As an important working condition in water conveyance projects, the water filling process of pipelines is a complex hydraulic transition process involving water–air two-phase flow with sharp pressure changes that can easily cause pipeline damage. In light of the complex water–air two-phase flow during pipeline water filling, this study explores the water filling process of right-angle elbow pressure pipelines using CFD numerical simulations and physical model experiments, analyzing changes in water phase volume fraction, water-gas two-phase flow patterns, and hydraulic parameters in the pipeline under low flow rate conditions of 0.6 m/s and high flow rate conditions of 1.5 m/s. Results show that under low flow rate conditions, there is more local trapped gas at the top of the pipeline, causing negative pressure at local high points in the pipeline and forming a vacuum. Under high velocity conditions, water-gas two-phase flow changes more frequently in the pipeline, with a large number of bubbles collapsing at the top, resulting in large fluctuations in pipeline pressure. Finally, through physical experiments, the main flow patterns during water filling in right-angle elbows are verified and analyzed. These results have certain reference significance for formulating safe and efficient water filling velocity schemes for pressurized pipelines.

Urban flooding, driven by extreme rainfall events and urbanization, poses substantial risks to urban safety and infrastructure. This study employed a neighborhood-scale InfoWorks ICM model to analyze the full-process impacts of urban flooding under six rainfall return periods in Haining, China. The results reveal distinct non-linear responses from the 3-year to 50-year rainfall return period: (1) the surface runoff volume increases by 64.3%, with peak timing advancing by about one minute; (2) the overflow nodes rise from 37.35% to 63.24%, with durations over 30 min increasing by 78.6%; (3) the inundation areas expand by 164.9%, with maximum depths increasing by 0.31 m, showing significant regional disparities; and (4) high-risk zones, such as Haining People’s Square and Railway Station, require targeted interventions due to severe surface overflow and inundation. This comprehensive analysis emphasizes the need for tailored and phased flood prevention measures that address each stage of urban flooding. It provides a strong framework to guide urban planning and enhance resilience against rainfall-induced urban flooding.

This study investigates the impact of cross structures on flood occurrences in mountainous rivers. The governing equations of open channel flow were formulated based on the Saint-Venant equations. The open channel was segmented, and a node equation was established at each section’s connection point. An overflow model of bridges and weir dams was also developed. The physical model of the open channel was simplified and modeled using actual building data and model calculation requirements. The study found that the primary impact of weirs and bridges on the open channel was the backwater effect on the water level. The influence of these structures on the water level in the Huang Stream urban section in the Yellow River Basin was assessed under various working conditions. The results showed that deleting the #1 weir could reduce the maximum backwater height by 1.14 m, and deleting the #2 weir could reduce it by 1.09 m. While reducing the weir height significantly decreased the backwater range and height, it did not enhance the river’s flood discharge capacity. The Huang Stream contains 17 bridges, 13 of which could potentially affect flood discharge. The eight flat slab bridges in the submerged outflow state had a significant impact on flood discharge, with a maximum water level change of 0.51 m. Conversely, the three single-hole flat slab bridges in the free outflow state downstream had a negligible impact on flood discharge. The study found that bridges had a greater influence on flood discharge capacity than weirs. This research provides valuable insights for the reconstruction of cross structures in mountainous rivers and for managing flood discharge capacity and flood control.

Trapped air bubbles in pipelines significantly affect the safety and efficiency of water supply systems, potentially inducing water hammer and leading to pipeline explosions and interruptions. This study investigates the sweeping velocity of air bubbles in horizontal pipelines by analyzing the forces acting on a single air bubble and deriving a theoretical expression for calculating the sweeping velocity. Physical model experiments and numerical simulations are conducted to examine the variations in sweeping velocity with respect to bubble volume and dimensionless bubble volume. Results indicate that the sweeping velocity increases with bubble volume, and the diameter of the pipeline significantly affects the movement of air bubbles. These findings contribute to a better understanding of air bubble behavior in horizontal pipelines and can inform strategies for mitigating risks associated with trapped air bubbles, ultimately improving the safety and reliability of water supply systems.

Due to the rapid acceleration of urbanisation and the increasing occurrence of extreme rainfall events, underpasses have become critical hotspots of urban flooding vulnerability. In this study, we investigated 36 underpasses in Hangzhou using the Urban Flood Inundation Model (UFIM) to systematically evaluate their drainage performance. A high-resolution hydraulic simulation framework was developed by integrating terrain data, drainage pipe networks, pumping stations, and land-use information. Based on the maximum tolerable hourly rainfall derived from multi-scenario simulations, the facilities were divided into high-, medium-, and low-vulnerability groups. Our quantitative and spatial analyses reveal a pronounced core–periphery disparity: 41.7% of the underpasses were highly vulnerable (drainage threshold ≈ 61.3 mm/h), exhibiting significant spatial agglomeration in the older urban core. In contrast, facilities in newly developed peripheral areas demonstrated better drainage performance (threshold up to 75.6 mm/h). Furthermore, the backwater effect from downstream rivers at flood stages significantly constrains pump efficiency by increasing the static head requirement. Based on these spatial vulnerabilities and thresholds, targeted infrastructure optimisation and spatial planning strategies are proposed, shifting the focus from uniform engineering upgrades to vulnerability-based drainage capacity enhancements.