Journal of Hydraulic Engineering

A linear stability analysis of flow in an open-channel partially covered with vegetation was performed. The differential drag between vegetated zones and adjacent nonvegetated zones is known to induce a lateral gradient of the streamwise velocity. The velocity gradient may result in flow instability in the shear layer around the edge of the vegetated zone causing the generation of discrete horizontal vortices. We assume that the base state flow field before the occurrence of instability is characterized by turbulence with a smaller length scale than the flow depth, which is mainly generated by the bottom friction. By introducing perturbations to the flow depth as well as the streamwise and transverse velocities in the base state, the conditions required for perturbations to grow in time were studied over a wide range of (1) Froude number, (2) normalized nonvegetated zone width, and three other dimensionless parameters that represent the relative effect of (3) bed friction, (4) vegetation drag, and (5) subdepth eddy viscosity. All parameters were found to have positive and negative growth rates of perturbations within their respective evaluated ranges. The characteristic vortex shedding frequencies associated with the maximum growth rate was compared with those observed in experiments. Although the analysis that employs a base state set without the large scale lateral motions was shown to be capable of predicting the order of magnitude of the frequencies, there is a systematic discrepancy between the predicted and observed frequencies, which may be due to the limitation of linear stability analysis.

This study experimentally investigates the effect of shallow water depth on the mixing of 30° and 45° inclined dense jets. Three different mixing regimes were identified, namely, the full submergence, plume contact, and centerline impingement regimes. The mixing characteristics in these three regimes, including the jet trajectory and minimum dilution at the water surface ($S_S$) as well as at the return point near the seabed ($S_r$), were quantified with respect to the densimetric Froude number ($\mathsf{F}$) and cover water depth ($H$). The nondimensional cover water depth, $H/D$, was found to be a suitable normalization parameter for shallow water scenarios. The transitional $\mathsf{F}·D/H$ among the regimes, the asymptotic limits of the minimum surface dilution at large $\mathsf{F}$, and the various linear coefficients were also determined. Overall, it was found that the surface constraint in the plume contact and centerline impingement regimes, lengthens the jet-spreading distances and reduces the surface dilution, while the bottom dilution remains relatively constant. The results enable the assessment of the mixing characteristics of the inclined dense jet in shallow coastal waters with possible surface contact for environmental impact assessment.

The numerical modeling of hyperconcentrated shallow flows is a challenging task because they exhibit special features, such as propagation over dry beds, profound bed elevation modifications owing to erosion or deposition phenomena, and flow discontinuities. In this paper, a novel depth-positivity preserving Harten, Lax, and van Leer—contact (HLLC) Riemann solver is devised in order to approximate the solution of the Riemann problem for the 1D (one-dimensional) hyperconcentrated shallow flows equations over horizontal beds. The solver is used as a building block for the construction of hyperconcentrated shallow flows (HCSF), a well-balanced finite-volume scheme for the solution of the hyperconcentrated shallow flows equations with variable elevation. HCSF is able to handle the case of dry beds, to take into account the variability of the topography also in the presence of bed discontinuities, considering the flow resistance and the mass exchange between the flowing mixture and the mobile bed. The numerical tests carried out confirm the well-balancing property of the scheme proposed, the robustness in the presence of dry beds, the ability to approximate the analytic solution of problems with smooth or discontinuous beds, and the ability to reproduce reasonably the results of a laboratory experiment.

In this two-part study, experiments are conducted to evaluate available topography measurement techniques for gravel beds in a laboratory flume and to study their suitability for statistical roughness analysis. The available instruments for this study include (1) an acoustic bed profiler; (2) a hand-held laser scanner; and (3) two digital consumer cameras forming a stereophotogrammetric system, and are employed to obtain digital elevation models (DEMs) of water-worked gravel beds. In the first part of the study, the three measurement techniques are reviewed and their feasibilities for future grain-scale roughness work assessed, based on the obtained elevation datasets. Water-worked gravel-bed topographies are measured with all three available measurement techniques. The analysis of the DEMs concentrates on using probability distribution functions (PDFs) and second-order structure functions of bed elevations. Roughness coefficients are determined and used as a benchmark for comparison of the three measurement techniques. Although, visually, differences in the DEMs obtained with different measurement techniques are observed, the results of the chosen statistical analysis do not disclose the visual differences to the same extent. It is shown that the used stereophotogrammetric system, although theoretically allowing a fast and high-resolution recording process, lacks behind in accuracy. Thus, the second part of the study identifies and presents steps to improve the quality of the obtained stereophotogrammetric DEMs. A checklist is provided, highlighting the improvements made in the follow-up study, in order to obtain a high-quality stereophotogrammetric DEM. The overview will be useful for other researchers to make use of available low-cost and high-quality consumer camera equipment, to setup their own, non-proprietary stereophotogrammetric system.

