Search

During the last decades, concerns regarding the ecological impact of standard culverts have led to some design evolution. The installation of baffles along the culvert barrel yields smaller velocities and larger water depths in the barrel, potentially more suitable for upstream fish passage, albeit with a decrease in discharge capacity. Small triangular corner baffles were proposed to facilitate the upstream passage of small-body-mass fish, without compromising the discharge capacity of the culvert at design flow. Although fish benefited from low velocity regions for resting and sheltering, a small fraction of small-body-mass fish were observed to become disoriented by the adverse effect of flow reversal regions in the wake of plain baffles. This study presents the hydrodynamic testing of small ventilated triangular corner baffles for standard box culverts. The baffle ventilation was introduced to reduce the impact of negative wake behind the baffles. Two designs were tested: a baffle with three holes and a brush baffle. Detailed modelling in a near-full-scale culvert barrel showed that the ventilated corner baffles created a smaller negative wake region. A lesser negative velocity magnitude was observed behind the ventilated baffles, in comparison to plain baffles, for the same flow rate, baffle height and spacing. With ventilated corner baffles, the longitudinal distribution of low-velocity zone was more uniform, yielding a better longitudinal connectivity for upstream passage, compared to plain baffles. A comparison between various boundary treatments suggested however that the requirements for continuous, sizeable low positive velocity zone suitable to small-bodied fish might be better fulfilled with an asymmetrically roughened culvert barrel than with triangular baffles, even with ventilation.

During very large rainfall events, 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 predict accurately the flow resistance associated with the steps. The authors investigate the flow resistance of skimming flows and associated form losses. New experiments were performed with channel slopes ranging from 5.7 degrees up to 55 degrees. The results provide a better understanding of the basic flow patterns and flow resistance mechanisms. They emphasise that form loss is dominant. Altogether more than 35 model studies and 4 prototype investigations (totalising more than 650 data) are re-analysed.

A culvert is a covered channel to pass streams and floodwaters through an embankment. The ecological impact of culverts has been recognised, in particular in terms of stream connectivity, but existing guidelines lead often to un-economical culvert design. Herein, a small triangular corner baffle system was tested physically in a near-full-scale fish-friendly facility of a box culvert barrel. Experiments were repeated with several configurations to characterise the flow properties for a range of less-than-design flows, baffle sizes and spacings. In presence of triangular corner baffles, the flow was asymmetrical, owing to the wake behind each baffle. The presence of triangular corner baffles had a moderate effect on the flow resistance and discharge capacity, albeit the data indicated the combined effect of relative baffle height and spacing on the friction factor. With triangular baffles, the surface area of slow velocity regions increased by a factor of two to three. Such low velocity regions are preferential swimming zones for fish, beneficial to small-bodied fish passage. Testing with small-bodied fish showed that fish preferred to swim upstream in slow-velocity regions, typically next to the sidewalls and in the left corner where the triangular baffles were located. The presence of small triangular baffles facilitated substantially the upstream passage of small fish, including in terms of endurance, compared to a smooth un-baffled box culvert barrel, when the baffle size was comparable to the fish length. The present findings highlighted the importance of physical modelling at near full-scale for the development of fish-friendly culvert designs.

Road crossings and culverts are common man-made structures along river courses, ranging from national highways to rural roads and urban networks. Present expertise in culvert hydraulic design is deficient because many empirically-based guidelines are often inadequate for fish passage. This project focused on the development of simple solutions for box culverts, with the aim to maximise slow flow regions suitable for small-bodied fish passage and to minimise the reduction in discharge capacity. Herein a physical study of box culvert was performed under controlled flow conditions, and seven designs were tested. In all the cases, the turbulence of the flowing waters was used to assist with fish migration. One baffle configuration presented promising results: i.e., small corner baffles. The triangular baffle system produced little additional afflux, while creating excellent recirculations both upstream and downstream of each baffle. Another configuration was based upon a rough bed and sidewall, enhancing secondary currents and recirculation in the corner region. This resulted in flow conditions that could be potentially used to enhance small-bodied fish passage, though further experiments involving detailed fish behaviour study are required for quantitative guidelines.

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.

In open channel flows, the transition from a rapid to fluvial flow motion is called a hydraulic jump. A related flow motion is a compression wave in a channel, such as a tidal bore or surge. A key feature of hydraulic jumps and breaking bores is the rapid spatial and temporal deformations of the free surface of the roller region, in response to the interactions between entrained air bubbles and turbulent structures. The flow structure in the roller remains a great research challenge due to large quantities of entrained air, bubble–turbulence interactions and the coupling between turbulent properties and free-surface deformations. Breaking bores and hydraulic jumps with a marked roller present a number of similar features that are discussed herein. Recent results have shown that the roller is a highly unsteady turbulent region, with both the roller toe and free surface constantly fluctuating with time and space, although the roller shape is quasi-two-dimensional on average. Downstream of the roller toe, air bubbles and vorticity are diffused in the shear zone at different rates. The double diffusive convection process leads to a complex interplay between instantaneous free-surface deformations, velocity fluctuations and interfacial processes including breakup and coalescence.

