Workshop: Advancements in Air-Water Flows in Outlet Structures of Reservoir Dams

Convenors: Simone Pagliara, Matthias Bürgler, David F. Vetsch, Robert M. Boes, Switzerland

Reservoir dams are vital hydraulic infrastructure, playing a key role in water resources management for irrigation and drinking water supply, hydroelectric generation, and flood mitigation, among others. The outlet structures of reservoir dams including low- and mid-level outlets and spillways are unique in terms of scale and dissipated power outputs. The operational safety of these structures critically depends on accurate predictions of high-velocity air-water flows, as inadequate design can lead to catastrophic consequences. Furthermore, with the aging of existing infrastructure and the necessity to adapt to evolving hydrological conditions driven by climate change, many dams will require significant refurbishment and upgrades in the near future. This highlights the strong need for robust design guidelines for high-velocity air-water flows in outlet structures of reservoir dams.

In recent years, considerable advancements have been made in this field. The research project "Safe Design of Hydraulic Structures for High-Energy Air-Water Flows" (2021- 2025), led by the Laboratory of Hydraulics, Hydrology, and Glaciology (VAW) at ETH Zurich, has made significant contributions through physical experiments conducted on large-scale models of low-level outlets and spillway chutes. This project, benefiting from international collaborations, has produced several outputs of scientific and practical relevance, including comprehensive high-quality air-water flow datasets. These have led to the development of empirical design equations for the prediction of air-demand in low-level outlets (Pagliara et al. 2023b, 2024b, 2024c), air-water flow development in low-level outlets (Pagliara et al. 2023a) and micro-rough spillway chutes (Bürgler et al. 2023a, 2023b). The project further advanced the knowledge on measurement errors and uncertainty of conductivity phase-detection probes (Bürgler et al. 2024, Pagliara et al. 2024a), which currently presents the state-of-the-art instrumentation for intrusive sampling air-water flow properties in both physical models and prototypes. In addition to these project-specific achievements, other researchers have made noteworthy progress in related areas. For instance, new methods have been developed for estimating uplift pressure heads on spillways (e.g., Wahl & Heiner 2023; Wahl 2024) and for mitigating the eiects of entrained air concentrations to prevent cavitation damage on spillways (Gessler 2024). Further advancements have been seen in non-intrusive air-water flow instrumentation, with the application of LiDAR technology for flow depth estimations in both model and prototype air-water flows (e.g., Montano et al. 2018; Kramer et al. 2020; Felder et al. 2021). Additionally, recent improvements in numerical air-water flow modelling techniques are enhancing the practical application of 1D (Wahl & Falvey 2022), 2D (e.g., Hohermuth et al. 2021), and 3D models (e.g., Zabaleta et al. 2023; 2024) for predicting air entrainment and aerated flows in hydraulic structures.

This workshop aims to convey the most recent scientific findings relevant for the safe design of air-water flows in outlet structures of reservoir dam, targeting both the research community and practitioners. The workshop program will be organised into three to five sessions, depending on the confirmed speakers, focusing on:

1. Overview on air-water flows at hydraulic structures and their significance for dam safety
2. Recent findings and design guidelines on air-water flow development along smooth/micro-rough spillways
3. Recent findings and design guidelines on flow patterns, air-demand and air-water flow development in tunnel outlets, including the eiect of wall roughness, profile transitions, and bottom aerators
4. State of the art of the design of stepped spillways and downstream energy dissipators
5. Overview of intrusive and non-intrusive instrumentation for investigating high-velocity air-water flows, including measurement errors and uncertainty of phase-detection probes

The target audience of the workshop includes both engineers and researchers dealing with aerated flows in hydraulic structures. The planned duration of the workshop ranges from 4 to 6 hours, depending on the confirmed number of speakers.

