Comparison numerical and theoretical discharge.

Background
High-speed flow of clean water or water with sediment released from aging spillways may cause abrasion and cavitation on the concrete surface gradually. The occurrence of irregularities on the concrete surface can exacerbate the erosion problem. Which might jeopardize the safety of dams constantly, hence the rehabilitation efforts become urgent tasks in dam safety projects.
Methods
This study employs a 3D Computational Fluid Dynamics (CFD) model to quantitatively analyze the cavitation risk on the aging concrete surface of the Chay 5 spillway in Ha Giang, Vietnam, under various operation scenarios. There are two standards used to measure cavitation: the cavitation index (σ) which indicates the danger due to the drop of pressure in rapid flow, and the new gasification index (β) which takes into consideration the formation and collapse of bubbles behind asperities.
Results
Three extreme flood cases may not result in potential cavitation because both σ and β exceed critical thresholds. Regarding the six controlled gate scenarios with normal water level, the σ profiles are approximated 1,0 showing a low likelihood of cavitation damage while the β values are smaller than 0.8, indicating a considerable risk of cavitation. Besides, the opening height of 100 cm poses the greatest risk of creating severe cavitation erosion in the concave area and slope portion. The flip bucket experienced the most vulnerable cavitation when the opening height is 400 cm. In addition, an approach to spillway surface rehabilitation involving specialized mortars has been presented.
Conclusion
For aging conveyance structure, gasification index (β) takes into account irregularities surface, providing a more comprehensive assessment of the likelihood of cavitation damage than cavitation index (σ). After rehabilitation with anti-shrinkage high abrasion resistance mortar, the entire spillway surface is smooth. This allows for reducing the cavitation risk and improvement of life service thereof.

背景
老化溢洪道泄出的清水或含沙高速水流，会逐渐对混凝土表面造成磨蚀与空化（cavitation）损伤。混凝土表面出现不平整缺陷会进一步加剧侵蚀问题，持续威胁大坝安全，因此溢洪道修复工作已成为大坝安全工程中的紧迫任务。
方法
本研究采用三维计算流体动力学（Computational Fluid Dynamics，CFD）模型，针对越南河江省Chay 5溢洪道的老化混凝土表面，在多种运行工况下开展空化风险的定量分析。本次研究采用两项标准评估空化风险：一是空化数（cavitation index，σ），用于表征急流中压力下降引发的空化危险；二是新型气化指数（gasification index，β），该指标考虑了粗糙凸起后方气泡的形成与溃灭过程。
结果
三项极端洪水工况下，空化数σ与气化指数β均高于临界阈值，因此不存在潜在空化风险。针对六项正常水位下的闸门控制工况，空化数σ分布近似为1.0，表明空化损伤可能性较低，但气化指数β均小于0.8，说明存在显著空化风险。其中，闸门开启高度为100cm时，凹面区域与斜坡段的空化侵蚀风险最高；当开启高度为400cm时，挑流鼻坎（flip bucket）区域最易发生空化。此外，本研究还提出了一种采用抗收缩高耐磨砂浆开展溢洪道表面修复的方案。
结论
对于老化输水结构而言，气化指数β考虑了表面不平整缺陷，相比空化数σ能够更全面地评估空化损伤发生概率。采用抗收缩高耐磨砂浆完成修复后，整个溢洪道表面变得平整光滑，可有效降低空化风险并提升其使用寿命。