Journal of Hydraulic Engineering

A low-risk, stream restoration design includes methods that validate design assumptions, incorporate uncertainty in the decision-making process during the project design phase, and reduce uncertainty by checking the final design. A two-step method of incorporating uncertainty and risk in stream restoration design has been developed as a combination of design failure modes and effects analysis (DFMEA) and risk quantification. As a first step, DFMEA is applied to identify risk in terms of ratings with respect to consequence of failure, the likelihood of occurrence of a failure, and the ability to detect a failure. Due to its evolutionary nature, the DFMEA can be revised to account for design modifications and relative ratings are reevaluated to examine reductions in uncertainty, and thereby, risk. The second step of the method is quantifying risk using initial and expected failure costs. The two-step, risk-based method is illustrated through application to a stream relocation project in Pennsylvania. Both sediment transport capacity and supply analysis and alluvial channel modeling design methods were shown to reduce uncertainty and risk by detecting design deficiencies that the initial design using incipient motion tests overlooked. However, the alluvial channel model method was favored due to its ability to simulate the combined effects of flow hydraulics, sediment transport, and river channel adjustment.

This paper presents a study on the reservoir sedimentation processes in response to changes in incoming flow at the upstream and changes in the pool level at the downstream for Sanmenxia Reservoir, which is located on the middle reach of the Yellow River in China and has experienced serious sedimentation problems even since its impoundment in 1960. The hysteresis effect in reservoir sedimentation was used as the basis for analysis and its behavior was fully investigated throughout this study. The research found that the rise in the elevation of Tongguan, which is located in the backwater region at a distance of $113.5\mathrm{km}$ upstream of the Sanmenxia Dam, had a time delay of about $2\text{years}$ compared with the sediment deposition in the reservoir area downstream of Tongguan. Moreover the accumulated sediment deposition in the reservoir area was closely related not only to the current year’s flow and dam operational conditions, but also to the preceding $3–4\text{years}$ ’ flow and dam operational conditions. Likewise the variation of Tongguan’s elevation was a function of $6\text{years}$ ’ linearly superimposed runoff, and the channel bed slope in the vicinity of Tongguan was determined by a moving average pool level over a $7\text{year}$ period. The research results are of practical importance in particular for optimizing the operation of Sanmenxia Dam, and the finding of the hysteresis phenomenon in the sedimentation process of the reservoir is of merit to the advancement of sedimentation science.

Sediment management in reservoirs with the help of water jets has motivated this work. Erosion caused by single and multiple submerged circular turbulent wall jets on a noncohesive sediment bed of finite thickness lying on a fixed boundary was studied with the help of laboratory experiments. Different combinations of jet diameter, jet separation, and sediment thickness to jet diameter ratio were tested. Results show a relation between dimensionless parameters characterizing the steady state bed profile and the densimetric particle Froude number ${\mathsf{F}}_0$ given by the velocity at the nozzle and the effective diameter and submerged specific density of the sediment. Evolution of scour with time confirms previous studies where the erosion was found to initially grow with the logarithm of time up to a certain reference time $t^{*}$ . This time, made dimensionless with a time scale $t_c$ involving the volume of sediment scoured and the rate of erosion, was also related to the densimetric Froude number. A comparison with studies regarding erosion of a semiinfinite layer of sediment is also presented.

New field data on cohesive sediment erosion is presented and discussed, with particular focus on partitioning the total erosion into resuspension and bed load. The data were obtained using a recently developed in situ flume of the National Institute of Water and Atmospheric Research, New Zealand. The erosion rate is estimated from direct measurements of bed surface elevations by acoustic sensors, whereas resuspension rate is obtained using data on sediment concentrations measured by optical backscatter sensors. The bed- load contribution to the total erosion rate is evaluated from the conservation equation for sediments. To test repeatability, the data from the in situ flume are compared with those from a previous version of the flume. The results show that comparative studies of in situ flumes and standardized deployment procedures enable direct comparison of experimental data on cohesive sediment erosion. Overall, the data show that a commonly used assumption that the erosion rate is equal to the resuspension rate is not always valid as bed load plays a significant role in cohesive sediment erosion. The data also highlight the importance of clay content and other sediment physical characteristics in the sediment mixture.

