Journal of Hydraulic Engineering

Improving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent flow and sand transport over a bed of cobbles with a median diameter of approximately 150 mm. This change in scale expands the generality of the previous experiments and broadens the range of sand transport and turbulence measurements. It was found that the same relationship between bed shear stress and sand elevation was valid for both gravel and cobble systems. Reductions in bed shear stress, relative to the clear-water case, were observed as the sand elevation was increased, although the highest sand elevation did not yield the lowest shear stress. Quadrant analysis showed that, for stronger turbulent events, there was an increase of sweeps and a decrease in bursts as the sand level was raised. This effect was observed for a region with a height of approximately 1.4 times the thickness of the roughness layer.

The hydraulics of stepped spillways with flat steps has been studied for the last three decades, including for embankment dam slopes, but studies of alternative stepped designs are limited. In this study, a pooled stepped spillway was investigated in a relatively large-size facility, and three different pool wall porosities were tested. The flow patterns, the macro- and microscopic air-water flow properties, and the energy dissipation performances were recorded; the results were compared with the flat stepped spillway design for the same chute slope ($\theta =26.6°$). The investigations highlighted a close agreement between air-water flow properties on the configurations in terms of void fraction, turbulence levels, bubble count rate, and chord sizes. The interfacial velocity distributions showed larger interfacial velocity on the pooled step configurations of approximately 5–10% linked with a reduced flow depth. On the porous pooled stepped spillways, the interfacial velocities within the cavity highlighted the flow through the pores and the reduction in cavity recirculation. The porous pooled weir reduced the form drag, and the residual energy was approximately 1.5–2 times larger on the porous pooled stepped chute with $Po=31\%$ and approximately 1.3 times larger on the porous steps with $Po=5\%$ compared with the flat stepped chute. The flat step design appeared to be the most advantageous in terms of flow stability and energy dissipation performance.

Transcritical flows in three and four branch channel intersections have been studied experimentally, and an empirical relationship is derived from the data to express the flow distribution as a function of the Froude number of the flow in the main channel downstream of the intersection, and in the case of the four-branch intersection, the ratio of the incoming flows in the two inlet channels. The empirical relationship agrees closely with the data from this study, and with data from other studies in comparable conditions. The results illustrate an important point which has been overlooked in some previous studies—the existence of a critical section in the lateral outflow channel is not sufficient to isolate flow in the intersection from the downstream control in the lateral channel. It is shown that if drowned flow occurs in the lateral channel, then the downstream control can affect the flow in the intersection.

An experimental investigation of supercritical uniform and gradually varied open channel flows is presented for a wide range of Froude numbers and flume width-to-flow depth aspect ratios. The instantaneous streamwise and vertical flow velocities were measured in a laboratory flume over the entire width using a two dimensional–laser Doppler anemometry (2D-LDA) system to determine turbulence intensities, and bed and Reynolds shear stresses. The mean velocity patterns show undulation across the flume, indicating the presence of counterrotating secondary current cells. These currents redistribute turbulence intensities and bed and Reynolds shear stresses across the flume. For aspect ratios $\le 4-5$, i.e., narrow open channel flow, the velocity-dip phenomenon is identified both in the streamwise velocity and the Reynolds shear stress distributions. For high aspect ratios, i.e., wide open channel flow, the strength of secondary currents diminish toward the flume center, resulting in a 2D flow farther away from the walls and no velocity-dip phenomenon. Froude number effects on the flow characteristics are less pronounced compared to the aspect ratio effects. At high Froude numbers, the results for narrow and wide open channel flows agree well with literature data. The log-law holds in the inner region across the entire flume width for all investigated Froude numbers and aspect ratios. The Reynolds shear stress distribution agrees well with the computed spanwise bed shear stress distribution. At the flume side walls, the bed shear stresses are $20–50\%$ higher than the mean values. These results are verified with an engineering example in which high sediment transport and corresponding deep abrasion patterns at the side walls were observed.

The Tai Hang Tung Storage Scheme (THTSS) is an essential part of the West Kowloon Drainage Improvement Project that aims to solve the flooding problem in Mongkok, a congested and densely populated district of the Hong Kong Special Administration Region of China. The heart of the flood detention scheme is a $\mathrm{100,000}\text{-}{\mathrm{m}}^3$ underground storage tank beneath the Tai Hang Tung Recreation Ground, a triple side-weir system, and transitions to the upstream inlets and downstream channels. During heavy storms, part of the flood flow will spill over the side weirs into the underground storage tank and be temporarily stored; the flow diversion serves to attenuate the flood peaks and prevent downstream flooding. The THTSS was commissioned in 2005, and flow monitoring has been conducted since 2007. The operation of THTSS has essentially resolved the historic flooding problem. Nevertheless, the THTSS performance was put to an extreme test on June 7, 2008, when the highest hourly rainfall on record occurred. The heavy storm led to rapid filling up of the storage tank and minor local street flooding. This storm event suggested the need to use hydrodynamic models to evaluate the performance of the THTSS system. One-dimensional (1D) and two-dimensional (2D) mathematical models have been developed for the THTSS system including the side weir channels and transitions. The complex hydraulics in the triple side-weir system is first studied via a 2D numerical solution of the shallow water equations using a shock-capturing finite-volume method; the flow behavior and weir overflow distributions are compared with laboratory experiments and field data. Measured flood levels in the surcharged system are also studied using a 1D unsteady flow model for key storm events. The calibrated models are used to analyze postcommission performance of the system and to interpret the complex turbulent flows measured during the field survey.

