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In recent decades, stepped chutes followed by a stilling basin became a standard for spillways of dams. The comprehensive knowledge of stepped chute approach flows on internal air–water flow properties of the hydraulic jump is still too limited for an appropriate design of stilling basins. Therefore, an experimental campaign was performed on a large-scale physical model of a plain stilling basin preceded by a 30° sloping smooth or stepped chute. Stepped chute approach flows induce lower deaeration rates along the jump roller as compared to smooth chute approach flows, indicating longer dimensionless jump lengths, normalized by the tailwater depth. Pronounced bottom air concentrations were observed within the first 40% of the jump length in the stilling basin downstream of the stepped chute. Thus, despite the significantly higher pressure fluctuations, this zone seems better protected against cavitation damage than in stilling basins following a smooth chute. [ABSTRACT FROM AUTHOR]

Local scouring is a leading cause of bridge collapses. To protect bridges against local scouring, different countermeasures have been proposed and tested in the literature. In this study, the performance of collars was evaluated for scour reduction at two tandem piers aligned with four skew angles ( θ) of 0 ° , 30 ° , 60 ° , 90 ° with respect to flow direction. For this purpose, long duration tests were conducted with uniform sediments under clear-water conditions with flow intensities close to the threshold of sediment motion. The results showed that increasing the skew angle increased the scour depth, while the performance of collars for scour reduction decreased. The maximum and minimum scour depths occurred at skew angles of θ = 60 ° and θ = 0 ° , respectively, around both the piers with and without collars. Analysis of the results indicated that the maximum scour depth at the upstream pier shifted to the downstream pier from θ ≥ 20 ° and θ ≥ 28 ° for the tandem piers with and without collars, respectively. In addition, as the skew angle increased, the equilibrium time of scour increased. Moreover, the performance of collars for scour reduction around the piers decreased over time. [ABSTRACT FROM AUTHOR]

Tsunamis, impulse-waves and dam-break waves have affected humanity in recent decades and the construction of vertical shelters can provide safety to people. However, for non-critical infrastructures, typically residential houses of lower height, overtopping is accepted during such events. This study experimentally quantifies the effect of building overtopping, i.e. water flowing over the roof, on the resulting loading process. Both surges and bores were investigated and the impact against buildings with two different heights was assessed. Detailed measurements of forces and moments allowed key differences to be captured between the scenarios with and without overtopping. Results showed that overtopping induced higher downstream water depths, leading to lower horizontal forces and a reduced resistance coefficient. Furthermore, cantilever arm, moment and impulse values were constantly lower in case of overtopping. Finally, this study presents an innovative methodology to assess the main loading features of buildings subject to overtopping, supporting engineers to design safer resilient structures. [ABSTRACT FROM AUTHOR]

Recently, Jafarnejad, Franca, Pfister, and Schleiss [2017. Time-based failure analysis of compressed riverbank riprap. Journal of Hydraulic Research, 55(2), 224–235. doi: ] presented an experimental study on the stability of compressed riprap, which means that it is composed of individually placed blocks with a single layer as riverbank protection. The relationship between the time to failure and dimensionless bed shear stress was presented. Here, the authors complement the previous investigation with results regarding the effect of a second riprap layer. In total, 49 tests (28 tests with one and 21 tests with two riprap layers) were performed using a single block size, three longitudinal channel slopes and three different riprap bank inclinations. Under similar conditions, the second layer can significantly delay the time to failure, whereas the block erosion rate was found to increase. The second layer has a more stabilizing role when the bank angle of the riprap is approaching the angle of repose of the blocks. [ABSTRACT FROM AUTHOR]

Large wood is often transported by rivers into reservoirs during heavy rainfall events. When a critical section like a spillway is blocked and discharge capacity reduced, an uncontrolled increase of the reservoir water level may occur. This study aims to statistically analyse the importance of repetitions for the accuracy of experimental campaigns when studying blocking probabilities at ogee crested spillways equipped with piers. Systematic and reliable estimations based on physical models are critical for developing preventive measures against large wood blockage. Two statistical methods have been described and applied to calculate confidence intervals. A minimum number of repetitions for a maximum acceptable error is recommended for blocking probabilities. The minimum number of experimental repetitions has been statistically justified in accordance with a reasonable use of resources for experimental campaigns. In addition, a maximum acceptable level of error is proposed as a common metric of accuracy in large wood studies. [ABSTRACT FROM AUTHOR]

