Journal of Hydraulic Engineering

Recent field work at the Wax Lake Delta (WLD) in coastal Louisiana indicates lateral outflow from channels to islands upstream of the receiving basin; in this region of the delta the flow transitions from confined to unconfined (i.e., from a uniform discharge profile to a nonuniform discharge profile). The hydraulics of this transition zone and the controls exerted by vegetation, topography, and river discharge fluctuations are analyzed in this work. The shallow water equations are numerically solved in two model domains: an idealized channel-island complex and the full domain of the WLD. In both domains, a significant fraction of the river discharge flows laterally from the channels to deltaic islands before reaching the receiving basin. Vegetation roughness within the delta islands significantly impacts the fraction and rate of lateral outflow from the channel, while river discharge fluctuations have a limited effect, due to the backwater control on the subcritical flow. The presence of vegetation in the islands tends to increase velocities within the channel, except in the region upstream of significant lateral outflow, where the velocity increases with decreasing vegetation roughness due to the lowered water level that reduces the flow cross-section. The topography establishes a lateral water surface gradient between the channel and the islands even with low vegetation roughness, which drives lateral flow. A velocity spreading angle is used to mark the transition from confined to unconfined flow; the angle generally increases up to the onset of unconfined flow and then decreases as the flow approaches the receiving basin and the flow in the island tends to align with that of the channel. The lateral outflow from the primary channels influences the hydraulics of the flow throughout the backwater length. The transition between confined and unconfined flow in coastal river deltas has a significant impact on the flow hydraulics and the resulting transport dynamics of solids and solutes.

This paper reviews the more widely used bed-load surrogate acoustic measuring techniques that were tested both in flume experiments and field settings. A fair number of these measuring techniques have been successfully calibrated for total bed-load flux, which generally requires contemporary direct bed-load transport measurements in the field. Essentially, linear or power-law relations were established between a simple metric characterizing the acoustic signal and bed-load mass. In some studies, further calibration relations were established to identify particle size, either based on signal amplitude and/or on characteristic frequency of that part of the signal associated with a single impact of a particle (e.g., for impact plate systems), or by determining a characteristic frequency for an entire grain-size mixture (for the hydrophone system). A few of the acoustic measuring techniques were used to determine bed-load transport by grain-size classes. Some studies examined how far findings from flume experiments can be quantitatively transferred and applied to field sites for which independent, direct calibration measurements exist.

Significant mass wasting into artificial or natural reservoirs generates so-called impulse waves, thereby transforming the potential energy of the sliding mass into kinetic wave energy. These waves run up onto shores and may overtop dams. The destruction of the shoreline due to run-up and the tailwater flooding due to dam overtopping are often devastating. This research deals with solitary waves to model the overtopping process in a two-dimensional (2D) laboratory-scale test setup. Three questions are answered: (1) How much water volume overtops? (2) What is the maximum overtopping depth at the dam crest? (3) How long is the overtopping duration? Dimensionless empirical relations between the governing parameters of still-water depth, relative wave amplitude, dam front face angle, dam crest width, and the previously mentioned variables are derived. Data including scale effects were eliminated. The present predictions are compared with an alternative model, thereby highlighting the major differences.

Experiments are reported on multiport diffusers typical of brine disposal from desalination plants discharging into flowing currents. Diffusers discharging from one side into coflowing and counterflowing currents and from both sides were tested. A three-dimensional laser-induced fluorescence system was used to determine bottom impact dilution, impact location, and rise height. The results were plotted in dimensionless form as functions of port spacing and current speed parameters and empirical equations were fitted. As the current speed increased, the impact location was swept farther downstream, rise height decreased, and impact dilution increased. The merging of closely spaced jets prevents intrusion into the space between the ascending and descending regions, which can alter the flow fields. The results did not become independent of port spacing, at least within the parameter ranges studied and the experimental uncertainty. Raising the diffuser above the bed, which allowed the flow to pass beneath it, can affect the flow patterns significantly. Dilutions were not significantly affected, however, possibly because the nozzles were elevated above the pipe with a spacing that allowed passage of entrained flow between them. The dependence of the observed flow patterns on relatively small changes in source conditions should be considered in diffuser designs to avoid constricting the diluting entrainment flow.

