Journal of Hydraulic Engineering

Contact erosion is potentially initiated at the interface between two soil layers by a groundwater flow within the coarser material. Once eroded by the flow, particles from the finer soil are transported through the pores of the coarser layer. Fluvial dikes are often exposed to this phenomenon. Small-scale experiments combining refractive index matching medium, planar laser-induced fluorescence, and particle image velocimetry were carried out to measure the flow characteristics in the vicinity of an interface between a model granular medium and a fine graded sandy layer. Longitudinal velocities and shear-stress distributions were obtained in Darcy flow conditions. They revealed a long tail toward large values, which reflects the spatial variability of the constrictions in the pores network. Taking into account these distributions can improve the modeling of contact erosion by going beyond the simple use of mean quantities, like Darcy velocity, as is usually proposed in the literature. This is done by successively considering the variability of the porous flow and also that of the critical shear stress. As a main consequence, the resulting global erosion rate is now nonzero for any value of the mean shear stress and there is no longer a stress threshold. Finally, the effect of paving at the sand surface can also be added to the statistical model and makes it possible to account very satisfactorily for previous contact erosion tests at the sample scale.

Gravel-bed rivers display a surface layer that is coarser than the substrate lining below this armored surface. The armored layer that is developed under certain flow conditions may be attributed as water-worked sediment bed. Both the threshold of incipient motion and the bed-load transport are affected by the presence of this layer. In this study, the coarse surface development and its effect on the sediment transport were investigated experimentally in a rectangular flume. Four antecedent flow rates were combined with an input hydrograph, without sediment feeding. The reference shear stress of individual fractions and the grain-size distribution were determined after the development of the coarse surface due to the antecedent flow. Then a triangular-shaped hydrograph was generated for both the intact surface and the coarse surface cases. It was seen that the maximum bed-load transport values obtained in unsteady flow experiments were highly dependent on the coarse surface formed by the antecedent flow. The interrelation between the armor ratio and the total bed load was also sought. It was revealed that there existed a nearly linear relation between the armor ratio and the dimensionless total bed load with a correlation coefficient of 0.99. This strong interdependence implies that the knowledge of the armor ratio is of basic and utmost importance to predict accurately the bed load to be transported. The values of the Einstein bed-load transport parameter were plotted versus those of the dimensionless shear stress. When the global sediment transport was considered, a clockwise hysteresis appeared in the case of the initial intact bed surface, whereas a counterclockwise hysteresis arose when the coarse surface was developed. As for the fractional sediment transport, in the case of the intact initial surface the clockwise hysteresis was encountered more frequently, but for the armored bed experiments the coarser the bed surface, the more dominant was the counterclockwise hysteresis, meaning that the quantity transported during the falling limb of the hydrograph was higher than that transported during the rising limb.

A one-dimensional (1D) finite-volume model is developed for simulating nonequilibrium sediment transport in unsteady flow. The governing equations are the 1D mass and momentum conservation equations for sediment-laden flow and the sediment continuity equation for both bed load and suspended-load transport. The Rouse profile is modified to consider the nonequilibrium transport of suspended sediment. The spatial lag between the instantaneous flow properties (e.g., velocity, bed shear stress) and the rate of bed load transport in unsteady flow is quantified by using an adaptation length, which is derived theoretically by applying the momentum principle in the bed load layer. This new method for calculating the adaptation length is verified using data from several experiments and yields better results than other empirical formulas for a wide range of shear stress. The nonequilibrium model is applied to simulate a series of laboratory dam-break flows over erodible beds, and the simulated results agree well with the experimental measurements.

This paper presents an integration procedure for the Reynolds-averaged Navier-Stokes equations for the determination of the distribution of the streamwise velocity using the vertical component. This procedure is dedicated to the outer region and central part of channels. The proposed model is applicable to both rough and smooth flow regimes, provided the velocity at the inner-outer boundary has been properly defined. To generate a simplified expansion, a number of hypotheses are proposed, focusing in particular on the analytical modeling of the vertical component by adopting a negligible viscosity. The proposed hypotheses are validated by the experimental data existing in the literature. The proposed simplified expansion is studied through a sensitivity analysis and proved consistent in regards to model experimental data. The proposed model seems capable of demonstrating different kinds of flows, including dip phenomenon flow patterns.

Stricter environmental regulations have led to new wastewater treatment plants and a centralization of existing wastewater treatment plants. Therefore, pressurized wastewater mains have become an indispensable link between the collection systems and the treatment plants. In urban areas in particular, these pipelines include many inverted siphons to cross other infrastructure, like railways, motorways, other pipelines, and buildings. Accumulation of air in downward sloping sections of these wastewater mains reduces the transport capacity significantly. A dominant air transport mechanism is the air-entraining hydraulic jump at the tail of an accumulated air pocket. The novelty of this paper is a systematic investigation of the influence of surface tension on the air discharge in downward sloping pipe sections. Experiments have been performed with clean water, surfactant-added water, and wastewater. The experiments with surfactant-added water confirm that the air discharge increases significantly at lower surface tension. However, the lower surface tension of wastewater does not enhance the air transport in comparison with the air transport in clean water.

