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.

In 1997, Toledo developed a numerical model to characterize the flow through the downstream shell of a rockfill dam for the overtopping scenario. He performed a parametric study focused on nondeformable, completely saturated dams and analyzed the effect of different geometric and rockfill parameters on the characteristics of pressure heads and throughflow, such as the infiltration and emergence lengths. This paper provides experimental validation of this theoretical work. Six physical models were tested in a flume, modeling the downstream shell of 1-m high rockfill dams and using three uniform gravels with D50 sizes varying between 0.0126 and 0.045 m. The conceptual frame developed by Toledo was confirmed, and the derived results allow the estimation of the minimum discharge needed for full saturation of the downstream shell, and also the determination of the pressure heads and the position of the first emergence point for that saturated state. Differences between the physical and numerical data varied around 23% for the saturation discharge and around 7.0% for the pressure head, and were reduced to 3.9% for the emergence length. Experimental observations were also used to fit new formulas for the phreatic surface profile. Future research is needed in areas related to the anisotropy of rockfill dams and the definition of nonlinear resistance laws based on the main characteristics of the rockfill.

In this paper, submerged soil excavation by high-velocity water jet was investigated by means of a two-fluid smoothed-particle hydrodynamics (SPH) method. A critical state theory was coupled to account for the dilatancy or compaction of the soil subjected to deformation. Numerical simulations for excavations by plane wall jet and vertical impinging jet are presented. The development of the crater size during the jetting process was obtained and found to be qualitatively in agreement with experimental observations and numerical simulations provided by other researchers. The effects of the nozzle width, impinging height, jetting velocity, and water flux on the formation of different crater patterns were examined. The effects of soil parameters, such as cohesion [in the range of $(0,25\text{kPa})$] and dilatancy, were also investigated. Computational results illustrate the critical role of dilatancy in the coupling evolution of the solid volume fraction and pore fluid pressure by modifying the Coulomb friction and thereby regulating the dynamics of soil deformation. The proposed method is robust and efficient, and can be applied to water-soil mixture flows in subsea engineering and geomechanics.

Recent research shows the potential of resonant frequency-based leakage detection methods. However, there is a disagreement in whether a leak shifts the normal modes (often called natural or resonant modes) and whether a leak introduces additional peaks to the frequency response function (FRF) of the pipeline. In this paper, the impact of a leak on the normal modes is investigated. The trajectories of normal modes in the frequency complex plane with varying leak size are studied. The key parameter that represents the leak size and controls the trajectories of the normal modes is the ratio of the acoustic impedance of the pipe to the resistance impedance of the leak. It is found that, as the impedance ratio increases from zero (i.e., no leak), each normal mode shifts toward the upper-half complex plane of frequency by a leak, where the imaginary part is a measure of the leak-induced damping of the wave. When the impedance ratio is less than the order of one, the leak-induced normal-mode frequency shift is negligible, which supports the theory put forward by proponents of the no-shift and no-additional-peak hypothesis. When the impedance ratio is of the order of one or larger, not only is the shift of the FRF’s peak significant, but also new peaks appear, which supports the theory raised by proponents of the leak-induced additional peaks hypothesis.

Due to random noise in real measurements, leak detection (estimation of leak size and location) is subject to a degree of uncertainty. This paper provides a framework to investigate the lower bound of the variance of a leak’s variable estimation and delineates the parameters upon which this lower bound depend. This is accomplished by applying the Cramer-Rao lower bound (CRLB) principle to the leak detection problem. For a given data set, CRLB gives the minimum mean square error of any unbiased estimator. The CRLB is evaluated using the Fisher information, which is evaluated from direct differentiation of the water-hammer characteristics equations. The results show that the CRLB of the leak-size estimate increases with time of closure and noise level but reduces with the duration of the measured signal. It is also shown that the CRLB is instrumental in the systematic design of efficient transient tests for leak detection. The error of leak-size estimates rises remarkably with setting distances between consecutive potential leaks of less than half the minimum wavelength of the probing signal. More conclusions are drawn on appropriate mesh-size for inverse transient analysis (ITA), maximum possible accuracy in successful localization, and its probability subject to the physical situation’s parameters.

Several pipeline blockage detection methods assume blockages have a regular geometric shape; that is, the blocked section is a pipe of smaller diameter than the original. These methods require a priori knowledge of the number of blockages in the pipeline. Restricting blockages to such special forms also raises identification issues such as nonuniqueness, nonconvergence, and computational inefficiencies. This paper develops a detection method that does not assume regularly shaped blockages and that reconstructs an internal pipe area of unconstrained form. The mathematical and physical bases of the proposed method are described, and a step-by-step solution algorithm is provided. A numerical example of a pipe with irregular blockages is considered to test the performance of the method. It is found that the proposed method accurately identifies multiple blockages of arbitrary shapes and sizes at a relatively low computational cost. The described method is compared with a method for area reconstruction as well as a method for blockage characteristic identification. The proposed method is shown to be more accurate and efficient than the other methods.