In-stream open check dams are essential for flood protection at mountain rivers that are characterized by steep slopes (>1%) and sediment-laden flood discharges. These structures comprise an opening acting as a lateral or vertical flow contraction to force sediment retention when floods occur. Otherwise, the contraction should not affect the run-off. Design criteria for the discharge capacity referring to the size and geometry of the opening were previously established. This experimental study reviews the existing formulae for the discharge capacity and analyses the beginning of sediment deposition, with varying channel slopes. The effects of the channel slope on backwater, sediment deposition and local head losses are relevant when free surface flow conditions persist in the opening. A channel slope-sensitive correction factor is introduced for calculating the discharge capacity. The sediment transfer rate through the contraction decreases with increasing backwater and is most sensitive for trans-critical flow conditions. The findings are validated against a case study in the Swiss Alps. [ABSTRACT FROM AUTHOR]

Orifices are largely used in pipeline systems. The most common application is to measure the discharge of the water flowing through. However, orifices may be used to adapt hydroelectric power plants submitted to moderate increase of installed generation capacity. For this application, the orifice is firstly characterized by a pair of head loss coefficients evaluated by a numerical simulations. Two different orifice shapes, chamfered and rounded orifices, were systematically studied in conduits with laboratory experiments. The main goal of the study is to evaluate the effects of orifice geometrical parameters on the head loss coefficients. A global empirical relationship has been found and can predict the head loss coefficients in both flow directions of a given orifice contrary to previous studies, which focused on sharp approach flow. These head loss coefficients depend on the inner diameter, thickness, inner thickness, chamfer angle and/or shape of orifices. [ABSTRACT FROM AUTHOR]

The geometry and hydrodynamics of river reaches are key ecohydraulic descriptors. Statistics of water depth and velocity measurements are usually taken as proxies for habitat suitability in rivers. However, little is known about the sufficiency of data to produce effective and representative results. In this research, 19 reaches with differences in terms of discharge, river width, substrate, reach length, cross-section spacing and geomorphology are investigated. Measurements of flow depth and velocity were taken at multiple, equally spaced cross-sections along each reach. Data were sub-sampled using different methodologies and analysed each time. The sets of sub-sampled data were then compared with those calculated with the full data set from a reach. The focus was put towards the hydro-morphological index of diversity (HMID), a combination of the classical ecohydraulic variables flow depth and velocity. It represents the spatial variability of hydraulic habitats in a reach. The results point out that, with a well-defined sampling strategy, 100 measurement points lead to a good estimation of the HMID value in a reach, if more than eight measurement points are taken per cross-section. For geomorphologies with small complexity or when the analysis only includes the estimation of mean flow depth or mean flow velocity, this number can be decreased according to the results presented here. These findings help both, aquatic ecologists and engineers to estimate their data reliability for hydraulic field measurements in a river reach and are herein discussed taking into account the different studied morphologies. [ABSTRACT FROM AUTHOR]

Tsunamis, landslide-generated waves, and dam failures are rare, but highly destructive phenomena, associated with extreme loading on infrastructure. Recent events showed that specific measures must be taken to guarantee safety of both people and the built environment. This experimental study investigates the forces and moments exerted on free-standing buildings that are induced by both surges and bores. The hydrodynamic impact was characterized by high splash, subsequently followed by a quasi-steady flow around the structure. For dry bed surges, the time history of the horizontal force was proportional to the momentum flux per unit width. For wet bed bores, an attenuation of the peak force due to the presence of an aerated front was observed and the introduction of a reduction coefficient was necessary to achieve a realistic force estimation. Additional force analysis in terms of peak time, wave height at maximum force and impulse also pointed out some key differences between forces exerted by dry bed surges and wet bed bores. The occurrence of the maximum tilting moment on the building coincided with the maximum horizontal force and an evaluation of the cantilever arm was possible. These findings provide engineers with practical information for the design of safer coastal structures. [ABSTRACT FROM AUTHOR]

