Beach erosion has occurred globally in recent decades due to frequent and severe storms.Dongsha beach,located in Zhujiajian Island,Zhejiang Province,China,is a typical embayed sandy beach.This study focused on the morphodynamic response of Dongsha beach to typhoon events,based on beach topographies and surficial sediment characteristics acquired before and after four typhoon events with varying intensities.The four typhoons had different effects on the topography and sediment characteristics of Dongsha beach.Typhoons Ampil and Danas caused the largest(-51.72 m3/m)and the smallest erosion(-8.01 m3/m),respectively.Remarkable alongshore patterns of beach profile volumetric changes were found after the four typhoon events,with more erosion in the southern and central parts of the beach and few changes in the northern part.Grain size coarsening and poor sorting were the main sediment patterns on the beach influenced by different typhoons.Typhoons that occurred in the same year after another typhoon enhanced the effect of the previous typhoon on sediment coarsening and sorting variability,but this cumulative effect was not found between typhoons that occurred during different years.A comparison of the collected data revealed that the topographic state of the beach before the typhoon,typhoon characteristics,and tidal conditions were possible reasons for the difference in the responses of Dongsha beach to typhoon events.More severe beach erosion was caused by typhoons with higher intensity levels and longer durations,and high tide levels during typhoons can determine the upper limit of the beach profile erosion site.Taken together,these results can be used to improve beach management for storm prevention.

The study of estuarine hydrodynamics within an isohaline framework has been well established in recent years. However, most studies of estuarine mixing and exchange flow using salinity coordinates have been conducted for idealized estuaries or realistic estuaries with simple bathymetry and constant runoff. In this study, a realistic numerical model is applied to the Changjiang River estuary, a complex multi -branched estuary with huge and variable runoff. The destruction of salinity variance is used to quantify salinity mixing, in which numerical mixing accounts for about one third of the total mixing. The results of total mixing per salinity class m ( S ) indicate that the universal law of estuarine mixing applies to realistic simulations under huge and variable runoff when averaged over two spring -neap cycles (28 days). During the flood season, volume integrated salinity mixing is approximately twice that of the dry season due to the double runoff. Salinity mixing at different channels displays distinct characteristics in both horizontal and vertical distributions. The patchy distribution of effective vertical diahaline velocity implies that the volume flux across an isohaline surface can exhibit significant spatial variability. The most vigorous salinity mixing occurs in the North Passage, where about 58 % of the freshwater enters the sea. We found that the estuarine mixing within different branches is linearly correlated with the corresponding water transport. Dikes fringing the navigational channel on both sides downstream of the North Passage significantly increase the length of saltwater intrusion in the North Passage, alter the distribution of horizontal and vertical flow velocities, and further cause significant changes in local mixing, especially near the mouth of the dikes.

岬湾砂、砾质海岸是海岛、海岸带重要的旅游资源,具有较高的社会经济和生态价值,长期以来备受关注.本文以浙江朱家尖岛东岸的5个代表性岬湾海滩为例,基于2019年台风季节早期、中期及晚期测量获得的海滩地形和沉积物数据,结合海滩近岸的水动力数据,分析了砂质海滩和砾石海滩这两种不同类型岬湾海滩的沉积地貌动态变化.结果表明,朱家尖岛东岸5个海滩在台风季节出现了不同的体积变化,大沙里、东沙、千沙、乌石塘和小乌石塘海滩的单宽体积变化分别为11.93m3·m-1、-54.41m3·m-1、-19.75m3.m-1、2.19m3·m-1和-1.96m3·m-1.砾石滩较砂质海滩更为稳定,无人类活动干扰的砂质海滩在台风季节侵蚀更少、变化更小.台风季节大沙里、东沙、千沙、乌石塘和小乌石塘海滩表层沉积物的平均粒径分别为2.47Φ、2.24Φ、2.64Φ、-5.96Φ和-6.03Φ,粒径粗化和离岸输运是5个海滩表层沉积物在台风季节的主要表现,砂质海滩的沉积物粒度特征变化比砾石海滩要大.沉积物粒径、台风强度及台风期间的主要波向与海滩走向之间的关系、海岸工程这3种因素都可能对海滩在台风季节的沉积地貌动态变化产生影响.本文研究结果可为台风季节的海滩管理提供参考.

Sand-mud transition (SMT) is an important boundary of the beach system, and its location and evolution have been increasingly studied in recent years. This study focuses on the location and migration of SMTs and their influencing factors on three representative embayed beaches on the east coast of Zhujiajian Island, Zhejiang Province, China, characterized by different levels of human interventions. Beach topographies, surficial sediment characteristics and nearshore hydrodynamic data were obtained through three field campaigns carried out during the early, middle and late stages of the 2019 typhoon season. This typhoon season included four typhoons with nearshore significant wave height Hs exceeding 3.71 m, and maximum Hs of 6.77 m (Super Typhoon Lekima). Results show that the three beaches were all impacted by the high-energy wave conditions, although with some different behaviors. Sediments of the three beaches all coarsened with worse sorting during the typhoon season, with the nearshore surficial sediments showing similar patterns. The SMTs of the three beaches were stable or migrated seaward during the typhoon season. During the typhoon season, offshore SMT migration was positively correlated with the beach profile volumetric loss at three embayed beaches in this study. The SMTs of beaches with less human intervention are more stable during typhoon season. By including 12 additional embayed beaches of eastern China in our analysis, we found that the SMT offshore distance increases with increasing nearshore average significant wave height, increasing headland offshore extent and decreasing tidal range. Our study suggests that SMT is a relevant indicator of beach sediment stability, which can help to increase the understanding of embayed beach dynamics and to guide coastal management and planning during typhoon seasons at such embayed beaches.

