Study on Environmental Hydrology and Hydrodynamic Characteristics of Basins, Estuaries and Offshore

1. Introduction

Freshwater from land flows into the ocean, mixing with saline water at the land–sea interface to form estuarine and adjacent nearshore ecosystems—one of the most productive and biologically active critical ecological zones [1,2,3]. Under the dual pressures of climate change and intensifying human activities, the entire aquatic system, from watersheds to the coastal ocean, is undergoing unprecedented changes. These changes directly impact the safety, food security, and livelihood sustainability of coastal communities, in addition to affecting the physical, chemical, and biological processes within all water bodies [4]. Understanding and quantifying the environmental hydrological processes and hydrodynamic characteristics in these regions make up the scientific foundation for predicting their future, safeguarding their health, and achieving sustainable management.

Precipitation-driven runoff from watersheds, complex tidal dynamics and freshwater–saltwater mixing in estuaries, and circulation and transport processes in the nearshore zone together form a “land–ocean continuum”. The hydrological and hydrodynamic processes within this continuum regulate the transport of nutrients and pollutants, manage the evolution of landforms, and crucially, determine the structure and function of ecosystems [5,6]. In recent years, advancements to observational technologies and improvements in numerical modeling capabilities have rapidly deepened our understanding of this continuum. From high-resolution satellite remote sensing to numerical models capable of capturing fine-scale turbulent details, scientists are now able to reveal complex processes that were previously difficult to observe.

This Special Issue aims to compile the latest research findings from this interdisciplinary field, focusing on hydrological and hydrodynamic processes in river headwaters and estuarine and nearshore environments, in order to provide scientific insights and solutions for addressing climate-induced changes in coastal zones.

2. Main Contributions to the Special Issue

Since the call for papers was announced in 2024, six original papers have been accepted for publication following a rigorous peer-review process. This Special Issue covers the land–ocean continuum, including topics from rivers to the continental shelf. The original research published in this issue comprises the following four key aspects.

2.1. Nearshore Hydrodynamic Characteristics

Alfonso Arrieta-Pastrana et al. [Contribution 1] proposed a hydrodynamic modeling method suitable for bay and estuarine systems. This method is based on the mass and momentum equations and requires the calibration of only one parameter—bed friction. Through a sensitivity analysis of bed friction, the authors evaluated the model’s response to achieving maximum water levels. The results were compared with linear theory, demonstrating that the proposed simulation method can effectively model the water phase; therefore, this modeling approach serves as a practical tool for shallow-water hydrodynamic modeling.

Mingkun Qiu et al. [Contribution 2] investigated a small tombolo on the west coast of Honghai Bay in Guangdong Province, China. This tombolo occasionally forms a lagoon behind the island, exhibiting a distinctive double tombolo morphology. The beaches on both sides of the tombolo are headland-bay beaches; the headlands at each end of these beaches effectively inhibit longshore drift on either side of the tombolo, as waves approach the shoreline at a perpendicular angle. The sediment sustaining the tombolo is derived from two primary sources: stream sands delivered by fluvial input and offshore sands transported onshore by wave action. The dynamic balance between these sediment supplies governs the formation of the tombolo. When the quantity of wave-driven onshore sediment transport exceeds the sediment input from streams, a tombolo develops. Conversely, when the fluvial sediment supply dominates, only a salient is formed. This detailed case study includes previously unreported phenomena and valuable insights into the hydrodynamic and sedimentary processes underlying the formation of double tombolos.

Hongqian Zhang et al. [Contribution 3] employed ensemble simulations using the non-hydrostatic SWASH model to quantify sources of uncertainty affecting the overtopping rates of vertical seawalls on coral reef platforms. The authors found that infragravity waves play a dominant role near the seawall, significantly increasing overtopping discharge. This finding distinguishes coral reef environments from typical continental coastlines and highlights the limitations of conventional design formulas that neglect low-frequency wave components.

2.2. River and Ocean Water Environment Monitoring

Stella Patricia Betancur-Turizo et al. [Contribution 4] investigated the utility of light absorption coefficients as indicators of optical water quality in Varadero Reef, an extreme coral ecosystem located in Cartagena Bay, Colombia. Absorption-based indicators offer more direct insights into light attenuation processes, which is particularly relevant for assessing the conditions affecting coral reef habitats.

The strong dependence of the optical absorption coefficient on the fluvial input from the Canal del Dique indicates that any modification to this freshwater sediment delivery system could alter the underwater light environment of Varadero Reef. The complete closure of the canal could reduce both particulate and chromophoric dissolved organic matter (CDOM) inputs, leading to enhanced water transparency. While this change may appear advantageous for light availability, it would also remove the spectral filtering effect provided by CDOM; consequently, corals could become more exposed to harmful ultraviolet (UV) radiation, potentially disrupting the ecological balance that currently sustains coral survival in this turbid environment [7,8].

