Coastal Engineering and Fluid–Structure Interactions

1. Introduction

Fluid–structure interaction (FSI) in coastal engineering is a core topic in the interdisciplinary fields of harbor, coastal, offshore, and structural engineering. It involves the dynamic effects of fluid loads, including waves, ocean currents, and winds on structures (e.g., harbors, breakwaters, cross-sea bridges, offshore wind turbine foundations, and subsea pipelines) and their responses and counteractions. Recently, significant progress has been made in theoretical and modeling techniques, experimental and monitoring technologies, and engineering applications.

Advancements in theory and model developments include the following: (1) traditional linear potential flow theories (e.g., the Morrison equation, diffraction and radiation theories), which remain widely used but have evolved into higher-order boundary element methods and fully nonlinear models (e.g., higher-order spectral methods), enabling more accurate simulations of wave breaking, overtopping, and highly nonlinear waves [1]. (2) Computational Fluid Dynamics (CFD) methods (e.g., VOF, SPH, LES) have gradually matured, enabling the processes of complex turbulence, vortex-induced vibrations, and coupled interactions between waves, structures, and sea beds (e.g., fluid–structure-soil coupling and gas–liquid–solid three-phase coupling) [2]. (3) High-fidelity CFD-FSI coupling employs the partitioned coupling method (e.g., MPCCI, Fluent + ABAQUS) or the immersed boundary method to achieve interactions between transient flow and large structural deformation [3]. The computational efficiency is significantly enhanced by the acceleration of GPU using the OpenFOAM integrated solvers with high-resolution [4]. Combining Large Eddy Simulation (LES) with potential flow theory achieve cross-scale simulations from wave propagation in open waters to the interaction with local structures in coastal engineering [5]. (4) Under stochastic wave loads, Proper Orthogonal Decomposition and deep learning (e.g., LSTM, CNN) methods are used in surrogate models to accelerate long-term dynamic response analysis [6]. Uncertainty quantification and reliability analysis are introduced for material and geometric uncertainties, structural failure probability, and fatigue life through Monte Carlo methods and polynomial chaos expansion [7].

Progress in experimental and monitoring technologies include (1) advanced experimental facilities: Large wave tanks (e.g., multi-directional wave generators), wind–wave-current interaction basins, and centrifuge model tests can simulate complex environmental loads and foundation conditions [8]; particle image velocimetry and laser Doppler velocimetry are employed for precise measurement of flow fields and vortex structures around structures [9]; and (2) on-site monitoring and digital twins: Long-term health monitoring of actual engineering structures (e.g., coastal and offshore wind turbines and bridges) are conducted based on fiber-optic sensing, the Internet of Things, and satellite remote sensing. Digital twin technology integrates monitoring data and numerical models to achieve real-time structural state diagnosis and predictive maintenance [10].

Applications of engineering fields primarily include three scenarios: (1) Offshore renewable energy structures: Coupled wave–current–wind dynamic analysis of fixed and floating coastal and offshore wind turbine foundations, particularly the stability of tension leg platforms and semi-submersible platforms under severe sea conditions. Studies include flow-structure interactions and energy capture efficiency optimization of tidal and wave energy devices [11]. (2) Coastal protection and ecological engineering: This includes research on the wave dissipation mechanisms, hydraulic performance of permeable and porous media structures (e.g., ecological reefs and vegetated wetlands), with a focus on disaster prevention and ecological restoration, and assessing the overtopping and breaching risks of breakwaters and seawalls in response to sea level rise and typhoon storm surges [12]. (3) Sea-crossing transportation infrastructure: This scenario focuses on the wave impact and local scour protection for deepwater bridge pile foundations, and hydrodynamic stability during the sinking and splicing of submarine tunnel pipe sections [13].

Studies on the coastal engineering and fluid–structure interactions face multiple aspects of challenges for future research. Under extreme environments and climate adaption (e.g., rouge waves, tsunamis, and tropical cyclones), research on the survivability and resilience design of structures is important [14]. Research needs to be further developed for analysis methods for multi-hazard coupling effects (e.g., complex disaster scenarios of seismic-wave interaction and sea–ice–wave coupling loads). Intelligence and sustainable development are key to coastal engineering and FSI. Therefore, one future direction could optimize FSI model parameters with AI and develop adaptive structures with autonomous control (e.g., intelligent damping) [15]. Another direction is to develop durability assessment methods for green materials (e.g., composites and eco-concrete) in complex fluid environments involving corrosion and biofouling.