Determining the roughness of riparian vegetation is often a challenge in river hydraulic modeling. A large number of quantitative methods have been developed, but none of them have been integrated into any one-dimensional (1D) hydraulic model for directly computing dynamic vegetation roughness. In this study, we evaluated the performance and applicability of eleven vegetation roughness computation methods by integrating them into a widely used 1D Hydrologic Engineering Center-River Analysis System (HEC-RAS). The enhanced HEC-RAS was applied to predict river stages of the San Joaquin River reach with dense and diverse riparian vegetation. Among the eleven vegetation roughness methods, the method of Freeman et al. clearly overestimates Manning’s roughness coefficient ($n$) values for flow depth greater than its original experimental flow depth (1.5 m). The method of Whittaker et al. considers the reconfiguration of flexible vegetation and uses measured vegetation projected area, resulting in a slightly more accurate prediction of river stages than all other methods. The methods that model vegetation as a rigid cylinder produce identical and reasonable river stage predictions. The Järvelä method includes the impact of velocity on vegetation roughness and uses an indirect metric leaf area index to represent the vegetation projected area, which produces a river stage prediction that is similar to other methods based on a rigid cylinder analogy. Compared with the original model with manually calibrated Manning’s $n$, the San Joaquin River HEC-RAS model integrated with dynamically computed roughness methods is able to predict observed river stages and performs notably better for flood flows exceeding the maximum calibration flow. The present study also demonstrates that methods that model vegetation as a rigid cylinder are applicable for computing dynamic $n$ values in 1D hydraulic simulation for the areas in which most riparian vegetation is not fully submerged and dominated by trees and shrubs.

Earth dike failure due to overtopping flow produces a significant displacement of bed-sediment mass. While sediment dynamics typically prevails as a nonequilibrium condition, the bed deformation is resulted from the simultaneous bed erosion and dike body displacement. However, most of the existing shallow water erosion models do not resolve the dynamic bed deformation. In this study, a depth-averaged nonequilibrium sediment transport model is developed and coupled with a set of Savage-Hutter type equations to characterize the dynamic bed deformation, leading to an innovative approach to tackle flows over erodible/deformable beds. The one-sided first-order upwind finite-volume method was adopted for the solution of the system of conservation laws of flow and bed deformation (granular mass transport). A static resistance condition for granular mass was thoroughly defined to preserve the numerical stability of the equations representing the bed deformation component of the model. The model was tested against the experimental data sets for dike overtopping flow, dam-break flow of dry granular mass on a slope, and the analytical solutions. The proposed model enhances the prediction capability by the existing shallow water equations-based equations due to the dynamic bed deformation modeling.

Nappe oscillation and its potentially undesirable impact associated with significant noise production can be an issue for free-overfall hydraulic structures. Although nappe oscillation has been observed on various prototype free-overfall structures, this instability behavior may not be evident during the design process in which experimental and/or numerical modeling may be utilized. In addition, previous studies regarding nappe oscillation scalability using similitude or other relationships has received very limited attention in the literature. An experimental study aimed at investigating the possible size scale effects on nappe oscillations was undertaken utilizing two experimental facilities: a prototype-scale linear weir (3-m fall height) and a geometrically similar 1:3-scale model (1-m fall height). The nappe oscillation occurrence assessment and oscillation frequency evaluation were performed using sound and image analyses. Experiments on both models showed that the nappe oscillation phenomenon generally occurs over a fixed range of unit discharge and is independent of size scale. Nappe oscillation can therefore not be reproduced at different model scales according to standard similarity laws. This study also highlights the secondary influences of the crest profile and the fall height on the oscillation characteristics.

The analysis of transients in a pipeline system has been primarily studied for a reservoir pipeline valve system with a single valve maneuver. This paper introduced a method to handle transients associated with multiple generation sources in the frequency domain. The combination of multiple transients can be effectively addressed through transient analysis of the multiple reservoir pipeline valve (MPRV). The impedance method was extensively developed to effectively represent the hydraulics of the MPRV. The impedance at any location along the MPRV system was derived using the relationships of complex head and discharge along the pipeline component, common head condition, and continuity condition for the junction. Both partially or fully closed and open boundaries of the control valve were implemented into the generalized impedance formulation. Several hypothetical examples were introduced to show the validity of the developed method through simulation comparisons with the method of characteristics. The impedance method provides a new capability in the pressure signal analysis, which allows the decomposition of the pressure head, depending on the pressure generation source. The generation of transients at different times can also be feasibly addressed using the proposed method.

