Tinglu Cai

Coastal reclamation reshapes tidal hydrodynamics in semi-enclosed bays by removing intertidal storage, modifying channel conveyance, and redistributing tidal exchange among connected sub-regions. This study quantifies the multi-decadal cumulative impacts of reclamation on tidal currents and tidal prism in Yueqing Bay, China, using shoreline and bathymetric reconstructions for 1978, 2002, 2013, and 2020; hydrological observations; and a two-dimensional MIKE21 FM tidal hydrodynamic model. Characteristic cross-sections were used to estimate bay-wide and sub-regional tidal prisms, and representative stations were used to diagnose current-speed responses. The bay-wide tidal prism decreased from 15.235 × 10⁸ m³ in 1978 to 12.316 × 10⁸ m³ in 2020, corresponding to a reduction of 2.919 × 10⁸ m³ (19.16%). The strongest loss occurred during 1978–2002, when large-scale reclamation and closure of the Xuanmen Channel removed tidal storage and redirected flow into the remaining main-channel system. Although reclamation intensity weakened after 2013, mean current speed still changed by −0.050 to 0.033 m/s and sub-regional tidal-prism shares continued to adjust, indicating delayed hydrodynamic reorganization rather than immediate stabilization. These results show that reclamation impacts cannot be explained by reclaimed area alone; they depend on project timing, spatial layout, and the connectivity with key tidal pathways. The findings support staged assessment and pathway-sensitive shoreline management in reclaimed semi-enclosed bays.

Coastal changes in China, notably in the Zhoushan Islands, have primarily been driven by coastal reclamation since the establishment of New China. This study conducted a comprehensive analysis of the Zhoushan Archipelago shoreline spanning four decades, employing remote sensing, aerial photographs, and shoreline data since 1984, along with GIS (Geographic Information System) technology. We assessed shoreline changes using the shoreline change index and shoreline artificialization index, as well as examined the influence of the Yangtze River’s suspended sediment and impoldering activities on Zhoushan’s shoreline. Furthermore, the correlation between local economic development and shoreline development was explored. The results revealed the following key findings: (1) From 1984 to 2018, the Zhoushan Archipelago shoreline decreased by 7.05 km. Temporally, the shoreline change index was −0.08%, with the most significant reduction occurring between 2008 and 2018. Spatially, differences among island groups were not pronounced. (2) The shoreline diversity index consistently increased, indicating greater diversity and complexity in shoreline use over the four decades. (3) The shoreline artificiality index steadily rose, particularly after 2000. It was highest in the south, followed by the center, and lowest in the north. (4) The intensity index of coastal land use continuously increased, with the southern island group having a higher index compared to the Zhoushan Islands. (5) The Yangtze River contributed significantly to sand inflow, influencing shoreline changes and beach shaping in Zhejiang. However, reclamation projects were identified as the primary and direct factor. (6) A positive correlation existed between Zhoushan City’s economic development and the intensity of coastal land use. This study emphasized the need for improving the control over reclamation projects and the better management of coastal protection and use. These measures could optimize resource allocation and establish a more scientific and rational coastal zone pattern.

Analyzing the processes and influencing factors of accretion or erosion in estuaries and tidal basins is important for coastal conservation and utilization, planning, and ecosystem maintenance. This study analyzed the process of topographic changes in Taizhou Bay, China over the past five decades by comparing bathymetric datasets from different years. The coastlines were extracted via remote sensing image interpretation, and the coastal reclamation process was analyzed. The results revealed that this bay experienced slow siltation from 1963 to 2004, which mainly occurred on mudflats and shore slopes. It turned into a feature dominated by erosion between 2004 and 2013, and erosion increased between 2013 and 2019. Coastal reclamation has occurred during every 10-year period since the 1960s. Reclamation occupied a portion of the intertidal flat, decreased the tidal prism, and changed tidal asymmetry, causing net sediment to be transported into the bay and accumulate on the intertidal flat and upper part of the subaqueous shore. The drastic reduction in sediment supply caused by the Three Gorges Dam may have been responsible for erosion from 2004 to 2019. A negative feedback process exists between tidal flat expansion and coastal reclamation. The compensatory expansion of the tidal flat is a much slower process than the advance of reclamation.

