Mangrove forest is one of the most important ecological and environmental resources by effectively promoting tidal flat deposition and preventing the coastal region from typhoon. However, there have been mass loss of mangrove forests due to anthropogenic activities. It is an urgent need to explore an effective way for mangrove restoration. Here, three rows of bamboo fences with hydro-sedimentary observation set over Aegiceras corniculatum mangrove tidal flat of the Nanliu Delta, the largest delta of Beibu Gulf, China, were conducted to analyze the hydro-sedimentary variations induced by bamboo fences. Results identified that the mean horizontal velocity U_m per burst (20 min) decreased by as much as 71% and 40% in comparison with those without bamboo fences in March and November, respectively, when the tidal current entering the bamboo area during flood. The maximum of mean horizontal flow velocity U_m-max at bamboo area was 50%–75% of that without bamboo fences during ebb tide. The suspended sediment concentration of bamboo area suggested a maximum reduction of 57% relative to bare flat during flood, and was 80% lower than bare flat at ebb peak. Moreover, the turbulent kinetic dissipation ε at flood tide was significantly higher than that at ebb tide, while the bamboo fences greatly increased the turbulent kinetic dissipation ε by 2 to 5 times relative to bare flat, resulting in an increase of the bed elevation by inhibiting the sediment incipient motion and intercepting suspended sediment. The siltation rate at the bamboo area was 140% and 29.3% higher than that at the bare flat and the region covered with A. corniculatum, respectively. These results highlight that bamboo fences can effectively attenuate tidal current and thus promote siltation over mangrove flat, which contribute great benefit to mangrove survival.

The horizontal structure of mangrove forests is an important characteristic that reflects a significant signal for coupling between mangroves and external drivers. While the loss and gain of mangroves has received much attention, little information about how the horizontal structure of mangrove forests develops from the seedling stage to maturity has been presented. Here, remote sensing images taken over approximately 15 years, UVA images, nutrient elements, sediments, and Aegiceras corniculatum vegetation parameters of the ecological quadrats along the Nanliu Delta, the largest delta of the northern Beibu Gulf in China, are analyzed to reveal changes in the horizontal structure of mangroves and their associated driving factors. The results show that both discrete structures and agglomerated structures can often be found in A. corniculatum seedlings and saplings. However, the combination of seedlings growing into maturity and new seedlings filling in available gaps causes the discrete structure of A. corniculatum to gradually vanish and the agglomerate structure to become stable. The aggregated structure of seedlings, compared to the discrete structure, can enhance the elevation beneath mangroves by trapping significantly more sediments, providing available spaces and conditions for seedlings to continue growing. Furthermore, by catching fine sediments with enriched nutrients, the survival rate of A. corniculatum seedlings in the agglomerated structure can be much higher than that in the discrete structure. Our results highlight the significance of the agglomeration of A. corniculatum, which can be beneficial to coastal mangrove restoration and management.

Mangroves are crucial for protecting coastal areas against extreme disasters such as tsunamis and storm surges. An experimental study was conducted to determine how mangroves can mitigate the tsunami wave propagation. The test was performed in a flume, where mangrove models were installed on a slope, and dam-burst waves were used to simulate tsunami waves. To study how mangrove forests reduce the impact of tsunamis, this paper measured the heights of the incoming waves under different initial conditions (tsunami wave intensity and initial water depth) and plant factors (arrangement and distribution density) and described the reduction process. The results show that, after passing through the mangrove, the tsunami bore height will decrease within a certain range as the initial water depth increases. However, there is no correlation between the increase of inundation level and the drop of water level. The bore height attenuation is more significant at higher density of mangroves, but after tsunami passing through the mangroves, the relative bore height will decrease. When the distribution density of mangroves is constant, the wave attenuation at different locations (before, on and after the slope) shows different relationships with the initial water depth and wave height for different models. The transmission coefficient ($ {K }_{i} $) shows a parabolic correlation with its density. The proportion of the energy loss caused by the mangrove resistance to the total energy ($ {E }_{b} $) is defined as $ {C}_{m2} $. The variation trend of $ {C}_{m2} $ corresponds to the tsunami wave energy attenuation rate ($ {C}_{a} $) and $ {K }_{i} $.

