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.

Modeling of hydraulic machines is fundamental in steady and unsteady analysis. The simulations carried out in different scenarios will define the need for protection devices or other mechanisms that ensure the operational safety of the installation with minimum investment. In addition, the employment of pumps operating as turbines (PATs) for small hydropower plants are very common. PATs are also being considered for use in water supply networks for pressure control and energy generation. In this case, modeling the reverse rotation is necessary to evaluate project feasibility. This study proposes a method to predict the complete curves of pumps using only their normal operation data and curves of other machines with similar specific speed. The complete curves are modeled as a trigonometric series, where the coefficients are fitted using the particle swarm optimization (PSO) technique. The results are compared with laboratory tests and a simulation of a pump shutdown is made to verify the differences using the modeled curve and a similar one.

A new jet-erosion test device has been developed to measure the erosion volume of soils under laboratory conditions. This device allows the determination of a complete erosion profile in addition to the depth of erosion below the jet. Jet-erosion tests using this new device were carried out on a silty soil to study the influence of the compaction dry density and water content on the erosion volume, critical shear stress (${\tau }_c$), linear erodibility coefficient ($k_D$), and volumetric erodibility coefficient ($k_D^{*}$). The results of these tests showed the major roles played by the compaction dry density and suction, and the important effects of the soil fabric and permeability on the penetration of water into the soil sample. An inverse relationship was observed between the volumetric erodibility coefficient and critical shear stress, which allowed an estimate of $k_D^{*}$ as a function of ${\tau }_c$. The relationships between $k_D$ and ${\tau }_c$, and between $k_D^{*}$ and ${\tau }_c$, are quasi-parallel lines with smaller values of $k_D^{*}$. Using the volumetric erodibility coefficient ($k_D^{*}$) rather than the usual linear erodibility coefficient ($k_D$) may lead to better prediction of erosion in hydraulic works because this coefficient is more-directly related to the loss of soil that must be quantified in practice.

Tracer tests were conducted using floaters equipped with a global positioning system (GPS) to investigate the turbulent mixing of the floating pollutant at the surface of natural rivers. The position data were collected by tracking the floaters at different reaches of the Nakdong River in Korea, and the diffusion coefficients were calculated by applying the moment method to the floater distribution data collected. While applying the moment method, the diffusion coefficient was calculated from the time evolution of variance of the particle distribution, which becomes the distance from the centroid of the particle cloud, based on the homogeneous turbulence assumption and the ergodic hypothesis of the turbulent mixing. Analysis of the trajectories of GPS floaters showed that the horizontal mixing in the initial mixing stage was hampered by the interaction and collision between floaters. The experimental results revealed that the dimensionless streamwise diffusion coefficients were larger, in the range of 0.11–0.56, while the spanwise component was in the range of 0.02–0.16. The simulation results using the Lagrangian particle tracking model of the Environmental Fluid Dynamics Code program showed that the discrepancy between the predicted values and the observed data in the initial time gradually decreased as time elapsed.

The distribution of water is an energy intensive process in which potential to recover energy currently wasted exists, via the installation of microhydro energy recovery systems. Pump-as-turbine (PAT) technology offers an economically viable solution; however, the lack of performance data for PAT technology is a major barrier to its widespread use. As a context, the water distribution network (WDN) also adds technical complications to the use of PATs: Pressure needs to be regulated to allow effective distribution of water to consumers and flow conditions are based on user demand and thus, are variable. This paper outlines the result of experimental investigations of this concept at laboratory scale, quantifying its potential as an energy recovery device for use within the WDN. Hydraulic arrangements where flow is fixed through the PAT and variable through the bypass yielded maximum potential energy recovery. Up to 39% of the available energy was shown to be recoverable using this arrangement.

Experiments using a 1∶30 scale physical model show that channel degradation on alluvial fans is dominated by lateral channel migration rather than vertical incision. The results are used to estimate the exposure probability during single flood events with peak flows up to twice the formative flow, considering both the burial depth beneath the channel bed and the setback from the channel banks. For the largest flows modeled, the exposure probability fell below the detection limits for this analysis when the burial depth was greater than approximtely 3.6 times the mean flow depth, and the lateral setback distance was greater than approxinately 0.9 times the mean flow width for the formative flow. The minimum depth-of-cover criterion accounts for the worst-case occurrence of net bed degradation during a single flood event, but does not consider the vertical degradation that could occur in channels because of knickpoint development and migration, or in channels with engineered banks that prevent channel width adjustments; they also do not consider the potential effects of debris flows. These hazards are driven by different processes and require different analyses to evaluate the potential exposure risk. Because the experiments evaluated the effects of single flood events, the results do not account for shifting channel position over time, and they are intended to guide monitoring of channel behavior for existing infrastructure. The results also can be used to guide infrastructure design, but in this case the design will need to consider the cumulative effect of channel migration and avulsion over the design life span of the infrastructure.