Beach nourishment is a proved effective protection approach which has been widely used in recent years. An Argus video monitoring system has been set up to monitor morphological changes and effects of continuous nourishments at Dongsha beach, an embayed beach in Zhoushan Archipelago, eastern China. Video-derived shorelines along with their morphological parameters, such as dry beach width, dry beach area, beach orientation and unit width volume were analyzed during the monitoring period from June 2016 to July 2017. Analysis of video monitoring data shows that shorelines retreated during autumn and winter when storms were intensive, while advanced in spring and summer, with a lot of bulges occurred after nourishment projects. Abrupt variations in the beach orientation were always followed by gradual recoveries to the average beach orientation, while continuous counter-clockwise rotation occurred after March 2017 when storm events were sparse. Comparing the different beach responses to individual storm events, we found that small-scale and short-interval sand nourishment implemented timely after storms can compensate for sediment loss more effectively on this beach. This study can provide a reference for local beach management.

Limited studies have investigated the modification of tidal currents by mangrove trees. In particular, the impacts of mangrove trees on a vertical velocity profile remain unclear. An automatic system is developed to observe the vertical velocity profiles within a mangrove forest composed of artificially introduced Kandelia obovata located on the Ximen Island, the southern Zhejiang Province. The results reveal low flow velocities throughout the vertical profile within the mangrove forest. The vertical profile shows an overall decrease of the velocity with an increasing height above the bed. This pattern is due to the vertical increase of the drag force by the trunks and tree canopies. In addition, the turbulent energy density also varies vertically, corresponding to the vertical structure of the mangrove trees. In comparison with the vertical structure of flows within bare mudflats (semi-logarithmic) and salt marshes (nearly J-shape), the mangrove trees are shown to have a considerable impact on the vertical velocity profile in different ways.

Current rehabilitation efforts to restore degrading mangroves mainly include afforestation with nursery grown plants. Hydrodynamic processes associated with sediment deposition and erosion play major roles in determining seedling establishment success. However, only a few detailed reports of the influence of seedlings on tidal flows are available. In situ measurements were conducted in this study to compare tidal flows between a restored mangrove flat covered by seedlings and a bare mudflat, located within Yueqing Bay, southeast China. High frequency 3D flow velocity variations and vegetation attributes were recorded on comparative sites to determine the effects of vegetation characteristics on flows at a small-scale. The vertical velocity profile displayed an S-shape distribution within the seedlings that was closely related to the vertical distribution of vegetation biomass. However, the turbulent kinetic energy within the seedlings varied minimally in the vertical direction, except for an abrupt increase at the top of the canopy. Comparing the mangrove seedling site and the bare mudflat site, the horizontal flow magnitude in the mangrove seedlings was reduced only limitedly from the mudflat, while the turbulence kinetic energy density had been noticeably reduced. Thus, mangrove seedlings were found to mainly modify the vertical velocity profile and the turbulent energy magnitude in terms of their influences on tidal flows. These findings are able to provide more detailed mechanisms for biogeomorphological modeling and further apply into mangrove restoration.

Saltmarsh and mangrove are common coastal wetland types and their ability to enhance deposition has been investigated extensively, but rarely compared directly. This study carried out in situ observations to compare the sediment transport processes between a bare mudflat, a mangrove stand and a saltmarsh stand within a subtropical estuary. Turbidity variations over the latter portion of a spring tide were recorded, alongside measurements of flow data, to estimate sediment trapping by hydraulic forces under similar hydroperiods. In addition, vegetation was transplanted to compare the direct sediment trapping by high-standings and short-seedlings. The suspended sediment concentration (SSC) time series showed an overall reduction between the bare mudflat and the vegetated flats. Suspended sediment flux estimates revealed that a considerable amount of sediment was trapped by the saltmarsh and the mangrove edges. The flux estimates find that the saltmarsh edge is more efficient than the mangrove edge in trapping sediments transported normal to the edge. The sediment trapping mechanisms were considered based on two approaches: the hydrodynamic related sediment settling and direct trapping by vegetation. The calculation of deposition tendency showed that the presence of vegetation altered the flow direction and the tidal asymmetry of the deposition process, resulting in a higher deposition tendency during the flood phase to enhance sediment settling. In addition to sediment settling, vegetation surfaces were found to trap sediments directly. In combination with rinsing by precipitation, these trapped sediments accumulated on the bed and contributed to the deposition. Against the background of similar inundation periods, the saltmarsh grass showed a greater sediment trapping ability than the mangrove trees, in terms of both the hydraulic sediment trapping and the direct trapping by vegetation surface.