Dam-break flood waves are associated with major environmental disasters provoked by the sudden release of water stored in reservoirs. Ritter found in 1892 an analytical solution to the wave structure of an ideal fluid released during an instantaneous dam failure, propagating over initially dry horizontal terrain. This solution, though ideal, hence frictionless, is widely used to test numerical solutions of the Shallow Water Equations (SWE), and as educational tool in courses of fluid mechanics, given that it is a peculiar case of the Riemann problem. However, the real wave structure observed experimentally differs in a major portion of the wave profile, including the positive and negative fronts. Given the importance of an accurate prediction of the dam break wave, the positive and negative wave portions originating from the breaking of a dam with initially dry land on the tailwater reach are revisited in this work. First, the propagation features of the dry-front are investigated using an analytical boundary-layer type model (Whitham/Dressler/Chanson model) constructed matching an (outer) inviscid dynamic wave to an (inner) viscous diffusive wave. The analytical solution is evaluated using an accurate numerical solution of the SWE produced using the MUSCL-Hancock finite-volume method, which is tested independently obtaining the solution based on the discontinuous Galerkin finite-element method. The propagation features of the negative wave are poorly reproduced by the SWE during the initial stages of dam break flows, and, thus, are then investigated using the Serre–Green–Naghdi equations for weakly-dispersive fully non-linear water waves, which are solved using a finite volume-finite difference scheme.

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.

The flow structure in the aerated region of the roller generated by breaking waves remains a great challenge to study, with large quantities of entrained air and turbulence interactions making it very difficult to investigate in details. A number of analogies were proposed between breaking waves in deep or shallow water, tidal bores and hydraulic jumps. Many numerical models used to simulate waves in the surf zone do not implicitly simulate the breaking process of the waves, but are required to parameterise the wave-breaking effects, thus relying on experimental data. Analogies are also assumed to quantify the roller dynamics and the energy dissipation. The scope of this paper is to review the different analogies proposed in the literature and to discuss current practices. A thorough survey is offered and a discussion is developed an aimed at improving the use of possible breaking proxies. The most recent data are revisited and scrutinised for the use of most advanced numerical models to educe the surf zone hydrodynamics. In particular, the roller dynamics and geometrical characteristics are discussed. An open discussion is proposed to explore the actual practices and propose perspectives based on the most appropriate analogy, namely the tidal bore.

In high-velocity chute flows, free-surface aeration is often observed. The phenomenon is called self-aeration or white waters. When the turbulent shear stresses next to the free-surface are large enough, air bubbles are entrained throughout the entire air–water column. A rapidly-varied flow region is observed immediately downstream of the inception point of free-surface aeration. An analytical solution of the air diffusion equation is proposed and the results compare well with new experimental data. Both experiments and theory indicate that the flow bulking spans over approximately 3–4 step cavities downstream of the inception point of free-surface aeration on a stepped chute. Further downstream the void fraction distributions follow closely earlier solutions of the air diffusion equation. The application of the diffusion equation solution to prototype and laboratory data shows air bubble diffusivities typically larger than the momentum transfer coefficient. The result highlights however a marked decrease in the ratio of air bubble diffusivity to eddy viscosity with increasing Reynolds number. The finding might indicate some limitation of laboratory investigations of air bubble diffusion process in self-aerated flows and of their extrapolation to full-scale prototype applications.

In open channel, canals and rivers, a rapid increase in flow depth will induce a positive surge, also called bore or compression wave. The positive surge is a translating hydraulic jump. Herein new experiments were conducted in a large-size rectangular channel to characterise the unsteady turbulent properties, including the coupling between free-surface and velocity fluctuations. Experiments were repeated 25 times and the data analyses yielded the instantaneous median and instantaneous fluctuations of free-surface elevation, velocities and turbulent Reynolds stresses. The passage of the surge front was associated with large free-surface fluctuations, comparable to those observed in stationary hydraulic jumps, coupled with large instantaneous velocity fluctuations. The bore propagation was associated with large turbulent Reynolds stresses and instantaneous shear stress fluctuations, during the passage of the surge. A broad range of shear stress levels was observed underneath the bore front, with the probability density of the tangential stresses distributed normally and the normal stresses distributed in a skewed single-mode fashion. Maxima in normal and tangential stresses were observed shortly after the passage of a breaking bore roller toe. The maximum Reynolds stresses occurred after the occurrence of the maximum free-surface fluctuations, and this time lag implied some interaction between the free-surface fluctuations and shear stress fluctuations beneath the surge front, and possibly some causal effect.

In an estuary, a tidal bore is a hydraulic jump in translation generated at the leading edge of the flood tide during the early flood tide under spring macrotidal conditions in a narrow funnelled channel. After formation, the bore is traditionally analysed as a hydraulic jump in translation and its leading edge is characterised by a breaking roller for Fr1 > 1.3–1.5. Herein new unsteady experiments were conducted to investigate in details the upstream propagation of breaking bore roller. The toe perimeter shape fluctuated rapidly with transverse distance and time. A characteristic transverse wave length of the toe perimeter was observed. Both the standard deviation of toe perimeter location and characteristic transverse wave length were comparable to field observations. The celerity of the roller toe fluctuated rapidly with time and space. The instantaneous longitudinal profile of the roller free-surface showed significant temporal and spatial fluctuations. Although the bore propagation may be analysed in an integral form in first approximation, the rapid fluctuations in roller toe perimeter and free-surface profiles indicated a strongly three-dimensional turbulent flow motion.

