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River floods and human activities would impact land‐to‐sea sediment transport, which is essential for understanding the evolution of the global sediment cycle in the Anthropocene era. This study focused on investigating how river floods and a dike constructed in 2005 along a river channel influence sediment transport from the Ou River Estuary to the East China Sea. A validated three‐dimensional sediment transport model based on the Finite Volume Community Ocean Model was utilized for this study. The presence of the dike obstructs alongshore currents and fish migrations, leading to negative effects on marine ecology. Therefore, selective dismantling and ecological restoration measures are deemed necessary. Three scenarios were considered in this study: no‐dike, dike‐constructed, and dike‐partially‐removed conditions, along with various types of river floods. The findings indicate that the monthly sediment flux to the sea decreased by 7.7% from 8.11 × 106 to 7.49 × 106 t following dike construction, while the proportion of cross‐shore sediment flux to the total flux increased from 37% to 59%. The dike consistently has a greater impact than river floods in their interactions. However, partially removing the dike reduces its influence and restores sediment transport to pre‐dike levels, the effectiveness of which is more pronounced with more frequent floods, larger volumes, and rising sea levels. This study provides valuable insights into the interplay between river floods and dikes on sediment transport, thereby enhancing our understanding of the repercussions of human interventions on sediment dynamics. Plain Language Summary: Global warming is expected to cause an increase in sea levels and a rise in the frequency and volume of river floods, which will change the delivery of riverine sediment to the ocean. Human construction of dikes for various purposes has also changed modern estuarine sediment transport patterns. These alterations are significant as they affect the natural sediment cycling of the Earth, influencing marine geology and marine ecosystems. Through a case study of the Ou River Estuary in southeastern China, numerical sensitivity tests have shown that dikes constructed along river channels disrupt natural flow patterns, resulting in decreased sediment transport to the sea and redirection toward deeper oceanic areas. The presence of dikes is crucial in their interaction with river floods on sediment transport. However, removing sections of dikes can help restore sediment transport to conditions without dikes. The increasing frequency, larger volumes, and faster discharge rates of river floods, as well as dam‐controlled river floods and rising sea levels, may all play a role in restoring natural sediment transport dynamics. This study examines how floods and dikes collectively impact sediment transport in estuaries with drainage areas larger than the estuary area. Key Points: A dike redirects sediment transport from the alongshore to the cross‐shore from the Ou River to the East China SeaMetrics are proposed to quantify the interaction between various river flood conditions and dike configurations on sediment transportFrequency, pattern, and duration of river floods, as well as partial dike removal and sea‐level rise, impact the interaction [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Spring tide amplifies more than neap tide in Qiantang Estuary. • Tidal dynamics is strongly influenced by morphological evolution. • A conceptual model of river-tide-morphology interaction is proposed. 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. [ABSTRACT FROM AUTHOR]

Abstract: The Hangzhou Bay is a macro-tidal bay located to the south of the Changjiang estuary in China. Along its northern shore, a large-scale tidal channel system has developed, which includes a main northern tidal channel, with a length of more than 50km and a width up to 10km, and a secondary southern tidal channel. A process-based morphodynamic model, incorporating the cohesive sediment transport module of Delft3D, is used to analyze the physical processes and mechanisms underlying the formation and evolution of this tidal channel system. The results show that spatial gradients of flood dominance, caused by boundary enhancement via current convergences, is responsible for the formation of the channel system, due to a combination of the various factors such as funnel-shaped geometry hindering associated with the presence of islands, and flow deviation by the southern tidal flat and so on. The model results agree well with the real morphological features. This study also indicates that the reclamation of the southern tidal flat imposes a profound influence on the morphological evolution of the tidal channel system in the Hangzhou Bay. It is feasible to use the model to simulate long-term estuarine morphological changes with cohesive sediment settings. [Copyright &y& Elsevier]

In this contribution, the morphodynamics of open-sea tidal channels eroded into sandy seabed in regions of flow constriction is simulated by a one-dimensional model using the Bagnold formula for bedload transport rate, and accounting for the effect of bed slope. The results show that equilibrium conditions for such channels can be reached over a period of 102–103 years. Sediment eroded from the channel floor is transported in the direction of the dominant current, and deposited beyond the regions of flow constriction where the current looses competence due to spreading. In this way, the material remobilized from older strata in the channels is deposited in younger sand banks near the channel heads. Where several successive channels are incised along the current axis, they interact in their morphological evolution. The morphodynamic equilibrium of a tidal channel is reached once the combined interacting sedimentological and hydrodynamic factors, such as sediment particle diameter, tidal current velocity and flood/ebb dominance, are balanced. The model output shows that the equilibrium shape of the tidal channels appears to be related mainly to flow field characteristics and, to a lesser extent, to particle size. A positive correlation exists between the depth of the channels and their response times. The equilibrium water depths of the channels are more sensitive to current speed than to either particle size or the time–velocity asymmetry of the flow field. The response times for overall morphological equilibrium are sensitive to all of the above-mentioned parameters. In particular, sediment characteristics associated with critical current velocities have far-reaching effects on the morphodynamic behaviour of tidal channels. [ABSTRACT FROM AUTHOR]