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This White Paper grew out of the collaborative work conducted byseveral IAHR Technical Committees, with a focus on the TechnicalCommittee on Experimental Methods and Instrumentation.The outcome is a community vision on future pathways aimed atleveraging conventional and emerging (unconventional) methodsto facilitate access to hydro-environment observations in the nearfuture. It has been formulated in collaboration with a broad spectrumof experts from different backgrounds.

Dams are important for flood mitigation, water supply, and hydroelectricity. Every dam has a water conveyance structure, such as a spillway, to safely release extreme floods when needed. The flows down spillways are often self-aerated and spillway design has typically been investigated in laboratory experiments, which is due to limitations in suitable full scale flow measurement instrumentation and safety considerations. Prototype measurements of aerated flows are urgently needed to quantify potential scale effects and to provide missing validation data for design guidelines and numerical simulations. Herein, an image-based analysis of free-surface flows on a stepped spillway was conducted from a top-view perspective at laboratory scale (fixed camera installation) and prototype scale (drone footage). The drone videos were obtained from citizen science data. Analyses allowed to remotely estimate the location of the inception point of free-surface aeration, air–water surface velocities, and their fluctuations, as well as the residual energy at the downstream end of the chute. The laboratory results were successfully validated against intrusive phase-detection probe data, while the prototype observations provided proof of concept at full scale. This study highlights the feasibility of image-based measurements at prototype spillways. It demonstrates how citizen science data can be used to advance our understanding of real world air–water flow processes and lays the foundations for the remote collection of long-missing prototype data.

Wall shear stress modification in self-aerated flows has been observed in boundary layer–type flows mainly in the form of drag reduction. Due to its complexity, drag reduction has predominantly been studied using experimental techniques and empirical analyses, whereas the development of conceptual approaches is limited. In this study, two-phase open-channel flow equations with variable density are revisited and bottom air concentration is identified as a key parameter for drag reduction in air-water flows. The application of this new theory to smooth and stepped chute flows is introduced, and implications for the determination of friction factors are discussed. A modified version of the Manning-Strickler formula is proposed to model friction factors across a wide range of roughness and bottom air concentration levels.

Gas–liquid flows occur in many natural environments such as breaking waves, river rapids and human-made systems, including nuclear reactors and water treatment or conveyance infrastructure. Such two-phase flows are commonly investigated using phase-detection intrusive probes, yielding velocities that are considered to be directly representative of bubble velocities. Using different state-of-the-art instruments and analysis algorithms, we show that bubble–probe interactions lead to an underestimation of the real bubble velocity due to surface tension. To overcome this velocity bias, a correction method is formulated based on a force balance on the bubble. The proposed methodology allows to assess the bubble–probe interaction bias for various types of gas-liquid flows and to recover the undisturbed real bubble velocity. We show that the velocity bias is strong in laboratory scale investigations and therefore may affect the extrapolation of results to full scale. The correction method increases the accuracy of bubble velocity estimations, thereby enabling a deeper understanding of fundamental gas-liquid flow processes.

Previous research has shown that the inflow conditions upstream of hydraulic jumps can affect flow aeration, bed pressures and turbulence. However, studies of the effects of inflow conditions on free-surface properties have not been conclusive and have lacked a comparison of partially and fullydeveloped inflow conditions. Detailed measurements of the free-surface features in fully aerated hydraulic jumps with partially and fully developedinflow conditions have been conducted using a LIDAR. The results showed that the free-surface profile, the roller length, the free-surface and jumptoe movements are strongly influenced by the inflow conditions. Hydraulic jumps with fully developed inflow conditions were of comparativelyshorter length compared to partially developed hydraulic jumps, while they were characterized by stronger fluctuations, jump toe motions andfrequencies as well as stronger energy loss across the hydraulic jump. Inflow conditions must be considered in determining the envelope of hydraulic jump motions with consequences for dissipator design.

Dual-tip phase-detection intrusive probes are the most widely used experimental technique for measuring interfacial velocities in highly aerated flows. Recently, we have introduced an adaptive window cross-correlation (AWCC) technique, which allows for the estimation of pseudo-instantaneous velocities and associated velocity statistics. Several potential limitations of this technique were suggested by Chanson (2020). In a direct response, the AWCC is clarified and shortcomings of conventional signal processing as well as general limitations of dual-tip phase-detection probes for velocity measurements are discussed. A comparison and validation of the AWCC against event detection techniques, image-based velocimetry and stochastic signals demonstrates the method's novel opportunities for air–water flow research.

