Artificial reefs are being implemented around the world for their multi-functions including coastal protection and environmental improvement. To better understand the hydrodynamic and morphodynamic roles of an artificial reef (AR) in beach protection, a series of experiments were conducted in a 50 m-long wave flume configured with a 1:10 sloping beach and a model AR (1.8 m long × 0.3 m high) with 0.2 m submergence depth. Five regular and five irregular wave conditions were generated on two types of beach profiles (with/without model AR) to study the cross-shore hydrodynamic and morphological evolution process. The influences of AR on the processes are concluded as follows: (1) AR significantly decreases the incident wave energy, and its dissipation effect differs for higher and lower harmonics under irregular wave climates; (2) AR changes the cross-shore patterns of hydrodynamic factors (significant wave height, wave skewness and asymmetry, and undertow), leading to the movement of shoaling and breaking zones; (3) the beach evolution is characterized by a sandbar and a scarp which respectively sit at a higher and lower location on the profile with AR than natural beach without AR; (4) the cross-shore morphological features indicate that AR can lead to beach state transformation toward reflective state; (5) the scarp retreat process can be described by a model where the scarp location depends linearly on the natural exponential of time with the fitting parameters determined by wave run-up reduced by AR. This study demonstrates cross-shore effects of AR as a beach protection structure that changes wave dynamics in surf and swash zone, reduces offshore sediment transport, and induces different morphological features.

Submerged Reefs (SRs) are a kind of artificial fish habitat that can protect coasts and maintain ecological biodiversity. In this study, the flow field of the SR is simulated by solving a Reynolds-averaged Navier–Stokes equation closed with the Realizable k-ε model based on the finite volume method. The turbulent characteristics of SRs under different inflow velocities and submergences in the vicinity of the SR are analyzed. The wake vorticities are the primary turbulent pattern within and around the SR. The back wake and vorticity are chosen as critical indicators to quantitatively assess the hydrodynamic characteristics induced by the SR. The results show: (1) as the main flow passes through the SR, the upwelling is produced in front of the SR and a large-scale wake region is formed behind the SR which contains a clockwise vortex; (2) the length of the wake region formed behind the SR is positively and linearly correlated with both the inflow velocity and submergence; (3) the dipole-type vorticity patterns are induced within the compartment of the SR, where the area and average value of high vorticity have a positive correlation with the flow velocity and a negative correlation with the submergence, respectively.

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.

Groins are one of the popular manmade structures to modify the hydraulic flow and sediment response in river training. The spacing between groins is a critical consideration to balance the channel-depth and the cost of construction, which is generally determined by the backflow formed downstream from groins. A series of experiments were conducted using Particle Image Velocimetry (PIV) to observe the influence of groin spacing on the backflow pattern of two bilateral groins. The spacing between groins has significant effect on the behavior of the large-scale recirculation cell behind groins. The magnitude of the wake flow induced by a groin was similar to that induced by another groin on the other side, but the flow direction is opposite. The spanwise velocity near the groin tip dictates the recirculation zone width behind the groins due to the strong links between the spanwise velocity and the contraction ratio of channel cross-sections between groins. Based on previous studies and present experimental results, quantitative empirical relationships are proposed to calculate the recirculation zone length behind groins alternately placed at different spacing along riverbanks. This study provides better understanding and a robust formula to assess the backflow extent of alternate groins and identify the optimum groins array configuration.

Tidal-flat reclamation and coastal stabilization projects are widely implemented in the tide-dominated estuaries of eastern China, where intensive human intervention has profoundly altered sediment dynamics and morphological evolution. The Nanbei Lake reclamation area, located on the northern coast of Hangzhou Bay and the transitional reach of the Qiantang Estuary, China. This zone experiences strong semidiurnal tides, rapid current variations, and frequent typhoon impacts that shape highly dynamic geomorphic patterns. To mitigate erosion and promote siltation, a main embankment and seven rockfill groynes were constructed in 2007. However, long-term monitoring indicates that high crest elevations of the groynes have suppressed cross-shore water exchange, weakened tidal flushing, and promoted excessive sedimentation—patterns likely exacerbated by increasing storm-tide levels and evolving tidal asymmetry under climate change. To address these issues, this study evaluates a groyne-lowering scheme designed to enhance hydrodynamic connectivity while maintaining shoreline protection. A two-dimensional hydro–morphodynamic model (MIKE 21 FM) was developed using high-resolution bathymetry, tidal observations, and sediment data. The computational domain (~4000 km²) employs an unstructured mesh (minimum grid size 5 m) with 20 s time steps. Model calibration achieves strong agreement with measured tidal levels and velocities. The proposed scheme lowers the groyne crests by 0.2–2.5 m, increasing overtopping frequency during spring tides and enabling reactivation of intertidal exchange pathways. Model results reveal that groyne lowering significantly modifies the nearshore flow structure: bottom velocities increase by 0.005–0.050m/s, residual circulation strengthens between groynes, and previously stagnant zones behind the structures become reconnected. Morphodynamic responses over a spring–neap cycle indicate 0.2–0.4 m reduction in sedimentation near groyne heads, accompanied by mild accretion on the inner tidal flat, leading to a smoother, more gradually sloping intertidal profile. These changes reflect a shift toward a more dynamic and resilient morphodynamic state capable of better accommodating extreme water levels. This study highlights groyne lowering as an adaptive and nature-based intervention to counteract human-induced hydrodynamic restriction and climate-driven pressures. The findings contribute to improved understanding of eco-morphodynamic adjustment processes and offer guidance for sustainable coastal management in tide-dominated estuaries such as the Qiantang River delta.