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.

Sediment-laden turbulent buoyant jets are commonly found in natural and engineered environments. Examples include volcanic eruptions, deep sea hydrothermal vents, discharge of partially-treated wastewater, and dredging operations. A horizontal sediment buoyant jet is characterized by a horizontal momentum jet, rising plume, and a horizontal surface gravity current. The settling of particles from a sediment buoyant jet depends on the complex interaction of particles with turbulent fluctuations and the mean flow—in particular particle re-entrainment due to the external irrotational flow induced by the jet. A three-dimensional (3D) stochastic particle tracking model is proposed to predict the sedimentation from an arbitrarily inclined sediment-laden buoyant jet in a stagnant ambient. The simple model predicts the entire 3D jet flow field via a coupling of semianalytical models for the mean flow as well as turbulent fluctuations. The mean flow for the turbulent buoyant jet and the surface gravity current are determined using well-validated integral models, while the external jet-induced irrotational flow field is computed by a distribution of point sinks along the jet trajectory. Turbulent velocity fluctuations are modeled by a Lagrangian particle velocity autocorrelation function that mimics the trapping and loitering of sediment particles in turbulent eddies. The turbulent fluctuations are stochastically generated from a self-similar profile of the turbulent kinetic energy derived from computational fluid dynamics (CFD) solution of jets and plumes. Predictions of the particle tracking model are in excellent agreement with experimental data over the entire jet-plume regime for a wide range of particle sizes (58–621 μm) and properties. The model provides physical insight in delineating the modes of sediment fall out. The fraction of sediment mass fall-out from the jet lower boundary is found to be a function of the ratio between the jet momentum–buoyancy length scale $l_s=M_o^{3/4}/B_o^{1/2}$ and the momentum-settling length scale $l_m=M_o^{1/2}/w_s$, where $M_o$ and $B_o$ are the source momentum and buoyancy fluxes, and $w_s$ is the particle settling velocity in quiescent fluid. The particle distribution in the vertical centerline plane of the buoyant jet is also well-predicted. The model does not require any a priori adjustment of sediment settling velocity or particle re-entrainment rate.

Many riverine systems have been disconnected from their receiving basins by flood-protection levees and other engineered systems. Reconnecting these alluvial rivers with their receiving basins is a viable option to nourish and sustain existing coastal wetland systems as well as to build new land. This sediment nourishment can be accomplished through direct dredging and placement or through sediment diversions. Efficient design of sediment diversions is important to maximize the land building potential. This study’s objective is to quantitatively identify key design attributes of sediment diversions, influencing their ability to capture sediment. The outfall channel alignment angle ($\varphi$), intake invert elevation, and diversion size are hypothesized as key parameters. The analysis is limited to sediment grain sizes larger than 63 μm and has been performed using a validated three-dimensional numerical model. A time integrated sediment-water ratio was used as an indicator to reflect efficiency of sediment capture. Analyses indicate $\varphi$ has minor impact on total diverted sediment load and limited to the medium sand (M) fractions (250–500 μm) compared to the fine (125–250 μm) and very fine (63–125 μm) size classes. The sediment water ratio increases as intake invert elevation is deepened to a certain limit, then it plateaus around $y_{\text{Inv}}/y_{\text{Bar}}\sim 0.75$, implying that deeper invert beyond that limit may not be beneficial. The analysis also shows that the sediment water ratio sharply increases up to a water discharge extraction ratio of 0.1 as the size of the diversion is increased, and plateaus around 0.2.

The Swiss plate geophone system is a bed load surrogate monitoring technique that is used to quantify bed load transport in steep streams. The amplitude of the signal recorded by the Swiss plate geophone contains information about the grain-size distribution of the transported bed load. To extract this information, the authors computed the number of impulses and packets (representing a single particle impact) registered for different amplitude ranges, the so-called amplitude histograms, for 46 samples when bed load was measured independently with automatic basket samplers at the Erlenbach. Amplitude histograms can be interpreted as a statistical distribution of the signal’s amplitude over a given time interval. Using the number of bed load particles per unit mass, absolute bed load masses for each grain-size class can be calculated. The results show that the grain-size distribution of the transported bed load for particles larger than 9.5 mm can be continuously monitored at the Erlenbach with the Swiss plate geophone system.