A tidal bore is a hydraulic jump in translation, propagating upstream as the tide turns to rising and the flood flow advances in a funnel-shaped river mouth under spring tide conditions. This study focused on the unsteady turbulence induced by a breaking tidal bore. Detailed free-surface and velocity measurements were conducted with a high temporal resolution using non-intrusive free-surface measurement probes and acoustic Doppler velocimetry sampled at 200 Hz. The laboratory data were systematically compared with an earlier series of field measurements conducted in the breaking bore of the Selune River (France). Key findings include the agreement, in terms of dimensionless instantaneous free-surface and velocity data, between laboratory and field observations as well as the existence of a transient recirculation region near the bed.

A sudden decrease in water depth, called a negative surge or expansion wave, is characterised by a gentle change in free-surface elevation. Some geophysical applications include the ebb tide flow in macro-tidal estuaries, the rundown of swash waters and the retreating waters after maximum tsunami runup in a river channel. The upstream propagation of expansion waves against an initially steady flow was investigated in laboratory under controlled flow conditions including detailed free-surface velocity and Reynolds stress measurements. Both non-intrusive free-surface measurements and intrusive velocity measurements were conducted for relatively large Reynolds numbers with two types of bed roughness. The data showed that the propagation of expansion waves appeared to be a relatively smooth lowering to the water surface. The wave leading edge celerity data showed a characteristic trend, with a rapid acceleration immediately following the surge generation, followed by a deceleration of the leading edge surge towards an asymptotical value: $$(\mathrm{U}+\mathrm{V}_\mathrm{o})/(\mathrm{g}\times \mathrm{d}_\mathrm{o})^{1/2}=1$$ for both smooth and rough bed experiments. The results indicated that the bed roughness had little to no effect, within the experimental flow conditions. Relatively large fluctuations in free-surface elevation, velocity and turbulent shear stress were recorded beneath the leading edge of the negative surge for all flow conditions. The instantaneous turbulent shear stress levels were significantly larger than the critical shear stress for sediment erosion. The present results implied a substantial bed erosion during an expansion wave motion.

Steppedspillwayflowsarecharacterisedbysignificantfree-surfaceaerationdown- streamoftheinceptionpointofairentrainment.Thesteppeddesignisadvantageousforappli- cations which require large energy dissipation and strong flow aeration. While the energy dissipation rate for embankment stepped spillways was studied previously, the optimum design for aeration and air-water mass transfer is not known. Herein new air-water flow experiments were conducted on several stepped spillways with embankment dam slopes using with phase-detection intrusive probes. The present data were compared with previ- ous studies in terms of energy dissipation, flow aeration and air-water mass transfer. The air-water mass transfer was calculated based upon air-water flow measurements in terms of dissolvedoxygen.There-oxygenationrateswerecomparedwithpreviousstudiescomprising both conductivity and direct dissolved oxygen measurements. The comparison highlighted a monotonic increase of aeration efficiency with energy dissipation rate. All data were in good agreement independent of channel slopes, stepped configuration and sensor size. The data confirmed the effects of strong air-water interactions within the bulk of the flow for both energy dissipation and re-oxygenation performances.

Air-water flow experiments were conducted on some flat and pooled stepped spillways with slopes of 8.9° and 26.6° in transition and skimming flows. The study comprised the observations of the flow patterns, characteristic air-water flow properties, and energy dissipation performances. The air-water flow properties showed some differences in terms of interfacial velocity, bubble count rate, and turbulence intensity between the stepped chutes for the two channel slopes. These differences were also reflected in the residual energy data, highlighting a better energy dissipation rate for the pooled stepped spillway with slope of 8.9°. However, the aerated flows on the pooled stepped spillways exhibited some hydrodynamic instabilities, and a safe operation must be tested in physical models. The flat stepped spillway appeared to be the preferable design in terms of energy dissipation and flow stability.

The design floods of several reservoirs were recently re-evaluated and the revised spillway outflow could result in dam overtopping with catastrophic consequences for some embankment structures. Herein a physical study was performed on flat and pooled stepped spillways with a slope typical of embankments (θ = 26.6 ◦ ) and four stepped configurations were tested: a stepped spillway with flat horizontal steps, a pooled stepped spillway, and two stepped spillways with in-line and staggered configurations of flat and pooled steps. The focus of the study was on the flow aeration, air-water flow properties, cavity flow processes, and energy dissipation performances. The results demonstrated the strong aeration of the flow for all configurations. On the in-line and staggered configurations of flat and pooled steps, the flow was highly three-dimensional. The residual head and energy dissipation rates at the stepped chute downstream end were calculated based upon the detailed air-water flow properties. The results showed that the residual energy was the lowest for the flat stepped weir. The data for the stepped spillway configuration with in-line and staggered configurations of flat and pooled steps showed large differences in terms of residual head in the transverse direction. Altogether the present results showed that, on a 26.6 ◦ slope stepped chute, the designs with in-line and staggered configurations of flat and pooled steps did not provide any advantageous performances in terms of energy dissipation and flow aeration, but they were affected by three-dimensional patterns leading to some flow concentration.

