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.

Secondary circulation is a mechanism that increases vertical exchange between the horizontal layers in open channels. Vertical shear in curved flows induces secondary circulation in the plane normal to the primary flow direction. Models for secondary circulation have been developed that include the nonlinear advection terms in the momentum equations. A consequence of this is that secondary circulation deforms the vertical profile of the primary flow. This in turn reduces the vertical shear of the primary flow thus reducing the driving force of secondary circulation. Observations from a bend of moderate curvature in the Clutha river show flattening and deformation of the vertical profile of the primary flow due to the advection of momentum by secondary circulation. The observations are on a much larger river than previous studies so the increase in scale makes these observations robust. A counterrotating outer-bank cell of secondary circulation is associated with this deformation. A visual explanation for this outer-bank cell is proposed based around the forces of plane equilibrium and the deformation of the vertical profile of the primary flow. A suggested requirement for a counterrotating outer-bank cell is that the depth average velocity must be found at two depth locations on the vertical profile of the primary flow. The width of this outer-bank cell is twice that of previous flume experiments, perhaps due to outer-bank roughness. The strength of this outer-bank cell is approximately 0.5 of the main cell. Calculation of a bend parameter $\mathcal{B}=3.7$ classifies the curvature for this survey site as weak to moderate. The deformation of the vertical profile of the primary flow is greater than expected for a bend parameter of $\mathcal{B}=3.7$, perhaps due to the increase in size and strength of the outer-bank cell.

Sediment transport is a geophysical phenomenon in which sediment particles are driven to move in streamwise and vertical directions by various forces. Almost all existing formulas of sediment transport were derived without considering vertical flows $V$, resulting in a large discrepancy between measured and predicted transport rates, as has been reported in the literature. This paper investigates the effect of vertical motion on sediment transport. It was found that upward fluid velocity increases particles’ mobility, and downward motion increases particles stability. Furthermore, the investigation showed that decelerating flows can promote upward flow and vice versa. New equations were developed to express the influence of vertical motion on sediment transport. A reasonably good agreement between measured and predicted sediment transport rates was achieved.

Dual spillway interference refers to the loss of hydraulic performance of spillways when they are placed close together. Spillway interference is examined using both physical experiments and numerical simulations. Stage and discharge measurements from four physical models with dual spillways configurations are compared to the Flow-3D computational results at four dam sites in South Korea. The conjunctive use of two spillways is compared with the singular operation of each spillway. When both spillways are operated at the same time, the total flow rate through the two spillways is reduced by up to 7.6%. Interference coefficients are most significant when the stage $H_e$ exceeds the design stage $H_d$ and when the distance $D$ separating two spillways is short compared to the spillway width $W$. The parameter $D{H}_d/W{H}_e$ correlates very well with the calculated and measured interference coefficients. A flood routing example for the design discharge at Andong dam shows a 42 cm difference in reservoir water level with and without application of the interference coefficient. Consequently, the width of additional spillways (including the interference coefficient) should be increased for dam safety.

The collapse of the Fundão tailings dam in Brazil spilled 32 million cubic meters of mine waste, causing a severe socioeconomic and environmental impact in the Doce River. Approximately 90% of the spilled volume settled on floodplains, over 118 km downstream of Fundão Dam. A hyperconcentrated flow ($\approx \mathrm{400,000}\mathrm{mg}/\mathrm{L}$) reached the Doce River, where the floodwave and sediment wave traveled at different celerities over 570 km until the Atlantic Ocean. The one-dimensional advection-dispersion equation with settling was solved and calibrated to fit the observed concentration using an analytical solution to determine the longitudinal dispersion coefficient and the settling rate, both along the river and in the reservoirs. The longitudinal dispersion coefficient ranged from 30 to $120{\mathrm{m}}^2/\mathrm{s}$, and the sediment settling rate corresponded to particle sizes from 1.1 to 2.0 μm. Moreover, simulations with the calibrated model revealed that without the presence of three hydropower reservoirs on the Doce River, the sediment concentration at the watershed outlet would have been $\mathrm{64,000}\mathrm{mg}/\mathrm{L}$ instead of the observed $\mathrm{1,500}\mathrm{mg}/\mathrm{L}$. The temperature effects were also examined, and a decrease from 30°C to 5°C in water temperature would have caused a concentration 12 times higher at the outlet.

