Journal of Hydraulic Engineering

A series of experiments has been conducted to investigate recirculating shallow-water flow in the wakes of conical model islands with gently sloping sides. Four islands have been used with side slopes ranging from 33.1 to 8.0°. For all tests reported here the water depth was less than the island height; the islands are said to be surface piercing. Measurements of flow velocity have been made in the laboratory using a digital particle tracking velocimetry (PTV) system. This produces instantaneous, whole-field velocity vector maps of the wake flows. A “wake stability parameter,”S has been used to classify the island wakes into “vortex shedding” or “unsteady bubble” types. The stability parameter is a measure of the stabilizing effect of bed friction relative to the destabilizing influence of transverse shear. For small values of S (<0.2) vortex shedding was well organized and vigorous in the island wakes. Vortex shedding was found to cease for larger values of S (0.35–0.40). These values are similar to those found by other researchers for the shallow wakes of vertically sided circular cylinders. The influence of the island side slope angle on the wakes is discussed. Depth-averaged and three-dimensional (3D) numerical shallow-water flow models with the usual hydrostatic pressure assumption have been used to simulate the experimental flows. The models are based on a fully coupled semi-implicit Lagrangian numerical method. For cases where vortex shedding occurred, the 3D model was found to predict a more 3D far wake than was observed. The two-dimensional (2D) model produced good overall agreement with experimental results. For very shallow depths with the wake stabilized by bed friction, the 3D model demonstrated closer agreement to the measured results than the 2D model.

Experiments have been conducted to study the unsteady wakes of submerged conical islands. The islands used in the tests have side slopes ranging from 8.0 to 33.1°. Experiments in a shallow-water channel with a steady, subcritical free stream showed vortex shedding to occur in the wake when the water depth above the island apex was relatively small. Flow separation from positions near the island apex was found to be important in producing this unsteady wake. As the water depth was increased the shedding was observed to become less vigorous and eventually stop. All islands tested produced similar results with the angle of the island side slope exerting relatively little influence on the process. The results of wind tunnel visualization studies, which used a rigid top plate to produce the effect of fluid depth, support the results from the water channel. Pictures of the surface flow patterns produced on the islands by the wind action are presented. Two-dimensional (2D) and three-dimensional (3D) shallow-water numerical models with the hydrostatic pressure assumption have been run for comparison with the laboratory measurements. The complex 3D flow observed in the near wake provides a severe test for the models. Although both models were found to reproduce gross features of the submerged island wakes their mode of generation could be quite different in model and experiment.

Values of calibration parameters embedded in open-channel flow equations normally are ascertained using observation data that often contains Gaussian noise. These values subsequently suffer from induced errors even if they are determined by using optimization methods. If these parameters are to be considered a reliable representation of their true values, investigations are needed to establish their response to a range of factors, such as data errors, through statistical methods. A systematic study was carried out by a set of synthetic database for effective control of test conditions and by using typical flood events. The investigations showed that the identified parameters could be affected by such factors as data error, objective function, or gauge sites. However, through a better understanding of the behavior of the induced errors the mean of the identified parameters was found to lie within a prescribed confidence interval that contained the true value, even in the presence of high noise levels. Selection of objective function was found to be prone to undue biases affecting the identified parameters, which could be avoided through a careful consideration of the problem.

A monostatic acoustic Doppler velocity profiler (ADVP) for use in hydraulic research is presented including signal treatment algorithms. The 1-MHz ADVP used in the present study has an accuracy better than 2 mm$\dot{s}$s⁻¹ with a range of up to 100 cm for velocities up to 290 cm$\dot{s}$s⁻¹ and a resolution of 256 points in the profile. Results of ADVP measurements in uniform flow in two laboratory open channels with smooth and gravel beds compare favorably with established distribution laws for mean velocity, variance, and Reynold's stress, indicating that the turbulence scales are sufficiently resolved by the instrument. Measurements with the same instrument in a river and a lake illustrate that it is capable of producing excellent results in the field as well. It was demonstrated that, unaffected by water quality, the instrument is a good research tool for nonintrusive and accurate instantaneous profile measurements under turbulent flow conditions.