Negative surges can be caused by a sudden change in flow resulting from a decrease in water depth. In the present study, some physical experiments were conducted in a rectangular channel to characterize the unsteady free-surface profile and longitudinal velocity beneath a negative surge propagating upstream. The physical observations showed that, during the first initial instants, the celerity of the surge leading edge increased rapidly with time, while later the negative surge propagated upstream in a more gradual manner with a celerity decreasing slowly with increasing distance. The velocity data highlighted some relatively large turbulent fluctuations beneath the negative surge. The physical results were used to test the analytical solution of the Saint-Venant equations and some numerical models. The findings suggested that the negative surge propagation appeared relatively little affected by the boundary friction within the investigated flow conditions. DOI: 10.1061/(ASCE)HY.1943-7900.0000674. © 2013 American Society of Civil Engineers. CE Database subject headings: Storm surges; Open channel flow; Unsteady flow; Numerical models; Velocity. Author keywords: Negative surges; Unsteady open channel flow; Physical modeling; Numerical modeling; Water depth; Velocity; Negative waves.

In skimming flow on a stepped spillway, the upstream flow motion is nonaerated and a turbulent boundary layer develops until the outer edge of the boundary layer interacts with the free surface; that is, at the inception point of air entrainment. Herein, new experiments were performed in the developing flow region on a large 1 V∶1 H stepped spillway model with step height h=0.10 m. The flow properties in the developing flow region were carefully documented. In the developing boundary layer, the velocity distributions followed a 1/4.5th power law at the step edges. Detailed velocity and pressure measurements showed some rapid flow redistribution between step edges and above step cavities. The application of the momentum integral equation indicated an average friction factor of 0.18, which is close to the observed air-water-flow friction factor of 0.23, suggesting that the spatially averaged dimensionless shear stress was comparable in the developing flow and fully aerated flow regions.

On a stepped spillway, the steps act as macroroughness elements, contributing to enhanced energy dissipation and significant aeration. In a skimming flow, the upstream flow motion is nonaerated, and the free surface appears smooth and glossy up to the inception point of free-surface aeration. In this developing flow region, a turbulent boundary layer grows until the outer edge of the boundary layer interacts with the free surface and air entrainment takes place. The flow properties in the developing flow region were documented carefully in a large stepped spillway model (1V:1H; h=0.10 m). The upstream flow was controlled by a broad-crested weir and critical flow conditions were observed along most of the weir crest, although the pressure distributions were not hydrostatic at the upstream and downstream ends. Downstream of the broad crest and upstream of the inception point, the free surface was smooth, although some significant free-surface curvature was observed for all discharges. The boundary ...

On a gabion stepped chute, the steps contribute to the dissipation of turbulent kinetic energy, free-surface aeration may be intense, and there are complex interactions between the free-surface flow and seepage motion. Detailed measurements were conducted in a relatively large gabion stepped spillway model. Using a combination of high-speed movies and phase-detection probe measurements, the air–water flow properties in the step cavities and in the gabions were documented. Strong air–water exchanges between seepage and stepped cavity flows were observed. The data showed a complex bubbly seepage motion in the gabions associated with a high level of interactions between seepage and free-surface flows, leading to a modification of the step cavity recirculation and lesser flow resistance.

The stepped spillway design is associated with significant flow resistance and associated energy dissipation on the steps, yielding smaller, more economical downstream dissipation structures. A number of design guidelines were developed for steep stepped spillways typical of concrete gravity dams. The focus of this study is on embankment stepped spillways. A large set of air-water flow data is compared with reanalyzed data sets to provide a simple unifying design approach for the residual energy at the stepped chute’s downstream end and to highlight the uncertainties involved. The results provided some simple design criteria in terms of the dimensionless residual energy of stepped chutes with flat steps. It is believed that a stepped design with a 1V:2.5H slope (θ=21.8°) might be optimum in terms of energy dissipation performances. The Darcy-Weisbach friction factors were close for all stepped data ranging between 0.1≤fe≤0.4.

Basic experiments were conducted on a large-size broad-crested weir with a rounded corner. Detailed free-surface, velocity, and pressure measurements were performed for a range of flow conditions. The results showed the rapid flow distribution at the upstream end of the weir and next to the weir brink at large flow rates. The flow properties above the crest were analyzed taking into account the nonuniform velocity and nonhydrostatic pressure distributions. Introducing some velocity and pressure correction coefficients, it is shown that critical flow conditions were achieved above the weir crest for 0.1

A tidal bore is a positive surge taking place during the flood tide with a large tidal range and the bore corresponds to the leading edge of the tidal wave propagating upstream. In this study, some physical modelling was performed to investigate the upstream bore propagation over fixed and movable gravel beds. Both undular and breaking bores were tested. The free-surface and velocity measurements were complemented by some observations of particle motion beneath the bore front. In the initially-steady flow and beneath undular bores, no sediment motion was observed for the experimental set-up. Beneath breaking bores, on the other hand, some upstream gravel bed load motion was observed behind the bore. The gravel bed particles were de-stabilized by the roller toe passage and advected upstream. The unsteady velocity data showed some damping of the transient recirculation on the movable gravel bed.