To explore the nourishment effect and disaster reduction efficiency of a fully dissipative dry beach under the impact of storms, this paper uses the measured topography and hydrodynamic data to establish a one-dimensional numerical model of the XBeach beach profile. By numerically modeling the change in the nourished profile for different dry beach widths under normal waves and storm conditions and the recovery process of the profile after the storm, the degree of response in dry beach nourishment for the fully dissipative beach is analyzed. The results show that under normal wave conditions, the response of the nourished dry beach is obvious. Sediment on the dry beach erodes heavily, and the shoreline moves landward over a long distance. With the increase in the width and size of the dry beach, the wave height at the bottom of the backshore profile decreases, the wave height attenuation rate increases continuously, and the wave elimination effect is remarkable. When the storm incident wave intensifies, the wave height attenuation rate of the nourished dry beach decreases, indicating that the smaller the storm intensity is, the more significant the wave reduction effect of the nourished dry beach is. At the same time, different profile arrangements of nourished dry beaches suffer from different degrees of erosion under storm conditions, with significant changes in profile morphology. With intensified storm action, the intensity of sediment erosion in the nourished dry beach increases, the nourishment is weakened, and the recovery effect of the profile after the storm is not obvious. The results of the numerical modeling highlight that the dry beach nourishment method can resist storms to a certain extent, but the overall effect is relatively limited.

The cross-shore variation in wind speeds influenced by beach nourishment, especially the dramatic changes at the nourished berm, is important for understanding the aeolian sand transport processes that occur after beach nourishment, which will contribute to better beach nourishment project design on windy coasts. In this paper, the influencing factors and potential mechanism of wind speed variation at the edge of a nourished berm were studied. Field observations, together with the Duna model, were used to study the cross-shore wind speed distribution for different nourishment schemes. The results show that the nourished berm elevation and beachface slope are the main factors controlling the increase in wind speed at the berm edge. When the upper beach slope is constant, the wind speed at the berm edge has a positive linear correlation with the berm elevation. When the berm elevation remains constant, the wind speed at the berm edge is also proportional to the upper beach slope. Considering the coupling effects of nourished berm elevation and beachface slope, a model for predicting the wind speed amplification rate at the nourished berm edge was established, and the underlying coupling mechanism was illustrated.

The change of coastal wetland vulnerability affects the ecological environment and the economic development of the estuary area. In the past, most of the assessment studies on the vulnerability of coastal ecosystems stayed in static qualitative research, lacking predictability, and the qualitative and quantitative relationship was not objective enough. In this study, the “Source-Pathway-Receptor-Consequence” model and the Intergovernmental Panel on Climate Change vulnerability definition were used to analyze the main impact of sea level rise caused by climate change on coastal wetland ecosystem in Minjiang River Estuary. The results show that: (1) With the increase of time and carbon emission, the area of high vulnerability and the higher vulnerability increased continuously, and the area of low vulnerability and the lower vulnerability decreased. (2) The eastern and northeastern part of the Culu Island in the Minjiang River Estuary of Fujian Province and the eastern coastal wetland of Meihua Town in Changle District are areas with high vulnerability risk. The area of high vulnerability area of coastal wetland under high emission scenario is wider than that under low emission scenario. (3) Under different sea level rise scenarios, elevation has the greatest impact on the vulnerability of coastal wetlands, and slope has less impact. The impact of sea level rise caused by climate change on the coastal wetland ecosystem in the Minjiang River Estuary is mainly manifested in the sea level rise, which changes the habitat elevation and daily flooding time of coastal wetlands, and then affects the survival and distribution of coastal wetland ecosystems.