In pumped-storage reservoirs, turbulent kinetic energy input from plant operation can be considerably high. Flow velocities were measured by Acoustic Doppler Current Profilers near the intake/outlet structure to describe flow patterns induced by pumped-storage operation in an Alpine reservoir. Recorded data allowed reproducing 1D and 2D velocity profiles along the water column. The comparison between the main frequencies of the velocity signal and the discharge series from the plant reveals correlation between recorded flow patterns and pumped-storage operation. Numerical modeling enhanced the understanding of flow patterns developing near the intake/outlet structure. Both results reveal that water withdrawal by pumping only marginally affects flow patterns in front of the intake, whereas water injected during turbine mode leads to backflow areas and large-scale recirculation cells. Numerical modeling further revealed that steady flow patterns are developing only after some 2.5 h of continuous turbine operation. [ABSTRACT FROM AUTHOR]

Reservoir sedimentation is a worldwide problem hampering the sustainable use of reservoirs and the sediment balance of impacted rivers. Various techniques are applied for sediment mitigation. However, as turbidity currents are a major source of sediments in reservoirs, venting them through outlets reduces sedimentation. The sediment release efficiency associated with venting turbidity currents on a horizontal bed is experimentally investigated in this paper. The outflow discharge and duration of venting are the main parameters assessed. Venting efficiency is studied based on two different concepts: (1) a global venting efficiency comparing inflow and outflow sediment fluxes during the total venting operation and (2) a local venting efficiency comparing masses starting at the arrival of the turbidity current to the outlet and taking into account deposited sediment masses. An efficiency indicator accounting for water losses is also introduced. Results can be used to improve the efficiency of venting by employing adequate outlet discharges. [ABSTRACT FROM AUTHOR]

Several stepped spillways have been built in recent decades, mostly integrated in the downstream faces of roller compacted concrete dams. Among them, only a few have changing bottom slopes. Comprehensive information on the effect of a slope change on the flow features is lacking. This paper reports a systematic study of air entrainment and flow bulking in skimming flow along stepped spillways in the vicinity of abrupt slope changes. Physical modelling was conducted using a large facility with abrupt pseudo-bottom slope changes from 50° to 30° and from 50° to 18.6°. Air–water flow measurements were conducted at several flow cross-sections upstream and downstream of the slope change. The results indicate a significant influence of the slope change on the air pattern and flow bulking. Four main flow sub-regions were identified to describe the typical air–water flow patterns. The normalized length of the influence reach induced by the slope change was found to depend mainly on the critical flow depth, regardless of the slope change and step height. [ABSTRACT FROM AUTHOR]

Methods to design riprap-lined channels usually refer to dumped material. Large blocks placed individually by machinery are used when more stability is required. They offer additional resistance against flow erosion since space between blocks is minimized and interlocking increased. The behaviour of this protection has rarely been studied. An experimental investigation was carried out on the stability of compressed riprap as riverbank protection. Riprap was reproduced by uniform crushed limestones with three block sizes. Tests were conducted for three channel slopes under supercritical flow conditions and for constant bank slope. A time-based analysis allowed establishing relations among time to failure, friction velocity, and dimensionless bed shear stress. The results of 45 tests confirm that the rate of block erosion is significantly reduced with increase in the riprap diameter. The time to failure of the riprap protection depends strongly on the longitudinal slope and on the block sizes. An empirical prediction to estimate the riprap time to failure is shown. [ABSTRACT FROM PUBLISHER]

Artificial water storage, originated by the construction of dams, is essential for the sustainable health and welfare of civilizations since it supplies water for human consumption, irrigation and energy production. Furthermore, dam reservoirs are used for recreation, navigation and they provide safety in the downstream valleys against extreme flood events and droughts. All reservoirs are subjected to sedimentation which, without adequate prevention and mitigation counter-measures, threatens their sustainability. As well as the evident loss of storage capacity, the adequate and safe operation of water intakes and bottom outlets belonging to the vital outlet structures can be affected by the deposition of sediments in the reservoir. This Vision Paper first addresses the state-of-the-art and the main scientific advances in terms of prevention and mitigation measures against sedimentation in reservoirs. Then, the main research challenges, which result from the assessment of what are the remaining open questions, are pointed out. The emerging research methodologies to study sedimentation processes are also discussed. [ABSTRACT FROM AUTHOR]

