Journal of Hydraulic Engineering

A coupled flow and pollutant transport model based on the finite-volume method is developed and applied to predict the tidal transport of urban runoff in a southern California network of flood control channels that drain to near-shore bathing waters. Urban runoff in southern California contains elevated levels of indicator bacteria that signal the presence of fecal pollution and pose a risk to human health, and model predictions are used to understand the transport of these pollutants toward the coastline. The model is based on 1D conservation equations for fluid mass, momentum, and pollutant mass that are solved in integral form along channel reaches. A 2D formulation is solved at channel junctions. The model incorporates the monotone upwind scheme for conservation laws approach to give a high-resolution, nonoscillatory prediction of water level, velocity, and pollutant concentration. Model predictions and field measurements of water level, velocity, and a conservative urban runoff tracer are presented and compare favorably. This case study demonstrates that this finite-volume method–based scheme results in an accurate, stable, nonoscillatory and computationally manageable model. The nonoscillatory behavior is particularly beneficial in this study, since runoff enters the channels in pulses that create large gradients in pollutant concentration.

An analysis of the predictive capabilities of cross-section–integrated models of gradually varied, unsteady flow in channels initially highlights the physical characteristics of the conservation equations, with particular emphasis on those aspects that are operationally crucial or potentially challenged in numerical implementation. A sequence of benchmark test problems are introduced with the express purpose of evaluating all potentially important features of expected response patterns. This sequence of problems is designed to exercise the physically and numerically significant response patterns that are expected to be within the predictive capability of numerical models of flood and tide propagation in channels. An extensive and independent review of this nature should be a routine and automatic part of any numerical model study.

Detailed measurements of three-dimensional turbulent flows within a rectangular single-pump bay area of a right-angle water intake model with and without cross flow were obtained using an acoustic Doppler velocimeter (ADV) in order to elucidate swirling flow characteristics within the pump sump. Without cross flow, the pump-approach flow distributions were characterized by nearly uniform streamwise velocities in the pump bay and weak free-surface vortices near the pump column. With cross flow, the three-dimensional mean velocity measurements revealed the existence of a large recirculation zone upstream of the pump column such that strong streamwise velocities were present at higher depths and near the left sidewall, while the reverse current concentrated at lower depths along the right sidewall. Flow patterns in the latter case were also characterized by strong free-surface vortices in the vicinity of the pump column and a strong floor-attached subsurface vortex underneath the pump bell. Uncertainty analysis for ADV velocity measurements showed good quality data, with uncertainty in mean velocities varying from 2.5 to 6.4%. These experimental data were utilized in validating inviscid numerical solutions.

Flow over a developed bottom topography in a bend has been investigated experimentally. The measuring section is in the outer-bank half of the cross section at 60° into the bend. Spatial distributions of the mean velocities, turbulent stresses, and mean-flow and turbulent kinetic energy are presented. The cross-sectional motion contains two cells of circulation: besides the classical helical motion (center-region cell), a weaker counterrotating cell (outer-bank cell) is observed in the corner formed by the outer bank and the water surface. The downstream velocity in the outer half-section is higher than the one in straight uniform flow; the core of maximum velocities is found close to the separation between both circulation cells, well below the water surface. The turbulence structure in a bend is different from that in a straight flow, most notably in a reduction of the turbulent activity toward the outer bank. Both the outer-bank cell and reduced turbulent activity have a protective effect on the outer bank and the adjacent bottom and thus influence the stability of the flow perimeter and the bend morphology.

Field experiments were conducted to determine the oxygenation potential of gated sill structures in the Ohio River Valley. The objective was to determine operational procedures for water quality improvement. Oxygenation potential is typically characterized by gas transfer efficiency, with high transfer efficiency indicating a greater input of oxygen into the water. Direct oxygen measurement can be unreliable in determining transfer efficiency, and this is due to upstream stratification, relatively high background concentrations, and changes in saturation concentration when the bubbles are exposed to hydrostatic pressures within the stilling basin. As a result, in situ methane was used as an independent dissolved gas tracer. Methane is naturally present in measurable quantities with little stratification, and it does not experience an appreciable increase in saturation concentration with bubble depth. Therefore, methane measurements better reflect the true transfer efficiency of a structure. This paper focuses on the factors that influence transfer efficiency and how to achieve high quality field data through the use of methane and oxygen measurements. In addition, results from six-gated sill structures in the Ohio River Valley are presented.

In this study, fall velocities of 174 regularly shaped angular particles—namely cylindrical, cubic, wedge-shaped prisms, and box-shaped prisms—made of five different materials were measured in calm water conditions in a cylindrical settling column. Reynolds numbers based on particle fall velocity and characteristic length were varied, in the range from 10³ to 4.2 × 10⁴. A new shape factor and characteristic length were defined from a transformed form of the original particle. Variation of experimentally obtained data of particle drag coefficient, $\mathit{C}{*}_{\mathit{D}}$, with Reynolds number, R*, as a function of newly defined shape factor and characteristic length was analyzed. Verification was made by using experimental data of irregular angular particles. The results showed that the fall velocity of any irregular angular particle could be estimated by using graphical curves obtained from the analysis of the regular angular particles.

This paper describes and illustrates a technique for high resolution monitoring of the surface morphology of water-worked sediments. The monitoring uses close-range digital photogrammetry. While photogrammetry is a long-established technique, more recent developments in digital photogrammetry allow application in fluvial research to be highly cost effective in both flume and natural river channel studies. Results are presented that involve two scales of laboratory flume: a smaller-scale application associated with sediment sorting processes in a straight channel; and a larger-scale application involving sediment transport and bed material feedbacks in a meandering channel subject to overbank flows. A preliminary assessment of data quality is undertaken with encouraging results. The precision of elevation estimates corresponds to the scale of the imagery acquired and hence may be controlled by design of the image acquisition process.

Experiments show that a rough surface reduces the pressure drop and likelihood of cavitation downstream of the roughness. A rough surface upstream of an irregularity significantly reduces the pressure drop caused by the irregularity. The effect of the rough surface is a function of the roughness d/s and can prevent cavitation damage. If an irregularity is inevitable, increasing the roughness of the upstream surface in front of the irregularity will reduce the possibility of cavitation caused by the irregularity. For the curved open channel used in this study of the flow in a spillway, the critical height of the irregularity was increased six to eight times at the downstream tangential point of the curve, and the negative pressure caused by the irregularity was reduced enough to prevent cavitation.

A thorough understanding of the effect of raindrop impacts on the characteristics of a shallow flow is very helpful to properly predict and control water erosion. The turbulence properties of a shallow rain-impacted flow over a rough bed were studied using a two-dimensional fiber-optic laser Doppler velocimeter and an oscillating type raindrop generator. It was found that the value of momentum correction factor β for a rough boundary was much smaller than that for a smooth boundary at a given value of Reynolds number because of the vertical mixing effect of the boundary roughness. For the condition without rainfall, the vertical distribution of the longitudinal turbulence intensity became more uniform as the boundary roughness increased. For the “fully rough” condition with rainfall and constant roughness size, the effect of roughness on the distributions of turbulence intensities was small. Similar to the condition with smooth boundary, the rainfall-generated turbulence was greatest near the water surface and decreased downward at an exponential rate for the flow over rough bed. Two types of statistical correlations were developed to predict the distributions of both the longitudinal and the vertical turbulence intensities depending on whether the flow is completely penetrated by the raindrops.