Journal of Hydraulic Engineering

The numerical study in this paper aims at obtaining exact well-balance and improving the computational accuracy, efficiency, and the performance at wet–dry fronts, especially for the flows in ventilated sewer pipes. To this end, a novel numerical model for mixed flows in a circular-shaped sewer is proposed. The major contributions of this study are: (1) a MUSCL scheme (monotonic upstream-centered scheme for conservation laws) is designed so that second-order accuracy is achieved, which leads to accurate solutions even over coarse meshes; (2) a special reconstruction technique and novel source term discretization are proposed for pipe flows to guarantee the exact well-balance in the framework of a MUSCL scheme, thus numerical oscillations at stationary steady states are avoided; (3) the proposed new scheme produces reasonable solutions for kinetic flows in underresolved finite-volume grids with very low water volumes; and (4) it guarantees the positivity of the computed water volume without forcing a minimum wetted area value or reducing the global time step size. The performance and superiorities of the proposed new scheme for sewer flows are verified against previously reported well-balanced numerical models on a number of experiments.

The effects of the free surface on the behavior of inclined dense jets in shallow ambient were investigated in detail. The hybrid turbulence model scale-adaptive simulation (SAS), coupled with the volume of fluid (VOF) multiphase model, was used to detect the complex interaction of the jet and the free surface as well as displacements of the free surface and induced waves. The simulation results had reasonable accuracy compared with experimental data for dilution, geometrical characteristics, and complex structures of the jet, except the trend of return point dilution, which the increase of Froude number results in fluctuations in it. Moreover, it was shown that the dilution and geometrical quantities have the same trend whether the Froude number increases in the constant depth, or the depth level decreases in the constant Froude number, except the return point dilution, which is more sensitive to depth than to Froude number variation. The generation of waves due to the impingement of the jet on the free surface leads to more detrainment, and causes the emergence of an area with low dilution value beneath the jet on the bed before the return point. In addition, analyzing the three-dimensional (3D) concentration contours demonstrated that in shallower depths, the so-called lateral plumes impact the bed over a wider area. Therefore, based on the effects of waves and lateral plumes, the so-called return plate seems to give a better understanding of the environmental impacts of the jet on the bed than does the traditional return point.

The effect of the intense transport of lightweight sediment without turbulent suspension (Shields numbers of mixture flow from 0.34 to 1.80) on resistance and surface elevation of water flow in an open channel is experimentally examined. Flows over fixed and mobile plane beds, at the same water flowrate and bed slope, are compared. Detailed measurements—including distributions of solids and velocity—permit to identify flow interfaces and determine the role of sediment transport on flow resistance. Two friction factors for mobile bed flows are introduced: one for water only and another one for the mixture. The first seems to be insensitive to the water submergence, while the latter follows the classic formula for the hydraulically rough regime: both increase with respect to the equivalent clear water flow. The water surface elevation seems unaltered by the presence of sediment transport, thus reducing the need for the modelling of the evolution of an erodible bed surface in, at least, some routine applications in fluvial hydraulics.

The transient behavior of pipe systems is typically simulated using water hammer models [such as the method of characteristics (MOC)], or rigid water column (RWC) models depending on whether the hydraulic transition is fast or gradual. In this paper, an elastic water column (EWC) model for analyzing hydraulic transients in pipe networks is formulated using a novel graph-theoretic approach. The new method of modeling a network with a state-space representation inherits the advantages of the RWC model, such as its high computational efficiency and potential to integrate with modern control theory and signal analysis algorithms. Meanwhile, the proposed method incorporates water compressibility and is therefore significantly more accurate than the standard RWC models, and is shown to be equivalent to MOC models below a critical frequency. Another advantage of the new model is its elegantly simple formulation for an arbitrarily configured pipe network. The accuracy of the model was validated numerically on 6- and 51-pipe networks. The simulated results of the 51-pipe network demonstrate that the transient pressures in a large-scale pipe network dominate in the low-frequency range where the EWC model has high accuracy. These results demonstrate the utility of the proposed method to provide a flexible solution to optimize accuracy and efficiency for simulating hydraulic transient events in pipeline networks. The EWC model has great potential to be combined with other control and signal analysis techniques because of its state-space representation of a water network.

A physical model study was conducted on jet impingement in a plunging dropshaft with different drop heights of 3.38 and 1.88 m. Experimental observations show that an upward splash, annular flow, and downward bounced jet could be formed when the approaching jet hits the opposite shaft wall. The impinging pressure on the shaft wall was mainly dominated by the incoming flow velocity, and the mean pressure on the shaft wall can be reasonably well-predicted when considering the bounced flow. The pulsation of the impinging pressure on the shaft wall was significant, with the maximum pressure being approximately 1.4–3.8 times the mean value. The impinging pressure at the shaft bottom was primarily determined by the velocity at which the falling jet reached the pool. The mean value can be well estimated using the annular flow and bounced jet assumption. The pulsation intensity increased with an increase in drop height or flow rate, which could be alleviated by a plunging pool with a larger depth. However, for flows in dropshafts under study, the impinging pressure and the hydrostatic pressure of the cushion layer were within the same order of magnitude; thus, the increase of the pool depth leads to a reduction of the energy-dissipation rate.

