuary, the flood dominance is increased during the high flow periods, explaining the fast sediment input and bed recovery in the post high flow periods. A conceptual model of river-tide-morphology interaction of the estuary is proposed, which is also applicable for other shallow systems.

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Despite the increasing number of studies on the river-tide interactions in estuaries, less attention has been paid to the role of seasonal morphological changes on tidal regime. This study analyzes the seasonal interplay of river and tide in the Qiantang Estuary, China, particularly focusing on the influences of the active morphological evolution induced by the seasonal variation of river discharge. The study is based on the high and low water levels at three representative stations along the estuary and daily river discharge through 2015, an intermediate flow year in which a typical river flood occurred, as well as the bathymetric data measured in April, July and November, 2015. The results show strong seasonal variations of the water level in addition to the spring-neap variation. These variations are obviously due to the interaction between river discharge and tide but can only be fully explained by including the effect of morphological changes. Two types of the influences of the variation of the river discharge on the tidal dynamics in the estuary can be distinguished: one is immediately induced by the high flow and the other continues for a much longer period because of the bed erosion and the following bed recovery. Tidal range in the upper reach can be doubled after the flood because of bed erosion and then decrease under normal discharge periods due to sediment accumulation. Over a relatively short term such as a month or a spring-neap tidal cycle, there exist good relationships between the tidal range, tidal amplification in the upper reach and the tidal range at the mouth, and between the hydraulic head over the upper and lower reaches. Such relationships are unclear if all data over the whole year are considered together, mainly because of the active morphological evolution.@en

Many alluvial estuaries worldwide include an inside bar system, a large sediment deposit deeply stretched into the estuary. A good example of such a system is the large sediment deposit in the Qiantang Estuary, China. Its length and height reach 130 km and 10 m, respectively. Bathymetrical comparison reveals that the large bar has moved seaward by around 15 km over the last decades, probably related to the large-scale coastal embankment project. This motivated a quantitative investigation of the impact of estuarine planform on the inside bar development. The bar morphology is reproduced by means of an idealized 1-D morphodynamic model. Model results suggest that the bar movement is related to a decreasing tidal prism, increasing flood dominance in the lower reach and enhanced ebb currents in the upper reach, in response to the embankment. The timescale of the morphological response is only several years. The rapid response is related to the strong tidal currents and large sediment fluxes within the estuary. Sensitivity experiments show that the location and dimensions of the bar are related to the convergence length of the estuary. A decrease of the convergence length causes seaward movement and shortening and lowering of the bar. The bar dimensions also depend on the ratio between river and tidal discharges. When the ratio increases, the bar apex moves seaward and the elevation decreases. The bar movement has significantly influenced the tidal bore in the Qiantang Estuary.@en

The importance of seasonal variations in river discharge on the morphological development of estuaries has been recognized in recent years, yet in situ observations about such variations are rare. Here we report a long-term dataset of bathymetry in the middle reach of the Qiantang Estuary, China, characterized by the presence of a large inner bar. Moreover, a hydrographic survey was carried out in the Yanguan reach where one of the largest tidal bores in the world occurs, covering a spring-neap tidal cycle in 2015. Meanwhile, detailed seasonal bathymetric data together with daily river discharges of 2015 were collected. The bed morphology shows strong seasonal and inter-annual variations. During the high flow season, the river flow erodes the bed and transports a large amount of sediments seaward. A good power-law relationship exists between the high river discharge and the channel volume at the upper estuary. Flood tides dominate under usual river flow condition. In particular, the tidal bore during spring and intermediate tides is characterized by large current velocity and high suspended sediment concentration, and transports a large amount of sediment landward. Over a year, a dynamic morphological equilibrium can be maintained. Moreover, the estuary has also been significantly influenced by the large-scale embankment in recent decades, constraining the lateral thalweg migration, bank erosion and point bar deposition, which usually occur in natural sinuous estuaries.@en

We investigate the morphological responses of the Hangzhou Bay, China, located immediately south of the Changjiang Estuary, to the drastic reduction of the sediment load from the Changjiang River and the large-scale coastal embankment schemes over past decades. The spatial patterns of deposition and erosion, sediment volume changes, and the hydrodynamic and sediment dynamic feedback were analyzed, on the basis of historical bathymetric and hydrographic data. The results show that the sedimentation rates in the bay have generally increased rather than decreased over the past decades, despite bed erosion having occurred in the northern bay-mouth. This observation reveals that the influence of the reduction in the Changjiang River sediment supply on the morphological evolution of Hangzhou Bay has to date been insignificant, mainly due to the buffering effect of existing sediment in the outer Changjiang Estuary. The morphological change is mainly related to the implementation process of the coastal embankment. Sediment accumulation induced by progressive seaward coastal embankment has resulted in seaward aggradation from the Qiantang Estuary towards Hangzhou Bay. Analysis of the annually-averaged high and low tidal levels, and durations of rising and falling tides reveals that flood dominance in the inner bay has been increased, due to the coastal embankment and sediment accumulation. The ratio between annually-averaged rising tide and falling tide durations have decreased from 0.85 to 0.63. The tidal prism at the interface between the inner and outer bay has decreased by about 25% since the 1980s, while the net landward sediment flux has been intensified to a certain extent, which is responsible for the intensifying sedimentation in the inner bay. The local human activities have overwhelm the decreased sediment from the Changjiang River. Although the coastal embankment will cease in the near future, the morphological response to human activities is expected to continue on for a longer time.

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We investigate the evolution of a large-scale sand body, a unique type of sandbars in a convergent estuary. Specifically, we analyze and simulate the sand deposition system (defined as an inside bar) in the Qiantang Estuary (QE) in China. The deposit is 130 km long and up to 10 m thick, and is characterized by a dextral morphology in the lower QE. Numerical simulation is carried out using an idealized horizontal 2-D morphodynamic model mimicking the present QE settings. Our results indicate that the morphological evolution is controlled by the combination of river discharge and tides. The seasonal and inter-annual cycles of river discharges play a major role on the inside bar evolution. The bar is eroding during high river discharge periods but accretion prevails during low river discharge periods. Meanwhile, the highest part of the sand body can move downstream or upstream by several kilometers, modifying the seasonal sediment exchange patterns. We also show that the Coriolis force plays an important role on the dextral morphology patterns in wide, convergent estuaries. It induces a significant lateral water level difference and a large-scale gyre of residual sediment transport. Subsequently, the seaward tail of the inside bar shifts southward to help create a condition for the development of tidal flats in the lower reach of the estuary. The lateral bed level differences induced by Coriolis force are up to several meters. Coriolis effects also modify the behavior of flood and ebb tidal channels.
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