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Subaqueous deltas are facing growing threats of erosion. Tropical cyclones (TCs), which are predicted to affect mid‐latitude estuaries more frequently with higher intensity in the future, can play an important role in subaqueous delta geomorphology change. Nevertheless, direct in situ measurement of TC‐induced bed elevation change (BEC) was rarely reported before. In this study, four tripods were deployed at different parts of the Changjiang River subaqueous delta (CRSD) to capture BEC during TCs in July 2019. Rapid erosion (20 mm day−1) during TC Danas (2019) was observed by high precision bed elevation measurement, over 10 times faster than that during calm weather conditions. Moreover, the recorded rates of BEC varied substantially among these stations. A coupled three‐dimensional wave‐current‐sediment model was then developed for the CRSD to help understand the spatial variability of observed BEC. The model results showed two striking features of BEC during TC Danas: (a) enhanced sediment accretion near the sharp bottom salinity front offshore of the north channel; (b) marked erosion near the front of CRSD. Further analysis showed that the converging bottom currents in concert with enhanced sediment resuspension during TC Danas led to significant convergence of sediment flux near the bottom salinity front. In addition, the diverging isobaths along the front of the CRSD resulted in the onshore deviation of bottom current and diverging sediment flux under the downwelling‐favorable winds of Danas. Many TCs in the CRSD generate similar downwelling‐favorable winds and currents, growing TC threats in the future may therefore exacerbate erosion in this region. Plain Language Summary: Submerged river deltas are vulnerable to erosion, and this problem may worsen due to the increased frequency and intensity of tropical cyclones (TCs) in the future. However, the impact of TCs on delta erosion is not fully clear. To address this issue, this study deployed four tripods in different parts of the Changjiang River submerged delta during TC Danas (2019). The observed data showed that erosion occurred much faster during the storm than during calm weather. Interestingly, the erosion rate was different at each tripod. A computer model was built to help understand why the erosion rate varied in space. Model results showed that sediment built up in the region where fresh water meets seawater (known as front), since the front can act as a barrier to the seaward movement of sediment brought by the river. In contrast, strong erosion occurred at the offshore edge of the delta. This is caused by diverging ocean currents along widened submerged delta driven by north winds, which carried sediments away from this region. Computer simulations of more storm events confirmed that such erosion can frequently occur at the same location during TCs. This highlights the importance of protecting these areas in the future. Key Points: Fast erosion near the 30 m isobath of the Changjiang River subaqueous delta (CRSD) was captured by in situ measurement during Cyclone DanasConverging flows near the bottom salinity front led to continuous deposition around the 10 m isobath of the CRSDDiverging isobaths facilitate the erosion of subaqueous delta during downwelling‐favorable storm winds [ABSTRACT FROM AUTHOR]

Sediment transport within small tidal inlets is sensitive to natural processes, whilst large tidal inlets are relatively robust systems because of their large tidal prism. However, remarked morphological changes may be initiated even under the condition of low sediment supply, as illustrated by Jiaozhou Bay, a large coastal embayment on the Shandong Peninsula, eastern China. Jiaozhou Bay is characterized by its relatively slow rate of natural change, and while the embayment has a flood-dominated entrance channel and muddy seabed, the suspended sediment concentration is generally low due to the lack of abundant source material. Observations of sediment dynamics show that net suspended sediment transport is directed towards outside of the bay, with an order of magnitude of 10 3 t during a tidal cycle. The export of sediment associated with this flood-dominated environment implies that the net transport pattern is controlled by tidal exchange processes rather than the strength of the seabed shear stress. Sediment budget calculations show that supply of artificial sediment into the bay can account for up to 72% of the total input, which is in agreement with the 210 Pb and 137 Cs radioisotope geochronologies, and this leads to accumulation rates of 10 0 –10 1 mm yr −1 ; without this, the deposition rate would be low under natural conditions. The flood tidal delta area is also influenced by the input of anthropogenic material, and acts as a depocenter with relatively high accumulation rates. Furthermore, although the inlet system has not yet reached its equilibrium state (i.e., the entrance cross-sectional area is still larger than the equilibrium cross-sectional area), land reclamation activities have resulted in a rapid reduction of the embayment area (by 37%) over the last 80 years. Our findings indicate that the rapid changes observed in the tidal basin area and seabed morphology are mainly the result of human activity rather than natural processes. [ABSTRACT FROM AUTHOR]

Abstract: This paper focuses on the hydrodynamic and suspended sediment transport processes in response to sequential reclamations over the intertidal flat, Jiangsu coast, which used to be one of the widest intertidal flats in China. Investigations into the sediment transport patterns reveal that net suspended sediment fluxes increased from 102 kgm−1 on the upper intertidal flat to 103–4 kgm−1 on the lower intertidal flat, per tidal cycle. This variation was caused mainly by the phases of the current velocity, suspended sediment concentration (SSC), local bed geomorphology, inundation patterns, and extreme weather conditions. The longshore component of tidal currents intensifies seaward over the intertidal flat. Extreme weather conditions (e.g. storms) and strong winds were important factors affecting the current velocity structures, the intensity of residual currents, and the SSCs. The SSCs were characterized by several peaks throughout the tidal cycle. These peaks were associated with strong currents, combined wave-current interactions and intense turbulences due to the initial flood surge. Furthermore, the suspended sediment was finer and contained more clay materials during the flood phase of the tidal cycle, than during the ebb. Fine particles were transported within the upper water column by landward residual currents, but coarse particles were transported within the lower water column by seaward residual currents. Overall, residual currents were apt to transport fine-grained sediment landward and coarse-grained sediment seaward. This pattern indicates that the suspended sediment fluxes are highly variable in the water column, even for very shallow tidal environments. The cross-shore component accounted only for a small percentage of the total sediment flux. However, this part of sediment supply dominated accretion over the intertidal flat. The intertidal flat has become narrower and steeper, following sequential reclamations. A negative feedback process existed to maintain the convex profile shape, although the human activities (i.e. reclamation) could have modified the natural process. The surficial sediment tended to become finer on the mid-upper intertidal flat but coarser on the lower intertidal flat following reclamations, in response to a reduction in the tidal currents over the intertidal zone and enhancement of wave action over the lower intertidal flat. Accretion, with high rates, was maintained by landward transport of sediment. In such an accretional system with influences of reclamation, the width or slope of the intertidal flat is adjusted continuously. Before the cease of reclamation, the intertidal flat is unable to achieve an equilibrium shape. [Copyright &y& Elsevier]