Uplift of blocks from the rock mass at the bottom of plunge pools is the main process of the scouring phenomenon caused by high-velocity jet impact. Although the influence of water and rock on dynamic block ejection has been investigated, no systematic research has been done regarding the influence of air entrainment. This study presents theoretical developments and experimental investigations of the vertical displacements of a block embedded in the pool bottom impacted by aerated plunging jets. The theoretical model reproduces well the measured block vibrations, especially for high-velocity jets where the properties of the air–water mixture inside the fissures are steadier. Block displacements reduce with jet aeration for shallow pools, but, inversely, they increase with jet aeration for deep pools. This confirms that jet aeration reduces jet momentum due to a lower apparent density, but may also increase bottom pressures due to lower velocity decay along the pool depth. [ABSTRACT FROM AUTHOR]

Floodplains are regions of great interest for environmental assessment as they constitute important ecological reserves and contribute efficiently to natural flood attenuation. However, the implementation of a model describing the basic hydrological behaviour of floodplains is not an easy task due to the complexity of the processes included. Although several attempts have been made to simulate floodplain effects in global rainfall-runoff models, no satisfactory routines have been developed yet. In this study, an adapted version of the Soil and Water Assessment Tool (2009) reservoir model is proposed and applied to the Zambezi Basin at daily time step with the intention of adequately modelling floodplain behaviour. The model separates the outflow of the reservoir simulating the floodplain into main channel flow and flow over the floodplain area. The improved solution was compared with the original model regarding its potential to simulate observed discharges in terms of volume ratio, the Nash–Sutcliffe coefficient and hydrograph plots. These evaluation criteria attest, for both calibration and validation periods, that the modified model is superior to the original one for simulating the discharge downstream of large floodplains. A sensitivity analysis is carried out at two geographical levels: at the outlet of a floodplain and at the outlet of the entire basin. The results show that upper flow parameters are more sensitive than base flow parameters. [ABSTRACT FROM PUBLISHER]

Flow velocity and sedimentation patterns were investigated experimentally and numerically in shallow rectangular reservoirs with different asymmetric locations of the inlet and outlet channels. Velocity fields were measured in the entire reservoir, both for clear water flow and with suspended sediments. Thickness of sediment deposits was mapped in the whole reservoir by means of a laser light method. In one of the studied geometric configurations, injection of suspended sediments led to a complete change in the observed flow field. Experimental results were compared with numerical simulations performed with the depth-averaged flow model WOLF 2D, using a depth-averagedk–ϵ turbulence model. The simulations lead to accurate predictions of the velocity profiles and the change in flow pattern as a result of sediment deposits was successfully reproduced. [ABSTRACT FROM PUBLISHER]

Surge waves from upstream have been studied systematically in a 40 m long channel. Its banks were equipped with large-scale rectangular roughness elements. In addition to a prismatic reference channel, 41 different configurations of rectangular bank cavities have been investigated. The purpose was to study the effect of large-scale bank roughness on steady and unsteady flows. The macro-roughness of the channel banks increases the flow resistance. Consequently, the absolute celerity of the surge wave front is also reduced by between 5 and 25%. Based on the concept of an effective channel width for the computation of the mean flow velocity, the relative celerity of the surge wave can be predicted for macro-rough channel configurations. The attenuation of the wave front height is increased by the macro-rough flow resistance as well as by the partial reflections of the surge waves in the cavities and passive retention of the latter. [ABSTRACT FROM PUBLISHER]

Piano Key Weirs (PKWs) are an alternative to linear overflow structures, increasing the unit discharge for similar heads and spillway widths. Thus, they allow to operate reservoirs with elevated supply levels, thereby providing additional storage volume. As they are relatively novel structures, few design criteria are available. Hence, physical model tests of prototypes are required. This study describes comprehensive model tests on a sectional set-up of several A-type PKWs, in which the relevant parameters were systematically varied. Considering data of former studies, a general design equation relating to the head–discharge ratio is derived and discussed. The latter is mainly a function of the approach flow head, the developed crest length, the inlet key height, and the transverse width. To extend its application range, case study model tests were analysed to provide a design approach if reservoir approach flow instead of channel flow is considered. [ABSTRACT FROM AUTHOR]