A tidal bore is a series of waves propagating upstream in the river mouth as the tidal flow turns to rising in macro-tidal conditions. Some related geophysical processes include the tsunami-induced bores and uprushing bores on beaches. In the present study, the inception of sediment motion beneath tidal bores was investigated physically. No sediment motion was observed in the initially steady flow and beneath undular bores. A transient sheet flow motion was observed beneath breaking bores and the onset of sediment motion was closely linked with the passage of the roller toe. The forces acting on the movable gravel bed particles were estimated from the physical measurements. The results showed that the longitudinal pressure gradient force was the dominant contribution de-stabilising the particles and inducing the onset of sediment motion. The drag force added a sizeable contribution to maintain the upstream particle motion, although the entire sheet flow motion was brief.

A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide under macrotidal and appropriate bathymetric conditions. This paper presents a study that physically investigates the turbulent properties of tidal bores. Results from some experimental measurements of free-surface fluctuations and turbulent velocities conducted on smooth and rough beds are reported. The free-surface measurements were conducted with Froude numbers of 1-1.7. Both undular and breaking bores were observed. Using an ensemble-averaging technique, the free-surface fluctuations of breaking tidal bores are characterized. Immediately before the roller, the free-surface curves gradually upwards. The passage of the bore roller is associated with some largewater elevation fluctuations; the largest free-surface fluctuations are observed during the first half of the bore roller. The turbulent velocity measurements were performed at several vertical elevations during and shortly after the passage of breaking bores. Both the instantaneous and ensemble-averaged velocity data highlight a strong flow deceleration at all elevations during the bore passage. Close to the bed, the longitudinal velocity component becomes negative immediately after the roller passage, implying the existence of a transient recirculation. The height and duration of the transient are a function of the bed roughness, with a higher and longer recirculation region above the rough bed. The vertical velocity data presented some positive, upward motion beneath the front with increasing maximum vertical velocity with increasing distance from the bed. The transverse velocity data show some large fluctuations with nonzero ensemble average after the roller passage that highlight some intense secondary motion advected behind the bore front. DOI: 10.1061/(ASCE)HY.1943-7900.0000542. © 2012 American Society of Civil Engineers.

A tidal bore may develop in an estuary during the spring tide conditions when the tidal range exceeds 5–6 m and the flood tide is confined to a narrow funnelled estuary with low freshwater levels. The tidal bore is of great importance for the geomorphology of the estuarine zone. In this study, some physical modelling was performed to investigate the sediment motion inception beneath a tidal bore on a movable gravel bed. The results show the significant impact of breaking bore propagation on the gravel bed motion. The dominant contribution to sediment transport inception is the longitudinal pressure gradient force, while the transient recirculation motion next to the bed yields to a drag force acting in the upstream direction and contributing to sediment motion.

A positive surge is a unsteady open channel flow resulting from the rapid rise of the free-surface. The phenomenon may be observed in water supply canals and channels as well as in some estuaries during spring tidal conditions. The formation and development of positive surges can be predicted using the method of characteristics and shallow water equations. The paper is the second part of a study presenting the results from new experimental investigations conducted in a large rectangular channel. Detailed unsteady velocity measurements were performed with a high temporal resolution using acoustic Doppler velocimetry and non-intrusive free-surface measurement devices. Several experiments were conducted with the same initial discharge (Q = 0.060 m3/s) and six different gate openings after closure resulting in both non-breaking undular and breaking bores. A comparison between main features of the undular surges with literature theories demonstrated that the experimental data were mostly in agreement with Andersen’s theory. The analysis of unsteady flow field including Reynolds stresses confirmed and extended previous findings about positive surge hydrodynamics.

A positive surge results from a sudden change in flow that increases the depth. It is the unsteady flow analogy of the stationary hydraulic jump and a geophysical application is the tidal bore. Positive surges are commonly studied using the method of characteristics andthe Saint-Venant equations.Thearticle presents the results fromnewexperimental inves- tigations conducted in a large rectangular channel. Detailed unsteady velocity measurements were performed with a high temporal resolution using acoustic Doppler velocimetry and non-intrusive free-surface measurement devices. Several experiments were conducted with the same initial discharge (Q = 0.060m 3 /s) and six different gate openings after closure resulting in both non-breaking undular and breaking bores. The analysis of undular surges revealed wave amplitude attenuation with increasing distance of surge propagation were in agreement with Ippen and Kulin theory. Also, undular wave period and wave length data were relatively close to the values predicted by the wave dispersion theory for gravity waves in intermediate water depths.

A hydraulic jump is a sudden, rapid transition from a supercritical flow to a subcritical flow. At large inflow Froude numbers, the jump is characterized by a significant amount of entrained air. For this paper, the bubbly two-phase flow properties of steady and strong hydraulic jumps were investigated experimentally. The results demonstrate that the strong air entrainment rate and the depth-averaged void-fraction data highlight a rapid deaeration of the jump roller. The results suggest that the hydraulic jumps are effective aerators and that the rate of detrainment is comparatively smaller at the largest Froude numbers.

