Journal of Hydraulic Engineering

This paper investigates the lateral outflow over a short-crested side weir inserted in a converging rectangular channel with linearly decreasing width in the flow direction. This type of overflow device induces a spatially varied free-surface flow with decreasing discharge and, because of the convergent geometry, it can potentially improve the performance of the distribution channels commonly used in irrigation systems, sewer networks, wastewater plants, and hydropower facilities for flow diversion and control and for flood protection. A laboratory investigation is carried out to analyze the main hydraulic characteristics of this kind of flow and assess the improvement in outflow efficiency compared to the conventional rectangular side weir in a prismatic channel. Only subcritical steady flow conditions are considered. Experimental data show that the Froude number in the main channel downstream of the weir and the dimensionless weir height are significant variables to describe the outflow efficiency, and they confirm that the nonprismaticity of the main channel induces a noticeable increase in outflow performance. Furthermore, the experimental results show that the assumption of constant specific energy usually adopted in one-dimensional side weir flow modeling is reasonably valid, despite the considerable increase in the velocity-head and momentum correction coefficients along the side weir. Finally, the comparison between experimental data and numerical results shows that the classic one-dimensional modeling approach can adequately predict the main engineering aspects of side weir flows in nonprismatic channels.

Successful sediment management of streams and rivers downstream of dams requires a method for predicting sand transport across a range of conditions that can include sand transported below the top of an armored gravel layer through complete burial of the coarse substrate. Previous studies have shown that the elevation of sand relative to gravel along with shear stress at the elevation of the sand are needed to successfully predict sand transport. Experiments were conducted in a laboratory flume channel with increasing sand elevations to document the transition from supply-limited sand transport over gravel to capacity-sand transport. A modified Engelund-Hansen sand transport relation was developed by adding terms to scale the fraction of the bed covered with sand and to reduce the bed shear stress acting on the sand. This modified relation yielded good predictions for the range of conditions from very low rates of sand transport over an immobile gravel bed to 100% sand bed conditions for the data from this and two previous studies.

A supercritical open-channel flow can skirt an emerged obstacle by using two distinct forms of workaround: a detached hydraulic jump or a so-called wall-jet-like bow wave. These two forms stem from the properties of supercritical flow and are described in detail. Experiments assess the conditions of appearance of one form or the other, depending on both upstream Froude number and flow-depth to obstacle-width ratio. A conceptual model, based on mass conservation, reproduces and explains the corresponding transition. For the wall-jet-like bow wave, additional information is given regarding water-depth oscillations; the associated Strouhal number show they are caused by reverse spillage on the obstacle face. Implications of the present results on scouring and forces exerted by the flow on structures justify future works on the subject.

This study experimentally investigates the temporal and spatial development of the scour hole induced by a confined propeller jet in the presence of an open type quay. The longitudinal distance between the propeller face and toe of the quay slope, i.e., toe clearance, was systematically varied to examine its influence on the scour hole development under different test conditions. The scour depth along the centerline of the propeller axis was measured at predetermined time intervals and the three-dimensional scour profile was obtained at the end of each test when the asymptotic state was reached. The asymptotic centerline profiles at varying toe clearances, which were compared with published unconfined propeller scour data, show that evolution of the scour hole is significantly influenced by the toe clearance. The asymptotic scour profiles that formed at different toe clearances were classified into three types with reference to the toe clearance field in the vicinity of the slope toe, namely the near toe clearance, intermediate toe clearance and far toe clearance fields. Two scouring mechanisms, namely jet diffusion and quay obstruction, were found to govern the scouring process and dictate whether the scour hole forms in any one of these three fields. The influence of these two mechanisms on the development of the scour characteristic dimensions in each field were also investigated based on which quantitative demarcations among the fields were obtained.

This paper presents the results of laboratory experiments conducted to study the effects of varying ambient stratification strengths, characterized by the buoyancy frequency $N$, on the dynamics of a buoyant plume propagating in a linearly stratified environment. The buoyancy frequency was varied between $N=0.37–1.17{\mathrm{s}}^{-1}$, while the source velocity was constant at $W_0=26\mathrm{cm}{\mathrm{s}}^{-1}$, giving a source bulk Richardson number of ${\mathsf{Ri}}_b=(\pi /4{)}^{0.25}\sqrt{g^{\prime }{d}_j/W_0^2}=0.09–0.31$. Bulk parameters such as maximum height, $Z_m$, spreading height, $Z_s$, and radial propagation of the plume, $R_f$, were measured to characterize the plume and understand the flow dynamics. Experimentally, it was observed that the maximum height and spreading height of a plume decreases with increasing $N$, a trend attributed to the reduction in the momentum and buoyancy flux due to the vigorous entrainment between the plume and ambient fluid. The radial propagation of the plume, $R_f$ was insensitive to the variations in $N$ and $R_f\propto t^{0.75}$ for all values of $N$. An empirical parameterization involving ${\mathsf{Ri}}_b$ and $N$ was developed that gives the characteristic time for a plume to reach a steady-state spreading and maximum height. Further, the transient density profiles show that the mixing-layer thickness decreases with increasing $N$, thereby rendering the entrainment to occur in a shallow layer of fluid. Using empirical relations, it is shown that the entrainment coefficient is larger for higher values of $N$, thus becoming a function of the Richardson number. The results are expected to be useful in understanding the mixing and convective process in various environmental flows.