A two-dimensional (2D) layer-averaged model was previously developed to simulate turbidity current characteristics and its sluicing in reservoirs. It was calibrated and validated using the data from a down-scale physical model of the Shimen Reservoir with great success. This study aims to calibrate and validate the numerical model to predict turbidity currents in the Shimen Reservoir in the field-scale scenarios with the goal of using the model for real-time forecasts for reservoir operation. Field studies are also carried out to assist the model calibration and validation. Three field events surveyed include typhoons Soudelor and Dujuan in 2015 and Megi in 2016. The study serves three key points. First, it verifies that the previously developed 2D turbidity current model is also valid and practical for field applications, which has not been done before based on the authors’ knowledge. The model is capable of predicting turbidity current movement toward a reservoir dam face, as well as the sluicing out of outlets at different levels. The model results are compared with the available field data. Second, this study establishes guidelines on how to perform a practical turbidity current modeling in the field based on the model sensitivity study. It is found that the drag coefficient and mesh size are the primary parameters, and the proper ranges of these parameters are recommended. Third, the model is used to understand the turbidity current characteristics in the reservoir; they include the arrival time of the turbidity current, the concentration distribution in the reservoir, and sediment sluicing through the sluice gates. The results show that the developed 2D turbidity current model is mature enough that it can be combined with the real-time observation data to conduct the current rolling prediction and sluicing volume estimate. The modeling results provide the data necessary for the prediction of sediment turbidity current characteristics, as well as for the reservoir management.

Investigations are presented on the effects of grain-size composition on the flow resistance of debris flows via laboratory experiments with bidisperse granular materials. Two models focusing on whether the smaller particles behave as a completely solid or completely fluid phase are considered to describe the experimental flow resistance. To examine their performance, a blending factor is introduced that represents the relative contributions to the experimental flow resistance between these two models; results show that some cases are described by neither of the two models, which indicates that the smaller particles behave neither as a completely solid nor a completely fluid phase. As the blending factors are positively correlated with the suspension conditions of the smaller particles and are described by the ratio of the shear velocity or turbulent velocity to the settling velocity, the smaller particles behave in an intermediate way, with some as a solid phase and the rest behaving as a fluid phase, depending on the suspension conditions.

An investigation of the characteristics of live-bed scour at circular and skewed rectangular (slab) piers in comparatively shallow flows is presented. The 110 experiments were undertaken in a 2.4-m flume using two circular piers and a slab (long) pier. Based on flow shallowness, that is, the ratio of approaching flow depth ($y_0$) to pier diameter ($D$), the new data span the wide-pier ($y_0/D<0.2$) and transition-pier ($0.2\le y_0/D<1.4$) scour ranges. The Sheppard-Melville (S-M) method shows better performance than the other existing predictors and is more readily modified. Modification to the equation for the flow shallowness factor in the S-M method is proposed, based on both the data from the present study and those from literature. The results show the considerable influence of flow shallowness ratio on scour depth at piers in comparatively shallow water, and that, even for non–ripple-forming sediment, the scour depth at the live-bed peak may still exceed that at the clear-water threshold. The modified predictor provides better accuracy than the original Sheppard-Melville method for piers with $y_0/D<1$, and no significant underestimation of the maximum scour depth is observed.

This paper presents the results of an experimental study in which an acoustic Doppler velocimeter (ADV) was used to monitor the longitudinal velocity profiles and discharge proportions in a channel with a mobile bed and banks. The resultant channel bed elevations were also monitored. The physical model consisted of a channel with two branches and a midchannel bar. A spur dike was located at the upstream of the branches. The main variable parameters in this study were the length of the midchannel bar and the length of the spur dike. These variables were investigated by performing nine trials (with differing spur dike and channel bar lengths) with different corresponding discharges. Clear water conditions were maintained in the channel throughout this study. The results showed that the spur dike diverted and redistributed the discharge between the branches of the multithreaded channel. The highest discharge proportion was obtained when the spur dike’s length was one-third of the main channel’s width and when the midchannel bar’s length was short. In addition, the slope of the channel’s bed was steeper in the branch that carried the highest discharge proportion.