The Shields and the Isbash equations for predicting the stability of rocks exposed to a turbulent flow were both proposed in 1936, and since then, both equations have been used widely in the analysis of transport thresholds for coarse sediment. These two equations were obtained using two very different approaches, but as demonstrated in this paper, the equation developed by Isbash is consistent with the relation formulated by Shields for predicting the motion of sediments either in the flume or in the field. Comparison of the two equations suggests that standard approaches tend to oversize riprap in mountain streams.

Hydraulic jumps at the tail end of spillways are usually induced by baffle blocks or other obstacles. Such jumps can also be induced by jets that oppose the main flow. Another application is to back up turbidity currents in reservoirs by means of opposing jets. This measure can be adopted when transfer tunnels feed water into the reservoir at a higher elevation near the dam. Stopping the turbidity current increases the local sedimentation rate. To reconcile the shallow water equations for turbidity currents with those for open channel flows, mass-based scales for the depth and velocity of both types of flows are outlined. The continuity and momentum equation for flows opposed by jets are then stated in terms of these scales and expressed by a single curve for both gravity currents and free surface flows. The corresponding results for free surface flows agree well with those of experiments carried out for this study. An application to turbidity currents is provided as well.

Retrofitting embankment dams with stepped spillways has become a common design practice, particularly for those dams that change hazard classification from low to high. For embankment dams retrofitted with stepped spillways, the chute length is often insufficient for developing aerated flow or an inception point. The inception point is a key spillway design parameter used in energy dissipation, flow depth, and air entrainment prediction relationships. Original research for developing an inception-point relationship for stepped spillways was based on primarily gravity ($\theta \ge 26.6°$) stepped spillways, with the majority having an ogee crest control section. The resulting, inception-point relationship tends to overestimate the inception-point location for broad-crested weir stepped spillways ($\theta \le 26.6°$) when the Froude surface roughness (${\mathsf{F}}_{*}$) is less than 10. Consequently, research on broad-crested weir stepped spillways retrofitted for embankment dams has been conducted to provide an optimized inception-point relationship. This study provides additional data allowing further refinement (i.e., ${\mathsf{F}}_{*}\le 28$) of the inception-point relationship for broad-crested stepped spillways ($\theta \le 26.6°$), and it provides a new relationship for broad-crested weir stepped spillways for ${\mathsf{F}}_{*}>28$.

Debris flow in a wide open downstream channel has a significant transverse velocity component that strongly influences its mean longitudinal velocity. Investigation of observation data of debris-flow surges at Jiangjia Ravine in China shows the dependency of Manning resistance of debris flows on the ratio of flow width to depth. Regression fit reveals a power function relationship between the Manning resistance coefficient and the width-to-depth ratio. This derives a new formula of mean velocity incorporating the influence of flow width. The result indicates that the width-to-depth ratio can be viewed as a kind of shape roughness analogous to grain roughness in the Darcy-Weisbach resistance coefficient expression.

A simple and robust method is proposed for calculating reach-averaged bed-load transport in sand- and gravel-bed rivers with no calibration. The data required are the bed surface $D_{50}$ and $D_{84}$ measured with the nontruncated pebble-count technique, the slope, the width, and the flow depth or discharge. The method is illustrated by comparison with bed-load samples measured in 15 river reaches.

The depth-averaged equation of flow in a rectangular compound channel with secondary flows is established by analyzing the forces acting on the elemental water body and using Newton’s second law. The analytical solution to the transverse variation of depth-averaged velocity is presented that includes the effects of lateral momentum transfer and secondary flow in addition to bed friction. Different forms of boundary conditions at the internal wall between the rectangular main channel and the adjoining floodplain are presented. A comparison with the published experimental data demonstrates that the present model is capable of predicting the distributions of depth-averaged velocity and boundary shear stress. The results also indicate that the secondary flow and boundary conditions have influences on them. Finally, the key parameters in the model, such as the Darcy–Weisbach coefficient, the momentum transfer coefficient, and the secondary flow coefficient are also discussed and analyzed.

In urban drainage systems, it is usually necessary to split the storm water discharge among different sewer branches because of limited flow capacity of either hydraulic structures or sewer lines. For this purpose, sewer sideweirs and leaping weirs are generally used as diversion overflow structures. For supercritical approach flow, the former is not recommended because of the occurrence of hydraulic jumps, whereas the latter requires an outlet located below the approach flow sewer bottom. To overcome these constraints, a novel compact hydraulic structure is proposed. It consists of a frontal rectangular intake, representing the diversion sewer inlet, located in the approach flow sewer and parallel to its bottom. A 6.67:1 scale model was tested over a range of hydraulic and geometrical parameters to determine its head-discharge relationship, developing a dimensionless equation relating the diversion discharge to the geometrical properties of the intake under supercritical flow conditions. The limited hydraulic structure size provides cost-effectiveness and ease of installation and maintenance.

Sedimentation due to gravitation is applied extensively in water and wastewater treatment to remove suspended solids. Suitable baffle configurations may help form favorable flow fields and increase the efficiency of a primary sedimentation tank. Experimental tests were performed to determine the best position for a baffle in a rectangular primary sedimentation tank. The experimental measurements consisted of three parts. First, the velocity fields were measured using an acoustic Doppler velocimeter. The concentration at different vertical and horizontal points was then evaluated. Finally, the removal efficiency of the sedimentation tank was measured. The results of the present study indicate that a uniform flow field in the settling zone is enhanced when the baffle position provides a small volume of circulation regions. In this case, the maximum concentration of the suspended sediments inside the settling zone and the highest removal efficiency are achieved.