The failure of conventional methods for discharge prediction in compound channels results from their ignorance of the loss in conveyance arising from the complicated flow interaction between the main channel and its associated floodplains. In this paper, a new method is presented for assessing the stage-discharge relationship and the discharge distribution in compound channels by considering both the momentum transfer between the upper and lower main channel flows and that between the upper main channel and its adjoining floodplain flows. A calibration using data from the laboratory channels and three natural rivers demonstrates that the presented method can give predictions that agree better with the experimental and field data. The computed results also show that the method not only is well capable of predicting the discharge distributions of the floodplain and the whole main channel, but also of predicting the discharge distribution in the lower main channel. In addition, the momentum transfer coefficient introduced in the presented method is also further discussed.

Scour characteristics downstream of rock W-weir structures were studied in a horizontal straight channel and under clear water conditions. For each structure, different hydraulic conditions were considered, including densimetric Froude numbers, water drops, and tailwaters. Results showed that the tailwater depth plays an important role in predicting the scour parameters. Dimensional analysis was used to derive equations to predict the maximum scour depth and length and the maximum dune height and length. Selected tests were conducted with an open W-weir. In this case, results show that the maximum scour depth decreases and its location shifts downstream with respect to classical W-weirs. Three types of scour morphology were classified: Type A, characterized by one scour hole and two ridges; Type B, with two scour holes and two ridges; and Type C, characterized by only one scour hole and one ridge.

The writers examine possible remediation options to mitigate the dangers found at low-head dams, using computational fluid dynamics and physical model tests. The objective was to identify an option that could be relatively simple to implement, would reduce the strength, or eliminate the dangerous reverse roller found downstream of low-head dams. Two promising options were identified, consisting of (1) ramps spaced along the channel width, or (2) horizontal platforms protruding from the downstream face of the dam. Many designs were investigated, resulting in six that were rigorously tested for flat-topped and ogee-crested weirs. The effectiveness of the options was evaluated based on entrapment times of dummies introduced into the flow. One of the platform designs was the most effective option on both dam shapes. A ramp design also had good results, but was only tested on flat-topped dams.

A simple theoretical model is presented in order to predict the recirculation velocity in river embayments, also commonly referred to as eddies, gyres, cavities, and dead zones. This velocity is important as it sets the fundamental timescale of the motion in these ecologically important regions. Using only an equilibrium torque balance, basic geometrical properties, and resistance characteristics, the eddy velocity is determined as a percentage of the freestream velocity. The model is restricted to the single-gyre case, placing modest limits on the embayment aspect ratio. The result compares favorably with recent experimental, computational, and field studies of river embayments.

Rock riprap is one of the most widely used erosion control techniques for protecting embankments, levees, spillways, and in-stream structures subjected to overtopping flow conditions. The literature indicates that over 21 riprap design expressions exist to appropriately size the stone. However, each of these relationships was developed from limited databases with narrow bands of boundary conditions. A power regression analysis was performed using 102 overtopping observations from 10 separate investigations; the analysis related median stone size to slope, unit discharge, coefficient of uniformity, rock layer thickness, and stone specific gravity. A unique, riprap-sizing expression was developed with a coefficient of determination ($R^2$) of 0.97 and a variance of 11%. The resulting design relationship expands the boundary conditions of the independent variables beyond earlier efforts, thereby reducing the need for extrapolating previous relationships.

A one-dimensional (1D) discontinuous Galerkin morphodynamic model has been devised with application to simulate of dam-break flows over erodible beds with suspended sediment transport. The morphodynamic equations adopt the shallow-water equations (SWE) considering the interaction of sediment transport and bed changes on the flow. A local second-order Runge-Kutta discontinuous Galerkin (RKDG2) model has been reformulated to numerically solve the morphodynamic equations in a fully coupled manner and with a noncapacity sediment model. The model’s implementation is thoroughly detailed with focus on the discretization of the complex source terms, the treatment of wetting and drying, and other stabilizing issues pertaining to high-solution gradients and the transient character of the topography. The model has been favorably applied to replicate experimental dam-break flow over erodible sediment beds.