References
Bürgler, M., Valero, D., Vetsch, D. F., Boes, R., & Hohermuth, B. (2023a). Air-Water Flow Measurements on a Large-Scale Spillway Chute. In Proceedings of the 40th IAHR World Congress (pp. 2119-2127). International Association for Hydro- Environment Engineering and Research.
Bürgler, M., Valero, D., Hohermuth, B., Boes, R.M., & Vetsch, D.F. (2024). Uncertainties in measurements of bubbly flows using phase-detection probes. International Journal of Multiphase Flow, 181, 104978.
Bürgler, M., Vetsch, D. F., Boes, R. M., Hohermuth, B., & Valero, D. (2023b). Effect of invert roughness on smooth spillway chute flow. In Role of Dams and Reservoirs in a Successful Energy Transition (pp. 687-695). CRC Press.
Felder, S., Montano, L., Cui, H., Peirson, W., & Kramer, M. (2021). Effect of inflow conditions on the free-surface properties of hydraulic jumps. Journal of Hydraulic Research, 59(6), 1004-1017. Gessler, D. (2024). Mitigation Measures to Prevent Cavitation Damage in Concrete Spillways. In Proceedings of the 10th International Symposium on Hydraulic Structures (ISHS) 2024. ETH Zurich.
Hohermuth, B., Schmocker, L., Boes, R.M., & Vetsch, D.F. (2021). Numerical simulation of air entrainment in uniform chute flow. Journal of Hydraulic Research, 59(3), 378-391.
Kramer, M., Chanson, H., & Felder, S. (2020). Can we improve the non-intrusive characterization of high-velocity air–water flows? Application of LIDAR technology to stepped spillways. Journal of Hydraulic Research.
Montano, L., Li, R., & Felder, S. (2018). Continuous measurements of time-varying free-surface profiles in aerated hydraulic jumps with a LIDAR. Experimental Thermal and Fluid Science, 93, 379-397.
Pagliara, S., Felder, S., Boes, R.M., & Hohermuth, B. (2024a). Intrusive eiects of dual-tip conductivity probes on bubble measurements in a wide velocity range. International Journal of Multiphase Flow, 170, 104660.
Pagliara, S., Felder, S., Hohermuth, B., & Boes, R.M. (2024b). Preliminary Analysis on the Effect of Tunnel Profile Transitions on Air-demand and Flow Patterns of Low-level Outlets. In 10th International Symposium on Hydraulic Structures (ISHS 2024). Zurich, Switzerland.
Pagliara, S., Felder, S., Hohermuth, B., & Boes, R.M. (2024c). Air demand and flow patterns in low-level outlets with rough walls. Journal of Hydraulic Engineering (under review).
Pagliara, S., Hohermuth, B., Felder, S., & Boes, R.M. (2023a). Eiects of Wall Roughness on Air-water Flow Properties of Low-level Outlets. In Proceedings of the 40th IAHR World Congress (pp. 1945-1954). International Association for Hydro- Environment Engineering and Research.
Pagliara, S., Hohermuth, B., Felder, S., & Boes, R.M. (2023b). Eiects of wall roughness on low-level outlet performance. In Role of Dams and Reservoirs in a Successful Energy Transition (pp. 898-907). CRC Press.
Zabaleta, F., Damián, S.M., & Bombardelli, F.A. (2023). A novel three-phase mixture approach for the numerical modeling of self-aerated flows. Computer Methods in Applied Mechanics and Engineering, 408, 115958.
Wahl, T.L. (2024). Predicting Uplift Pressures and Joint Flows Along a Spillway Chute. In Proceedings of the 10th International Symposium on Hydraulic Structures (ISHS) 2024. ETH Zurich.
Wahl, T.L., & Falvey, H.T. (2022). SpillwayPro: Integrated water surface profile, cavitation, and aerated flow analysis for smooth and stepped chutes. Water, 14(8), 1256.
Wahl, T.L., & Heiner, B.J. (2024). Laboratory Measurements of Hydraulic Jacking Uplift Pressure at Oiset Joints and Cracks. Journal of Hydraulic Engineering, 150(4), 04024016.
Zabaleta, F., Bombardelli, F.A., & Márquez Damián, S. (2024). Numerical modeling of self-aerated flows: Turbulence modeling and the onset of air entrainment. Physics of Fluids, 36(4).