Using a Laser-Doppler anemometer, mean velocities are measured in developing and fully developed turbulent subcritical smooth open channel flows. Experiments are conducted in a rectangular laboratory channel for 12 different test conditions with Reynolds number ranging from 28,026 to 136,842. From the experiments it is found that the boundary layer along the centerline of the channel develops up to the free surface for a flow aspect ratio b/h≥ 3. Shear velocities are calculated using the measured velocity profiles in the viscous sublayer of the boundary flow. The experiments show that shear velocity varies in an oscillatory manner across the flow section around b/h= 3. In the turbulent inner regions of developing and fully developed boundary flows, the measured velocity profiles agree well with the logarithmic “law of the wall” distribution when the coefficients in the expression are 2.44 and 5.5, respectively. The “wake” effect becomes important in the velocity profiles of the fully developed boundary layers. A reasonable agreement between the modified velocity-defect law and the experimental profile in the inner and outer regions is obtained with a profile parameter of 0.1 in the Coles's “law of the wake.”

The methodology of combining subsamples into a single “composite” sample representative of an entire river reach is examined. Current methods of characterizing the overall grain-size distribution of a river reach are inadequate because they either require enormous amounts of material to be collected, are inherently biased, or fail to describe the local variations occurring within the sampled region. These problems can be overcome by employing the grid-by-number sampling technique. Criteria for obtaining an unbiased grid composite sample as well as its overall accuracy are provided. It is proposed that local variations in a river reach's grain-size distribution can be located by comparing the arithmetic means of adjacent subsamples through the use of statistical hypothesis testing. By systematically comparing the arithmetic means of adjacent subsamples throughout the area of interest and using a moving windows approach to locate sudden changes in a sediment's grain-size distribution more precisely, the local variability of the river reach can be described adequately.

The presence of sand moving at low transport rates over a flat bed modulates the production of turbulence when compared to clearwater flow at similar mean flow conditions. Phase Doppler anemometry is used to discriminate the turbulence characteristics of the carrier fluid from the sediment grains (0.22 mm diameter) and shows that the presence of mobile sediment increases the near-wall velocity gradient and shear velocity when compared with the clearwater values. This increased shear velocity is associated with a greater bed roughness height and near-bed turbulence intensities and smaller mixing lengths. Quantification of slip velocities between the fluid and sediment phases reveals particle Reynolds numbers that range from 1 to 30. Turbulence enhancement is shown to occur at lower values of both the Stokes number and ratio of the particle size-to-turbulent length scale than in past work. Several mechanisms of turbulence modulation may be invoked to explain these changes, including increased bed roughness, eddy shedding from large grains, grain inertial effects, and particle-coherent structure interactions. These mechanisms may be significantly influenced by both particle-particle and particle-wall interactions. Since mobile sediment modulates the carrier fluid turbulence, there is a need for modification of existing theories of sediment suspension and for caution when interpreting velocity profiles that are obtained without discriminating the fluid and sediment phases.

The results of new experiments on bed load transport on steep longitudinal slopes are presented. The experiments were carried out in a large tilting duct with a fine sand (d₅₀= 208 μm) covering the range of slope angles β of ± angle of repose for the sand ϕ_s (32°). Measurements included bed shear-stress, flow discharge, and the sediment pickup rate on slopes. The results suggest that existing theories for bed load transport on slopes are not adequate for all bed slopes and the new data are used to formulate a semiempirical relation that satisfactorily predicts the transport rate on horizontal as well as mild and steep slopes. The following are two important findings: (1) Correcting the threshold term (for slope) in the Meyer-Peter and Müller formula adequately describes the transport rate on positive slopes (elevation increasing in direction of flow) and on small negative slopes. On steeper negative slopes an extra correction term is required, although the value of β/ϕ_s at which this term becomes necessary is a function of shear stress; and (2) the influence of slope vanishes at high shear stresses.

A solution to the inverse problem in groundwater is presented with a geostatistical framework, using Kalman filtering and a nonlinear gradient-based search technique. The Kalman filtering recursions are based on a newly developed and linear state-space equation that relates aquifer head perturbations to stochastic perturbations of log-aquifer properties and effective recharge. The Davidon-Fletcher-Powell (DFP) search algorithm is used to identify the mean and the variance of log-aquifer transmissivity and storativity, by minimizing the joint negative log-likelihood function of the innovations (prediction errors). Application to a numerical experiment indicates that the methodology performs well for log-transmissivity integral scale smaller than aquifer dimensions. The results underline the need for conditioning on point measurements of transmissivity and storativity, if the objective is to estimate the variance parameters. While head measurements are instrumental for estimating the geometric means (large-scale parameters), they, however, may not be sufficient for inferring the variance of log-aquifer properties (small-scale parameters).