Experiments involving both a circular wall jet and an impinging jet were conducted, as occurring at irrigation control structures or at dams with combined crest and low-level spillways. The effects of the non-dimensional parameters on the scour hole dimensions have been investigated. It was found that the effect of the expansion ratio of the channel width to the jet diameter is negligible for values larger than 22.5 and 35, depending on the experimental conditions. The scour depth decreases with increasing tailwater depth and the horizontal distance between the impinging jet and the origin of the wall jet. The scour length and the distance between the end of the ridge and the wall jet origin increase with decreasing densimetric Froude number of the impinging jet. The opposite trend occurs for decreasing densimetric Froude number of the wall jet. An empirical relationship for estimating the scour depth due to the simultaneous jet action is given. [ABSTRACT FROM AUTHOR]

The effect of local drop of wall stiffness in pressurized waterways on the pressure wave speed and the wave attenuation during transients is investigated experimentally. A new signal-processing procedure to identify the presence of a weak reach is introduced and validated by physical experimentation based on assessing the pressure and vibration records acquired at both ends of a multi-reach steel test pipe. Water-hammer was generated by closing a downstream valve, whereas the weak reaches were simulated by replacing the steel portions with aluminium and PVC material. The wave speed and wave attenuation factor during transients are considered as global indicators of local and large changes in stiffness of the pipe wall. The method is capable of locating the stiffness weakness along the test pipe if one PVC reach is used. The error in estimating the position of such a reach relative to the real position of its mean value varies up to 23%. [ABSTRACT FROM PUBLISHER]

Block Stability in 3 Dimensions (BS3D) is an incremental-iterative algorithm and code to analyse general failure modes of rock blocks subjected to generic forces, including non-conservative forces such as water pressure. This algorithm is used to investigate the effect of high-velocity water jet impact on the stability of single rock blocks in plunge pools. After reviewing the literature, an approach is proposed to estimate the dynamic water pressure fluctuations at the pool bottom and in rock fractures. The approach is validated using an experimental study. In addition, by comparing the results of these analyses to additional experimental results, a scour threshold is introduced for plunge pools. It is found that if the maximum upward block displacement is larger than a quarter of the block height, the block is most probably removed from its mould. The proposed approach and threshold are validated using case histories that indicate excellent match between prediction and observed scour depth. [ABSTRACT FROM PUBLISHER]

Velocity fields in rectangular shallow reservoirs with different length-to-width and expansion ratios were investigated in an experimental study, to evaluate the effect of geometry on the flow field. A wide range of combinations of these two non-dimensional geometric parameters were tested at constant hydraulic conditions. Ultrasound velocity profilers were used to measure the horizontal velocity components across the entire reservoir surface, allowing for the visualization of streamlines and of the instantaneous and average velocities. Five different types of flow patterns were identified, depending on the values of the length-to-width ratio and expansion ratio of the reservoir. Asymmetrical flow patterns were found to develop for certain combinations of these geometric parameters despite the perfect reservoir symmetry. A critical comparison of these new experimental results with those of other works is provided. [ABSTRACT FROM AUTHOR]

Transverse or longitudinal movements of a water body are observed for flows along cavities, river embayments, groyne fields or harbours. They are significant for certain flow conditions and geometrical properties. To study the effect of large-scale roughness on banks, 36 geometries of axi-symmetric, rectangular cavities were investigated in a laboratory flume under subcritical, turbulent free surface flow conditions. Significant movements of the water body were detected. The frequency of these periodic movements, identified by level and velocity observations, is in agreement with the natural frequency of the water body in a rectangular basin assuming the first-order mode of sloshing. Major movements of the water body, which lead to significant and periodic oscillations of the water surface, are avoided by excluding Strouhal numbers near 0.42 and 0.84. For low aspect cavity ratios, the periodic water-surface oscillations are insignificant if the flow reattaches to the sidewalls of the widened channel reach. [ABSTRACT FROM AUTHOR]

This study addresses the design of riprap aprons as a scour countermeasure near abutments under clear-water conditions. It deals with the lateral extent of riprap aprons and the geometry of the scour hole prevailing at the apron edge. The study applies to riprap aprons acting as granular filters. The scour depth appears to be independent for a sufficiently long relative abutment length. Scour holes develop farther away from spill-through abutments than from vertical-wall abutments; the distance between the point of maximum scour depth and the abutment increases with the relative abutment length. The effect of contraction on this distance was not identified. The angle defining the position of the deepest scour point is close to 30°. Neither the abutment shape nor the flow contraction seems to influence the minimum stable apron width. [ABSTRACT FROM AUTHOR]

