Journal of Hydraulic Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

This study presents an analytical model of a steady turbulent flow over a two-dimensional downstream-skewed wavy bed of small amplitude. The mathematical framework rests on the time-averaged continuity and Reynolds-averaged Navier–Stokes (RANS) equations. The streamwise velocity profile is considered to follow a self-similar power law, whereas the Reynolds normal stresses are founded on the turbulent diffusivity hypothesis. The curvilinearity in flow streamlines induced by the wavy bed is introduced into the analysis via Boussinesq approximation. The analysis provides solutions to the free-surface, bed shear stress, and Reynolds shear stress profiles. As the flow Froude number varies, the free-surface and bed shear stress profiles change their phases. The phase shift of the free-surface profile with respect to the bed profile decreases with an increase in skewness factor, whereas the phase shift of the bed shear stress profile with respect to the bed profile increases with an increase in skewness factor attaining a constant value. The convex and concave shapes of the Reynolds shear stress profiles on the downslope and upslope of the bed profile are attributed to the decelerated and accelerated flows, respectively. The implementation of the key findings of this work to study the hydrodynamics of fluvial bedforms is discussed from the standpoint of sediment transport, as a future scope of research.

This study utilizes a detailed analysis of the first direct numerical simulation (DNS) of thermally stratified open-channel flows through idealized sine-generated meanders with varying sinuosity to enhance our understanding of the impact of thermal stratification on the mean flow, secondary currents, and boundary shear stresses. Four different channel sinuosities $Si=(1.07,1.34,2.12,3.70)$ at a moderate curvature of $R_{\min }/B=2.5$ and channel aspect ratio $B/H=10$ with friction Reynolds number $R{e}_{\tau }=200$ in neutral and density stratified flows are presented. The results indicate that thermal stratification has a significant effect on the characteristics of the mean flow, resulting in differences in the separated shear layers and separation zones for different sinuosity, as well as in the bed and sidewall shear stress distributions for both neutral and stratified flows.

The fix-grid method of characteristic (MOC) has been the main numerical scheme for modeling the transient pipe flows with an entrapped air pocket, where the Courant number $Cr$ usually equals one (i.e., $Cr=1$) to ensure its accuracy and stability. However, $Cr=1$ cannot always be guaranteed in each pipe of real pipe systems; thus, the MOC needs to be approximated by interpolation or wavespeed adjustment. This could lead to large accumulated numerical errors and serious shape distortion of simulated pressure curves. To address this problem, an alternative coupled scheme, which combines the second-order Godunov-type scheme (GTS) and the MOC, is developed. Specifically, the conservation equations with unsteady friction of the water column are numerically solved by the GTS, and the moving air-water interface is modeled and captured by the coupled GTS-MOC scheme. The simulated pressure curves by the GTS-MOC scheme are compared with both MOC results and laboratory experiments. The proposed scheme with unsteady friction can better reproduce the experimental pressure oscillations, and is more robust and efficient than the MOC. The MOC scheme with $Cr<1$ and coarse grids causes more obvious numerical dissipation during an intensive transient induced by relatively high inlet pressure, in which more high-frequency waves occur.

Any transient event in a pipeline system is an energy process, implying that the dynamic response can be interpreted and analyzed through energy transfer between different forms, including potential (internal), kinetic, and losses. This water hammer energy approach to investigating the impacts of leaks uncovers notable insights about transient-leak interaction in fluid-filled pipelines. In this paper, the energy exchanges in leaky conveyance pipeline systems, including elastic and viscoelastic pipes with various leak sizes and locations, are analyzed and compared with intact pipe cases, thus revealing the extent to which a leak can vary the transient internal and kinetic energy intensity. The results are supported by an extensive numerical analysis that describes transient energy in different cases and illustrates transient stability conditions from a new perspective. Finally, practical implications for developing transient-based leak detection methods are drawn specifically when data from multiple stations are available, and the limitations of this study are summarized in the end of the paper.

