Discharge Relation for Cutthroat Flume under Free-Flow Condition

A cutthroat flume is commonly used as flow measuring device for open-channel flow due to ease of fabrication and installation. In most of the cases it is difficult to calibrate the flume in the field. Therefore, accurate relation between discharge and upstream head applicable for all sizes of cutthroat flume is needed. Seven different sizes of cutthroat flumes, having different length to throat width ratios, are fabricated and tested in the laboratory under free-flow condition. Selecting groups of different variables describing flow through a cutthroat flume number of dimensionless parameters are formed. Regression analysis of experimental data is carried out between all possible combinations of pairs of dimensionless parameters and the pair giving the best correlation is selected. Using the selected pair, relation between dimensionless parameters of discharge and head is developed. The relation is simple and convenient to use and, at the same time, more accurate compared to methods available in literature for prediction of discharge.     

Simple Flume for Flow Measurement in Open Channel

There are various types of water measurement devices available. The type of flume used in a particular application depends on site-specific factors or limitations and the need for precision and cost. In measuring water in open drains, it is often desirable to prefabricate the flume to avoid the need for dewatering prior to installation. Traditionally, cutthroat flumes have been used in open drain systems or ditches due to ease of fabrication and installation. However, due to the extended transitional length and width associated with the cutthroat flume, transportation of the flume requires special facilitation or permits. A simple Venturi flume was proposed and tested that has several advantages over the traditional cutthroat flume. These advantages include lower construction cost, simplicity in fabrication, ease of transportation, and less width requirements where large flows are to be measured.     

Upstream Discharge Distribution in Compound-Channel Flumes

Common inlet design for compound-channel flumes does not ensure a proper upstream discharge distribution. As the total head in the upstream tank is the same for both main-channel and floodplain subsections, the velocity in the upstream section is also the same in both subsections. The floodplain discharge is therefore too large and a mass transfer towards the main channel occurs along the flume. This Technical Note investigates how long a compound-channel flume must be to ensure that equilibrium between subsection discharges is achieved. The required length is found to be significant compared to the actual length of experimental flumes reported in the literature.     

Field Testing of a Simple Flume (SMBF) for Flow Measurement in Open Channels

In this paper the stage–discharge relationship of a new flume named SMBF (Samani, Magallanex, Baiamonte, Ferro), originally proposed by Samani and Magallanez and tested by Baiamonte and Ferro, for measuring flow discharge in open channels is reviewed. The flume is obtained inserting two semicylinders in a rectangular cross section. The results of some experimental runs carried out using horizontal flumes characterized by different values of the contraction ratio (ranging from 0.17 to 0.81) are used for determining the two coefficients of the power stage–discharge equation. The stage–discharge equation is tested using flow measurements carried out in the period between December 2004 and March 2006 in the Sicilian experimental SPA1 basin. Field testing of the SMBF flume is developed using discharge measurements carried out by a Khafagi–Venturi flume placed in the field measurement channel.     

Supercritical Flow Measurement Using a Small Parshall Flume

An experimental program was conducted to determine if a Parshall flume, developed to accurately measure open-channel subcritical flow, could also be used to measure discharge in a supercritical flow regime. Fifteen experimental configurations were tested using two small Parshall flumes [6-in. (15.2-cm) and 9-in. (22.9-cm) crest width] with varying approach channel slopes, approach channel roughness, and flume convergence. It was determined that a single Parshall flume can be used to measure flow (within ±5%) for both supercritical and subcritical flow regimes for a specified range of flows. The original Parshall flume equation was then modified to incorporate crest width, channel slope, channel roughness, and convergence in the prediction algorithm. Unique expressions were developed for both supercritical and subcritical flow regimes to estimate the discharge. A single expression does not appear feasible for accurate discharge measurement for both flow regimes in a Parshall flume at this time.     