Commonly applied procedures for the evaluation of surface bed-material composition in rivers often include collection of particles by wax or adhesives. These collection procedures produce particle frequency distributions that are not directly applicable to sediment transport models. Appropriate conversion models are required to turn these distributions into volumetric frequency distributions. An empirical exponential formula has been widely used in the past, but it breaks down under some circumstances. Failures are due to conceptual limitations of the empirical approach and to a rough schematization of reality. These problems led to some recent discouragement towards wax sampling. A new conversion model based on the statistical properties of homogeneous mixtures is presented here. The model performs the conversion through evaluation of the sample layer thickness calculated from the total weight of the sample. Results from the proposed model are in good agreement with experimental results obtained from the literature, and compare well with an extensive set of new experimental data.

Sand wave development on an initially flat bottom under steady unidirectional flow is examined on the basis of laboratory measurements. A common scaling relationship P₀(t)/P_e= (t/T)^α is proposed for the length and height of waves [P₀(t) and P_e denote the average length or height of sand waves after time t and in an equilibrium state that is attained after time T]. A similar relationship σ_z(δx, t)/σ_z0= (t/T)^αf[(δx/δx₀)/(t/T)${}^{\alpha /\mathit{H}}]$ is proposed to describe the scaling behavior of the root-mean-square deviations σ_z(δx, t) of bottom elevation Z(x, t). Here, σ_z0 is σ_z(δx, t) when δx≥δx₀ and t≥T, and δx₀ is δx when the function σ_z(δx, T) becomes saturated, i.e., during the transition from the regime where σ_z∝δx^H to the regime where σ_z=σ_z0= const. The scaling exponents α and H equaled 0.28 and 1.0, respectively, for the laboratory experiments.

At issue is the common use of Gaussian distributed steps in producing the desired dispersive effects in contaminant transport modeling. Shown are alternative solutions to the governing transport equation describing mass displacements based on non-Gaussian, white (uncorrelated) increments. In particular, the use of uniform distributed steps can be employed without loss of accuracy in the solution (over all Péclet numbers) and may yield significant savings in the computational generation of the random deviates required in the Monte Carlo procedures of the random walk method. Although the results provided here are for one-dimensional problems, the methodologies may be easily extended to higher-order domains yielding even more efficient computational solutions.

Small sandy tidal inlets of coastal lagoons usually exhibit dramatic changes in morphology under the interaction of storm flow and tidal exchange. While storm flow may keep the inlet open by flushing, the oscillating tidal flow contributes to a net sediment replenishment, or recharge, of the inlet channel. A numerical model has been developed to simulate the fluvial processes during inlet recharge. It provides the time and spatial variations of flow characteristics, sediment transport, and changes in channel morphology for the inlet channel during the recharge period. The model was applied to the inlet channel for the San Dieguito Lagoon in southern California. The recharge is elucidated by the simulated flow pattern that contributes to a net transport of beach sand into the channel. Time variations of the recharge rate were obtained. The total duration of recharge before inlet closure was found to be sensitive to the recharge rate which is affected by the supply of littoral sand. The simulated changes in channel morphology were substantiated by measurements. The study provided inlet opening durations that were used in assessing the enhancement in marine resource value by keeping the inlet open permanently.

Check valves usually are fitted on pump-rising main systems to prevent the development of backflow when a pump cuts out. Frequently, air vessels are used in such systems to control transient pressures resulting from pump start and stop. The presence of an air vessel adjacent to a check valve may cause the valve to close, after pump cutout, with a sharp impact, typically producing an audible valve-slam sound. The phenomenon of check-valve slam is undesirable, because it can result in damaging pressure transients. A case study of a pump-rising main system, protected by an air vessel, which experienced the problem of check-valve slam, was investigated. Both field measurements and numerical modeling (using a spreadsheet) of the system were conducted.