A wide variety of hydraulic and coastal flows can be modeled using shallow water theories, where the so-called Serre-Green-Naghdi (SGN) equations constitute a fully nonlinear and weakly dispersive wave theory that has been successfully applied in fluvial and maritime contexts. In the present contribution, we show that SGN models with wave-breaking parameterizations can reproduce challenging nonlinear processes in the surf and swash zones including wave–wave interactions and infragravity wave generation from a narrow-band swell spectrum. The excellent performance of the model motivates us to explore its application to a simplified shallow bar-built river configuration where surf zone–generated infragravity waves may propagate upstream the river. We show that long-wave penetration is controlled by the Froude number over the bar, and that these nonlinear long waves may give rise to a solitonic dynamics. The power spectral density (PSD) signature of free surface time series with a slope of $\sim f^{-1}$ in the infragravity range is consistent with the latter, as also found in field observations. Transferring of energy into lower frequencies is observed in the numerical experiment as long waves propagate upstream; nevertheless, low-frequency energy also cascades back into the swell energy band. Standard linear Fourier analysis may fail in showing the hidden solitonic dynamics, so nonlinear techniques would need to be applied to fully elucidate the fate of the long-wave energy while propagating upstream of river mouths.

The last decade has seen the rapid emergence of nonhydrostatic modeling as an advanced tool for studies of tsunami processes and source mechanisms that warrants a critical assessment of the state of the art and value-added features in relation to contemporary approaches. Inclusion of depth-averaged vertical velocity and nonhydrostatic pressure in the nonlinear shallow-water equations enables description of long-wave dynamics in quasi three-dimensional flows. The governing equations involve first-order derivatives, but retain higher-order properties, as in the Boussinesq-type approach. The commonly-used staggered finite difference scheme continues to provide the surface elevation and horizontal velocity, which in turn are updated by the nonhydrostatic pressure evaluated from a Poisson-type equation. In addition to having dispersion properties complementary to the governing equations, the numerical framework allows implementation of time-varying seafloor excitation from earthquake rupture, a shock-capturing scheme for discontinuous flows, and a multilevel two-way nested grid system for dispersive and shock waves. A series of numerical and laboratory benchmarks as well as a case study of the 2011 Tohoku tsunami illustrate the model capabilities in describing tsunami generation, dispersion, shoaling, bore formation, and separation-driven currents with high precision across a wide range of temporal and spatial scales for general application. These capabilities have an important role in resolving effects of detailed earthquake rupture patterns and providing accurate tsunami impact predictions with implications for warning guidance, hazard assessment, and seismological research.

The current study presents a mathematical model to simulate a power-law fluid hammer problem in pipelines. The flow is considered two-dimensional, isothermal, weakly compressible, and laminar. The governing equations consisting of mass and momentum balances are solved by the method of characteristics. The model results are compared to other numerical model values and validated against measured data found in the literature. The comparisons show that the two-dimensional model agrees better with experimental data than the one-dimensional approach. Two dimensionless parameters (a dissipative parameter and the power-law index) are identified to govern the phenomenon, and a sensitivity analysis with respect to these parameters is conducted. It can be anticipated that the first pressure overshoot due to line packing and pressure wave dissipation increase with the dissipative parameter and with the power-law index.

Fish passage design could greatly benefit from knowledge of the movement and path choices of fish in complex, turbulent environments, but empirical information required to construct and validate agent-based models is generally not available. To address this issue, an image-based data analysis scheme is proposed to measure turbulent flow and fish motion simultaneously using a small keystone species to exemplify the procedures. Using the same video images collected in a laboratory channel, large-scale particle tracking velocimetry is employed to compute the instantaneous Eulerian flow field, and a fish detection model and background-foreground subtraction method are used to identify fish positions. The fish detection model uses deep learning with an architecture based on a faster regional convolutional neural network (faster R-CNN). This methodology and architecture are shown to have a success rate greater than 93% in identifying fish positions. Data are then presented to illustrate the utility of this technique as it relates to agent-based models for fish movement. These results show that when velocities surrounding a swimming fish are spatially averaged, the values derived from time-averaged velocities are markedly different when compared to those derived from instantaneous velocities, as evidenced by relatively large root-mean-square errors. A swimming fatigue analysis also shows that, in general, the fish would swim for relatively short durations when experiencing fatigue in contrast to swim trial data. The experimental and data analysis methods proposed here offer a new approach to examine fish-flow interactions in complex hydrodynamic environments and provide a potentially important tool for assessing and evaluating fish passage design and river restoration projects.

