Green tides have increasingly become a nuisance worldwide in recent years, and especially in China. Since 2015, green tides have started recurring in Jinmeng Bay, Qinhuangdao, western Bohai Sea of China, and have severely deteriorated the tourism environment there. In order to investigate the migration process of the green tides in Jinmeng Bay, a hydrodynamic model and a particle-tracking model were applied based on the latest green tide event in August 2021. The hydrodynamic model was applied with triple-level 2DH meshes with different refinements and scales, which provided the hydrodynamics to drive the green macroalgae into the particle-tracking model. From the model results, the semi-enclosed waters surrounded by multiple artificial structures are a low-energy hydrodynamic environment, which is not helpful for water exchange and thus the dispersal of nutrients. The green macroalgae are distributed substantially within the semi-enclosed waters, and few are transported out with low biomass. The effects of wind and artificial structures both increase the coverage of the green macroalgae trajectories; the effect of wind plays a more important role. A sensitivity analysis of the effect of wind showed that 6 m/s wind in ENE led to the maximum coverage of the green macroalgae trajectories in the cases of different magnitudes and directions of winds. This study can provide references for the pre-warning and mitigation of green tides in Jinmeng Bay and other similar places.

Complex turbulent flow structures are formed within and around box-type artificial reefs (ARs). This study utilizes Particle Image Velocimetry (PIV) and numerical simulation to investigate the flow field within and around ARs of various configurations, such as single- and dual-row arrays. It was found that a small space between reefs causes loading to concentrate on the first reef, while a large space enhances vortex intensity and reduces interference among reefs and promotes vortex development within individual reefs. An optimal space may enlarge the recirculation zone, increase vortex numbers and size, alter the flow distribution, and intensify turbulence, ultimately reshaping the flow characteristics at the reef array. The experimental data show that vortices within ARs attain their maximum strength at an overall reef length to height ratio (L_r/h_r) of 3 and reef width to height ratio (W_r/h_r) of 0.68. A further increase in L_r/h_r weakens the dipole, while an increase in W_r/h_r expands the area of high-vorticity and strong turbulence behind the stoss-face openings. These findings provide new insights for the optimum layout of artificial reefs for coastal defense design.

Recurrent green tide has been widely studied due to its severe damage to coastal ecosystem. Jinmeng Bay, a popular resort in northeastern China, has suffered from green tide events since 2015, after the constructions of artificial islands and submerged reefs. To investigate the potential impacts of artificial islands and reefs on the water quality in Jinmeng Bay, a MIKE 21 numerical model was established by coupling a hydrodynamic model with a transport model of Chemical Oxygen Demand (COD) and nitrate ion (NO₃⁻). The changes in the water quality in Jinmeng Bay by the initial, first-stage and current construction conditions of artificial islands and submerged reefs were simulated. The model results indicated that: (1) The artificial islands and reefs in Jinmeng Bay hinder the tidal currents and weaken the tidal actions. (2) The weakened tidal actions at the estuary lead to the accretion of COD and NO₃⁻. The neap tides generate a littoral zone with the high concentration of COD and NO₃⁻, and the spring tides maintain the zone at the estuary. (3) NO₃⁻ is more sensitive than COD to the variation of hydrodynamic conditions. The NO₃⁻ concentration in the north of Conch artificial island is altered significantly, where the construction of the artificial structures decrease the concentration by ~30%, while the demolition of the connection road increases the concentration beyond its initial values by 16~21%. (4) Under the current construction conditions, the rising concentrations of COD (up to 2%) and NO₃⁻ (up to 40%) increase the frequency and scale of green tides in Jinmeng Bay considerably. Therefore, continuous monitoring of water quality is required for this region.

Field observations were combined with a coupled hydrodynamic and water quality model to investigate the spatial and temporal variation in Chlorophyll-a (Chl-a) in Jinmeng Bay, China. The relatively high Chl-a values were distributed in the inshore waters, mainly due to the abundant nutrient inflow from the Tanghe River. The model’s results indicate that the Chl-a concentration was much higher in seaweed beds surrounded by artificial islands and reefs under the southeasterly wind, largely due to the fact that pollutants are prone to accumulate in coastal areas where flow is attenuated by the presence of natural and artificial marine structures. It was also found that the southwesterly winds suppress the inflow of nutrients from the Tanghe River to the coastal areas, and, therefore, lower the Chl-a levels. River input and wind forcing are the major factors that influence Chl-a concentrations in the anthropogenically influenced bay. This finding provides useful guidance for the prediction and mitigation of green tides in Jinmeng Bay in the future.

Better understanding of the hydro- and morphodynamic processes within vegetated sandbar-lagoon coasts is important for assessing the coastal protection capability of vegetation meadow for the coastal environments. Eighteen flume tests were conducted in a mobile-bed sandbar-lagoon with mimicked submerged vegetation under different water depths and wave conditions. It was found that wave attenuation by submerged vegetation near the breaking point is significant. An empirical linear expression for the total wave energy change ratio is proposed with a determination coefficient of 0.84. Moreover, the quantitative formulae for the erosion volume and maximum erosion thickness of sandbars and foredunes, as well as the total sediment transport volume, were proposed to demonstrate the implications of submerged vegetation meadows. These findings provide scientific references for coastal management and conservation planning, especially for sandbar-lagoon coasts. Nevertheless, additional physical experiments or field data are necessary to further validate those formulae.