2. Findings Reported in This Special Issue

Coastal engineers have designed and constructed protected structures to cope with coastal dynamics, including wave behavior, storm surge, sediment transport, erosion, and sea level changes. Therefore, understanding coastal hydrodynamic environments and fluid–structure interaction is an important issue in coastal engineering. The research topics in the field of coastal engineering include broad scopes such as (1) coastal dynamic environments of winds, waves, currents, sea ice; (2) sediment transport in the changing morphology of coastal, estuarine, and offshore regions; (3) the technical and functional design of coastal and harbor structures; (4) fluid–structure interactions, including conventional hard and nature-based soft structures; (5) innovations in research methods and techniques, including mathematical and numerical modeling, laboratory and field observations, and experiments. This Special Issue invites papers including, but not limited to, the abovementioned topics. This Special Issue comprises 13 articles, including 12 research articles and 1 review article.

Wang et al. (contribution 1) apply the Volume of Fluid (VOF) method in OpenFOAM to study the impact of a submerged shell dike on dissipating broken waves before a breakwater. When the broken wave heights decrease and the dike’s radii increase, the peak pressure decreases. The relationship is established between the broken wave pressures and the dimensionless parameters as a function of broken wave and breakwater heights and the dike’s radii. They propose the equations for estimating broken wave pressures on various points along the breakwater.

Zhao et al. (contribution 2) use OpenFOAM v2206 to discuss model simulations from four turbulence models and three mesh types. It reveals that the stabilized $k-\omega$ shear stress transfer turbulence model better simulates the complex wave evolution process on the cube and effectively captures the wave free surface, wave run-up, and reflection coefficient, and it is selected in the hydrodynamic model with new ecological hollow cubes.

Chen et al. (contribution 3) study the incipient motion and sediment scouring near submarine cables with five grain sizes. They find that the relative flow velocity, scour rate, and sediment erosion increase with the increasing Froude number. The modified formula is reliable in assessing the submarine cable exposure risks, providing insights into developing effective protection strategies, and enhancing cable stability in complex marine conditions. Understanding sediment dynamics and submarine cable stability is important to developing effective protection strategies in the dynamic marine conditions.

Peng and Yin (contribution 4) analyze the distribution and anomalies of total suspended sediment with the satellite-derived residual surface currents. Coastal water masses show obvious seasonal variations. It varies spatially for the offshore transport pathway, which extends to the shelf edge restricted by the Taiwan Warm Current, and shifts from northeastward to eastward for the persistent transport pathway. The Hangzhou Bay’s pathway is related to the tidal mixing, and the Yangtze River estuary’s path follows Yangtze River Diluted Water. These crucial observational insights are beneficial to the material cycling model in the East China Sea shelf.

Qian et al. (contribution 5) use the commercial finite element method to model the submarine cable’s pullback process with the horizontal directional drilling technology. The cable’s tension increases with the incident angle and crossing length. During its pullback operation, the cable could be locked by the extremely large velocity. Further investigations indicate that the permissible values could be important for similar engineering projects.

Hou et al. (contribution 6) use satellite observations to study the effects of consecutive typhoons on chlorophyll-a concentrations in the South China Sea. In 2006, after Typhoon Durian, strong vertical mixing and upwelling rapidly enhanced chlorophyll-a concentrations. These enhancements were significant in the path for Typhoon Utor. The changing marine environment brought by the first typhoon modulates the effects of successive typhoons on marine ecology, and this provides insights into understanding a typhoon’s effects on marine productivity.

Ghaderi et al. (contribution 7) use CFD simulations in a rectangular channel to examine the effects of four distinct converging configurations of guide-banks on the unsteady flow’s propagation. The flow experiences a depth increase as it encounters the converging geometries. This forms a hydraulic jump for the resulting waves’ progress upstream. When floods interact with the topography, the contracting channel results in pronounced initial water elevation rises and deeper reflected waves. Less time is needed to influence upstream for the waves generated by the trapezoidal configuration.

The increased water elevation becomes wider with longer configurations in the converging zone. This work helps understand hydraulic processes during dam failure scenarios.

Fan et al. (contribution 8) use the physical system to investigate the mechanical properties of soft soils and geotextile pipe and bags. The soil’s compression curve shows strong nonlinearity and high apparent compressibility from silt. The deviatoric stress in the soft soil increases when the consolidation pressure rises, indicating a typical behavior of strain-hardening. Based on the modeling results, the structural horizontal displacement is substantially impacted by the reclamation construction process. It centers at the toe of the slope for the significant stress, while the maximum tensile stress occurs in the central part of the pipe-bag structure. The drainage board’s installation effectively speeds up the pore pressure dissipation, achieving near-complete consolidation in one year. This work lays a theoretical foundation and practical guidance for the assessment of the stability and safety of the geotextile pipe and bag structures on soft soil foundations.