The Integrated, Section Surface Difference Over Time, version 2 (ISSDOTv2) method provides a means of quantifying bedload in large sandbed rivers. Like all measurements, it is important to understand the uncertainty associated with the method before making management decisions based on its results. A methodology is presented for quantifying and combining the uncertainty for each component of the ISSDOTv2 method including particle density, bed porosity, acoustically measured bed topography, the timing of measurements, sand wave identification, and regression analysis used for geometric correction of bedload measurements. The approach provides an indication of the relative magnitude of each source of uncertainty in addition to the uncertainty in the final resultant transport rate. Laboratory flume measurements were used to evaluate the uncertainty limits and verify the approach. The greatest contributor to uncertainty was found to be the bathymetric uncertainty, and, at the highest transport rates, cumulative relative uncertainty was found to be approximately 10%. Cumulative relative uncertainties grew rapidly with decreasing flow rates, driven primarily by the higher relative contribution of the effect of bathymetric uncertainty on the smaller bedforms that are typically present at lower transport rates. The approach documented here will be transferrable to real-world systems to determine the uncertainty in measured bedload sediment transport rates using the ISSDOTv2 method.

By incorporating horizontal sediment transport and bed erosion into the two-layer depth-averaged shallow water system, one can construct a hydro-sediment-morphodynamic mathematical model in which a variable sediment concentration in a denser layer and interaction between underflows and ambient water are taken into account. Numerically solving such a promising system presents challenges due to its inherent mathematical properties of being conditionally hyperbolic and nonconservative. These properties may result in instability and incorrect results near sharp hydraulic gradients in numerical solutions. The major contribution of this paper is to develop a novel numerical scheme to overcome these issues and to allow the system to be extended to a wider range of engineering applications. To this end, for a one-dimensional two-layer hydro-sediment-morphodynamic system, (1) its mathematical model is first reformulated to a novel relaxation format, which is mathematically equivalent and unconditionally hyperbolic, by relaxing interlayer surface level, and (2) this relaxation system is then solved using a novel path-conservative numerical algorithm with new discrete formulas for the fluxes and nonconservative terms. Thus, the proposed numerical scheme ensures the well-balanced property, and has a major advantage in its ability to accurately and stably handle sharp hydraulic/density gradients with second-order accuracy. Several numerical and laboratory tests are conducted to demonstrate the performance of the proposed new scheme.

Local scour processes can be divided into initial, progressing, and equilibrium stages. Due to its short duration and high scour rate, the initial stage has not been well investigated to date. By applying laser-based technique and particle image velocimetry, this study conducted a series of measurements of scour hole profile and flow field at the beginning of scour hole development for the case of plane jet–induced scour. The results show that the scouring process in the initial stage is dominated by the wall shear, and thus can be classified as shear scour, whereas that in the progressing stage largely is affected by the turbulent vortex, and therefore can be referred to as vortex scour. The two modes of scour could be characterized by different mechanisms of sediment erosion. A bedload model is proposed to analyze the sediment transport for the shear scour, and the results agreed well with the experimental data.

This paper presents the results of a hydrodynamic experimental study of the hydraulic characteristics of flow in different vertical slot fishway (VSF) designs with L-shaped, polygonal, and semicircular baffles. A biological experiment was conducted using endemic fish to analyze the efficiency of the upstream passage of different VSF designs. Based on the results, a significant difference in water depth, slot velocity, turbulence kinetic energy, and efficiency of the upstream passage was observed. The L-shaped baffle had better energy dissipation; however, the fishway with a polygonal baffle had better efficiency of the upstream passage. The results indicated that the present VSFs are all fish-friendly for Chinese endemic fish, but VSFs with polygonal baffles may be a better choice.