Side weirs and overtopable levees are widely used to increase flood routing along a channel or river. The lateral loss of water reduces the sediment transport capacity leading to the formation of a local sediment deposit close to the overflow. Since the extent of the morphological bed changes is not known a priori, the design discharge is increased by this flow-sediment transport interaction in an uncontrolled way. Based on a systematic flume study, a semi-empirical model to predict the three-dimensional bed evolution of the aggraded channel reach in the vicinity of the overflow is developed. The shape of the deposit is modelled by an adapted Maxwell-type distribution function. The main input parameters of the model are expressed in terms of dimensionless parameters accounting for main channel and side overflow geometry as well as flow and sediment transport characteristics. The application of the empirical model in numerical flow calculations predicts 90% of the measured overflow. [ABSTRACT FROM AUTHOR]

The sustainable use of reservoirs for irrigation, flood protection, water supply and hydropower may be endangered due to unavoidable reservoir sedimentation. Turbidity currents are the main process for the transport and deposit of sediments in reservoirs, especially in the deepest part near the dam where vital structures such as power intakes and bottom outlets are located. Besides other measures such as solid or permeable obstacles, turbidity currents can be influenced by means of an inclined water jet screen. Physical experiments of a turbidity current flowing through a water jet screen were carried out. Velocity profiles, front velocities, and deposit evolutions were determined. The results indicate that in certain configurations, turbidity currents can be partially stopped by the jet screen. Furthermore, the deposits downstream of the screen may be reduced up to a factor of two as compared with deposits of a free-flowing turbidity current. [ABSTRACT FROM AUTHOR]

Depositional and flow patterns are first described to gain an understanding of the erosion patterns during hydraulic flushing in a reservoir. Considering the importance of this issue, two modes of flushing operation for control of sedimentation were performed in several experiments with different reservoir geometries. In order to investigate the effect of flushing and the effectiveness during free and drawdown flows, ten experiments have been conducted. The final bed morphology formed previously was used as the initial bed topography for the two modes of flushing. The entire experiments lasted for two days. Investigations of the flow pattern and the associated bed topography for free flow with normal water depth and drawdown flushing in various shallow reservoir geometries are presented. To effectively apply the flushing processes for the removal of sediment deposits, the location, depth and width of the flushing channel can be changed by modifications to the reservoir geometry. The channel formed during flushing attracts the jet and stabilizes the flow structures over the entire surface. Empirical formulae to describe the relationship between the reservoir geometry and flushing efficiency for the two modes of flushing were developed. Flushing at normal water level allows only a relatively small part of the deposited sediment to be evacuated. As deposits could be flushed out of the basin. [ABSTRACT FROM AUTHOR]

The article presents an experimental study on Unstructured Block Ramps (UBR) and their structural design which allow passing of fish species. Topics addressed include meander type ramp which creates meandering flow of the river, failure response in case of hydraulic overload, and automatic adjustment of the UBRs during hydraulic overload.

The article discusses the highlights of a discussion on the discharge coefficient for free and submerged flow over Piano Key weirs. The participants limited their data sets to two scenarios, which includes tests not respecting the scale effects criterion that were ignored while all other tests were considered. Head-discharge measurements of prototype related case study model tests were taken into account.

The article presents a response to the article "Closure Problem To Jet Scour," by Gijs J.C.M. Hoffmans, which is published in the first issue of Journal of Hydraulic Research's volume 47 in 2009. It notes the difficulty in using a critical block stability factor accounting for quasi-steady and turbulent forces that act on a rock block in a plunge pool. It also provides thoughts about the influence of air on scour depth.

In this article, the author discusses the comments by Professor, Hubert Chanson on the paper "Propagation of surge waves in channels with large-scale bank roughness" which appeared in the 2013 issue of the periodical. It mentions the impact of large-scale bank roughness, on the propagation speed and the height of the surge wave front. It highlights the local transitions in the contour of channel which could result from substructures of bridges, and piers.