Hydraulic scour is one of the major causes of bridge and embankment failure during typhoon-induced or monsoon-induced floods in steep intermittent rivers in Taiwan. In this study, field data for sediment load and riverbed scour, measured in gravel-bed and sand-bed reaches between 2003 and 2013 are analyzed to determine their differences under nonequilibrium sediment-transport conditions. The riverbed scouring potential was extremely high during the fast-rising stage of a large flood in the bimodal gravel-bed reach. By contrast, the unimodal sand-bed reach with a higher suspended load typically demonstrated a lower scouring rate, although it was still higher than that of a perennial river. Existing bridge-scour formulas suitable for intermittent rivers were examined by separating short-term general scour and localized scour. A maximum general scour-depth formula is developed in this study for use as a potential tool for emergency evacuations and bridge closure purposes.

Dam safety programs are informed by numerical model solutions of dam-break flood depth, extent, and timing that are uncertain owing to an imperfect mathematical and numerical representation of system dynamics as well as uncertain model parameters and input data, yet model uncertainty is rarely reported. The most extreme and damaging events of greatest interest are also the most infrequent to occur and, thus, seldom studied and poorly understood from a model uncertainty perspective. Here an extraordinary event is considered that occurred in Gangneung, South Korea, on August 31, 2002, when Typhoon Rusa dropped nearly 90 cm of rainfall over a 24 h period, causing two dams in tributary valleys to fail only a few moments apart. A two-dimensional (2D) hydrodynamic flood model is developed using the best available data, calibrated, and validated, and uncertainty is systematically examined. Sources of uncertainty include topographic and bathymetric data, breach geometry data, precipitation data, storm surge data, resistance parameters, and structural model errors that represent deficiencies in the flood model formulation. An uncalibrated solution yields a flood height root-mean square error (RMSE) of 0.4 m and a flood-extent agreement of 86% when the best available measurements are used as data input, including precipitation. Additionally, the calibration of resistance parameters reduces the flood height RMSE to 0.33 m, which is close to the estimated uncertainty in flood height measurements (0.2 m) and RMSE of the best available topographic data (0.2 m). Results show that many factors contribute to solution uncertainty and in different ways depending on whether the focus is flood height, flood extent, or flood discharge and timing, and strategies to reduce uncertainties are presented.

Laboratory experiments were carried out to investigate the water surface characteristics of submerged flow over sharp-crested weirs. Interesting observations were made on the water surface profiles near the weir as weir submergence sets in as well as for various stages of submergence. Submerged flow was divided into four regimes: (I) impinging jet, (II) surface jump, (III) surface wave, and (IV) surface jet. It was found that a surface jump turned into a surface wave when the wave trough became level with the tip of the weir. The surface jet regime began as the surface waves faded out. A condition was set to quantify the boundary between the surface wave and surface jet regimes. Based on flow observations, the surface jet regime sets in as the amplitude of the surface wave becomes small (i.e., less than $0.1h$, where $h$ is the head). An empirical equation was developed based on the literature data to predict the discharge reduction factor in the generally used discharge equation. This formulation was compared with other formulations in the literature. It was found that the proposed formulation provides a slightly better estimation of flow discharge.

On a gabion stepped chute, the steps contribute to the dissipation of turbulent kinetic energy, free-surface aeration may be intense, and there are complex interactions between the free-surface flow and seepage motion. Detailed measurements were conducted in a relatively large gabion stepped spillway model. Using a combination of high-speed movies and phase-detection probe measurements, the air–water flow properties in the step cavities and in the gabions were documented. Strong air–water exchanges between seepage and stepped cavity flows were observed. The data showed a complex bubbly seepage motion in the gabions associated with a high level of interactions between seepage and free-surface flows, leading to a modification of the step cavity recirculation and lesser flow resistance.

Saint-Venant Equations (SVE) are often simplified for the sake of practicality, faster computational times, or physical representation. The Gravity Wave Model (GWM), or Local Inertial Equations, is a simplification of the SVE whereby the convective terms are neglected. The aim of this work is to present an analytical solution for the dam-break problem based on the GWM set of equations without source terms using the method of characteristics (MOC) and compare it with the analytical solution of SVE. The formulas for the depth and velocity are derived and explained along with the wave propagation characteristics. Conclusions are drawn about the propagation and analytical solution.