In an open channel, a sudden rise in water level induces a positive surge, or bore, that may develop as a hydraulic jump in translation. When the surge propagates against an adverse slope, it decelerates until it becomes a stationary hydraulic jump. Both hydraulic jumps and decelerating surges induce some intense turbulent mixing and have some major impact on the sediment transport in natural systems. Herein, a physical investigation was conducted in a relatively large rectangular channel. Hydraulic jumps and surges were generated by the rapid closure of a gate at the channel downstream end. The turbulent shear stresses were measured with high temporal and spatial resolution (200 Hz sampling rate) in the jump flow. A comparison between the stationary hydraulic jump, hydraulic jump in translation and decelerating surge measurements showed some marked differences in terms of turbulent mixing. The results highlighted some intense mixing beneath the jump front and roller for all configurations. The levels of turbulent stresses were one to two orders of magnitude larger than a critical threshold for sediment motion. The findings provide some insights into the hydraulic jump migration processes in mobile bed channels, and the complex transformation from a moving jump into a stationary jump. Copyright (C) 2010 John Wiley & Sons, Ltd.

An open channel flow with a flow depth close to the critical depth is characterised by a curvilinear streamline flow field that results in steady free surface undulations. Near critical flows of practical relevance encompass the undular hydraulic jump when the flow changes from supercritical (F > 1) to subcritical (F < 1), and the undular weir flow over broad-crested weirs where the flow changes from subcritical (F < 1) to supercritical (F > 1). So far these flows were mainly studied based on ideal fluid flow computations, for which the flow is assumed irrotational and, thus, shear forces are absent. While the approach is accurate for critical flow conditions (F = 1) in weir and flumes, near-critical flows involve long distances reaches, and the effect of friction on the flow properties cannot be neglected. In the present study the characteristics of near-critical free-surface flows are reanalysed based on a model accounting for both the streamline curvature and friction effects. Based on the improved model, some better agreement with experimental results is found, thereby highlighting the main frictional features of the flow profiles. © 2010 Springer Science+Business Media B.V.

A tidal bore is a wave propagating upstream as the tidal flow turns to rising. It forms during spring tide conditions when the flood tide is confined to a narrow funneled channel. To date, theoretical and numerical studies rely upon physical experiments to validate the developments, but the experimental data are limited mostly to visual observations and sometimes free-surface measurements. Herein turbulent velocity measurements were obtained in a large-size laboratory facility with a fine spatial and temporal resolution. The instantaneous velocity measurements showed rapid flow deceleration at all vertical elevations, and large fluctuations of all velocity components were recorded beneath the bore and secondary waves. A comparison between undular (nonbreaking) and breaking bores suggested some basic differences. In an undular bore, large velocity fluctuations were recorded beneath the first wave crest and the secondary waves showing a long-lasting effect after the bore passage. In a breaking bore, some la...

A tidal bore may occur in a macro-tidal estuary when the tidal range exceeds 4.5–6 m and the estuary bathymetry amplifies the tidal wave. Its upstream propagation induces a strong mixing of the estuarine waters. The propagation of undular tidal bores was investigated herein to study the effect of bridge piers on the bore propagation and characteristics. Both the tidal bore profile and the turbulence generated by the bore were recorded. The free-surface undulation profiles exhibited a quasi-periodic shape, and the potential energy of the undulations represented up to 30% of the potential energy of the tidal bore. The presence of the channel constriction had a major impact on the free-surface properties. The undular tidal bore lost nearly one third of its potential energy per surface area as it propagated through the channel constriction. The detailed instantaneous velocity measurements showed a marked effect of the tidal bore passage suggesting the upstream advection of energetic events and vorticity “clouds” behind the bore front in both channel configurations: prismatic and with constriction. The turbulence patches were linked to some secondary motions and the proposed mechanisms were consistent with some field observations in the Daly River tidal bore. The findings emphasise the strong mixing induced by the tidal bore processes, and the impact of bridge structures on the phenomenon.

A tidal bore is a natural and fragile phenomenon, which is of great importance for the ecology of an estuary. The bore development is closely linked with the tidal range and the river mouth shape, and its existence is sensitive to any small change in boundary conditions. Despite their ecological and cultural value, little is known on the flow field, turbulent mixing and sediment motion beneath tidal bores. Indeed, some striking features can be highlighted in two-dimensional simulations, such as large velocity fluctuations and flow recirculation structures. Using Large Eddy Simulation method, the numerical results emphasised the complicated turbulent structures and their unsteadiness under a tidal bore.

With the re-evaluation and revision of a number of design floods, several embankment overtopping protection systems have been developed and a common technique is the construction of a stepped spillway on the downstream slope. For such moderate slope stepped channels, detailed air–water flow measurements were performed in a large facility with a focus on the rate of energy dissipation, flow resistance, air–water interfacial areas and re-aeration rates. Past and present experimental results showed a significant aeration of the flow. The median dimensionless residual head was about 3 × dc for the 21.8° sloping chute and smaller than that for flatter slopes (θ = 3.4° and 15.9°). The flow resistance results yielded an equivalent Darcy friction factor of about 0.25 implying a larger flow resistance for the 21.8° slope angle than for smaller slope angles. The re-aeration rate was deduced from the integration of the mass transfer equation using measured air–water interfacial areas and air–water flow velocities. The results suggested an increasing re-aeration rate with increasing rate of energy dissipation. The stepped invert contributed to intense turbulence production, free-surface aeration and large interfacial areas. The experimental data showed however some distinctive seesaw pattern in the longitudinal distribution of air–water flow properties with a wave length of about two step cavities. While these may be caused by the interactions between successive adjacent step cavities and their interference with the free-surface, the existence of such “instabilities” implies that the traditional concept of normal flow might not exist in skimming flows above moderate-slope stepped spillways.

