Journal of Hydraulic Engineering

The objective of this study was to determine the contribution of a plant’s foliage to the total plant’s hydrodynamic drag. Experiments were conducted in a laboratory flume using samples of vegetation with different physical forms and biomechanical properties: Branches of pine (Pinus sylvestris) and ivy stipes (Glechoma hederacea). The drag force was measured directly using a strain gauge technique and determined for a series of velocities for each vegetation species with and without foliage. Experimental results revealed a distinct contribution of foliage to the total plant drag. For both plant types, this was particularly marked at lower velocities where the foliage is not streamlined and compressed and, hence, the frontal projected area of the plant is at a maximum. It was found that the flexibility of the plant’s foliage and its ability to streamline with the flow may reduce the overall drag considerably. There was a distinct difference in the $C_d{A}_P$ parameter-velocity squared relationship between the “with” foliage plants and nonfoliage counterparts due to the streamlining effect of the foliage with the flow and, hence, the reduction in overall drag associated with the new compressed plant form.

One-dimensional numerical sediment transport models (DREAM-1 and DREAM-2) are used to simulate seven experimental runs designed to examine sediment pulse dynamics in a physical model of forced pool-riffle morphology. Comparisons with measured data indicate that DREAM-1 and -2 closely reproduce the sediment transport flux and channel bed adjustments following the introduction of fine and coarse sediment pulses, respectively. The cumulative sediment transport at the flume exit in a DREAM-1 simulation is within 10% of the measured values, and cumulative sediment transport at flume exit in a DREAM-2 simulation is within a factor of 2 of the measured values. Comparison of simulated and measured reach-averaged aggradation and degradation indicates that 84% of DREAM-1 simulation results have errors less than $3.3\mathrm{mm}$ , which is approximately 77% of the bed material geometric mean grain size or 3.7% of the average water depth. A similar reach-averaged comparison indicates that 84% of DREAM-2 simulation results have errors less than $7.0\mathrm{mm}$ , which is approximately 1.7 times the bed material geometric mean grain size or 11% of the average water depth. Simulations using measured thalweg profiles as the input for the initial model profile produced results with larger errors and unrealistic aggradation and degradation patterns, demonstrating that one-dimensional numerical sediment transport models need to be applied on a reach-averaged basis.

Incision and ensuing widening of alluvial stream channels is widespread in the midsouth and midwestern United States and represents an important form of channel adjustment. Streambanks have been found to contribute as much as 80% of the total suspended load. The location, timing, and magnitude of streambank erosion are difficult to predict. Results from field studies to characterize the resistance of fine-grained materials to hydraulic and geotechnical erosion, the impact of pore-water pressures on failure dimensions and shearing resistance, and the role of riparian vegetation on matric suction, streambank permeability, and shearing resistance are used to enhance the channel evolution model CONCEPTS (conservational channel evolution and pollutant transport system). This paper discusses the conceptualization of the above-mentioned physical processes, and demonstrates the ability of the derived model to simulate streambank-failure processes. The model is tested against observed streambank erosion of a bendway on Goodwin Creek, Miss. between March 1996 and March 2001, where it accurately predicts the rate of retreat of the outside bank of the bendway. The observed change in average channel width within the central section of the bendway is $2.96\mathrm{m}$ over the simulation period, whereas a retreat of $3.18\mathrm{m}$ (7.4% larger) is simulated. The observed top-bank retreat within the central section of the bendway is $3.54\mathrm{m}$ over the simulation period, whereas a retreat of $3.01\mathrm{m}$ (15% smaller) is simulated.

To improve knowledge of the flow and scour processes associated with spur dikes, three-dimensional flow velocities were measured at 2,592 points using an acoustic Doppler velocimeter over a fixed flat bed with a trapezoidal shaped submerged spur dike in a laboratory flume. General velocity distribution and detailed near field flow structures were revealed by the measurements and numerical simulations performed using a free surface turbulent flow model with a $k–\epsilon$ closure scheme. The three-dimensional flow separation characterized in this study was found to yield forces on the bed that were significantly different from nonsubmerged vertical obstructions that have been measured in other studies. Values of bed shear stress derived from both measured and simulated values were similar but indicated that local scour would be initiated in one rather than in the two locations of initial local scour measured in previous experiments with a similar flow.

