Journal of Hydraulic Engineering

The flow characteristics in a rock-ramp fish pass can be significantly altered by the boulder arrangement. This study used a three-dimensional computational fluid dynamics solver to investigate flow characteristics for variations of discharge, channel slope and boulder size, spacing, and pattern. This study derived relationships for estimating water depth and velocity in a rock-ramp fish pass as a function of discharge and structure geometry. The flow resistance varies significantly with emergent boulder spacing and is nearly constant for submerged boulders. This study derives relationships for the drag coefficient and submergence ratio to forecast average flow velocity using a flow-resistance equation. Based on maximum velocity and a slow velocity zone, this study recommends effective boulder spacing in the longitudinal and transverse directions for two different boulder patterns. Finally, this study developed a design procedure for designing a rock-ramp fish pass.

A two-phase smoothed particle hydrodynamics (SPH) mixture model to simulate water-soil interactions is presented. In this model, each phase of the mixture satisfies its own conservation equations of mass and momentum. The water is considered as a Newtonian fluid and the soil is modeled as an elastic-perfectly plastic material. Drucker-Prager criterion is employed to test the yielding of the soil and an associated flow rule is adopted to describe the soil behavior after yielding. Interactions between water and soil are modeled by the viscous drag force according to Darcy’s law. With this mixture model, it is possible to investigate the temporal and spatial evolutions of the volume fractions of both phases. This study first examines the proposed SPH mixture model for two single-phase flows, i.e., water dam break and sand column collapse, respectively. The drag force model is also tested using the problem of flow in porous media. Then its application to the problem of soil excavation by high-velocity impinging water jets is illustrated. The flow pattern, profile of excavation hole, evolutions of pressure, volume fraction, and plastic shear strain during the impinging process are obtained and found to be qualitatively good compared with previous experimental observations and numerical simulations. Effects of hydraulic conductivity and initial volume fractions of water and soil on the excavation are revealed. Numerical simulation shows that the proposed method is robust and efficient, and can be applied to water-soil mixture flow problems in hydraulic engineering and geotechniques, especially to those cases where volume fractions play important roles in the mixture dynamics.

Continuous sedimentation of fine sediments in dam reservoirs triggers considerable economic and ecological impacts worldwide. The definition of appropriate reservoir management procedures requires accurate tools for predicting the sediment dynamics both in the reservoir and downstream reaches. In this article, a one-dimensional numerical model aiming to reproduce suspended sediment dynamics is presented. To take into account water intake locations and vertical distribution of suspended sediments in areas of larger water depths close to dams, an additional module is developed. The model is applied to a reach of the French Upper Rhône River, including two reservoirs. The calibrated model provides accurate flux estimates and reproduces erosion and deposition patterns observed in the reservoir for several scenarios. The additional module allows to improve the estimation of the sand load released by low-level outlets. Eventually, the model could be used either to provide insights on the outcome of past flushing operations or as a predictive tool to prepare future operations.

A storm water pond’s effectiveness to treat particulate bound pollutants in urban runoff depends on its hydraulic behavior. Departures from the ideal plug flow, such as short-circuiting and mixing, pose hindrances to pond design. The goal of this study was to assess the importance of wind on storm water pond hydraulics. High resolution acoustic velocity measurements were conducted in a 0.3 ha, 2 m deep pond during dry weather. Results suggest that winds drive a turbulent, three-dimensional flow regime, including a lateral circulation and vertical exchange flows. A fully developed surface layer representing $1/3^d$ of the water depth, with drift scaling as 0.004 on wind speed at 10 m above ground, was observed in the downwind section of the pond. Wind-induced vertical mixing, short-circuiting, and basin scale mixing were estimated to occur faster than the typical nominal residence time in storm water ponds. Wind is therefore an important hydraulic driver in small water systems, which may potentially reduce their treatment performance. Local landscaping may be used as a wind mitigation measure.

Step-pool systems occur naturally in steep mountain streams and as man-made structures in steep channel sections where they serve as energy dissipating structures. Three different flow regimes may occur. The dominant flow regime is the cascading “nappe flow regime.” At higher discharges the “skimming flow regime” develops with an almost plane water surface. In between these flow regimes a “transition flow regime” occurs. A physical model was used to measure the pressure distribution around a single step for a particular step-pool geometry and for different flow regimes (nappe/transition/skimming). To achieve this, 14 simultaneously recording piezoresistive pressure transducers were distributed around the step. The results showed that the regime change from nappe to transition flow is associated with a discontinuity of the average Froude number of a step-pool unit as well as drops in pressures and water depth. Relative instantaneous drag forces reached a maximum for this regime change. This implies that the regime change from nappe to transition flow is a critical loading case with respect to the stability of the steps. On the other hand, the mean drag forces showed little systematic variation as the flow rate changed with a large spread in results for measurements repeated at the same flow. Thus, for the nappe and transition flow regimes, the mean drag force is not a good criterion to use to predict step stability. The occurrence of the different flow regimes could be related to the average Froude number of a step-pool unit. The results have practical implications for the design of step-pool systems.

