Journal of Hydraulic Engineering

Bendway weirs are linear structures similar to spurs but designed to be overtopped by typical streamflows, and have recently attracted attention as an environmentally sensitive means of protecting the outer banks of stream bends. A laboratory study was conducted to examine the effectiveness of these type of structures, constructed following FHWA (U.S. Federal Highway Administration) design-manual guidelines, in protecting the outer bank of a 90° bend in a narrow channel with both erodible bed and banks. Experiments were performed at a single nominal (initial) depth, and so characterized by a single (initial) ratio of radius of curvature to top width of 3.3, with and without a field of five weirs. Measurements of erodible boundary elevations as well as the magnitudes of the time-averaged point horizontal velocities at various cross sections were taken. Although a definite overall deflective effect by the weirs was observed, the outer bank remained susceptible to significant erosion even in the presence of weirs. Effects of increased velocities and increased weir heights, with other conditions held constant, were also examined. For the weir-channel configuration and flow conditions studied, increased velocities tended to be associated with increased rates of erosion, while higher (relative to the depth) weirs gave some added local protection, but could also result in increased overall erosion at certain channel sections.

In this study, sediment transport is considered as a typical multiphase flow in numerical simulation using a particle-based method and efforts are focused on reducing interface instability between phases. A new multiphase model, a particle-method based rheology model, and a higher order viscosity smoothening scheme are used in a particle-based method to reproduce the sediment transport. Model applications including open channel flow scouring on sand beds and various water-sediment dam break flows are simulated. This new particle-based multiphase method is able to predict both free surface and sediment profiles. Additionally, the simulated velocity distributions of water-sediment dam break flow also show good agreement with measured data, which has seldom been considered in previous studies using a particle-based method. The successful implementation of the sediment transport simulation confirms the strong capability of this particle-based Lagrangian method for predicting multiphase flow and provides an alternative numerical tool in sediment transport study.

A new mathematical framework of the steady unseparated turbulent flow over a wavy (sinusoidal) boundary is derived by treating the Reynolds averaged Navier-Stokes (RANS) equations. Theoretical formulations for the flow profile, boundary shear stress and Reynolds shear stress distributions are obtained assuming a power law of streamwise velocity and accounting for the effects of curvilinear streamlines induced by the wavy boundary. The flow profiles, and distributions of boundary shear stress and Reynolds shear stress are computed for different flow parameters and presented in graphical forms. In subcritical flow, the flow profile is out of phase with the wavy boundary; while the boundary shear stress distribution is almost in phase with the wavy boundary. Instead, the flow profile and the boundary shear stress distribution in supercritical flow are opposite to those in subcritical flow. Upstream of the crest of wavy boundary the vertical distribution of the Reynolds shear stress is characterized by a concave shape because of an accelerated flow; while downstream of the crest it is a convex shape because of a decelerated flow. The theoretical results show a satisfactory agreement with the experimental data. The analysis is then extended to the mobile boundary flow to determine the phase lag distance between the locations of the maximum sediment flux and the maximum boundary shear stress. The influence of the flow Froude number on the phase lag is more pronounced than the resistance parameter.

Side weirs are hydraulic structures widely used for flow control in rivers and canals. Whenever the water level rises above the side weir crest elevation, a fraction of the main flow is diverted so that the water discharge flowing downstream in the main channel is reduced. In movable bed channels, the lateral outflow may have significant interactions with the sediment transport processes affecting the side weir flow. The spilled discharge creates a reduction of the downstream sediment transport capacity with a consequent deposition in the side weir proximity. In addition, sediment in the main channel can be diverted into the lateral branch. To investigate these interactions, experimental data at the laboratory scale have been collected and analyzed. The increase of the spilled discharge induced by the local sediment deposition is described according to the classical De Marchi hypothesis. Furthermore, the sediment transport leaving the main channel through the lateral structure is modeled in terms of the stream power associated with the bed-shear stress. Results allow for prediction of the bed dynamics and for the development of new design criteria.

Typically, the analysis and design of fluvial flood defence schemes is based on a single $N$ year extreme flow event using a single survey of the river channel and flood plains. Adopting this approach assumes that the channel capacity is identical for all subsequent $N$ year events. If one assumes that the typical design life for a flood defence scheme is of the order of 50 years, then such an approach is flawed because river channel morphology, and hence flood conveyance, may change considerably over this time scale. Therefore, to provide a more robust estimate of future flood inundation, a sensitivity analysis of these changes should be undertaken. This paper proposes a modeling methodology that combines a stochastic model, for estimating streamflow throughout the design period, and a 1D sediment transport model (HEC-RAS), to enable this sensitivity to be included in flood inundation modeling and defence scheme design. The methodology is demonstrated through conceptual implementation to evaluate the change in water surface elevation (WSE) along an alluvial river (River Caldew, England) reach after 50 years of sediment transport. Changes in WSE are assessed when the reach is natural (no flood defences) and modified (with idealized flood defences). Results show that the construction of the flood defence scheme does not alter the overall morphological pattern of the reach but can significantly increase (260%) local aggradation. Additionally, 50 years of morphological change have the potential to increase WSE such that high flows, previously confined within the channel, can overtop the banks and become flood events; and that, the standard freeboard levels of the flood defence scheme may be insufficient to prevent overtopping when morphological change is considered. The method can be considered as a semiquantitative modeling methodology to account for the sediment-related sensitivity of flood risk management; and provides valuable insights into the potential magnitude that this has on future flood inundation.