Zhang et al. (contribution 9) apply a two-dimensional model to analyze the effects of the submerged dike orientation and height on flow and flux. For the alongshore flow, the obvious velocity variations occur at 1/5 of the dike length. Flow velocity changes with the increase in its distance to the submerged dike. The dividing value of 0.7 is the ratio between the submerged dike height and water depth. The cross-dike flux increases with the orientation angle. These findings support the establishment of a theoretical basis supporting future integrated management of coastal zones.

Fang et al. (contribution 10) analyze rainfall characteristics from 1986 to 2017 at three typical irrigation stations in East China. High utilization is observed during extreme rainfall events, and larger variability during moderate rainfalls. The Support Vector Regression model is used to predict daily effective precipitation based on rainfall, antecedent precipitation index, drought days, and extreme rainfall indicators. Results indicate that the model captures the nonlinear relationship satisfactorily between effective precipitation and rainfall characteristics. They conclude that the machine learning method can be used as an alternative tool for the existing estimation models for water-saving crop cultivation and irrigation scheduling.

Liu et al. (contribution 11) collect the long-term Sentinel-2 Multispectral Instrument and Sentinel-1 Synthetic Aperture Radar data, constructing a high-resolution time series of Apparent Inundation Frequency obtained through Unmanned Aerial Vehicle surveys. Results show that the average inundation probability in the Luanhe Estuary shows a fluctuated, but overall, upward trend from 2016 to 2025. The quantification demonstrates that microtopography is the major controlling factor influencing fine-scale tidal inundation variations. This work suggests a reliable framework for the assessment of coastal vulnerability. The high-resolution quantitative data offers scientific support for the strategies of geomorphological management and disaster mitigation.

Ma and Zhang (contribution 12) apply the Reynolds-averaged Navier–Stokes (RANS) model to investigate the correlation between the disturbance parameters and wave features. The $k-\epsilon$ model and immersed boundary method (IBM) are used to resolve the flow turbulence and fluid–structure interaction (FSI), and the free surface is tracked using the VOF method. Results indicate that the proposed model captures various wave patterns satisfactorily. The wave evolution strongly depends on the disturbance duration and its width. Shorter durations trigger earlier soliton fission, and longer widths accelerate phase celerity. This work concludes that the disturbance parameters are critical in governing soliton formation and energy propagation patterns that are vital in disaster forecasting.

Jiang et al. (contribution 13) review sandy beach resilience (e.g., resistance, recovery, and adaptation). They examine the Deep Learning (DL) method in monitoring and forecasting external forcing, which includes typhoon tracks and storm surge peaks. The DL is applied to forecast beach processes, including medium- and long-term shoreline evolution. Governance options are expanded by DL methods and multi-scenario generation. From the perspective of management and decision support, policy adoption and risk communication are enhanced by interpretable features with uncertainty quantification. The DL method surpasses traditional models by reducing the observation–model–decision cycle, expanding analysis for various scenarios, and improving the governance transparency. However, it remains as a challenge for the cross-domain generalization, robustness in extreme situations, and the data governance. This work reviews DL as a potential technology to improve sandy beach resilience while offering a theoretical basis for future studies.

3. Conclusions and Future Directions

The articles collected in the Special Issue “Coastal Engineering and Fluid Structure Interactions” include keywords of coastal and nearshore engineering, fluid–structure interactions (FSIs), extreme events (e.g., typhoon and seismic loads), numerical model (e.g., CFD, VOF, and OpenFOAM) and observations, Deep Learning (DL) method, Immersed Boundary Method (IBM), nonlinear process, climate change, human activities, submerged cables, and coastal protection. These keywords are aligned with the theme of this Special Issue but have some extensions related to coastal engineering and FSI. Studies of coastal engineering and FSI have evolved from deterministic analysis to stochastic, nonlinear, and multiscale coupling, with increasingly close integration of numerical simulations, experimental techniques, and field monitoring. Future trends will focus on extreme climate adaptability, intelligent operation and maintenance, and interdisciplinary integrated innovation, providing critical support for the safety, economy, and sustainable development of coastal and marine engineering.