2.3. Evolution of Marine Heatwaves in the East China Sea

Wenjing Xu et al. [Contribution 5] conducted an analysis of sea surface temperature (SST) using the OISST v2.1 dataset, covering the period from 1982 to 2024. Their findings revealed that six indicators of marine heatwaves (MHW) in the East China Sea demonstrated significant increasing trends: annual occurrence frequency, annual total number of days, annual duration, annual intensity, annual cumulative intensity, and annual maximum intensity. The authors introduced an innovative approach to anticipate marine heatwaves and provide timely alerts, significantly improving the precision of forecasts for severe marine weather phenomena. The East China Sea serves as an excellent area for examination due to its notable spatial variability in the occurrence of marine heatwaves. The high-frequency band is observed over three times annually, primarily located on the Northern Slope of the Taiwan continental shelf, adjacent to the Dongsha Islands and westward of the Ryukyu Islands. The topography of these areas is closely related to the branches of the Black Current. In contrast, the low-frequency bands, that occur once or twice a year, are found in the northern part of the East China Sea and the surrounding waters, and could be caused by the outflow and inflow of fresh water. Notably, the variability characteristics of marine heatwaves in the East China Sea are closely related to regional climate warming.

2.4. The Ecological Impact of Coastal Zone Development on the Nearshore

Lianyi Zhou et al. [Contribution 6] analyzed monitoring data from the Liaohe estuary and discovered that the distribution of inorganic nitrogen and active phosphate correlates with shoreline type. The phytoplankton diversity index in the aquaculture zone is inferior to that in the mixed estuary zone. The research indicates that a comprehensive policy comprising “total nitrogen and phosphorus regulation plus ecological compensation” is essential for the port area, and the construction of artificial reefs in the aquaculture zone is necessary to sustain benthic species diversity.

3. Conclusions

This Special Issue is on “Study on Environmental Hydrology and Hydrodynamic Characteristics of Basins, Estuaries and Offshore”.

① It focuses on the characterization and cross-scale links of hydrodynamic processes: for bays and estuaries, efficient hydrodynamic simulation tools based on core physical equations with simplified parameters were developed, enhancing the practicality and accessibility of shallow-water environment modeling. The study elucidated the evolutionary mechanisms of sandbar (including tombolo) morphology under the dynamic balance of fluvial sediment supply and wave-driven sediment transport, connecting watershed processes with coastal geomorphological responses. In coral reef coastlines, research confirmed the dominant contribution of low-frequency infragravity waves to seawall overtopping discharge, revising traditional engineering understandings based on sandy coasts and emphasizing the critical role of environmental specificity in hazard assessment.

② It highlights the key regulatory role of terrestrial inputs for the health of nearshore ecosystems: it was demonstrated that estuarine inputs of suspended particulate matter and chromophoric dissolved organic matter (CDOM) have a greater impact than only affecting coral photosynthesis by reducing water clarity, since their spectral filtering effect serves as a crucial ecological buffer for coral survival in turbid environments. Altering watershed inputs would disrupt the existing ecological balance, proven by different types of coastal development and utilization (e.g., ports, aquaculture) significantly altering nutrient distribution patterns and reducing plankton biodiversity, confirming that human activities directly impact ecological structure and function due to modifying the aquatic environment.

③ This study reveals new characteristics of marine extreme events under climate change: spatiotemporal analysis of marine heatwaves (MHWs) in the East China Sea indicates significant increasing trends in their frequency, duration, and intensity, with spatial distribution closely related to regional ocean dynamic processes such as the Kuroshio Current. This provides scientists with a method to guess the magnitude of thermal stress that the marine ecosystems will face in the future and devise a plan to manage it.

④ It suggests adaptive management strategies based on a systemic understanding: for port areas, a combined policy of “total pollutant control and ecological compensation” is recommended. For aquaculture zones, ecological restoration through nature-based solutions, such as building artificial reefs, is suggested. These findings advocate that management strategies should be grounded in an understanding of the interconnected mechanisms within the “watershed–estuary–nearshore” system.

This Special Issue methodically enhances the comprehension of the complexities of the land–sea continuum across several domains, including hydrodynamic simulation, geomorphological evolution, hazard mechanisms, eco-hydrology, and climate response. The research findings bolster the scientific foundations of relevant disciplines and provide essential technical support and a decision-making framework for achieving scientific regulation and sustainable development in coastal regions confronting the dual challenges of global climate change and anthropogenic activities. Research in this domain must persist in advancing interdisciplinary, multi-scale investigation that is closely integrated to tackle the escalating issues confronting coastal environments more effectively.