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Hydraulic jumps are induced in hydraulic facilities for the purposes of energy dissipation or flow aeration. Presently there is no means for a simple estimate of void fraction distribution and air entrainment flux, without detailed physical modelling. This paper presents a semi-theoretical model to simulate the void fraction and velocity distributions in hydraulic jumps characterized by partially-developed inflow conditions. Relationships were established between the inflow Froude number, jump roller length and key parameters that determine the full expression of void fraction and velocity profiles. The proposed model enables accurate prediction of void fraction, longitudinal velocity, and air flux using the inflow Froude number. The results indicated considerable air flux contribution of free-surface aeration, in addition to the singular air entrainment at the jump toe, for moderate to large Froude numbers. A Froude number between 8 and 9 tended to achieve highest aeration rate with maximum total air flux in the roller.

A tidal bore is a hydrodynamic shock, surging upstream in some shallow-water bays and estuaries during the flood tide under large tidal range. This study investigates experimentally the propagation of tidal bores over a large cylindrical roughness element, representative of damaged bridge pier foundation. In the initially steady flow, the large cylindrical element generated a wake region, with extents comparable to steady flow literature. During the tidal bore propagation, the presence of the element had negligible effect on the free-surface properties, but a significant impact in terms of the instantaneous velocity and Reynolds stresses. This resulted in longer transient recirculation both upstream and downstream of the element and larger maximum velocity recirculation magnitudes, as well as enhanced turbulent stress levels and potential bed erosion around the large element, within two diameters from the element centre. The results showed the potential development of a large scour hole around the cylindrical element.

Air–water flows on stepped spillways were investigated experimentally in the last decades with a focus on steep slope chutes equipped with flat horizontal steps. Detailed air–water flow properties were recorded herein with three stepped geometries down a slope of θ = 8.9° with: flat horizontal steps, pooled steps, and a combination of flat and pooled steps. The data included the distributions of basic air–water flow properties, as well as the energy dissipation and flow resistance data deduced from the air–water flow measurements. The results on the flat slope showed that the pooled stepped design enabled a greater rate of energy dissipation, but the pooled stepped geometries were affected by some flow instabilities and unsteady flow processes for a range of flow rates.

The Authors presented the outputs of some numerical modelling of a spillway aerator conducted using a commercial package (Aydin and Ozturk 2009). A spillway aeration device is located on the spillway bottom and sometimes on the sidewalls. It is designed to deflect high-velocity flow away from the chute surface (and walls) to artificially introduce air into the flow and protect the spillway invert from cavitation erosion. The main flow regions are: (a) the approach flow region, which characterizes the initial nappe flow conditions; (b) the transition region, which coincides with the length of the deflector; (c) the aeration region; (d) the impact point region; and (e) the downstream flow region (Chanson 1989a, 1989b; Kramer et al. 2006). A key feature of the flow abovestream and downstream of an aerator is the strong mixing of air and water yielding a complicated high-velocity airnwater flow. Herein, it is argued that the validation and certification of the commercial CFD package were ill-performed and that the physical modelling technique based upon a proper dimensional analysis remains the basic design tool.In summary, we believe that the validation and certification of the Authorsr numerical results were improper. These should be conducted on the detailed airnwater flow properties of the flow abovestream and downstream of the spillway aerators, and several extensive datasets are freely accessible (e.g., Chanson 1988; Kramer 2004; Toombes 2002). The physical modelling and theoretical calculations remain the basic tools of the design engineers to size the spillway aeration devices on a large spillway, although the physical model dimensions must be carefully selected to address the potential scale effects (e.g., Kramer 2004; Chanson 2009).

The stepped spillway design is characterized by an increase in the rate of energy dissipation on the chute associated with a reduction of the size of the downstream energy dissipation system. This study presents a thorough investigation of the air–water flow properties in skimming flows with a focus on the turbulent characteristics. New measurements were conducted in a large-size facility (θ = 22°; step height, h = 0.1 m) with several phase-detection intrusive probes. Correlation analyses were applied to estimate the integral turbulent length and time scales. The skimming flow properties presented some basic characteristics that were qualitatively and quantitatively in agreement with previous air–water flow measurements in skimming flows. Present measurements showed some relatively good correlation between turbulence intensities T u and turbulent length and time scales. These measurements also illustrated large turbulence levels and large turbulent time and length scales in the intermediate region between the spray and bubbly flow regions.