This study characterizes the shear layer and associated vortex shedding around an isolated submerged pebble cluster in a gravel-bed river. The approach combines flow visualization and high frequency three-dimensional velocity (acoustic Doppler velocimeter) measurements. Two vortex shedding modes in the wake of the cluster were identified: A small scale high frequency initial instability mode and a lower frequency mode that scales with cluster height. The lower frequency mode arose from the intermittent interaction and amalgamation of the small-scale instability vortices. Reynolds shear stresses, velocity spectra, and coherence functions indicated a dominance of longitudinal-vertical shedding vortices in the wake of the cluster. Simultaneous flow visualization was required to determine the nature and behavior of the shedding modes. Quadrant analysis revealed that Q2 and Q4 events contributed 80% of the local longitudinal-vertical component Reynolds shear stress, and demonstrated a dominance of ejection events in the wake of the cluster. Through flow visualization, the behavior of the shear layer was seen to vertically expand and contract with the passage of Q2 and Q4 events, respectively.

This paper deals with the underlying mechanism of flow resistance in an alluvial channel: The effects of sidewall and bed form on flow resistance, Einstein’s divided hydraulic radius approach and Engelund’s energy slope division approach are reexamined. These two approaches assume that the shear stress on a mobile bed is the summation of shear stresses caused by skin friction and bed form. Using a different approach, this paper presents a theoretical relationship between the total bed shear stress with grain and bed-form shear stresses. The contribution of sidewall on the total bed shear stress is also discussed. The writers found that the size of bed form plays a significant role for the flow resistance, and developed relevant expressions for the length of the separation zone behind the bed forms. In addition, a systematical approach has been developed to compute the flow velocity in an alluvial channel. This approach is tested and verified against 5,989 flume and field measurements. The computed and measured discharge/velocity are in good agreement and 83.0% of all data sets fall within the $\pm 20\%$ error band.

The flow patterns of various dropping flows were investigated and classified experimentally for subcritical approach flows passing a vertical drop pool. A wave gauge was used to measure the free surface fluctuations in the pool. A flow visualization technique was employed to reveal the flow structure of the dropping flows qualitatively. It was discovered that, under certain conditions, the dropping flow forms a switching jet that oscillates up and down periodically and impinges on the bed and the downstream pool wall alternately. The switching jet switches between an impinging jet (a napped flow) and a sliding jet (a skimming flow), causing it to oscillate periodically with a unique period. The primary frequency of the periodic oscillation was determined by applying spectral analysis to the time series of the gauge measurements. Since a large number of air bubbles was entrained in the oscillatory flow, particle image velocimetry, and bubble image velocimetry were used for quantitative velocity determination in the liquid region and the aerated region, respectively. The mechanism causing the periodic oscillation in the pool was elucidated, and variables affecting the oscillation frequency were identified. An empirical relation between a weighted Strouhal number and a grouped dimensionless parameter was proposed to predict the primary frequency of the periodic oscillatory flow.

Surge protection devices, such as surge tanks and air chambers, have been modeled with the impulse response method for transient analysis of water distribution systems. The lumped inertia model and continuity equation are used to represent nonpipe hydraulic elements. Results of pressure or discharge variations obtained by using the impulse response method and the method of characteristics are in good agreement. The impulse response method provides total pressure and discharge along any pipeline segment by direct integration of the ratio of complex head or complex discharge to a complex downstream discharge, respectively. A modification is proposed so that transition between turbulent and laminar flows can be considered. The representation of hydraulic devices has been incorporated into the impedance matrix method, which was developed for heterogeneous and multilooped pipe network systems. The potential advantages of the proposed method over other conventional approaches were investigated by applying the proposed method to hypothetical pipe network systems.

This paper focuses on the formulation and assessment of a second-order accurate finite volume (FV) shock-capturing scheme for simulating one- and two-phase water hammer flows. The two-phase flow model is based on the single-equivalent fluid concept. The proposed scheme for one- and two-phase flows is the same, except for the Riemann solvers used to evaluate fluxes between computational cells. For one-phase flows, the accuracy and numerical efficiency of the proposed scheme is contrasted against the fixed-grid method of characteristics (MOC) and a recently proposed FV scheme. For two-phase flows, the accuracy and numerical efficiency of the proposed scheme are compared to the fixed-grid MOC scheme. The results for one-phase flows show that, when a Courant number (Cr) very close to 1.0 (around 0.99 or higher) is used, the MOC scheme is more efficient than the proposed scheme and the other FV scheme. In this case, the latter two schemes have similar numerical efficiency. When Cr drops below about 0.95, the proposed scheme is more efficient than the MOC scheme and the other FV scheme, especially for smooth transient flows (no discontinuities). For two-phase water hammer flows, all the simulations were carried out using a maximum Courant number of 0.95 to avoid numerical instability problems. The results for two-phase flows show that the proposed scheme is much more efficient than the fixed-grid MOC scheme. The fixed-grid MOC and the proposed scheme are also used to reproduce a set of two-phase flow experiments reported in the literature. Good agreement between simulated and experimental data is found.