Results are presented for a computational study of free overfall from a smooth, horizontal, circular pipe that is flowing full upstream. The limiting discharge value that delineates the partially full and full pipe flows was established. Two different flow regimes for partially full pipe outflows were observed: cavity outflow in which an air bubble intrudes into the pipe and the water has a horizontal surface at some distance from the outfall, and bubble washout flow in which the cavity is shorter and the water surface is never horizontal. The outflow water depth and cavity length were expressed as functions of the discharge. The simulated data for several controlling parameters provided good agreement with available data in the literature. The end-depth ratio (EDR), i.e., the ratio of the brink depth to the critical depth (the depth at which the Froude number is one for the given flow rate and pipe diameter), was found to be 0.75 for the cavity outflow regime. For the bubble washout regime, the EDR is shown to vary linearly with the dimensionless critical depth. The simulation results were used to calculate several governing parameters, i.e., the Froude number at the brink, upstream and downstream pressure coefficients, and the minimum slope of the water surface in the cavity. Each of these parameters behaved differently in the two flow regimes. However, the nondimensional pressure distribution at the brink was the same for both flow regimes. The momentum equation was applied to the flow using appropriate pressure and momentum coefficients derived from this study to accurately predict the discharge as a function of the brink depth for the bubble washout regime. These findings provide insights into the mechanics of a free overfall from partially full pipes at the outlet, and explain the transition between the cavity flow and bubble washout regimes.

Bed-load transport rate in gravel-bed rivers shows large non-Gaussian fluctuations, even under steady flow conditions. The development of high-resolution measurement techniques during the last two decades creates research opportunities to study the intermittent character of bed-load transport and the significance of its fluctuations. In this paper, the use of an impact plate as an alternative to video-based technology for laboratory applications is investigated. The objective is to develop a simple and robust technology that can be run continuously over several hours. The impact plate is mounted vertically at the flume outlet. This is a novel nonintrusive configuration, which reduces the uncertainties in the particle transport rate measurement compared with the classical configuration: in that configuration, the impact plate is indeed parallel to the bed and the vibrations recorded by the sensor depend a great deal on the features of particle motion (e.g., the mode of transport, the angle of impact, and the particle velocity). Two key variables were monitored in different flume experiments: the number of moving particles and the bed-load transport rate. They were measured using the vertical plate sensor and image processing. The impact plate and the camera were found to reach the same level of accuracy. The vertical impact plate can therefore be an efficient measurement technique, which requires reduced costs and computational resources.

The interaction between fully nonlinear Miyata-Choi-Camassa (MCC) internal solitary waves and topographic slopes are modeled by direct numerical simulation. The immersed boundary method is employed to describe the no-slip boundary of the slopes and a novel iterative Neumann boundary condition (INBC) enforcement strategy is proposed to ensure local mass conservation. The wave Reynolds numbers $R{e}_w$ used in the present work (${\sim 10}^4$), although less than those of field scales (${10}^6{-10}^7$), but are an order of magnitude greater than those in most laboratory scale experiments and previous numerical simulations (${\sim 10}^3$). In the present study, three main internal wave breaking types (collapsing, plunging, and surging) together with two mixed mode breaking types (collapsing-plunging and plunging-surging) are observed for MCC wave shoaling process. The different breaking mechanisms are found to be related to the internal Iribarren number. New breaking regimes, breaking location criterion, and the maximum velocity prediction are also proposed.

One important environmental issue of hydropower in the Columbia and Snake River Basins (Pacific Northwest region of United States) is elevated total dissolved gas (TDG) downstream of a dam, which has the potential to cause gas bubble disease in affected fish. Gas supersaturation in the Columbia River Basin primarily occurs due to dissolution of bubbles entrained during spill events. This paper presents a physically based TDG model that can be used to optimize spill operations in multireservoir hydropower systems. Independent variables of the model are forebay TDG, tailwater elevation, spillway and powerhouse discharges, project head, and environmental parameters such as temperature and atmospheric pressure. The model contains seven physically meaningful experimental parameters, which were calibrated and validated against TDG data collected downstream of Rock Island Dam (Washington) from 2008 to 2012. A sensitivity analysis was performed to increase the understanding of the relationships between TDG downstream of the dam and processes such as air entrainment, lateral powerhouse flow, and dissolution.

