Case Study of Contaminant Transport Using Lagrangian Particle Tracking Model in a Macro-Tidal Estuary

Tidal waves from the open ocean undergo various transformations as they enter estuaries and are influenced by various factors, such as freshwater inputs, water depth, and topography [1,2,3,4,5]. These transformations create asymmetry in tidal curves, leading to significant differences in sediment transport during flood and ebb tides. This variation affects the overall estuarine hydrodynamic system, making an understanding of the flood and ebb dominance phenomenon crucial in regions where tides are a significant driving force. Historically, estuarine areas have undergone development for various coastal projects, such as dyke construction, airport development, and bridge construction. More recently, they have become key sites for offshore wind and tidal current power generation. These developments alter the adjacent coastal topography and water flow characteristics, necessitating predictions of marine environmental changes due to the ongoing coastal development pressures. To assess and predict these impacts and establish conservation strategies, various numerical models have been employed. In [6], the authors explored spatiotemporal changes and annual variability in residual currents through long-term fixed-point observations, indicating that freshwater flux affects residual current characteristics. In [7], the authors observed that tidal propagation characteristics in coastal waters vary spatially in amplitude due to bottom friction and topographic convergence near narrow tidal straits. Furthermore, [8] estimated material transport amounts during neap and spring tides using the two-dimensional POM (Princeton Ocean Model).

The research focusing on the numerical simulation of intertidal zones has made notable contributions. In [9], the authors employed MIKE21 to analyze the impact of intertidal simulation on numerical model results in domestic waters. This study identified that the inclusion of the intertidal zone significantly affects overall seawater flow in the models. Further extending this research, [9] applied three different hydrodynamic models in Gomso Bay, Korea—EFDC (Environmental Fluid Dynamics Code), ESCORT (Efficient Support for Coastal Ocean Research and Test), and MIKE21; each of these models involved the employment of distinctly different methods and intertidal simulation techniques. This research entailed a comparative analysis of the intertidal simulation characteristics of each model and provided insights into their respective efficiencies and accuracies in simulating intertidal zones. Despite the advances, the existing research often inadequately represents the complex coastal features and dynamic interactions between environmental factors. Most models face challenges in accurately simulating intricate processes like sediment transport, contaminant dispersion, and ecological responses in estuarine environments. Additionally, the computational demand and extensive data required for model calibration and validation pose challenges, especially in modeling dynamic estuarine systems with significant spatial and temporal variability.

The primary objective of this research is to comprehensively analyze and understand the dynamics of contaminant transport in estuarine environments, with a specific focus on the role of tidal creeks and flats. By employing advanced hydrodynamic models that integrate both irregular grid systems and detailed residual currents analyses, this study aims to provide a more accurate and nuanced understanding of how contaminants move and disperse in complex estuarine systems. To achieve this objective, the paper is organized into the following sections: After Section 1. the introduction, the methodology is detailed, encompassing the employed hydrodynamic model, the simulation setup, and the specific parameters for residual currents and particle tracking. The Section 3 then offers a comprehensive examination of residual currents and particle movements in both ‘Creek’ and ‘No Creek’ scenarios under varying tidal conditions. Following this, the Section 4 interprets these findings and considers their implications for estuarine sediment dynamics, contaminant transport, and the overall ecological health of these environments. This section further explores the wider relevance of the study’s outcomes in the context of estuarine management and conservation. Then, the paper concludes with a summary of the principal findings, emphasizing the importance of incorporating detailed environmental features and residual currents in estuarine models for the precise prediction and effective management of contaminant transport. The conclusion highlights the contribution of our research to a broader understanding of estuarine dynamics and the advancement of sustainable environmental strategies.

This study significantly contributes to the understanding of estuarine hydrodynamics by examining the interplay between the geomorphological features and hydrodynamic processes within macro-tidal flat environments. By integrating detailed residual current analyses and employing unstructured grids, this research offers a nuanced approach to the modeling of estuarine systems, which is crucial for predicting contaminant transport accurately. The ‘Creek’ case demonstrates the profound influence of tidal creeks on estuarine hydrodynamics, enhancing particle retention and potentially boosting nutrient cycling within these environments. This highlights the necessity of incorporating the complex interactions of Stokes drift and Eulerian and Lagrangian residuals in coastal models, especially in macro-tidal environments where tidal forces play a significant role in shaping the sediment and nutrient dynamics. Comparative studies from different estuaries, such as the Yellow Sea [14], highlight the impact of environmental factors, like solar radiation, wind forcing, river discharge, tides, and water exchange, on coastal morphology and oceanographic conditions. The Yellow Sea’s macro-tidal nature, with significant seasonal variability influenced by the Asian monsoon system, shares similarities with our study area, emphasizing the importance of considering such dynamic environmental conditions in estuarine modeling. Furthermore, the deployment strategies for monitoring systems in estuaries, as discussed in reference [14], underscore the critical need to strategically monitor location selection to capture changes in water quality and pollutants effectively. Based on spatial and temporal optimizations, these strategies align with our approach of using advanced modeling techniques to understand and predict the complexities of estuarine environments. Our study’s findings, set against the backdrop of these comparative analyses, highlight the unique aspects of our research area and the importance of accurately representing tidal channels in hydrodynamic models. The enhanced capacity for material transport observed in the ‘Creek’ case during spring tides underscores the critical role of tidal creeks in estuarine systems, necessitating accurate tidal channel representation for reliable transport predictions and a deeper understanding of coastal estuarine transport mechanisms. By drawing insights from various studies [14], our research contributes to a broader understanding of estuarine dynamics and the development of more effective environmental management and conservation strategies tailored to the macro-tidal flat environments’ specific needs and characteristics.