The method of large eddy simulation (LES) was employed to investigate the flow and turbulence structure around bridge abutments of different lengths placed in a compound, asymmetric channel. The simulations were faithful representations of large-scale physical model experiments that were conducted in the hydraulics laboratory at the Georgia Institute of Technology. The experiments are considered idealized hydraulic models of the Towaliga River bridge at Macon, Georgia, consisting of flat horizontal floodplains on both sides of a parabolic main channel, two spill-through abutments with varying lengths [long-set back (LSB) and short-set back (SSB)], and a bridge spanning across the abutments. In the LES, a free flow scenario was simulated where the water surface was not perturbed by the bridge at any point. The Reynolds numbers, based on the bulk velocity and hydraulic radius, were 76,300 and 96,500 for LSB and SSB abutments, respectively. Validation of the simulation results using data from the complementary experiment is presented and agreement is found to be reasonably good. A thorough comparison of various flow variables between LSB and SSB scenarios to highlight the effect of flow contraction was carried out in terms of flow separation and instantaneous secondary flow, streamwise velocity, streamlines, stream traces, and turbulence structures. Further flow instability and vortex shedding generated in the shear layer downstream of the abutments were quantified by analyzing time series of the instantaneous velocity in the form of the probability density function, quadrant analysis, and power density spectra.

Acknowledged as a significant cause of bridge failure is abutment scour around bridge foundations. Because of the complexity of flow structures and scour mechanisms around an abutment, abutment scour remains the focus of extensive research. In this study, to elucidate the complex flow physics and resulting sediment transport around an abutment, time development of abutment scour in three flow types of free, submerged orifice, and overtopping were examined by conducting laboratory experiments in a compound channel. Different lengths of erodible abutment were used to explore the effect of flow contraction on the abutment scour. Detailed bed contours at selected time intervals, as well as velocities, for the initial conditions were measured with acoustic Doppler velocimeters during each experiment. From observations, the scouring process and the resulting scour hole shape and scour depth around an abutment over time were found to be inconsistent depending on the flow type and degree of flow contraction. Tracking the maximum scour location over time explained the complex scouring process around an abutment, which is a simultaneous effect of local turbulence and flow contraction.

Velocity correction methods are often used in numerical models simulating fluid flows. This paper presents a new method to evaluate the velocity correction coefficient: an equation constructed from the coefficient matrix of the linearized momentum equations is simultaneously solved for the velocity correction coefficient. In addition, the implicit equation of correction coefficients (ECC) method includes a smoothing mechanism, which makes it numerically more stable than the commonly used Semi-Implicit Method for Pressure-Linked Equations-Consistent (SIMPLEC) method because larger relaxation factors and time steps can be used. The ECC method was integrated into a two-dimensional (2D) depth-integrated unstructured finite-volume method (FVM) model based on a hybrid mesh system (triangle and quadrilateral). It was demonstrated using one experimental case and a field case. According to the numerical tests, the proposed ECC method can not only enhance the numerical stability but also improve the computing efficiency due to the capability of using large relaxation factors and large time steps for the pressure correction–type models.

Detrending is required to enable robust parameterization of surface roughness by removing large topographic trends from fluvial data. A range of detrending methods exist in the literature; however, there is limited presentation of the effect of these methods on output roughness statistics. Here, the results of non-detrended data compared with flat-surface detrending (e.g., removal of bed slope and setup misalignment) and moving-window detrending (e.g., removal of nonlinear bed undulations) are presented for a wide suite of roughness statistics. Digital elevation models (DEMs) of gravel surfaces obtained from the field and the laboratory are analyzed. Roughness statistics include the coefficient of variation of the standard deviation of elevations, standard deviation of elevations, skewness, kurtosis, inclination index, and horizontal roughness lengths from second-order structure functions. Similarities exist between non-detrended data and the data detrended using flat-surface detrending. The moving-window detrending method, which removes larger topographic signatures and roughness scales (e.g., the influence of bed forms), results in lower roughness statistics in the coefficient of variation, standard deviation of elevations, and horizontal roughness lengths. In contrast, higher values of skewness and kurtosis are observed with the moving-window detrending method. These observed differences in roughness statistics for studied detrending methods support the use of a moving-window detrending method for robust grain-roughness parameterization. Furthermore, it is highlighted that comparisons between grain-roughness studies need to ensure that the same procedure of detrending has been applied.

Studying the flow through a dam breach is important because it is characterized by a great deal of uncertainty and can be associated with loss of lives and properties. In the present study, planar dam breach erosion due to overtopping is simulated using a three-dimensional (3D) computational fluid dynamics (CFD) model, FLOW-3D. The model parameters are calibrated by comparing the results of the dam surface elevations at different time instants with those of an earlier experimental study. Also, the results for maximum velocity and turbulence characteristics [turbulent kinetic energy (TKE), turbulence intensity (TI), and the excess shear stress (ESS)] are presented for the calibrated model. Sensitivity analysis of the parameters (dam material, $d_{50}$, dam height, $H_0$, downstream slope, $s$, width of the crest, $b$, and inlet discharge, $Q$) affecting the dam breach is performed to study the erosion rate and peak discharge through the breach section. Empirical equations for the peak breach discharge and erosion rate are presented using the simulated results. The study reveals that the dam material, inlet discharge, and dam downstream slope are among the most influential factors affecting the breaching process.