In the last two decades, research on the hydraulics of skimming flows down stepped chutes was driven by needs for better design guidelines. Skimming flows are characterized by significant momentum transfer from the main stream to the recirculation zones. Investigations are difficult because of the complex nature of the flow, the strong flow aeration, and the interactions between entrained air and turbulence. This study provides a comprehensive database on main stream and cavity flow interactions in skimming flows down a stepped chute. Measurements were conducted on a large facility (α = 15.9°, h = 0.05 and 0.1 m, W = 1 m) with precise instrumentation based upon a Froude similitude. Air–water velocity and turbulence measurements demonstrated a well-defined mixing layer developing downstream of each step edge in which the velocity profiles had the same shape as classical monophase flow results. A comparative analysis of air–water flow properties for Froude similar flow conditions showed a good agreement between the two step heights in terms of dimensionless distributions of air content, velocity, and turbulence intensity, but dimensionless bubble count rates, turbulence levels, and bubble chord sizes were improperly scaled.Key words: stepped chute, skimming flow, flow recirculation, momentum exchange, physical modelling.

P.A. Manso and A.J. Schleiss presented a challenging and detailed study of embankment overflow spillway systems. There are indeed numerous applications worldwide for both dam refurbishments and new structures. During the last two decades, a number of embankment dam spillways were designed with concrete overtopping protection shaped in a stepped fashion (Chanson 2001, pp. 220-230). A very different concept of overflow embankment spillway is the minimum energy loss (MEL) weir design.

Dams and weirs must be equipped with adequate flood-release facilities for a safe dissipation of the kinetic energy of the flow. With stepped spillway design, it is essential to accurately predict the flow-resistance contribution of the steps. The authors investigate the flow resistance of skimming flows and associated form losses. Recent laboratory experiments were systematically performed with channel slopes ranging from 5.7° up to 55° and the results are compared with existing laboratory and prototype data. The results provide a better understanding of the basic flow patterns and flow-resistance mechanisms and emphasize that form loss is dominant. Simple analytical models provide a reasonable order of magnitude of the pseudo-boundary shear stress and the recirculation cavity ejection frequency. Altogether, more than 38 model studies and four prototype investigations (for a total of more than 700 data points) are reanalysed.Key words: stepped spillway, flow resistance, skimming flow, form drag, physical and analytical modelling.

Stepped spillways have been used for about 3500 years. The last few decades have seen the development of new construction materials, design techniques, and applications, for example, embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes (α ~ 50°). This paper presents an experimental study conducted in a large-size stepped chute (α = 22°, h = 0.1 m, W = 1 m). Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air–water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterization is developed for the distributions of void fraction, bubble count rate, and velocity, and flow resistance data are compared with other studies. Transition flows exhibit significantly different air–water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls.Key words: stepped spillway, air entrainment, two-phase flow properties, skimming flow, transition flow.

Completed in 1885, the Gold Creek dam is an earthen dam located on the outskirts of Brisbane, Australia. The spillway system was refurbished three times, each time to increase the maximum overflow capacity. In 1890, a concrete stepped waterway was built to replace the damaged unlined-rock channel. This staircase chute is an unique structure: it is the first large man-made waste waterway built in Queensland, it is the only stepped weir built in Queensland before 1900, and it is, so far as the writers are aware, the first concrete-stepped spillway built for a large dam. The characteristics of the dam and its unusual stepped spillway are reviewed in a historical context. The design is compared with contemporary structures and present knowledge in stepped spillway design. The authors believe that the Queensland engineers gained expertise from overseas and within Australia for the stepped spillway design. However, the selection of concrete for the step construction was made by the local engineers and the reasons behind the decision are not yet understood.Key words: spillway design, engineering heritage, concrete construction, stepped weir, spillway operation, spillway refurbishment, 19th century structure.

