Journal of Hydraulic Engineering

Combining sewer junctions with a lateral inflow at $90°$ angle are commonly used in our sewer systems. A computational fluid dynamics (CFD) model based on Ansys CFX 10.0 was established to simulate fully surcharged flow at a $90°$ combining sewer junction. The model was carefully assessed by comparing its results with the measurements of detailed physical experiments. Good agreement was obtained between results of the computational model and of the laboratory experiments. The computational model was proved to be capable of simulating surcharged combining junction flow in the aspects of water depth, energy losses, velocity distributions, and turbulence. The verified CFD model was also used to investigate air entrainment and effects of the size of the junction chamber on the flow. Such CFD models can be used to optimize the design of sewer junctions and will also be useful in studying sediment transport at sewer junctions.

This paper describes the flow structures that occur in a 90° junction of four open channels with supercritical flow in two orthogonal inlet channels. An experimental facility was constructed to permit the measurement of flow rates, water depths, and the positions of hydraulic jumps in the channels. The various flow patterns which appear depend on the characteristics of the incoming flows and can be classified into three main types, depending on the location and shape of the hydraulic jumps that develop. These jumps can either be normal to the flow and located in the upstream channels, or can be oblique and confined within the junction. The explanation for the existence of various flow patterns is derived from previous studies of the rapid deflection of supercritical flow. A detailed description of each flow regime is provided, with information on the surface elevations, the behavior of the hydraulic jumps, the deflections of the incoming flows, and the formation and characteristics of recirculation zones.

Godunov-based shallow-water models utilize a discontinuous reconstruction of data at cell faces even for smooth flow, which can cause energy dissipation and degrade accuracy. Analysis of discrete equations shows that jumps (and therefore error) can be minimized by adaptively selecting either primitive or conservative variables for slope limiting and reconstruction according to the local Froude number. Therefore, a Godunov-based model with an adaptive scheme of slope limiting and variable reconstruction is presented. Two practical flood modeling applications are used to compare the performance of the adaptive scheme against two nonadaptive schemes. In addition, performance of second-order accurate schemes is compared to first-order schemes that utilize a second-order accurate description of terrain. Results show that the first-order adaptive scheme possesses the best combination of robustness, efficiency, and accuracy of the models tested.

This study presents a novel, simple, but rather accurate approximation of the eigenvalues of the system formed by the Saint-Venant–Exner equations, based on the comparison between eigenvalues for the complete system and eigenvalues for the water phase only. Moreover, a strategy is proposed to compute efficiently the intercell fluxes by properly adapting a Harten, Lax, and van Leer scheme for each equation. Two examples of transient transcritical flows are developed: the erosive migration of a knickpoint induced by an increase in the bed slope, and the evolution of a hydraulic jump over a mobile bed.

A numerical model is proposed to compute one-dimensional open channel flows in natural streams involving steep, nonrectangular, and nonprismatic channels and including subcritical, supercritical, and transcritical flows. The Saint-Venant equations, written in a conservative form, are solved by employing a predictor-corrector finite volume method. A recently proposed reformulation of the source terms related to the channel topography allows the mass and momentum fluxes to be precisely balanced. Conceptually and algorithmically simple, the present model requires neither the solution of the Riemann problem at each cell interface nor any special additional correction to capture discontinuities in the solution such as artificial viscosity or shock-capturing techniques. The resulting scheme has been extensively tested under steady and unsteady flow conditions by reproducing various open channel geometries, both ideal and real, with nonuniform grids and without any interpolation of topographic survey data. The proposed model provides a versatile, stable, and robust tool for simulating transcritical sections and conserving mass.

A robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors is presented. The physically based model simulates the three main processes that shape incised streams: hydraulics, sediment transport, and streambed and bank adjustments. A generalized implicit Preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals. The solution method introduces several enhancements that increase its robustness, specifically to simulate flashy flows. Transport of cohesive or cohesionless graded bed material is based on a total-load concept, and suspended and bed load transport modes are accounted for through nonequilibrium effects. The model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material. The present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds, and reports simulations of published experiments on degrading and aggrading channels with graded bed material. Description and validation of the model’s streambank erosion component and the application of the model to incised stream systems are presented elsewhere.

This paper focuses on an experimental study of the two-phase flow downstream of a laboratory model fish bypass. Experiments were performed on a 1:24 scale laboratory model of a fish bypass under consideration for construction at Wanapum Dam, on the Columbia River in Washington. The model was operated at the design condition of skimming flow regime and at the possible off-design plunging and surface jump regimes. Void fraction data were collected using an optical phase detection probe on a three-dimensional grid, and the phase indicator function was recorded at selected locations. It was found that in the laboratory model, the skimming flow regime effectively prevents bubbles from reaching deep into the tailrace, resulting in a considerably lower void fraction than plunging and surface jump regimes. For this geometry, the surface jump regime entrains air deeper than the plunging regime. To observe trends, the instantaneous source of total dissolved gas was estimated for the three regimes using the model data and several simplifying assumptions. Time distributions of the indicator function are also reported.