The motivation for the present work comes from the fact that a few recent publications describing separation flows report a counter-gradient diffusion phenomenon (CDP) along the separating streamline. This CDP is related to (1) a change of sign of the Reynolds shear stress near the separating point and (2) an opposite sign between the Reynolds shear stress and the mean shear rate, leading to a negative eddy-viscosity coefficient. Such CDP is only reported for configurations where the angle between the separating streamline and the main flow is large (referred to as group 1), whereas no CDP was reported for low angle configurations (referred to as group 2). All these flows were analyzed using Cartesian or curvilinear frame axes. The aim of the present paper is to propose a more intuitive frame for analysis, namely the Serret-Frenet frame axis based on the local flow direction, for which the CDP disappears for group 1 flow configurations and that highly improves the characterization of the mixing layer. The recirculation zone occurring in the lateral branch of an open-channel bifurcation is chosen as a separating flow configuration belonging to group 1 and measured using particle image velocimetry (PIV). The characteristics of the mean flow and of the horizontal Reynolds stress are first analyzed using a Cartesian frame axis, to retrieve the CDP, for which extension is enhanced compared with the literature cases. Then, the local Serret-Frenet frame axis is introduced and used for a second analysis of the data. The flow characteristics at the interface between the main flow and the recirculation zone accurately match the characteristics of the well-documented mixing layers available in the literature. Moreover, the eddy-viscosity concept nicely applies using this Serret-Frenet frame axis. A simple geometrical analysis is finally performed to confirm that the negative eddy-viscosity coefficient obtained when using a Cartesian frame axis is only attributable to a nonadequate frame of analysis.

Tools for engineers who assess and optimize hydropower intakes are provided to help them measure and quantify the characteristics of free surface vortices (characteristic radius, bulk circulation, tip depth, nominal depression slope) that form at the intakes. Accessible methods are proposed for measuring and modelling vortex characteristics and the processes that affect their generation and strength. Common mechanisms that produce and strengthen the vortices (flow separation, shear, asymmetric approach flow) are discussed. An analytical model, based on Burgers’s vortex model and laboratory measurements, is described that incorporates the effect of the approach flow and intake geometry on vortex characteristics. Simple measurement techniques (acoustic Doppler velocimetry and surface particle tracking velocimetry) are presented by which the flow and vortex characteristics can be documented, allowing the model to be adjusted to the particularities of the specific intake under consideration. The analytical model is then used to help understand how the different processes affect the scaling of vortex characteristics.

This paper focuses on pressurized wavefront behavior and an analysis of trapped air pockets. Flow behavior, trapped air-pocket pressure, and different pressurized wavefront shapes in addition to their propagation conditions were investigated to improve modeling of mixed flows with trapped air pockets in stormwater systems. Pipe slopes, inflow, and reservoir water-levels were considered in an experimental analysis of the initiation, propagation of the wavefront, and flow behavior. Results show that the formation, propagation, and stability of the wavefront shape are highly dependent on the reservoir water-level and pipe slope. Pressurized wavefronts take on different shapes depending on manhole water level, pipe slope, and air pressure. The shape of the pressurized wavefront is sharper when the initial pipe depth $y^{*}$ is set between 0.5 and 0.85. With low inflow, the pipe-filling phase is progressive and is accompanied by undulations with stronger curvature and short length on the free-surface flow zone. For mild slopes with low flow-rates, the pipe-filling process is achieved through progressive undulations with trapped air pockets. A two-phase numerical model is developed, combining the method of characteristics (MOCs) for solving the free-surface flow conditions, the rigid column for the pressurized flow conditions, and the ideal gas law. Numerical results are achieved by applying of the continuity equation taking into account any type of flow regime in the interface vicinities. The four equations that consider the direction of wavefront propagation and the type of pressurized or depressurized wavefront are then taken into account. In these equations, a minimum distance between the moving wavefront and the considered cross section on the fixed grid is imposed. Compliance with this minimum distance allows avoiding numerical instabilities that may affect the stability of the model. Good agreement is obtained between numerical and experimental results. This agreement demonstrates that pressure variations due to pressurized waves combined with trapped air-pockets can be reproduced by implementing the numerical model as a shock-fitting approach.