Data Interpretation for In Situ Measurements of Cohesive Sediment Erosion

Methods for interpreting data from in situ flume measurements of cohesive sediment dynamics are evaluated and a technique for estimating various erosion parameters using in situ measurements is proposed. There is currently a lack of uniformity in analysis techniques for cohesive erosion data collected in flumes and with in situ instruments and the proposed technique resolves some of these inconsistencies. The data set used in this study was derived from field experiments conducted with a straight benthic in situ flume in different aquatic environments in New Zealand. The experiments with stepwise increases in flow velocity revealed that peaks in the erosion rate at the beginning of each velocity step are most likely associated with heterogeneous bed structure, as transient hydrodynamic effects due to the experimental procedure were found to be insignificant. The field data showed an exponential decay of the erosion rate with time that is indicative of depth-limited erosion. These data are used to illustrate methods for the parameterization of the proposed semiempirical erosion equation, taking into account the time dependency of the erosion process.     

Distribution of Bed Shear Stress in Rotating Circular Flume

Distributions of bed shear stress across the width of a rotating circular flume with smooth and rough bed surfaces were obtained by measurement and model prediction. Results with flows over smooth beds showed that the flow in the central part may be considered to be two-dimensional and that effects of flow depth over the operating range of the flume are minor for flow depths not exceeding 0.14 m. For rough beds, the bed shear stress distributions were found to be skewed toward the inner wall. This can be corrected if a compensating roughness is added to the bottom of the ring. Such measures are also effective for flumes with smooth beds. Measured bed shear stress distributions agreed well with the predicted distributions for smooth beds and reasonably well for rough beds. The modified Preston tube, for measurement of bed shear stress in flows over rough beds, was found to give promising results. Further tests are required to completely define the uncertainty in bed shear stress measurements made with this instrument.     

Numerical Simulation of Flows in Cut-Throat Flumes

A numerical simulation is presented to obtain the flow characteristics of cut-throat flumes in rectangular open channels. Cut-throat flumes with a horizontal floor are used as simple devices for flow measurement in open channels. Since the flow in the throat section is highly three dimensional and curvilinear, the three-dimensional turbulence Reynolds stress model was applied in the present study to obtain the flow parameters such as the water surface profiles, the pressure distributions, and the mean velocity distributions. The volume of fluid scheme was used to determine the shape of the free surface by computing the fraction of each near-interface cell of a fixed grid that is partially filled with water. The previously published experimental data as well as data based on a new test related to cut-throat flumes were used to validate the simulation results.     

Cutthroat Measurement Flume Calibration for Free and Submerged Flow Using a Single Equation

A series of detailed laboratory measurements were made under steady-state flow conditions through a 0.914-m (3-ft) Cutthroat flume in an attempt to more accurately define transition submergence for four standard throat widths. It was found that the change from free to submerged flow, and vice versa, is gradual and that there is no easily observable transition point. The gradual transition between the flow regimes suggested a new calibration approach in which a single equation could more elegantly and more accurately fit the laboratory measurements, eschewing the need for separate free- and submerged-flow equations, and obviating the need to define transition submergence. Such an equation was found, providing greater calibration accuracy up to 95% submergence in 0.914-m Cutthroat flumes.     

Erosion of Cohesive Sediments: Resuspension, Bed Load, and Erosion Patterns from Field Experiments

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.     

Flow over Ogee Spillway: Physical and Numerical Model Case Study

A study was completed to compare flow parameters over a standard ogee-crested spillway using a physical model, numerical model, and existing literature. The physical model was constructed of Plexiglas and placed in a test flume. Pressure taps were installed along the entire length of the spillway. Discharge and pressure data were recorded for 10 different flow conditions. A commercially available computational fluid dynamics (CFD) program, which solves the Reynolds-averaged Navier-Stokes equations, was used to model the physical model setup. Data interpolated from U.S. Bureau of Reclamation and U.S. Army Corps of Engineers design nomographs provided discharge and pressure data from the literature. Nondimensional discharge curves are used to compare the results from the different methods. Pressures are compared at low, mid, and high flow conditions. It is shown that there is reasonably good agreement between the physical and numerical models for both pressures and discharges. The availability and power of existing numerical methods provides engineers with another tool in the design and analysis of ogee spillways.     