A physical model study was performed in which wood dowels were used to model rigid vegetation. The dowel configurations used in the flume were intended to simulate the effects of willow post systems (i.e., collections of rigid cylinders placed along a streambank to reduce streambank erosion). In addition, an analytical model is presented for predicting depth-averaged velocity distributions in straight trapezoidal or rectangular channels with newly constructed willow post systems. The analytical model is founded on wake theory and is applicable to channels with submerged and unsubmerged rigid cylinders. Data from three independent physical model studies were used to validate the analytical model. Depth-averaged velocities predicted using the analytical model, $U_{\mathrm{pred}}$ , were compared to velocities observed in the physical models, $U_{\mathrm{obsd}}$ , and yielded discrepancy ratios, $U_{\mathrm{pred}}∕U_{\mathrm{obsd}}$ , that were typically between 0.80 and 1.20. Results from this study are that significant variables for reducing local velocities are cylinder height, diameter, and density (number of cylinders per unit area); and the arrangement of the cylinders (i.e., rectangular versus staggered grid) is inconsequential.

The flow characteristics around short, bottom-mounted cylinders were examined with a three-dimensional nonhydrostatic model for Reynolds numbers of 300–3,900 and different obstacle geometries. Simulations using a large-eddy simulation closure scheme reproduce the vortex shedding in the wake, with reasonable Strouhal number comparisons to available laboratory data. The length of the recirculation region is also in good agreement with the data. The sensitivity of the flow to obstacle cross section and length is investigated for a Reynolds number of 500 with square and circular cylinder geometries for nondimensional lengths of 1–15. Model predicted upstream separation was found to be most sensitive to cross section. The predicted downstream recirculation lengths reproduced the nondimensional length dependency of available observations. To highlight the processes relevant to scour, the time-dependent bed shear was evaluated with the mean and peak Shields parameters. Significant wake variability was found which would not be described by the mean alone, indicating that sediment transport models assuming a mean shear will underpredict the transport in the wake.

Analytical solutions are developed to predict the behavior of inclined negatively buoyant discharges and these solutions are compared with data from previous studies and from experiments described here, where light attenuation and laser-induced fluorescence techniques are employed to study these flows. Comparisons show that the analytical solutions provide reasonable predictions of the flow’s path for initial discharge angles ranging from 0 to 75° and initial Froude numbers ranging from 14 to 99. The solutions also predict the maximum height of the outer edge of the jet with reasonable accuracy, and thus the outer spread of the jet is well predicted. However, the inner spread is underestimated and minimum dilution predictions are shown to be conservative by approximately 18% at the centerline maximum height and 34% at the return or impact point. Predictions from the CorJet and VisJet numerical models are also shown to be conservative.

A new model for the critical shear stress and the transport of graded sediment is presented. The model is based on the size distribution of the bed surface and can be used to compute sediment transport rates in numerical simulations with an active layer model. This model makes a distinction between unimodal and bimodal sediments. It is assumed that all size fractions of unimodal sediments have the same critical shear stress while there is selective transport for the gravel fractions of bimodal sediments. A recently published laboratory transport data set is used to calibrate our model.

This note presents an experimental study of seepage effects on the dimensions of a dune: its height, length and lee-side slope. The critical slope of a cohesionless material was found to be related to the ratio of the hydraulic gradient of seepage to its critical value under the quick condition. The theoretical relationship is successfully used to predict seepage effects on the lee-side slope of dunes. Measured laboratory data support the assumption that the effect of seepage on the lee-side slope of dunes is the same as that on the critical slope of the sediment. The data reveal that the slope is reduced and increased with injection and suction, respectively. Additionally, the study explores seepage effects on the development of dune height and length. Experimental results show that suction and injection increase and decrease the equilibrium height of dunes by 25 and 40%, respectively. On the other hand, the dune length is found to increase by approximately 30% and decrease by 20% under the influence of suction and injection, respectively.

Experimental results on scour below a high vertical drop (drop height/critical depth $>1$ ) in uniform sands and gravels are presented. The experimental results are used to describe the effects of important parameters, identified from the dimensional analysis, on equilibrium scour depth. The important observations are that the equilibrium scour depth increases with increase in densimetric Froude number, whereas the scour depth decreases with increase in sediment size and tailwater depth. The time scale of scour depth that follows an exponential law is determined. The nondimensional time scale decreases with increase in densimetric Froude number.

For a given discharge and channel geometry there are generally unique curves for specific energy and specific momentum, but for a particular section the governing equations for these depths may be satisfied for arbitrary depths. As the governing equations do not yield a unique solution, this section is singular. Reported herein is the equation of the singular section having constant specific energy and specific momentum.

Section elements of a new class of curvilinear bottomed channel whose boundary maps along a circle onto the hodograph plane have been investigated in the study. The perimeter of this type of channel always lies between an ellipse and a parabola of the same top width and flow depth. Presented herein are optimal section properties from least area and minimum seepage loss point of view for the curved channel section. The study also addresses the constraints on the channel dimensions and the velocity of flow. A nondimensional parameter approach has been used to simplify the analysis. Design procedures for different cases have been presented to demonstrate the simplicity of the method.