Liquid column separation (LCS) in pressurized pipelines may occur if a water hammer event drops the local pressure to the liquid’s vapor point. Numerical simulations of LCS have traditionally been based on one-dimensional (1D) transient flow theory; here, a two-dimensional (2D) computational fluid dynamics (CFD) model is used to investigate the complicated nature of LCS and to help characterize the limitations of the traditional 1D models. To this end, the Schnerr–Sauer cavitation model with a shear-stress transport (SST) $k-\omega$ turbulence model is employed, whereas the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are solved for the mixture of liquid and vapor. 2D model results are compared to both experimental data and to those of the 1D discrete vapor-cavity model (DVCM), thus demonstrating that the 2D method effectively simulates the pressure variations while helping to visualize the associated physical processes. More specifically, the 2D simulations vividly reveal the growth and the collapse of the cavity, including the formation of an intermediate cavity and both the location and shape of the region undergoing distributed vaporous cavitation.

To alleviate problems caused by reservoir sedimentation, the hydrosuction removal of sediment deposits is an attractive option. In this technique, pumping or siphoning is used to draw water and sediment into the inlet of a pipe and convey the slurry to settling ponds. A key performance target is then to maximize the delivered sediment concentration. For fine sediment (silt and clay), deposit behavior largely depends on its degree of consolidation. To investigate how this parameter affects hydrosuction performance, a series of laboratory experiments were conducted using a small-scale suction pipe equipped with an online densimeter to measure the time evolution of the outflow sediment concentration. To help interpret the resulting signals, the evolving shape of the deposit surface near the pipe inlet was recorded. Measurements were then compared with predictions from inviscid selective withdrawal theory. Depending on the duration of self-weight consolidation prior to hydrosuction, a transition was observed between two highly contrasted sediment responses. Up to a degree of consolidation of approximately 90%, loose deposits respond much like a dense inviscid layer. Beyond this degree, consolidated deposits resist withdrawal, causing rapid freshwater breakthrough and an abrupt drop in hydrosuction performance.

A physical model study was conducted to investigate the mechanisms of air entrainment in a tall plunging dropshaft of 7.7 m in height. The water flow in the dropshaft was observed to break up into small drops of approximately a few millimeters diameter at a drop height of 5 m. The dominant water drops were found to be approximately 2 mm and they fell at an average speed of approximately $6\mathrm{m}/\mathrm{s}$. An analytical model was developed to predict the amount of air drag and air flow needed on the basis of the momentum transfer from the water drops. The drag force induced a vertical air pressure gradient, and the model prediction compared well with the measurements. The entrained air flow rate increased with the water flow rate, but the ratio of the air to water flow rate decreased from approximately 20 to 5 times when the water flow rate increased from 3.9 to $47.7\mathrm{L}/\mathrm{s}$. The air pressure was negative at the top of the dropshaft to allow the ambient air to be entrained, and it increased to the atmospheric pressure at the bottom at which point the outlet was open to the atmosphere. Additional experiments also were conducted with a weir of two different heights at the outlet pipe. At a large water flow rate, the weir would cause a significant blockage in air passage, resulting in a reduced air entrainment.

Prediction of annual changes in braided-river morphology is important to assess the performance of river-training works and channel navigability. A two-dimensional (2D) depth-averaged numerical model was used for a 12-km-long reach of the Brahmaputra River in India. Major issues addressed in the modeling included approximation of boundary conditions, design of a flood hydrograph, and evaluation of river morphology. Simulated flow depth and velocity were calibrated with the observed data under bankfull discharge conditions, and predicted bed levels were validated with observed bed levels during low-flow conditions. The model predictions of thalweg shifting for two consecutive flood seasons agreed well with observed changes obtained from satellite imagery. The calibrated model was subsequently used to assess morphological changes in different groyne fields. The best groyne field was determined based on overall performance in scouring, deposition, channel alignment, and dredging volume.

At base-flow conditions, ephemeral streams with mobile beds may inherit bed topography caused by previous higher flow events, resulting in bed topography that is not in equilibrium with flow conditions. Major flow events can generate large bedforms that result in hydrodynamics and sediment transport that are not in equilibrium with subsequent lower flow conditions. To simulate a large storm runoff–driven stream flow followed by lower base-flow conditions, changes in sand bed topography and sand transport caused by a simultaneous reduction in discharge and flow depth in a laboratory flume with an equilibrium sand bed and steady, approximately uniform, flow were investigated. Bed topography was evaluated using second-order structure functions, semivariograms, and statistical analyses. Sediment transport responded rapidly to a near-instantaneous negative step in mean flow depth and discharge. The relative contributions of reduced bedform height and celerity to the decreased transport rate are quantified, and bedform celerity was the primary contributor to reductions in transport rate. To evaluate the effect of initial bed topography on equilibrium topography, a flat bed initial condition was subjected to the same flow as was applied after the negative step. The equilibrium bed topography for the flat bed initial condition was similar to the topography that was formed after the negative step. Semivariogram analysis provided a reliable estimate of mean bedform height without the need for detecting individual bedforms. Total sediment load following a negative step in discharge and flow depth followed an exponential decay model that resulted from the combined exponential decay of bedform celerity and bedform height.