In dam break waves, the surge front is a sudden discontinuity characterized by extremely rapid variations of flow depth and velocity. In this study, simple analytical solutions for instantaneous dam break wave are developed using the method of characteristics. The solutions are obtained for initially dry horizontal and sloping channels with turbulent motion, and they are compared with several data sets obtained in large-size facilities.A main feature of the development is its simplicity that is well-suited to students and young professionals.

In open channel flows, the relationship between specific energy and flow depth presents a singularity at critical flow conditions. Although the concept of critical flow was first introduced as a singularity of the backwater equation (Belanger 1828), it was associated with the idea of minimum specific energy by Bakhmeteff (1912). This approach is now commonly used (Henderson 1966, Chanson 2004) including herein. Numerous solutions of the critical flow conditions were proposed for flat channels with hydrostatic pressure distributions. Herein solutions for non-hydrostatic pressure distribution flow situations are developed, and compared with experimental measurements.

In macrotidal estuaries, a tidal bore may form during spring tide conditions when the flood tide is confined to a narrow channel. Most field occurrences showed well-defined undulations behind the leading wave, that is, an undular bore process. Herein, detailed free-surface and turbulence measurements were performed beneath undular bore fronts using side-looking acoustic Doppler velocimetry and nonintrusive free-surface measurement devices in a laboratory channel. Undular bores were observed for Froude numbers less than 1.7, which compared favourably with past studies. Velocity measurements with a temporal resolution of 50 Hz showed a marked effect of the bore passage. Longitudinal velocities were characterised by rapid flow deceleration at all vertical elevations, while large fluctuations of transverse velocities were recorded beneath the front. Turbulent Reynolds stress data highlighted high levels in the lower flow region, including next to the bed. Maximum normal and tangential turbulent stres...

In hydraulic structures, free-surface aeration is commonly observed: i.e., the white waters. The air bubble entrainment may be localised (hydraulic jumps, plunging jets) or continuous along an interface (water jets, chutes). Despite recent advances, there are some basic concerns about the extrapolation of laboratory results to large size prototype structures. Herein the basic air bubble entrainment processes are reviewed and the relevant dynamic similarities are discussed. Traditionally, physical studies are conducted using a Froude similitude which implies drastically smaller laboratory Reynolds numbers than in the corresponding prototype flows. Basic dimensional analyses are developed for both singular and interfacial aeration processes. The results are discussed in the light of systematic investigations and they show that the notion of scale effects is closely linked with the selection of relevant characteristic air–water flow properties. Recent studies of local air–water flow properties highlight that turbulence levels, entrained bubble sizes and interfacial areas are improperly scaled based upon a Froude similitude even in large-size models operating with the so defined Reynolds numbers up to 5 E+5. In laboratory models, the dimensionless turbulence levels, air–water interfacial areas and mass transfer rates are drastically underestimated.

On a spillway chute, a stepped design increases the rate of energy dissipation on the chute itself and reduces the size of a downstream energy dissipater. Up to date, the effects of step roughness on the flow properties remain unknown despite the practical relevance to damaged concrete steps, rock chutes and gabions weirs. New measurements were conducted in a large-size laboratory facility with two step conditions (smooth and rough) and three types of step roughness. Detailed air-water flow properties were measured systematically for several flow rates. The results showed faster flow motion on rough step chutes. Although the finding is counter-intuitive, it is linked with the location of the inception point of free-surface aeration being located further downstream than for a smooth stepped chute for an identical flow rate. In the aerated flow region, the velocities on rough-step chutes were larger than those of smooth chute flows for a given flow rate and dimensionless location from the inception point of...

Hydraulic engineers and researchers deal with the scientific challenges presented by turbulent flow and its interactions with the surroundings. Turbulent flows are characterised by unpredictable behaviour, and, as yet, little systematic research has been conducted in natural systems. This paper discusses the implications of recent developments in affordable instrumentation, which was previously characterised by intrinsic weaknesses that adversely affected the quality of the signal outputs. A challenging application is the unsteady turbulence field in tidal bores. The interactions between open channel flows and movable boundaries and the atmosphere illustrate another aspect of our limited knowledge. Rapid siltation of reservoirs and air entrainment in turbulent freesurface flows are discussed. In both applications, hydraulic engineers require some broad-based expertise. In turn, the education of future hydraulic engineers is of vital importance.