Recently, spillways with a stepped profile have regained interest and favor among design engineers to pass flood waters over the dams. The stepped geometry enhances the energy dissipation above the spillway and reduces the size of a downstream stilling basin. In this paper, the author shows that the technique of stepped channels has been developed since Antiquity. Spillways and irrigation channels with stepped profiles were developed by several civilisations around the Mediterranean sea and in America. The main characteristics of the stepped spillways along the ages suggest a regular evolution rather than a revolution. Present stepped spillways are designed to pass similar discharges as 200 years ago. Key words: stepped channels, spillway, irrigation system, history, design techniques, energy dissipation.

Aeration devices are introduced along chute spillways and at bottom outlets to prevent cavitation damage in high velocity flows. Bottom aerators are characterized by large quantities of air entrained along the jet interfaces and also by a strong deaeration process near the impact of the water jet with the spillway bottom. In this paper, the aeration and deaeration occurring respectively in the aeration region and in the impact region are reviewed. A reanalysis of air concentration data obtained on models provides information on the flow characteristics at the end of the impact region. These results enable an accurate initialization of the downstream flow calculations using the method developed by Chanson. Key words: bottom aeration devices, aerators, spillways, air entrainment, detrainment.

A main technical issue seems to be associated with the structure of the two-phase air–water flow skimming over the pseudo-bottom formed by the step edges. This is a difficult topic. In the air–water skimming flow, the microscopic structure of the gas–liquid mixture is closely linked with the interactions between turbulent vortices and air–water entities (bubbles, droplets, packets) (e.g., Chanson and Toombes 2002; Carosi and Chanson 2008; Gonzalez and Chanson 2008). In the high-velocity free-surface flow, the strong interactions between the turbulent waters and the atmosphere lead to a complete deformation at the interface. Through the free-surface, air is continuously trapped, and the resulting air–water mixture extends through the entire flow (Rao and Kobus 1971; Wood 1991; Chanson 1997). The air–water flow is characterized a complicated two-phase turbulent motion with void fractions ranging from some small, often nonzero values close to the invert to 100% above a pseudo "free-surface" that is usually defined as the location where the void fraction equals 90% (Cain and Wood 1981; Wood 1991; Chanson 1997; Matos 2000). The notion of "effective homogenous flow" contradicts detailed measurements conducted with accurate instrumentations in large-size facilities with well-controlled flow conditions and it is obsolete. Recent findings (Carosi and Chanson 2008) demonstrated in fact a marked changed in flow properties for C ~ 0.95 to 0.97, that is consistent with earlier studies suggesting the use of Y95 or even Y98 as the characteristic air–water flow thickness (Jevdjevich and Levin 1953; Aivazyan 1986).

of the flow patterns were thoroughly documented. The location of the nappe impact was reasonably predicted by a simple trajectory equation (Chanson 1995; Toombes 2002). Downstream of the nappe impact, flow was characterised by a highly fragmented spray. The spray and splashing appeared to be concentrated towards the centreline of the channel. Sidewall standing waves, similar to ship bow waves, formed on both sidewalls downstream of the nappe impact. Properties of these standing waves compared reasonably with the properties of waves observed on the opposite mitre bends and channel junctions, and at abrupt channel expansions (Chanson and Toombes 1997, 1998). The flow was supercritical in the downstream channel. No hydraulic jump was oberved. Shock waves were observed in the downstream channel, originating from the sidewalls at or close to the nappe impact. The angle of the crosswaves was inversely proportional to the inflow Froude number. The shock waves intersected on the channel centreline and continued to propagate towards the opposite wall (Fig. 1). Energy dissipation at the overfall was the result of friction losses, jet disintegration, nappe impact on the downstream invert, and recirculation within the pool of water beneath the free-falling jet. The energy dissipation was roughly equal to the drop height within the range of the experimental flow conditions (Table 1). The discussers are concerned by some broad conclusions contained in the original paper, which ignored the three-dimensional nature of the flow, the existence of sidewall standing waves, and strong splashing and spray in the centre of the downstream channel. These three-dimensional flow patterns have direct implications in terms of channel design. The discussers observed a maximum sidewall standing height y M that satisfied

The writer would like to congratulate the authors for their interesting investigations. For completeness, he wishes to comment on the results' application. Flows through rockfill embankments can be used in some cases to reduce the peak flow as discussed in the article. There are however other applications of seepage flows through rockfill structures.