Journal of Hydraulic Engineering

A general mathematical model is developed to solve unsteady, depth-averaged equations. The model uses boundary-fitted coordinates, includes effective stresses, and may be used to analyze sub- and supercritical flows. The time differencing is accomplished using a second-order accurate Beam and Warming approximation, while the spatial derivatives are approximated by second-order accurate central differencing. The equations are solved on a nonstaggered grid using an alternating-direction-implicit scheme. To enhance applicability, the equations are solved in transformed computational coordinates. The effective stresses are modeled by incorporating a constant eddy-viscosity turbulence model to approximate the turbulent Reynolds stresses. As is customary, the stresses due to depth-averaging are neglected. Excluding recirculating flows, it is observed that in most cases the effective stresses do not significantly affect the converged solution. The model is used to analyze a wide variety of hydraulics problems including flow in a channel with a hydraulic jump, flow in a channel contraction, flow near a spur-dike, flow in a 180° channel bend, and a dam-break simulation. For each of these cases, the computed results are compared with experimental data. The agreement between the computed and experimental results is satisfactory.

Hydrographs of ephemeral streams in an arid region usually decrease significantly in magnitude, downstream, unless they are augmented by tributary flows. Estimation of outflow hydrographs from a channel reach is essential for the development of water-resources systems. A hydrologic-flow-routing technique for arid ephemeral channels is developed. The hydrologic-routing equations for the movement of flood waves are modified by the inclusion of a nonlinear volumetric transmission loss rate term, which is empirically estimated from the observed inflow-outflow data for a channel reach. The predicted outflow hydrographs match better with the observed than the outflow hydrographs generated through the lumped regression model for ephemeral channels. The hydrologic-flow-routing technique may also be used for ungaged channel reaches through parameterization.

Initiation of bed-load transport of uniform spherical sediment particles on a horizontal bed in an open-channel flow is studied. On the basis of micromechanical and fluid dynamical considerations, two separate criteria for the initiation of motion are derived: one for rolling and one for lifting. Fluid forces such as drag, shear lift, Magnus lift, and lift due to centrifugal force are included in the derivation. The formulation of the lift force is theoretical. No empirical coefficient is used other than the drag coefficient C _d , which is well established in the literature. In the low particle Reynolds number regime, where u * d/ν ≤ 1, the dimensionless stress required to initiate lifting is found to be much higher than that of rolling, the former substantially above the Shields' curve while the latter is substantially below it. The theory is compared with Vanoni's 1964 data, with meaningful results. For higher particle Reynolds number, where u * d /ν> 10, the dimensionless stress needed to initiate lifting is closer to that of rolling. The Shields' curve for the most part lies between the two theoretical thresholds.

Hydrosuction sediment-removal systems (HSRS) remove deposited or incoming sediments from reservoirs using the energy represented by the difference between water levels upstream and downstream from a dam. HSRS are briefly described and compared to other reservoir-sediment management options. Hydraulic principles and design procedures are explained and applied to a field test carried out at Lake Atkinson, on the Elkhorn River, in Nebraska. The field study demonstrated that several different inlet shapes are capable of removing deposited sediment at the rate that it enters the reservoir on an annual basis. A ten-step procedure to determine the feasibility of using an HSRS is presented.

A simplified model of streambed armoring is used to extend Laursen's well-known equation to include the limiting effects of a coarse surface layer when calculating clear-water contraction scour at bridges. An empirical relation for selective entrainment of gravel from naturally sorted riverbed material is used to estimate the smallest nontransportable particle in the armor layer. If the smallest nontransportable particle approaches the largest-sized particles in the bed-material mixture, evidence indicates that the armor layer will be unstable and particles of all sizes will be nearly equally mobile. However, if the armor layer is stable, clear-water contraction-scour depth estimates might be significantly less than for nonarmored conditions. To calculate armor-limited contraction-scour depths, two equations need to be solved simultaneously, one for the depth of the so-called active layer provided by the armor layer model, and one for depth of clear-water scour in a long constriction. The simple computational procedure will be especially useful for evaluating clear-water contraction scour at flow relief bridges crossing floodplains composed of well-graded alluvial till where a coarse protective surface layer is likely to form.

Aquatic habitats in a deeply incised sand-bed channel were modified by adding 1,380 t of stone and planting dormant willow posts. Restoration structures (groin extensions and longitudinal toe protection) were designed as complements to existing channel stabilization works. Prior to restoration, base-flow aquatic habitats were characterized by uniform conditions, little woody debris or riparian vegetation, shallow depths, and sandy bed material. The stage-discharge relationship, channel geometry, and bed material size were unaffected by restoration, but the average depth of scour holes adjacent to extended groins increased from 32 cm to 72 cm, and pool habitat in the lower half of the study reach increased from 2.9% to 14% of water surface area. Median water depth at base flow increased from 9 cm to 15 cm. Woody vegetation cover on one side of the channel increased from 38% to 78%. Fish numbers tripled, median fish size increased by 50%, and the number of species increased from 14 to 19. Groin extensions experienced partial failure due to erosion of sand from underneath stones.

The time variation of the phase velocity of a high-frequency shear wave was measured using a miniature rheometer in a laboratory mud bed subjected to monochromatic waves. The fall in the velocity, starting with an undisturbed bed, is shown to correlate with a corresponding decrease in bed rigidity as the bed becomes fluidized. The in-situ rheometry may be an effective means to track mud fluidization in the prototype environment.

A new catchment calibration approach based on the fractional-factorial and central-composite designs (FFD-CCD) is presented in this paper. The proposed method requires fewer simulations (almost 75% less) than the approach based on the factorial and central-composite designs (FD-CCD) (Ibrahim and Liong 1992). However, the reduction in the number of simulations does not significantly affect the prediction accuracy of the model. The calibration model has been applied to the Upper Bukit Timah catchment in Singapore. Results show that the prediction errors in peak flow and runoff volume in the proposed model are as small as those errors resulting from the original FD-CCD approach.