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Stricter controls on destructive human activities in recent years have improved the protection and management of sandy coasts in China. Marine-driven geomorphic changes have become the predominant process influencing future beach evolution. However, in the complex geomorphic conditions of mixed artificial and natural coastlines, the mechanisms and contributions of various marine-driven factors to beach changes remain unclear. Using methods of field observations and numerical simulations by FVCOM model, this study reveals the sedimentary dynamic mechanism of straight sandy beach in the southern Shandong Peninsula (Wanpingkou beach, WPK beach). From 2005 to 2016, the northern section of WPK Beach eroded at a rate of approximately 2-3 m/year, the middle section at about 0.5-1.5 m/year, and the southern section accrued at an average rate of about 1-2 m/year, demonstrating an overall pattern of "northern erosion and southern accretion." During winter, the average grain size of beach sediments is about 0.5 mm with minimal variations, indicating an onshore transport trend; in summer, the average grain size is about 0.7 mm with larger variations, indicating an offshore transport trend. Offshore sediment transport correlates mainly with seasonal changes in wind direction, while alongshore sediment transport is influenced by tidal currents, wind-induced currents, and wave-induced currents. Wave-induced currents are the primary force in alongshore sediment transport with a velocity of 0.1-0.3 m/s toward southwest. Followed by tidal currents with a velocity of 0.05-0.1 m/s, and wind-induced currents with a velocity of 0-0.1 m/s, which have a relatively minor impact. Therefore, seasonal changes in beach morphology are primarily controlled by waves, while interannual variations is mainly influenced by a combination of wave-induced currents and tidal currents. With increasing efforts in beach maintenance and coastal ecological restoration in recent years, understanding the sedimentary dynamics of beaches remains of vital theoretical and practical value. [ABSTRACT FROM AUTHOR]

A wave–current coupled numerical model with 3D radiation stress formalism is used to study baroclinic effects, specifically stratification and baroclinic pressure gradient force (BcPGF), on wind-wave-driven flows, separated as vertically sheared flows and vertically homogeneous flows. Numerical experiments are conducted under ideal offshore winds of 8 m s−1. Results show that stratification enhances the two-layer structure of the wave-driven sheared flow, while BcPGF has minimal impact. Stratification and BcPGF have limited influence on the direction of the wave-driven depth-averaged circulation, but they can enhance the total circulation by promoting the upper one. In the upper layer where form drag dominates, the competition between the vertical form drag divergence (FDD) and turbulent stress divergence (TSD) plays a critical role in shaping the wave-driven flow. Stratification reduces vertical eddy viscosity Km, suppressing the growth of TSD associated with the vertical shear of velocity, which facilitates the FDD driving the upper outflow. Likewise, stratification can enhance the effect of FDD curl on the upper horizontal circulation by limiting the growth of TSD curl related to the vertical shear of vorticity. Additionally, low Km reinforces geostrophic effect and therefore enhances the lateral circulation. Baroclinic torque contributes to reducing the vertical variation of vorticity and the growth of TSD curl, indirectly improving the effectiveness of FDD curl in driving the upper circulation. Stratification undergoes fortnightly adjustments due to the spring–neap cycle, leading to the synchronized variation in the strength of wave-driven flow, particularly its vertically sheared component. During neap tides with weak tidal forcing and well-developed stratification, wave-driven sheared flow, horizontal circulation, and lateral circulation are stronger. Significance Statement: The aim of this study is to enhance our comprehension of the impact of baroclinic processes on wind-wave-driven flows in wind-dominated regimes. During summer, wave-driven flows exhibit noticeable vertical shearing, which differ from those observed in winter. This is particularly important because during summer, river runoffs generate strong stratification and introduce large quantities of terrigenous materials into coastal oceans. Consequently, wave-driven flows may play a crucial role in bay-shelf exchange, which has been given in few studies. Our findings highlight the influence of baroclinic processes on turbulent stress divergence, which can enhance wave-driven flows, both the vertically sheared and the vertically homogeneous components, especially during neap tides. [ABSTRACT FROM AUTHOR]

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]