Supersaturated total dissolved gases (TDGs) generated by dams can cause gas bubble trauma and mortality in fish in downstream waters. In this study, we evaluated air entrainment and generation of supersaturated TDG at two submerged low-level outlets (LLOs) at Hugh L. Keenleyside Dam, British Columbia, Canada. Specifically, we determined how air entrainment (less than 1% volume fraction) through the gate well, discharge level through the outlets, and geometry of the stilling basins at the south and north ends of the dam influenced supersaturated TDG generation. A mathematical formulation was developed, incorporating physical processes including air entrainment, bubble breakup, and gas transfer. Numerical modeling was also adopted to validate turbulence and flowfield downstream of the submerged low-level outlets. Despite being 8-m shallower, significantly higher TDG levels were measured in the stilling basin of the south LLO (about 120%) compared to that of the north LLO ($\le 110\%$). Results show that turbulence in the stilling basin can produce smaller bubbles and increase the mass transfer coefficient across bubbles, which will substantially enhance gas transfer and TDG generation. Higher TDG generated in the shallower LLO was therefore attributed to the water depth generating stronger turbulence flow, with more efficient gas transfer. This study improves TDG prediction and helps inform the development of operational alternatives during periods of high TDG generation to mitigate impacts on the aquatic environment.

Previously published field data, from groundwater tracer studies in Hawaii, are used to develop a theory for dispersion based on the probability density function for tracer particle velocities in the aquifer. The density function is derived from tracer breakthrough curves associated with wastewater injection wells at the Lahaina Waste Water Reclamation Facility in Maui, HI. The aquifer dispersion coefficient is found to be proportional to the product of the time since tracer release and the pore velocity variance, developed empirically from the breakthrough data. Although particle velocity data are well described by Fickian diffusion with this dispersion coefficient, an empirical Weibull distribution provides an even better fit to the data, thereby enabling determination of tracer residence time in the aquifer. Based on the data, two independent methods of determining the cross-sectional area of the aquifer occupied by the $\mathrm{12,000}{\mathrm{m}}^3/\mathrm{day}$ of injectate plume are found to be in close agreement at approximately $\mathrm{26,500}{\mathrm{m}}^2$. It is also estimated that treated wastewater arriving at the two tracer measurement sites is less than 2% of the injectate.

This technical note reassesses the prevailing understanding of lateral-contraction scour at bridge waterways. Herein, lateral contraction (or simply contraction) implies a reduction of flow width. Nonuniform flow to and along a channel contraction, the formation of a vena contracta within a contraction’s entrance, and complexities in alluvial bed dynamics to and along a contraction serve to complicate estimation of contraction scour depth. The reassessment uses data from flume experiments and explains that the aforementioned complexities invalidate the commonly assumed concept of long-contraction scour. These considerations cause trends in the depth of contraction scour, especially live-bed scour, to deviate from the trends suggested by common guidelines. Consequently, this note suggests adjustments to the standard equations, but it also suggests flow in alluvial, open-channel contractions requires further investigation. Also, this note briefly discusses the practical implications of the reassessment.

Abstract

This paper reports a new experimental method applying medical X-ray computed tomography (CT) to estimate the bed load in sand transport. A set of current-generated sand ripple experiments were conducted in a small hydraulic flume inserted in the CT scanner. The methodology is based on the measurements of height, velocity, and density of bed forms to estimate bed load. A temporal series of bed topography is first extracted from the CT scan images. The velocity is estimated by tracking the displacement of bed forms from two successive bed topographies. The sand bed density (${\rho }_{sb}$) is estimated from the CT scan measurements using a calibration technique. The method measuring ${\rho }_{sb}$ to calculate bed load is validated comparing measurements made with sand traps. The advantages and limitations of the CT method applied to bed-load transport are discussed.

Practical Applications

Sediment transport is a fundamental physical process in Earth Sciences. It refers to the movement of sediment grains transported by water currents and deposited where or when water flow ends. This cycle seems at first inoffensive but could impact millions of human lives all around the world. River floods, sea-level rise, and storms are likely to modify the landscape of many populated areas located nearby in the next decades. A better understanding of sediment transport processes would greatly benefit our capacity to determine the impact of those extreme events on river and coastal morphology. To achieve this, physical models are used in laboratories to simulate sediment dynamics at a smaller scale. These results help researchers developing numerical models of sediment transport to better predict river dynamics as well as the movement of coastlines. This study is an insight on the application of new laboratory techniques using advanced three-dimensional imaging technique as an effort to contribute to our knowledge of sediment dynamics.