The radial distributions of velocity components need to be resolved in quasi-two-dimensional laminar water hammer problems. In a collocation spectra method, the radial distributions are approximated with Chebyshev expansions and the equations are assumed valid at the collocation points. The traditional collocation method requires an equal number of equations and unknown expansion coefficients, which is sometimes difficult to implement. The proposed model adopts extra collocation points to provide extra equations for expansion coefficients to construct an overdetermined system. Singular value decomposition is used to solve the overdetermined system. In the new method, the boundary conditions can be naturally incorporated into the system. However, the accuracy of the boundary condition equation is not acceptable because of least-squares approximation. Large multipliers are introduced to enhance the accuracy of the boundary condition equations. Spatial variation in the axial direction and time advancement are treated using the method of characteristics.

Unsteady friction and its modeling have been widely studied in transient pipe flows for its influences and modification effect on pressure waves. Such a feature is of great importance particularly in pipe systems where the extreme pressure values are due to the overlapping of pressure waves generated in different sections. This paper investigates the relevance of unsteady friction term by considering different models available in the literature. In particular, attention is focused on the following two commonly used one-dimensional (1D) models: the weighting function-based (WFB) model and the instantaneous acceleration-based (IAB) model. The investigation is executed using laboratory experiments and field tests as well as 1D/2D numerical simulations in simple pipeline systems. Realistic ranges of both initial (pretransient) conditions and geometrical characteristics are considered. The data collected in experimental tests are first used to calibrate the unsteady friction models under investigation. The validated models are then applied to identify their respective domains of applicability and the limitations. The differences between models and data are measured using the local transient analysis (LTA) norm and the integral total energy (ITE) norm along the pipeline, and the reasons for such discrepancies are explored in the paper. The practical implications of the use and improvements of different unsteady friction models for transient pipe flow simulations are discussed in the paper.

The near-field flow and mixing properties of vertical buoyant jets subjected to lateral confinement are studied numerically for different cases, including different confinement indexes and jet densimetric Froude numbers. The performances of different turbulence models are investigated, such as the standard $k\text{-}ϵ$ turbulence model and buoyancy-modified $k\text{-}ϵ$ model. The modeled results are compared to previous and present experimental observations. The present paper confirms that the universally accepted model ($k\text{-}ϵ$ turbulence model) can be satisfactorily accurate, eliminating the need for an advanced modeling approach, as long as suitable modifications are performed. In contrast to previous studies, which used one single and constant value of $P_{tr}$ and $\mathsf{P}\mathsf{r}$ numbers, the present study links these two numbers to the $\mathsf{F}$ number, which is more practical and can produce very good results. This study also makes it possible to roughly quantify the rate at which the jet concentration spread width grows and identify the location where impingement occurs, which enables engineers or researchers to perform a quick estimation of the evolution and profile of a laterally confined vertical buoyant jet.

The pier scour caused by flood waves is analyzed, introducing the dimensionless effective work by the flow on the sediment bed around the pier, $W^{*}$. The three-parameter exponential function is adopted to relate the normalized scour depth $Z^{*}$ with $W^{*}$. A novel experimental installation able to reproduce any hydrograph with high precision in the laboratory flume is described and used to carry out four series of scour experiments in order to calibrate and validate the proposed relation. The first series consists of experiments with constant discharge until advanced stages of scour. The second and third series of experiments use single flood waves of different shapes and durations, respectively. The fourth series consists of scour experiments caused by more realistic flow hydrographs with multiple peaks. Results show that the relation between $W^{*}$ and $Z^{*}$ is unique and thus $W^{*}$ represents a reliable concept for the prediction of the flood wave scour because it appropriately integrates the effects of the hydrograph properties, duration, peak discharge, and shape, on scour. The proposed relation allows a straightforward prediction of maximum scour depth after a flood wave with high precision. A good agreement between measured and computed scour was observed in all cases.

Erosion of a saturated sand bed by a vertically upward water jet was studied experimentally. Direct observations on the behavior of sand-bed erosion were obtained by increasing the flow rate at a small increment until fluidization developed to the surface of the sand layer. With the increase in flow rate, the sand bed underwent different stages: no cavity, a stable cavity above the jet, an unstable cavity propagating upward, and full fluidization. The mechanism for the cavity evolution was investigated, and an analytical model was developed to predict the critical flow rate for full fluidization. The model predictions compared well with the measurements in the experiments with different sand-layer thicknesses.

As sewer pipes age, groundwater and the surrounding soil can enter into the sewer system through various defects on the pipe wall. Groundwater infiltration and soil erosion through cracked sewer pipes can result in increased sewage loading and cause sinkhole problems. A reliable model is needed to assess the soil and water erosion rates. By incorporating the related resistances during the erosion process, this paper presents improved expressions for calculating soil and groundwater erosion rates through the crack into a sewer pipe. Good agreement between experimental and predicted results on soil erosion rate is obtained. The analytical models indicate that the soil and water erosion rates are mainly controlled by the following factors: groundwater table or saturated soil height above the crack, particle size, particle shape, submerged repose angle, void ratio, and defect size.