This paper presents the comparison of two algorithms for water distribution network resolution in terms of computational efficiency: the Newton-Raphson Global (NR-GA) and the Newton-Raphson Loop Flows (NR-LF). Both algorithms use the hydraulic equations linearized by the Newton-Raphson method; however, whereas NR-GA solves the equations projected onto network nodes and pipes, the NR-LF solves the equations projected onto network loops and then requires the loop matrix to be determined prior to its application. In particular, the computational efficiency of the latter algorithm turns out to be maximized when reference to the sparsest possible loop matrix is made. In a bid to apply efficiently the NR-LF to high complexity case studies, a new automatic procedure for the identification of the basis of minimum loops from the topological viewpoint (i.e., of the basis of independent loops made up of the lowest number of pipes) is presented. The comparison between the NR-GA and NR-LF points out the slight superiority of the latter, which offers shorter computation times above all for case studies of low-intermediate topological complexity. However, an increase in network topology complexity affects the performance of the NR-LF more than that of the NR-GA, thus leading to an almost identical performance in case studies of very complex topology.

Laboratory tests were conducted with brown trout to evaluate their ability to pass through a small, baffled prototype-scale culvert under a variety of culvert slopes and discharge conditions. The culvert was 18.3 m long and 0.60 m in diameter with $0.15D$ baffle height and $0.9D$ spacing, where $D$ is the culvert inside diameter. An inverse relationship was observed between fish passage success and flow rate and/or culvert slope. The influence of the sample fish population and the length of the individual fish on passage rates were investigated; the data showed that the brown trout fish passage sample size evaluated in this study (25 per test) was sufficiently large to minimize sample size dependency. The elapsed time required for fish to traverse the culvert decreased with increasing hydraulic difficulty primarily owing to diminishing resting zones. The behavior of fish traversing the culvert was observed and reported, including resting/staging zone locations.

In this paper we experimentally investigate the final scour depth around cylindrical piers under flood waves. Only clear-water conditions are considered. An approach to estimate the final scour depth under a flood wave is proposed, based on the local scour depth calculated with steady flow equations under peak flow conditions. The new experimental results are also used to investigate the performance of existing expressions or approaches to predict time evolution under steady and unsteady flow conditions.

A new approach is proposed to treat the bottom boundary conditions for suspended sediment transport modeling, which works with turbulence models using wall functions. New boundary conditions for suspended sediment are specified directly at the sediment bed, not at some reference height. It is numerically equivalent to the traditional approach of specifying boundary condition at a reference height. The new approach eliminates the need of any special treatment for the gap between the bed and the reference height. It also reduces the loss of accuracy due to the interpolation of solution variables between the meshes for fluid and suspended sediment. The associated near-bottom grid resolution requirement is discussed and a general guideline is proposed. The application of the new near-wall treatment is demonstrated.

Gravity waves propagating over free-surface flows with shallow depth are well-known phenomena. The small-amplitude sinusoidal wave and Korteweg-de Vries (KdV) equations are based on potential-flow theory being widely used to describe water-wave propagation. The KdV equation leads to two basic flow patterns, as follows: (1) cnoidal waves, and (2) solitary waves. However, in case of a real Newtonian fluid, the bed resistance and the rapid motion of fluid generate turbulence (eddies) in the medium. The effects of turbulence are taken into account in this paper by using the equations for the surface elevation $\eta$ and depth-averaged flow velocity $U$ developed previously. These equations are based on the Reynolds-averaged Navier-Stokes (RANS) equations for turbulent flow in open channels. The wave profile can be approximated by a form $a\cos k\hat{\xi }/(1-b\cos k\hat{\xi })$, where $a$ and $b$ are constant amplitudes; $k$ = wave number; $\hat{\xi }$ = dimensionless horizontal distance given by $(x-ct)/h$; $x$ = horizontal distance; $c$ = wave velocity; $t$ = time; and $h$ = undisturbed flow depth. Such a profile has the characteristic that the peaks are narrower but higher compared to wider but shallower troughs. The effects of streamflow on wave propagations are also considered. If the waves travel in the direction of the streamflow, there is a lengthening effect on the peaks and troughs, whereas if the waves travel against the direction of streamflow, they become shorter.