The propagation of gravity currents is analyzed in the presence of regular surface waves, both experimentally and numerically, by using a full-depth lock-exchange configuration. Full-depth lock-exchange releases have been reproduced in a wave flume, both in the absence and in the presence of regular waves, considering two fluids having densities ${\rho }_0$ and ${\rho }_1$, with ${\rho }_0<{\rho }_1$. Boussinesq gravity currents have been considered here (${\rho }_0/{\rho }_1\sim 1$), with values of the reduced gravity $g^{\prime }$ in the range $0.01–0.1\mathrm{m}/{\mathrm{s}}^2$, while monochromatic waves have been generated in intermediate water depth. The experimental results show that the hydrodynamics of the density current are significantly affected by the presence of the wave motion. In particular, the front shows a pulsating behavior and the shape of the front itself is less steep than in the absence of waves, while turbulence at the interface between the two fluids is damped out. In the present test conditions, the average velocity of the advancing front may be decreased in the presence of the combined flow as a function of the relative importance of buoyancy compared with wave-induced Stokes drift. Moreover, a new numerical model is proposed, aiming at obtaining a simple, efficient, and accurate tool to simulate the combined motion of gravity currents and surface waves. The model is derived by assuming that surface waves are not affected by gravity current propagation at the leading order and that the total velocity field is the sum of velocities forced by the orbital motion and those forced by buoyancy. A Boussinesq-type wave model for nonstratified fluids is solved, and its results are used as input of a gravity current model for stratified flows. Comparisons of the numerical results with the present experimental data demonstrate the capability of the model to predict the main features of the analyzed phenomena concerning propagation of the density current (e.g., averaged velocities, front height), the increase of entrainment of the ambient fluid into the density current in the presence of the waves, and the intrawave pulsating movement of the heavy front.

The collection of time-sensitive data on real-life embankment failures and their analysis are essential steps to model breach processes and the consequential flood, which are of great importance for preparing emergency action plans. In this paper, data on 14 earthen embankments that failed or were damaged in the Midlands of South Carolina resulting from the historic 1,000-year storm during October 2015 are presented and analyzed. The investigation includes measurement of breach dimensions, collection of undisturbed soil samples, soil classification, embankment erodibility tests using the submerged jet erosion test (JET) method, calculation of peak discharge, estimation of maximum height of overtopping, and calculation of maximum reservoir volume behind each embankment at the time of failure. Using this information and results, single-variable and multivariable parametric breach models for the breach depth and breach width are developed and compared with selected breach models available in the literature. All the models except for one estimate the breach width satisfactorily, with the proposed single-variable equation giving a relatively better prediction. The collected data set of this study may be used by others for model verification.

Measurements of turbulent open-channel flow subjected to upward seepage (injection) were performed in a rectangular laboratory flume using a two-dimensional electromagnetic velocimeter. The influence of vertical groundwater flow on open-channel hydrodynamics was analyzed. The output of these experiments demonstrates that the induced seepage affects the dynamics of turbulence in open-channel flow, even though the magnitude of the groundwater flow velocity is significantly smaller than the magnitude of the free surface water flow velocity. Upward seepage affects the instantaneous water flow velocity of the open channel and its turbulence characteristics. It is observable that upward seepage modifies the shape of the profiles of the time-averaged shear stresses and turbulent kinetic energy. In most cases, the studied open-channel flows do not follow the $-5/3$ law of isotropic turbulence.

The bedforms on a biofilm-coated sediment (biosediment) bed are different from those on a noncohesive sediment bed under an identical flow condition and sediment particle size without biofilm. Bedforms on biosediment beds can therefore modify the resistance to flow. In this study, experiments were conducted for the bedforms on both cohesive and biosediment beds. The biofilm coat around the fine cohesive sediment particles of a median size less than 0.1 mm was cultivated within a laboratory flume. The bedforms on biosediment beds are identified as dunes by comparing the Froude number and the Shields parameter. The equations of bedform steepness, bedform height, and equivalent roughness for biosediment beds are empirically derived. The maximum variations of equivalent roughness, Chézy coefficient, and Manning coefficient for a biosediment bed are 96.7, 60.5, and 44.3%, respectively, as compared to those for a noncohesive sediment bed under an identical flow condition and sediment size without biofilm.