The monitoring of beach topographical changes and recovery processes under typhoon storm influence has primarily relied on traditional techniques that lack high spatial resolution. Therefore, we used an unmanned aerial vehicle light detection and ranging (UAV LiDAR) system to obtain the four time periods of topographic data from Tantou Beach, a sandy beach in Xiangshan County, Zhejiang Province, China, to explore beach topography and geomorphology in response to typhoon events. The UAV LiDAR data in four survey periods showed an overall vertical accuracy of approximately 5 cm. Based on the evaluated four time periods of the UAV LiDAR data, we created four corresponding DEMs for the beach. We calculated the DEM of difference (Dod), which showed that the erosion and siltation on Tantou Beach over different temporal scales had a significant alongshore zonal feature with a broad change range. The tidal level significantly impacted beach erosion and siltation changes. However, the storm surge did not affect the beach area above the spring high-tide level. After storms, siltation occurred above the spring high-tide zone. This study reveals the advantage of UAV LiDAR in monitoring beach changes and provides novel insights into the impacts of typhoon storms on coastal topographic and geomorphological change and recovery processes.

Coastal evolutions are expected to have a significant impact on storm tides, disproportionately aggravating coastal flooding. In this study, we utilize a nested storm tide model to provide an integrated investigation of storm tide responses to changes in coastline and bathymetry along the Zhejiang coasts. We selected coastline and bathymetry data from 1980 and 2016, as well as data from three typical typhoon events (i.e., Winnie, Haikui, and Chan-hom) for simulating the storm surge processes. The results indicate that changes in the coastline and bathymetry from 1980 to 2016 have resulted in an increase in storm tides in the northern part and a decrease in the central part of Zhejiang. Specifically, storm tides in Hangzhou Bay have increased significantly, with an average increase of about 0.3 m in the maximum storm tides primarily attributed to coastline changes. On the contrary, in smaller basins like Sanmen Bay, while reclamation itself has reduced peak storm surges, rapid siltation has consequently exacerbated the storm surge. By decomposing storm tides into astronomical tides and storm surges, we discovered that the change in tidal levels was twice as significant as the surge change. Moreover, the nonlinear tide–surge interaction was nearly four times that of the pure surge, significantly contributing to storm surge variation. Alterations in the momentum balance reveal that the water depth-induced bottom friction and wind stress increase contributes to the local enlargement of storm tides at the bay head, while the coastline changes exaggerate nearshore storm tides through an increase in the advection term.

Sea level rise (SLR) due to climate change is expected to alter tidal processes and energy transport, disproportionately affecting coastal communities. Utilizing a nested hydrodynamics model, we provided an integrated investigation of tidal responses to SLR in the Hangzhou Bay (HZB). The scenarios of SLR in the next hundred years count for both non-uniform trends based on historical altimetry data and uniform trends from the latest IPCC projections. In a comparison of model results under different SLR scenarios, we found that the tidal range is amplified by SLR in HZB with stronger amplification at the shallow southern coast. Tidal range change generally increases with the SLR scale; however, neglecting the heterogeneities in the spatial distribution of SLR tends to overestimate the SLR effects. The harmonic analysis illustrates that SLR exaggerates the dominated semidiurnal tides (M₂ and S₂) but dampens their overtides and compound tides (M₄, M₆, and MS₄), of which M₂ amplitude amplification explains 71.2–90.0% of tidal range change. SLR tends to promote tidal energy entering HZB through the Zhoushan Archipelago (ZA) compared to the prototype, while dampened sea-bed roughness and reduced tidal velocity come with a less dissipative environment in HZB, resulting in 6–18% more tidal energy exported upstream. Numerical experiments indicate ZA has significant effects on tidal responses and energy flux generation, therefore, its quantitative influences and physical mechanism are also discussed in this paper.