We present a generalized integral model for multiphase plumes in stratified ambient conditions based on the double-plume approach, where the plume is composed of a rising, multiphase core plume surrounded by a counterflowing outer ring plume of dense fluid. The generalized model captures as limiting cases the current approaches in the literature, including two-fluid and mixed-fluid equations, continuous and discrete detrainment, dispersed-phase mass transfer, and two models for entrainment in the counterflow region. These modeling approaches are compared and validated against both laboratory and field-scale data. In unstratified conditions, all model formulations perform equally well. In stratification, entrainment in the counterflow region is best represented by correlation to the inner plume velocity instead of the difference between the inner and outer plume velocities. The vertical distribution of the exchange between the inner and outer plumes in the models differs from that measured in the prototype due to enhanced entrainment at the detrainment zone and forced entrainment from the collapsing intrusion layer. Nonetheless, the models predict well the length scales and volume fluxes at the detrainment zone and intrusion layer. Applications are demonstrated for reservoir air bubble plumes. The mass transfer and near-field mixing in the double-plume integral model prove sufficiently accurate to predict the depth of maximum plume rise (both the locations of total dissolution of the bubbles and the maximum height of the decelerating plume) and the volume flux, dissolved constituent mass flux, and trap height of the intrusion.

Flow in discrete networks can be observed in biological, geological, or technical systems. Often, the number of individual segments is very large. Therefore, discrete pressure and flow calculation in each single segment becomes very time consuming, and a continuum model becomes attractive. We apply a global upscaling method based on a spatial average to investigate a porous media network model with heterogeneous pore length distribution. For this purpose, the porous media was modeled by triangular networks. For such networks, we characterize the representative elementary volume (REV) size and show that using window sizes smaller than the REV requires an heterogeneous Darcian description. We find that the permeability has to be distributed according to a log-Gaussian distribution with variance and correlation length depending on the window size and the porous microstructure. Finally, we apply the procedure to an anisotropic regular network for which an analytical solution can be found and show that using this method leads to the correct analytical behavior.

Seepage erosion phenomena can arise when particles of soil are transported underneath any hydraulic structure by the seepage flow, as a consequence of the difference in the surface elevation upstream and downstream of the hydraulic structure. To assess the stability of hydraulic structures against seepage erosion computational models are available varying from simple empirical rules to advanced models. In this paper, the safety coefficient is assessed by comparing the exit gradient with the critical gradient: the former represents the hydraulic gradient at the exit point along the downstream ground surface, whereas the latter is the gradient at which upward drag forces on the soil particles equal the submerged weight of the soil particles and a quicksand condition originates. An experimental research was carried out to analyze different thickness sheet piles embedded in sandy soils, resulting in agreement between experimental data and theoretical values if the thickness of the sheet pile and the ground surface deformation are properly taken into account.

Laboratory tests using a large-scale model of a spill-through bridge abutment led to important findings about the performance of a riprap apron as an abutment scour countermeasure. Riprap stone is widely used for protecting side slopes of embankments against erosion, and several design guidelines are available in the literature. In contrast, only a few guidelines exist for the design of a riprap apron around an abutment. These guidelines focus only on the armoring effect of riprap, neglecting other effects. This study shows that apron performance involves several mechanisms: armoring the bed, dissipating large-scale turbulence shed from the abutment, reducing the peak unit discharge, reducing the average shear stress, and shifting the scour region away from the abutment. Together, these mechanisms substantially reduce the maximum scour depth. The test findings are compared with those from a much smaller model of riprap-apron performance.

An understanding of bridge scour mechanisms during floods in a fluvial river is very important for cost-effective bridge foundation design. Reliable bridge scour data for flood events are limited. In this study, field experiments were performed at the Si-Lo Bridge in the lower Cho-Shui River, the longest river in Taiwan, to collect scour-depth data using a sliding magnetic collar, a steel rod, and a numbered-brick column. By separating each scour component, a methodology for simulating the temporal variations of the total scour depth under unsteady flow conditions is proposed. The proposed total-scour model integrates three scour components, namely general scour, contraction scour, and local scour. The collected field data, comprising both general scour and total scour depths, are used to validate the applicability of the proposed model. Based on the peak flow discharges during floods, a comparison of the local scour depths calculated using several commonly used equilibrium local scour formulas indicates that most equations may overestimate the local scour depth.

