Advanced Research on Hydraulic Engineering and Hydrological Modelling

1. Introduction to the Special Issue

Hydraulics and hydrology are ancient disciplines that play a crucial role in ensuring water security, safeguarding water environments, and maintaining water ecology. In recent years, the construction of high dams, such as the Baihetan Project, has not only resolved numerous problems but also generated a wealth of practical theories, advanced techniques, and valuable practical experiences in the fields of hydraulics and hydrology [1,2]. Physical models, numerical models, and prototype measurements are traditional research methods [3,4,5]. However, with the rapid advancement of emerging technologies like information technology [6] and big data [7], the integration of these technologies with hydraulics and hydrology has emerged as a prominent research topic in the current academic landscape. It is essential to summarize and disseminate the above-mentioned related research findings to further promote the development of this area of interest.

Consequently, this Special Issue of Water was launched. Since the call for papers was announced, twelve original articles and one review article have been accepted for publication following a rigorous peer-review process (Contributions 1–13). These papers can be categorized into several domains: high dam hydraulics, channel hydraulics, cylinder flow, air–water two-phase flow, and hydrological modeling. To provide a better understanding of this Special Issue, we hereby summarize the key highlights of the published papers.

2. Overview of the Contributions to This Special Issue

Regarding hydraulic research on high dams, Zheng et al. (Contribution 1) investigated the emergency discharge capacity of ultra-high dams under extreme events such as floods and earthquakes by reviewing dams over 200 m worldwide. The study revealed inadequate discharge capacity, poor structural safety at high heads, excessive hoisting requirements, and complex sealing technologies. To overcome these challenges, strategies such as multi-stage gates and siphon layouts were proposed, emphasizing enhanced gate pressure resistance, intelligent control, and integrated risk management to ensure continuous safe drawdown and strengthen disaster resilience. High-altitude hydraulic projects face increased cavitation risk due to reduced atmospheric pressure, and conventional aeration designs perform poorly under such conditions. Guo et al. (Contribution 2) conducted decompression chamber tests simulating various pressures and flow rates to analyze air discharge, cavity characteristics, and air–water concentration. Results showed that lower pressure significantly reduces aeration and air–water content. A correction formula was developed to accurately predict aeration performance under low pressure, providing a theoretical basis for ventilation design in high-altitude hydraulic structures. Cavitation is a major safety concern in high-head spillways. Dong et al. (Contribution 3) addressed the severe backflow and ventilation failure of conventional aerators in mild-slope chutes under low Froude numbers by proposing a curved aerator with locally increased slope to reduce impact angle. Shape parameters were derived and validated through numerical simulations. Results show that appropriately lowering the jet impact point eliminates backflow, significantly outperforms traditional designs, and effectively resolves cavitation protection challenges under mild slopes and high discharge conditions. Overtopping and seepage are the primary causes of tailings dam failures. Gao et al. (Contribution 4) conducted a 1:100 physical model test and combined numerical simulations to investigate the breaching process and downstream hazard evolution. Results show that failure progresses through seepage stabilization and flow-slide development stages before full crest breach. Measures such as controlling the phreatic line, adding drainage systems, and improving flood regulation were proposed to enhance dam safety and provide technical guidance for emergency management.

Regarding channel hydraulics, Smith et al. (Contribution 5) used the River2D model combined with field data to simulate velocity and shear stress distributions in a shallow river reach under ice-covered and open-water conditions. The results show that ice cover reduces overall flow velocity but enhances local recirculation and alters high shear stress distribution, while narrowing the main channel and increasing water depth, which may affect fish habitats. This study provides an important basis for understanding winter hydraulic characteristics and ecological impacts. China’s water resources are unevenly distributed between the north and south, making inter-basin water transfer projects highly demanding and risky. Zhang et al. (Contribution 6) addressed the issues of complex channel response and insufficient scheduling safety in such projects by developing a hydraulic model for the Yangtze–Huaihe Water Transfer Project. They proposed a safe regulation range based on the earliest and latest adjustment times and identified flow variation as the key factor through the Sobol sensitivity analysis, further deriving a power function-based rapid prediction formula. Results indicate high prediction accuracy, enabling the replacement of traditional models, improving scheduling efficiency, and ensuring safe project operation. Climate change has increased flash flood risks in steep-slope watersheds, while existing hydrodynamic models often neglect canopy interception and litter storage, reducing simulation accuracy. Ámon et al. (Contribution 7) integrated these processes into the HEC-RAS model to improve flash flood simulation in steep basins and compared two approaches for representing litter: as rainfall loss and as a high-porosity soil layer. Results show that litter significantly reduces surface runoff and peak discharge. This study provides a scientific basis for small watershed flood modeling and risk management.