Considering the limitations of unit discharge for stepped chutes, a ski-jump-step energy dissipater, which is a new type of energy dissipater, is first proposed for increasing the maximum available unit discharges. In this system, preaerations are achieved through jets generated by ski-jumps and further aerations are achieved via aeration basins. It provides fully aerated flows for the stepped chutes with large unit discharges, and the problems caused by high-speed flows like energy dissipation, cavitation, and atomization are well solved. Experimental investigations about hydraulic characteristics of the ski-jump-step energy dissipater were conducted, including the flow regimes, the principle and effectiveness of energy dissipation and the characteristics of the air entrainment and the pressure distribution. Even under conditions of large unit discharges ($q=118.00{\mathrm{m}}^2/\mathrm{s}$ on the prototype), results obtained demonstrate energy dissipation as high as 75.8% and air concentrations of at least 3.6% along the whole stepped chute. The method developed herein provides a new approach for energy dissipation in stepped chutes at large discharges.

Our note presents laboratory findings on the breaching of the earthfill embankments of bridge abutments at river crossings and the consequent scour at exposed abutment columns, the structural element supporting the end span of a bridge. Current United States design guidelines, based on Load and Resistance Factor Design, expressly recommend that abutment columns withstand scour if exposed during embankment breaching. The findings describe how breaching occurs for two abutment forms (spill-through and wingwall), and how pier-like scour may occur at their exposed columns. The findings include data regarding the time to breach, scour depths at breached abutments, and have significance for designing scour countermeasures.

As the flow expands at expansions, generation of turbulence by shear layers results in the dissipation of energy of the mean flow. Based on simplified momentum equations, this head loss was previously estimated to be comparable to the velocity head of the incoming flow. In this paper, an approach based on turbulent jet theories is used to more accurately evaluate head loss at outlets. This approach is based on the similarity between the generation of a shear layer at an outlet to that of a turbulent jet at its origin. A general equation is derived for the variation of the kinetic energy of the flow from outlets of any geometry and the corresponding head loss coefficient. The equation is simplified for rectangular outlets, which are more common in practice.

Untreated water can intrude into water distribution systems through pipe cracks and/or openings when negative pressure events occur inside drinking water pipelines. It is important to determine the magnitude of the possible intrusion event. To estimate the intrusion flow rate more accurately, the effects of surrounding soil must be considered. In this paper, an analytical relationship is derived by combining the seepage in soil and the flow through an orifice for predicting the intrusion flow rate for a circular orifice under steady-state conditions. The effect of the pipeline on reducing the flow area is incorporated in the model. An experimental study that simulates a buried pipeline was carried out to validate the accuracy of the improved model, and the analytical results fit well with the experimental results.

Laboratory experiments were conducted in order to investigate the effectiveness of a new scour countermeasure consisting of a horizontal extension plate (EP) downstream of a gated spillway-stilling basin structure. These experiments were performed on a 1:30 scale model of one of the South Florida Water Management District (SFWMD) gated spillway structures, the S65E gated spillway, because it has experienced worrisome downstream scour. Currently, riprap is the only scour countermeasure documented for these structures. EPs of various lengths were installed downstream of the stilling basin, and the equilibrium scour was measured. It was found that a plate length of approximately 1.88 times the exiting flow depth was the shortest length that resulted in significant scour reduction and downstream scour translation. Measured flow patterns reveal that the EP altered the flow patterns by transferring the plunging flow downstream and eliminating the near-bed reattachment and surface recirculation, possibly due to a decreased interaction between the plunging flow and the stilling basin face.

An experimental study on the breakup of a turbulent round-water jet in still air with a nozzle Reynolds number of 145,600 and Weber number of 10,400 is reported. The visual structure of the falling water jet was recorded by a high-speed camera, and the characteristics of the falling jet were investigated. As the jet travelled from the nozzle, initial surface disturbances grew, and the lateral oscillation of the jet surface was amplified. The jet thickness initially increased and then decreased because of the combined effects of lateral turbulence fluctuation and gravitational acceleration. The amplitude of the surface disturbance grew in an exponential form. Based on the averaged transverse-water distribution, the water jet spreading rate was found to vary between 0.5 and 1.8%, and the decay of the jet water core had an average value of 0.7%. The onset of jet breakup was found at a distance of approximately 100 times of the nozzle diameter. A theoretical model was developed for predicting the onset of jet breakup by comparing the dynamic air pressure with the restraining surface tension pressure. The velocity of the water drops released after breakup was measured and found to be approximately 0.8 times of the local jet velocity.