A new decomposition concept of the network graph according to its connectivity properties is introduced in this paper that allows various applications in the field of systems analysis of water supply networks. In this case, the network graph consists of two main components. The outer branched component of the network is called a forest. The inner complement is denoted as the core of the network. A forest consists of a number of trees, while the core network is composed of distinct biconnected blocks that are connected by bridges. Forest and core components of the network overlap at the so-called root nodes. Calculations for the simulation or the optimization of the single components can be done, independently, thus decreasing matrix sizes and calculation time. Control actions that are necessary to efficiently operate a water supply system result in changes of the connectivity of the network graph. By using the proposed decomposition approach the in-time identification of the different supply areas according to the actual state of the network is straightforward. It also includes measures of network vulnerability and several stages of network simplification that are able to enhance the understanding of the single network components and their interaction. The simplified network model can be applied, for instance, to the development of strategies for efficient operation and control of a water distribution network.

Ambient flow field and circulation patterns induced by circular bubble plumes and water jets in tanks of different sizes were studied in rectangular and square water tanks. A nonstationary nature of the flow was observed in all experiments and its dominant oscillation frequency was found to directly relate to the tank size. The flow circulation patterns were similar for bubble plumes and water jets, but changed significantly with tank size and geometry. Strong three-dimensional effects were observed in a rectangular tank, resulting in flow entraining in the longer plane and flow detraining in the shorter plane, especially for the bubble plume tests. A relationship was developed to relate the tank size to the patterns of circulation cells. Nearly isotropic turbulent flow conditions were obtained in all experiments, but the effect of tank size and geometry on the magnitude of the turbulent stresses was more pronounced in the bubble plume tests.

An economical methodology is proposed by which distinct air bubbles released at the bottom of a channel may be utilized for determining the local flow discharge $q$ per unit width. Simple theoretical analysis shows that $q$ is linearly dependent on the rise length $L$ of bubbles released at the bottom. This length is the horizontal displacement of the bubbles between the release cross section and the cross section where they emerge. The theoretical findings were compared with measurements in three laboratory flumes and in an irrigation canal. Based on the above, a relationship between $L$ and $q$ has been established. The empirically proposed relationship is very useful for fast discharge measurements in channels and natural streams.

The relation between the equivalent roughness and different grain size percentiles of the sediment in gravel-bed rivers was determined under the hypothesis that the vertical distribution of the flow velocity follows a logarithmic law. A set of 954 data points was selected from rivers with gravel size sediment or larger, with a nonsinuous alignment and free of vegetation or obstacles. According to the results, the $k_s$ roughness is equivalent to approximately $2.4D_{90}$ , $2.8D_{84}$ , and $6.1D_{50}$ . No correlation was detected between the sediment sorting and the sediment mobility index on one hand and, on the other, the coefficient of proportionality of each grain size percentile.

The hydraulic performance of pipelines can be significantly affected by the presence of biological growth on internal surfaces. The change in wall roughness brought about by the biofilms has been studied by the use of headloss tests, precleaning and postcleaning of the pipelines in three Tasmanian hydroelectric schemes. Results of the headloss testing show that improvements to hydraulic efficiency can be achieved from the cleaning of biofouling material. The data, when plotted as a Moody diagram, shows that the friction law for conduits roughened by biological growths may not always follow a Colebrook-White type relationship, although the results are too narrow in Reynolds number to be conclusive. It was found that bacteria made up the majority of the biofilm biomass in the pipelines studied. Based on molecular analysis, members of the class Alphaproteobacteria were the most frequently detected followed by members of the phylum Chloroflexi.

The analysis of the dynamic response of a pressurized water pipeline system is important for the design and also the integrity monitoring of these systems. An efficient method for summarizing the behavior of a pipeline system is through the determination of their system response functions. These functions can be extracted by injecting a pressure signal with a wide bandwidth that persists over the length of a pipeline system. Unlike electrical and mechanical systems, generating such signals in pressurized water systems is difficult. Valves capable of generating a signal against the system back-pressure often lack the necessary maneuverability to ensure the signal is sharp (and hence with high-frequency content) and the generated transient is often large in amplitude, risking damage to the system. A method for generating a small amplitude transient signal with a wide band of frequencies is desirable. This paper presents the design for a side discharge valve for generating a pseudorandom binary sequence of pressure changes that are of a small magnitude in relation to the steady state head of the pipeline. The pseudorandom pressure sequence is used to provide an estimate of the system response function. The continuous form of the signal allows the amplitude of each individual pulse within the signal to be small while maintaining the same signal bandwidth. The valve has been tested experimentally and was found to provide a good match with the theoretical response of the pipeline. The method provides a practical alternative to frequency sweeping using sinusoidal signals or sharp valve closures for the extraction of the response functions. Once determined, the system response function can be utilized to detect system faults such as leaks and blockages.