Current expertise in air–water turbulent flows on stepped chutes is limited mostly to laboratory experiments at low to moderate Reynolds numbers on chutes with flat horizontal steps. In this study, highly turbulent air–water flows skimming down a large-size stepped chute were investigated with a 1V:2.5H slope. For some experiments, the cavity recirculation was controlled using triangular vanes, or longitudinal ribs, to enhance the interactions between the skimming flow and cavity recirculating region. New experiments were performed with seven configurations. The results demonstrated the strong influence of the vanes on the cavity recirculation patterns and on the air–water flow properties. An increase in flow resistance was observed consistently with maximum rate of energy dissipation achieved with vanes placed in a zigzag pattern.

In high-velocity open channel flows, free-surface aeration is commonly observed. The effects of surface waves on the air-water flow properties are tested herein. The study simulates the air-water flow past a fixed-location phase-detection probe by introducing random fluctuations of the flow depth. The present model yields results that are close to experimental observations in terms of void fraction, bubble count rate and bubble/droplet chord size distributions. The results show that the surface waves have relatively little impact on the void fraction profiles, but that the bubble count rate profiles and the distributions of bubble and chord sizes are affected by the presence of surface waves.

Rectangular dropshafts, commonly used in sewers and storm water systems, are characterized by significant flow aeration. New detailed air–water flow measurements were conducted in a near-full-scale dropshaft at large discharges. In the shaft pool and outflow channel, the results demonstrated the complexity of different competitive air entrainment mechanisms. Bubble size measurements showed a broad range of entrained bubble sizes. Analysis of streamwise distributions of bubbles suggested further some clustering process in the bubbly flow although, in the outflow channel, bubble chords were in average smaller than in the shaft pool. A robust hydrophone was tested to measure bubble acoustic spectra and to assess its field application potential. The acoustic results characterized accurately the order of magnitude of entrained bubble sizes, but the transformation from acoustic frequencies to bubble radii did not predict correctly the probability distribution functions of bubble sizes

A hydraulic jump is a rapid transition from a high-velocity open channel flow to a slower fluvial motion. It is commonly experienced in streams and rivers, in industrial channels, during manufacturing processes and downstream of dam spillways. The air is entrapped at the jump toe that is a discontinuity between the impinging flow and the roller. The impingement perimeter is a source of vorticity. The air-water shear layer is characterised by a transfer of momentum from the high-velocity jet flow to the recirculation region above, as well as by an advective diffusion of entrained air bubbles. Herein the theoretical and experimental modelling of hydraulic jumps is presented with a focus on the two-phase flow properties. Later the hydraulic design of hydraulic jump stilling basin is developed and prototype experiences are discussed.

Recent advances in technology have permitted the construction of large dams, reservoirs and channels. This progress has necessitated the development of new design and construction techniques, particularly with the provision of adequate flood release facilities. Chutes and spillways are designed to spill large water discharges over a hydraulic structure (e.g. dam, weir) without major damage to the structure itself and to its environment. At the hydraulic structure, the flood waters rush as an open channel flow or free-falling jet, and it is essential to dissipate a very signifi cant part of the flow kinetic energy to avoid damage to the hydraulic structure and its surroundings. Energy dissipation may be realised by a wide range of design techniques. A number of modern developments have demonstrated that such energy dissipation may be achieved (a) along the chute, (b) in a downstream energy dissipator, or (c) a combination of both. The magnitude of turbulent energy that must be dissipated in hydraulic structures is enormous even in small rural and urban structures. For a small storm waterway discharging at a 4 m3/s mm high drop, the turbulent kinetic energy flux per unit time is 120 kW! At a large dam, the rate of energy dissipation can exceed tens to hundreds of gigawatts; that is, many times the energy production rate of nuclear power plants. Many engineers have never been exposed to the complexity of energy dissipator designs, to the physical processes taking place and to the structural challenges. Several energy dissipators, spillways and storm waterways failed because of poor engineering design. It is believed that a major issue affecting these failures was the lack of understanding of the basic turbulent dissipation processes and of the interactions between free-surface aeration and flow turbulence. In that context, an authoritative reference book on energy dissipation in hydraulic structures is proposed here. The book contents encompass a range of design techniques including block ramps, stepped spillways, hydraulic jump stilling basins, ski jumps and impact dissipators.

The authors presented a very interesting contribution to the topic of stepped spillway hydrodynamics. Based on a water-tunnel study, they presented some detailed flow features, for example, in terms of the strain rate, which are valuable to designers. The authors stated that “stepped spillways have no documented damage.” It is partially correct. Indeed no cavitation damage and sign of cavitation pitting were reported on stepped spillways after more than 3,000 years of operation, but there were documented damages and failures of stepped spillway systems (Marsh 1957; Jansen 1983; Pravdivets 1993; Chanson 1995b, 2000, 2001a). In this discussion, a number of prototype stepped spillway tests are reviewed and the operational experiences are discussed with a focus on the stepped chute performances...

Surge waves resulting from dam breaks have been responsible for numerous losses of life. A related situation is the tsunami surges advancing on dry coastal plains. Herein a simple dam break wave solution is presented. The results yield some simple expressions of the instantaneous free-surface profile and flow depths that compare well with well-known dam break wave data. The results are applied to tsunami surges on dry coastal plains and compared with some data set. The present development offers simple analytical expressions that compare well with both experimental data and more advanced theoretical solutions, and that are further well-suited for pedagogical purposes, computational model validation, and accurate real-time predictions of tsunami surges.