A numerical model has been developed employing an orthogonal stream-tube gridding system for the investigation of mixing patterns in meandering rivers. The proposed model couples field data and modeling. It uses a measured velocity field to solve the advection-diffusion equation. This equation provides a practical simulation with greater accuracy and lower computational cost for estimation of mixing in meandering channels with a long, full mixing length. The model has been used to estimate the transverse mixing coefficient in surveyed subreaches of the North Saskatchewan River in Canada. Good agreement was observed between the numerical and measured tracer concentrations using reasonable values for the transverse mixing coefficient. The average dimensionless transverse mixing coefficient from the calibrated numerical model for the surveyed reach was 0.39, which was consistent with values estimated using empirical formulas for transverse mixing coefficients in natural meandering rivers. The estimated transverse mixing coefficient was more than an order of magnitude higher in a subreach with a sharp bend than in other subreaches. For comparison, the transverse mixing coefficient was also calculated analytically from standard and general methods of moments between some sections.

The paper presents the results from detailed experiments of the statistical structure of turbulence pressure fluctuations at the bottom of hydraulic jumps, with special reference to the spillway stilling basins lining design. Here, the whole spatial correlation structure of the fluctuating pressure field is required in order to evaluate slab stability. This is computed via simultaneous acquisition of the point pressure fluctuations on a dense grid in the hydraulic jump region, requiring a severe experimental work. As an alternative, one can evaluate the pressure spatial correlation structure via autocorrelation using one point pressure acquisition and applying the Taylor hypothesis. To adopt the Taylor hypothesis, one must know the pressure propagation celerity in space that can be obtained by comparing the whole spatial pressure correlation with the pivot point pressure autocorrelation. The experiments were performed by simultaneous pressure acquisitions at the bottom of a hydraulic jump for Froude numbers of the incident flow ranging from 4.9 to 10.3. From experiments, a criterion to define the pressure celerity as a function of the incident flow velocity is presented. The results highlight a good agreement between the relevant pressure statistical parameters as measured and the ones computed using the Taylor hypothesis. The comparison between the slab thicknesses, as computed via Taylor hypothesis, with the ones retrievable in literature, as obtained by direct force measurement on instrumented slabs in laboratory conditions, highlights the accuracy of the proposed approach that presents undeniable practical advantages.

In this study, a bedload model for nonuniform sediment is developed considering the density function of sediment entrainment with no active layer. The model is formulated based on physical considerations, dimensional reasoning, regression analysis, and laboratory data. Four parameters are incorporated in the model formulation, namely the Shields stress ${\tau }_{*g}^{\prime }$, the critical Shields stress ${\tau }_{*cg}$, the Kramer coefficient of uniformity $c_u$, and the relative grain size (size of the $j$th grain fraction to the geometric mean size, $d_j/d_g$). The model collectively accounts for the hiding effects and the variation of the transported bedload material fractions from the surface and subsurface material. The findings of the study show that at low values of ${\tau }_{*g}^{\prime }/{\tau }_{*cg}$ fractions within the range of $0.3<d_j/d_g<3.0$ are present with larger amounts in the transported bedload material than in the substrate, whereas fractions finer or coarser than that range are present with larger amounts in the surface layer than in the substrate. The study shows also that only fractions of $d_j/d_g>3.0$ could contribute to the formation of the armor layer, whereas fractions close to the geometric mean value would contribute the most to the bedload transport rate at low values of ${\tau }_{*g}^{\prime }/{\tau }_{*cg}$. The armor layer would completely obliterate at high values of ${\tau }_{*g}^{\prime }/{\tau }_{*cg}>3.5$. Further, the model provides more insight in the expected errors in the predictions of both the substrate-based and surface-based models. Model results show that errors in the predictions of some fractions in the sediment mixture can be as large as 100% and 350% for the substrate-based and surface-based models, respectively. For both types of models, the errors were obvious at low shear stresses and decline with the increase of ${\tau }_{*g}^{\prime }/{\tau }_{*cg}$. By comparing the predictions of selected models with measured bedload data, it has been found that the proposed bedload model provides better predictions over the other models examined herein, highlighting the importance of considering the interaction between the surface and subsurface material when estimating the bedload rate material.