Acoustic Doppler velocity profilers (ADVP) measure the velocity simultaneously in a linear array of bins. They have been successfully used in the past to measure three-dimensional turbulent flow and the dynamics of suspended sediment. The capability of ADVP systems to measure bedload sediment flux remains uncertain. The main outstanding question relates to the physical meaning of the velocity measured in the region where bedload sediment transport occurs. The main hypothesis of the paper, that the ADVP measures the velocity of the moving bedload particles, is validated in laboratory experiments that range from weak to intense bedload transport. First, a detailed analysis of the raw return signals recorded by the ADVP reveals a clear footprint of the bedload sediment particles, demonstrating that these are the main scattering sources. Second, time-averaged and temporal fluctuations of bedload transport derived from high-speed videography are in good agreement with ADVP estimates. Third, ADVP based estimates of bedload velocity and thickness of the bedload layer comply with semi-theoretical expressions based on previous results. An ADVP configuration optimized for bedload measurements is found to perform only marginally better than the standard configuration for flow measurements, indicating that the standard ADVP configuration can be used for sediment flux investigations. Data treatment procedures are developed that identify the immobile-bed surface, the layers of rolling/sliding and saltating bedload particles, and the thickness of the bedload layer. Combining ADVP measurements of the bedload velocity with measurements of particle concentration provided by existing technology would provide the sediment flux.

There are a plethora of analytical methods that can account for the effect of unsteady flow (i.e., hysteretic behavior) on steady-state stage-discharge rating curves (hQRCs) used to continuously estimate discharge in open-channel flow. One of the most complete and accurate methods for wide rectangular channels is the Fread method. This paper presents an extension of the method that can be applicable to arbitrary geometry of the channel cross section by replacing the hydraulic depth with the hydraulic radius and adjusting formulations associated with wave celerity coefficient and kinematic wave equation in the original Fread method. The modified Fread method improves the accuracy associated with the estimation of conveyance factor and energy slope. It is particularly useful for low-aspect-ratio (i.e., the width-to-depth ratio) channels where the original method’s assumption does not properly hold. These improvements are tested at two sites, each representative of small streams and midsize rivers, to quantitatively illustrate the significance and performance of the refined method.

The quantification of the breach hydrograph of an overtopped earth dam remains a challenge, even in controlled laboratory conditions. In this work, the breach hydrograph is calculated as the product of the velocity normal to a breach cross section and the estimated area of that cross section. Although theoretically simple, because it is based on the very definition of discharge, this direct approach is an absolute novelty in dam-breach studies. To illustrate the application of the method, a laboratory model of an earth dam, built with cohesive sediments, was closely monitored. Competent velocities were estimated from surface velocity maps measured with large-scale particle image velocimetry (LSPIV). The breach area was estimated by analyzing images of the traces of a laser sheet on the free surface and the breach bottom. The direct breach hydrograph was compared with an estimate based on the mass balance in the reservoir and with the discharge obtained through a rating curve of a downstream spillway. The novel method was shown to reproduce the main expected features of the breach hydrograph. The potential to provide estimates free from experimental artifacts caused by improper description of inertial effects is underlined. The advantages and the difficulties inherent to the method are discussed.

The comparability of grain sizes emerging from different methods are discussed, including image-based grain-size analysis. Waterworked gravel-bed surfaces from laboratory and field experiments are analyzed in detail. Grain sizes estimated using freely available object-detection software are compared with grains measured with calipers by hand. On the basis of laboratory and field data, the pebble dimensions determined by square-hole sieving are demonstrated to underestimate real pebble dimensions by a factor of 0.83–0.86, and pebble dimensions derived from images underestimate the pebble-count measurements by a similar amount. Thus, for the present extensive data set, the software-detected grain sizes can be directly compared with grain sizes gained by square-hole sieving, as they are of the same order of magnitude. These results support future wide-spread use of image-based sieving for grain-size distribution analyses for both hydraulic research and engineering.