The significance of salinity stratification in estuarine systems, particularly during neap tides, is a pivotal factor influencing vertical mixing and the distribution of biological and chemical components. This study’s ‘Creek’ case highlights how pronounced stratification could shape the vertical dispersal of larvae, nutrients, and pollutants, which significantly impact the estuarine ecosystem and sediment dynamics. Such stratification effects on species distribution, behavior, and sedimentary processes underscore the need for models that intricately integrate geomorphological features and their hydrodynamic influences for precise material flux predictions in estuaries. Drawing parallels from field observations in the Geum River estuary [15], where artificial gate operations modulate stratification and tidal amplitudes drive mixing, our study underscores the complexity of the stratification processes influenced by natural and anthropogenic factors. As with the dynamics observed in the Geum River, our study’s ‘Creek’ case presents a scenario where tidal creeks significantly modulate the estuarine stratification, thereby affecting the estuarine dynamics and ecological balance. The exploration of advection, straining, and vertical mixing in estuarine stratification [15] resonates with our findings and emphasizes these processes’ critical roles in shaping the estuarine water body’s vertical density structure. The FVCOM application in the Seomjin River estuary highlights how straining and mixing govern the flow and stratification structures, which are akin to the stratification dynamics observed in our ‘Creek’ case. This parallel draws attention to the importance of considering the balance between mixing and straining in determining the stratification type in estuarine channels and further validates the nuanced approach of our study in capturing these intricate interactions. Moreover, the study on the impact of artificially discharged freshwater in a Korean estuary [15] provides insights into how discharged freshwater cyclically forms stratified layers during ebb tides and mixes during flood tides, aligning with the stratification and mixing patterns observed in our ‘Creek’ case. This cyclic pattern, characterized by the gradient Richardson number, offers a quantitative framework to assess the interplay between stratification and mixing in estuarine environments, reinforcing the necessity of incorporating such dynamic processes in estuarine modeling for effective environmental management and strategy formulation. The integration of the insights from these studies [15] with our research findings elucidates the multifaceted nature of estuarine stratification and its implications for material transport and ecosystem dynamics. The comparative analysis not only highlights the unique aspects of our study area but also emphasizes the overarching need for comprehensive hydrodynamic models that reflect the intricate interplay of geomorphological features and hydrodynamic processes in estuarine systems and ensure accurate predictions and effective management strategies in these dynamic and complex environments.

By integrating LPT models, this study advances the comprehension of particle dynamics by delving into the complexities of contaminant transport within macro-tidal flats. The influence of tidal and freshwater forces on salt intrusion and suspended sediment dynamics is assessed, echoing the necessity for high-resolution modeling, as demonstrated in the Changjiang River study [16]. This underscores the critical need to capture complex nonlinear interactions and to integrate high grid resolution for accurate hydrodynamic behavior modeling in estuarine systems. Our findings reveal distinct particle movements in the ‘Creek’ and ‘No Creek’ cases, emphasizing the geomorphological impact on coastal circulation. This is similar to the observations in the southeastern North Sea, where human-made structures affected sediment dynamics [17]. The study suggests that tidal creeks act as natural modulators of flow and sediment deposition, significantly impacting estuarine geomorphology and habitats [18]. This is further supported by the alignment of our study with the investigations into Lake Erie’s harmful algal blooms; these investigations highlight the utility of both Lagrangian and Eulerian models in forecasting ecosystem responses and the potential for hybrid approaches that can enhance future models [19]. The research integrates advanced modeling techniques and insights from various studies, highlighting the significant role of geomorphological features and their interaction with hydrodynamic processes [20,21,22]. The analysis of ‘Creek’ and ‘No Creek’ cases provides new insights into the protective role of tidal creeks in material transport within estuarine systems. This novel approach enhances our understanding of estuarine dynamics and aids in the development of effective environmental management and conservation strategies. The FVCOM framework used in our research corresponds with the latest advancements in coastal modeling, like the FESOM-C application in the southeastern North Sea [20]. The unstructured grid design of the FVCOM is crucial for simulating complex estuarine dynamics and offers refined meshing to capture small-scale processes effectively. Adjusting mesh resolution according to specific geographical and process requirements, validated against high-resolution observational data [20], highlights the efficacy of such models in capturing the environmental intricacies of estuarine systems. While high-resolution wave coupling models provide precision in short-term forecasting through the Navier–Stokes equations [23,24], incorporating them into three-dimensional flow-based LPT studies presents a challenge. Our methodology, which harmonizes Lagrangian and Eulerian perspectives, offers a promising avenue for the enhancement of the predictive accuracy and depth of analysis for contamination distribution studies in estuarine contexts. This integration of modeling techniques affords a comprehensive understanding of contaminant dynamics, informed by the pollutants’ final positions and the broader hydrodynamic interactions within estuarine systems.

The presence of tidal creeks significantly influences particle dispersion and material transport, implying a more intricate and dispersed transport mechanism provided by the additional pathways within the ‘Creek’ case. This complexity highlights the need for hydrodynamic models to accurately account for such features to make precise material transport predictions in estuarine environments. Suppose the tracked particles are representative of pollutants. In that case, the ‘Creek’ case demonstrates a swift dispersion to downstream river areas, underscoring the pivotal role of tidal channels in substance transport and distribution. This dynamic is crucial for environmental management, emphasizing the necessity for targeted strategies to mitigate pollution risks in riverine and coastal ecosystems [25,26,27]. The results of this study provide valuable insights for environmental management and policy making, particularly in the context of mitigating pollution risks in estuarine and coastal areas.