Maintenance or repair of water mains may require the interruption of flow. Reinitiation of flow may present opportunities for the interaction of air and the accelerating water, resulting in undesirable pressure transients. This study focuses on air intrusion during flow startup in an initially filled horizontal pipeline supplied by a constant-head reservoir. Flow was initiated by opening the downstream end, allowing a free discharge at that point. During the initial phases of flow startup, an air cavity intruded in the upper portion of the pipe cross section while water exited the pipeline through the lower portion. The initial reservoir head controlled the rate of water velocity increase and, thus, the air intrusion advance. Air intrusion occurred until the water velocity was sufficient to arrest the intrusion. Higher reservoir heads resulted in the subsequent expulsion of the intrusion from the pipeline. However, for lower reservoir heads, the intrusion was not expelled, but readvanced to a final position that was controlled by shear forces. Criteria are developed for the flow conditions necessary to arrest the air intrusion in a horizontal pipeline as well as the intrusion lengths to be expected.

The free surface flow of two conceptual fish passage designs are investigated numerically by a level-set finite-element method. One design has a right-angled entrance from the reservoir to the fish passage chute, and the other has a curved-shaped entrance. The numerical results are validated with hydraulic experiments through comparison of the free surface location and the pressure distribution in the spillway. It is found that the right-angled design yields a curved free surface and pressure distribution in the vicinity of the entrance due to large strains, whereas the curved-shaped design yields a smooth flow transition with small strains. The negative pressure distributions near the ogee crest for both designs exhibit similar characteristics. Further downstream the free surface is elevated near the wall and is associated with counterrotating vortices. It is concluded that the curved-shaped design is favorable for fish passage because of the feature of a smooth flow transition with small strains.

A study utilizing a three-dimensional, 1:22 scale, physical model was conducted to evaluate the flow characteristics in the spillway with varying training wall convergence angles. The wall height required to contain the flow for conditions tested varied from critical depth at a $15°$ convergence angle to three times the critical depth at a $52°$ convergence angle. The results of the study may be used to estimate minimum training wall height for conditions where bulking due to air entrainment may reasonably be neglected.

Circulation regions always exist in the settling tanks. These regions reduce the tank’s performance and decrease effective volume of the tank. Recirculation zones would also result in short-circuiting and high flow mixing problems. Inlet position would have an effect on these, too. Use of good baffle configuration may increase performance of settling tanks. One method for comparison performance of different tanks with each other is to use flow through curves (FTCs). In this note using FTCs, effects of inlet position and baffle configuration on the hydraulic performance of primary settling tanks are studied. The best position of baffle is also determined.

The ability to accurately simulate sediment transport processes in natural and engineered systems is crucial to both engineers and scientists. Many researchers have worked on the development of methods that yield reliable predictions of both sediment transport and morphological change with a minimum of computational burden, so that the long prototype time scales necessary to appropriately investigate these processes can be simulated within a reasonable amount of time. This includes the development of tools for approximating the complex behavior of the sediment concentration in the water column. In this technical note, the classic equilibrium sediment profile first introduced by Rouse is revisited. Following the development of the Rouse profile introduced by Einstein, a generalized nonequilibrium sediment concentration profile is generated. The principal assumption is that the convective term of the nonequilibrium vertical sediment flux can be approximated with the relationship that arises from purely convective transport. The method is compared to numerical solutions for depositional profiles, and appears to yield promising results.

Unsatisfactory features of the equation that is recommended in a standard reference on bridge scour (commonly known as HEC-18) for predicting pressure-flow scour are pointed out. These are found to stem from the basic model of the original regression analysis, which is argued to have suffered from a spurious correlation. An alternative best-fit and a separate design equation are proposed, together with a definition of the region where such equations may be expected to be applicable.