Gas–liquid flows occur in many natural environments such as breaking waves, river rapids and human-made systems, including nuclear reactors and water treatment or conveyance infrastructure. Such two-phase flows are commonly investigated using phase-detection intrusive probes, yielding velocities that are considered to be directly representative of bubble velocities. Using different state-of-the-art instruments and analysis algorithms, we show that bubble–probe interactions lead to an underestimation of the real bubble velocity due to surface tension. To overcome this velocity bias, a correction method is formulated based on a force balance on the bubble. The proposed methodology allows to assess the bubble–probe interaction bias for various types of gas-liquid flows and to recover the undisturbed real bubble velocity. We show that the velocity bias is strong in laboratory scale investigations and therefore may affect the extrapolation of results to full scale. The correction method increases the accuracy of bubble velocity estimations, thereby enabling a deeper understanding of fundamental gas-liquid flow processes.

Grass-lined spillways represent an environmental friendly solution for flow conveyance in mildly-sloped water ways. The grass linings act as roughness elements that facilitate the stability of the soil and the safe conveyance of flows. Velocity profiles of grass-lined open channel flows have been extensively studied under subcritical flow conditions, whereas experiments in supercritical flows are limited. Herein, velocity profiles and flow resistance on a mildly sloped chute with artificial grass were experimentally investigated using a Prandtl Pitot tube. The velocity profiles exhibited the development of the boundary layer along the chute achieving uniform flow conditions towards the chute's end. The results revealed similarities of the velocity profiles to a previously proposed velocity decomposition and provided insights into point flow resistance.

Energy dissipation is an important aspect in the design of hydraulic structures. Stilling basins are a common type of energy dissipator in which a hydraulic jump dissipates large amounts of the flow energy. The energetic processes in hydraulic jumps are closely linked with air entrainment and non-stationary interactions of roller and free-surface motions. Free-surface motions may provide information on the overall performance of a stilling basin. Recently LIDAR technology was pioneered as a method to measure highly aerated hydraulic jumps providing continuous non-intrusive free-surface measurements with high temporal and spatial resolution. In the present study, the LIDAR technology was applied to a hydraulic jump in a laboratory-scale stilling basin. A LIDAR measured the time-varying free-surface features at seven longitudinal and six transverse cross-sections across the stilling basin. A three-dimensional free-surface map was created providing a detailed and integrated free-surface image. Three dimensionalities of free-surface profiles and fluctuations are presented and opportunities of free-surface mapping discussed.

Dual-tip phase-detection intrusive probes are commonly used to characterize gas-liquid flow velocities in hydraulic structures. Traditional post-processing is based upon the cross-correlation analysis of the raw voltage signals of the two probe tips, providing a time-averaged velocity for the full sampling duration. A recent advancement of the traditional signal analysis, the adaptive window cross-correlation (AWCC) technique, allows the computation of pseudo-instantaneous velocities for small groups of particles. A different analysis approach is the particle event detection technique, which has the potential to directly estimate instantaneous particle velocities. Event detection (ED) techniques have received little attention in highly aerated flows and this work provides a first proof-of-concept. Air-water flow data on a stepped spillway have been re-analysed and it was found that a novel particle event detection technique was able to provide reliable particle velocities and velocity fluctuations across the full air-water flow column, with higher uncertainties close to channel invert linked to the three-dimensionality of the flow. A comparative analysis of the event detection technique and the adaptive cross-correlation method showed good agreement in mean velocities and velocity fluctuations, suggesting that event detection techniques may provide a valuable alternative.

Dual-tip phase-detection probes can be used to measure flow properties in gas-liquid flows. Traditionally, time-averaged interfacial velocities have been obtained through cross-correlation analysis of long time-series of phase fraction signals. Using small groups of detected particles, a recently developed adaptive window cross-correlation (AWCC) technique enables the computation of pseudo-instantaneous interfacial velocities and turbulence quantities in highly aerated flows, albeit subject to some smoothing which is due to the use of a finite window duration. This manuscript provides guidance on the selection of optimum processing parameters for the AWCC technique, additionally addressing shortcomings such as velocity bias correction and extrapolation of turbulence levels to single particles. The presented technique was tested for three self-aerated flows: smooth and rough-wall boundary layers (tunnel chute and stepped spillway), as well as breaking shear layer flows of a hydraulic jump. Robust estimations of mean velocities and velocity fluctuations were obtained for all flow situations, either using dual-tip conductivity or fiber optical probe data. The computation of integral time scales and velocity spectra is currently limited by the data rate and must be treated with caution.