In terms of cylinder flow research, Wang et al. (Contribution 8) developed a coupled internal wave–cylinder–topography model using large eddy simulation to examine how topography influences cylinder loading. The results show that irregular topography significantly alters the direction of horizontal forces, enhances shallow water effects, and greatly increases reverse forces on the lower section, while higher internal wave amplitudes intensify flow strength and vortex activity. This study reveals the hydrodynamic response of structures under complex terrain, providing valuable guidance for the safe design of hydraulic structures. Most studies on cylinder flow neglect boundary effects and lack the systematic analysis of how confinement influences flow structures. Xu et al. (Contribution 9) used two-dimensional numerical simulations to examine the impact of boundary confinement on cylinder flow characteristics. The results show that confinement does not alter the four fundamental flow regimes but reduces vortex size, shifts vortex centers toward the boundary, and significantly increases drag and vortex shedding frequency when G/D < 3.5. Prediction formulas for drag and shedding frequency were proposed, providing reference for the design and vibration control of nearshore structures.

In terms of high-speed air–water two-phase flow research, Luo et al. (contribution 10) conducted fluent-based numerical simulations to analyze concentric double-nozzle non-submerged cavitation jets, focusing on the influence of nozzle geometry and pressure parameters on the flow field. Results indicate that contraction and expansion sections significantly enhance cavitation intensity, with the expansion section enlarging the cavitation zone, and smaller contraction angles yielding better performance; moderately increasing throat diameter and nozzle pressure further improves impact pressure. The study proposes an optimized nozzle design method, providing theoretical guidance for large-scale surface strengthening applications. Composite cavitation nozzles can significantly enhance cavitation performance, yet the influence of key structural parameters and effective optimization strategies remain unclear. Huang et al. (Contribution 11) employed large-eddy simulation combined with the response surface method to optimize nozzle structural parameters. The results indicate that the optimized design substantially increased the vapor volume fraction compared with conventional nozzles, with model prediction errors of only 2.04% and impact force test errors ranging from 1.5% to 2.7%. This study introduces a systematic optimization approach, providing theoretical and technical support for cavitation jet nozzle design.

In terms of hydrological modeling research, Jin et al. (Contribution 12) proposed an improved vertical exchange approach based on node water balance, integrating vertical flow computation into the 1D hydraulic equation with iterative solutions to resolve unreasonable backflow calculations and instability in conventional models. The results indicate that the improved method achieves coupling results highly consistent with the ICM model while significantly enhancing computational efficiency, providing a high-accuracy, low-subjectivity solution for urban flood simulation and drainage system optimization. The Jakarta government plans to construct a coastal reservoir at the river mouth to alleviate water shortages and flood risks, but feasibility assessment methods tailored to Indonesian conditions are lacking. Soekarno et al. (Contribution 13) proposed a pre-feasibility analysis framework for coastal reservoirs, incorporating hydrological calculations, operational simulations, and flood and sediment assessments. The results show that the reservoir provides limited flood regulation, lowering upstream water levels only in the early stages. This study offers a scientific basis for planning inter-basin water supply and flood control projects in Indonesia.

3. Conclusions

This Special Issue features multidisciplinary scholarly works that aim to provide an in-depth comprehension of the scientific principles and mechanisms, encompassing critical technologies, challenges, and concepts, in the fields of hydraulic engineering and hydrological modelling. The guest editors anticipate that the papers published in this Special Issue will appeal to researchers, designers, and practitioners involved in the design and management of dams and reservoirs and will also assist in identifying future research directions. The research findings and methodologies presented in this Special Issue, including hydraulic research on high dams, channel hydraulics, cylinder flow, air–water two-phase flow, and hydrological modelling, hold great research significance. These technological contributions can aid relevant scholars and project managers in analyzing and managing the safety of the major structures in reservoir dams.