Low tide terrace beach is a main beach type along South China coasts with strong tidal actions. How strong tides affect wave transformations on low tide terrace beach still remains unclear. In this study, in-situ measurements are conducted on the low terrace beach at Xisha Bay to provide quantitative descriptions of wave shoaling and shore-breaker phenomena under the tidal effects. It is found that wave breaking is unsaturated on the low tide terrace beach at Xisha Bay. Magnitudes of wave skewness and asymmetry increase as wave shoals and achieve the maximum value at the shore-breaker, and then decrease rapidly. Mean energy dissipation rates of shore-breakers are tide-modulated since the bottom slope changes at the shoreward boundary of wave propagation in a tidal cycle. The remaining wave energy flux at the initialization of the shore-breaker is 1%–12% of offshore wave energy flux, and the energy flux ratio decreases with increasing offshore wave heights. Wave attenuation at shore-breakers can be estimated directly from offshore wave conditions based on findings in this study, favoring designs of seawalls or beach nourishment projects. Field datasets on wave transformations can also be used for verifications of wave numerical models.

Fences have been widely used in coastal protection engineering for their low cost, simple deployment, and easy integration with ecosystems. The effects of fence porosity and height on dune development have been investigated while not much attention has been paid to the effects of fence opening configurations, such as opening size and geometry, and porosity distributions. In this study, we deployed eight fences with same height and similar porosity, but different opening configurations on a sandy beach in Pingtan, Fujian Province. Results indicate that there is a similar two-dune-one-trough pattern for all fences at the beginning of dune development, and opening size, orientation, and geometry, and porosity distribution control the leeward dune peak locations. Fences with small openings and non-uniform porosity have high trapping efficiency, and upper denser porosity may be the optimal design as these fences have the highest trapping efficiency and capacity. The conclusions from this study can provide guidance on practical fence design.

How coral reefs with high productivity and biodiversity can flourish in oligotrophic tropical oceans has inspired substantial research on coral reef ecosystems. Increasing evidence shows that similar to water in an oasis in the desert, there are stable nutrient supplies to coral reefs in oligotrophic oceans. Here, with emphasis on the fluxes of organic matter, we summarize at the ecosystem level (1) the multiple input pathways of external nutrients, (2) the storage of nutrients in reef organisms, (3) the efficient retaining and recycling of dissolved and particulate organic matter within coral reef ecosystems, (4) the distinctly high phytoplankton productivity and biomass inside and near oceanic coral reefs, and (5) the export of reef-related organic carbon to adjacent open oceans. These properties enable coral reefs to function as ecological “pumps” for gathering nutrients across ecosystems and space, retaining and recycling nutrients within the ecosystem, supporting high phytoplankton productivity, and exporting organic carbon to adjacent open oceans. Particularly, the high phytoplankton productivity and biomass make waters around coral reefs potential hotspots of carbon export to ocean depths via the biological pump. We demonstrate that organic carbon influx is vital for coral reef ecosystems’ carbon budget and carbon export. The concept of the coral reef ecological pump provides a framework to improve the understanding of the functioning of the coral reef ecosystem and its responses to disturbance. Prospects of the coral reef ecological pump in coral reef studies are discussed in changing oceans driven by human activities and global change in the Anthropocene.

A series of experiments are designed to propose a new method to study the characteristics of convex mode-2 internal solitary waves (ISWs) in optical remote sensing images using a laboratory-based optical remote sensing simulation platform. The corresponding wave parameters of large-amplitude convex mode-2 ISWs under smooth surfaces are investigated along with the optical remote sensing characteristic parameters. The mode-2 ISWs in the experimentally obtained optical remote sensing image are produced by their overall modulation effect on the water surface, and the extreme points of the gray value of the profile curve of bright-dark stripes appear at the same location as the real optical remote sensing image. The present data extend to a larger range than previous studies, and for the characteristics of large amplitude convex mode-2 ISWs, the experimental results show a second-order dependence of wavelength on amplitude. There is a close relationship between optical remote sensing characteristic parameters and wave parameters of mode-2 ISWs, in which there is a positive linear relationship between the bright-dark spacing and wavelength and a nonlinear relationship with the amplitude, especially when the amplitude is very large, there is a significant increase in bright-dark spacing.