Computing Nonhydrostatic Shallow-Water Flow over Steep Terrain

Flood and dambreak hazards are not limited to moderate terrain, yet most shallow-water models assume that flow occurs over gentle slopes. Shallow-water flow over rugged or steep terrain often generates significant nonhydrostatic pressures, violating the assumption of hydrostatic pressure made in most shallow-water codes. In this paper, we adapt a previously published nonhydrostatic granular flow model to simulate shallow-water flow, and we solve conservation equations using a finite volume approach and an Harten, Lax, Van Leer, and Einfeldt approximate Riemann solver that is modified for a sloping bed and transient wetting and drying conditions. To simulate bed friction, we use the law of the wall. We test the model by comparison with an analytical solution and with results of experiments in flumes that have steep (31°) or shallow (0.3°) slopes. The law of the wall provides an accurate prediction of the effect of bed roughness on mean flow velocity over two orders of magnitude of bed roughness. Our nonhydrostatic, law-of-the-wall flow simulation accurately reproduces flume measurements of front propagation speed, flow depth, and bed-shear stress for conditions of large bed roughness.     

Subcritical Contraction for Improved Open-Channel Flow Measurement Accuracy with an Upward-Looking ADVM

Acoustic Doppler velocity meters (ADVMs) provide an alternative to more traditional flow measurement devices and procedures such as flumes, weirs, and stage rating for irrigation and drainage canals. However, the requirements for correct calibration are extensive and complex. A three-dimensional computational fluid dynamics (CFD) model was used to design a subcritical rapidly varied flow contraction that provides a consistent linear relationship between the upward-looking ADVM sample velocity and the cross-sectional average velocity in order to improve ADVM accuracy without the need for in situ calibration. CFD simulations validated the subcritical contraction in a rectangular and trapezoidal cross section by showing errors within +1.8 and ?2.2%. Physical testing of the subcritical contraction coupled with an upward-looking ADVM in a large rectangular flume provided laboratory validation with measurement errors within ±4% without calibration.     

Coupling of U-Tube and Shear Layer Oscillations in an Interconnected Flow Compartment

Results of experiments conducted in a 2?m high flume at large Reynolds numbers are reported in this paper. The flume was partitioned into two compartments. Flow entered the bottom of the upstream test compartment as a wall jet, at jet Reynolds number ranging from 11,000 to 170,000. Periodic oscillations of the free surface in the two compartments resembling the oscillatory flow in a liquid-filled U-tube, and large coherent structures formed above the potential core of the wall jet were observed. Coupling of the U-tube oscillations and vortex shedding is attributed to fluid-dynamic and fluid-resonant feedback processes. For test compartment length, Lc = 0.8?m, fluid-resonant feedback was found to be dominant, and the shear layer was observed to oscillate at the natural frequency of the two-compartment, U-tube system. The observed U-tube oscillations are initiated by the oscillations of the shear layer at a frequency equal to the subharmonic component for the U-tube. The flow oscillations were generally weaker for Lc = 1.2 and 2.0?m with oscillation frequencies governed by fluid-dynamic feedback, verified from a comparison with the results from a previously reported study.     

Inception Point and Air Concentration in Flows on Stepped Chutes Lined with Wedge-Shaped Concrete Blocks

Stepped channels lined with wedge-shaped concrete blocks are a promising solution to provide overtopping protection for embankment dams if the discharge capacity of existing spillways is not adequate. The paper addresses the characteristics of the two-phase transition and skimming flows in stepped channels lined with this type of block. An experimental setup was developed with two flumes designed with a relative scale of 1:2.5. Air concentration was measured with an optical probe in several cross sections of both flumes. The scale effects are analyzed. An expression for the location of the inception point is proposed. The vertical air concentration profiles and their longitudinal variation are studied, considering data and models proposed by other researchers. The establishment of the uniform flow regime is analyzed.     

Simple Flume for Flow Measurement in Sloping Open Channel

First, this paper presents a new flume for measuring flow discharge in sloping channels, originally proposed by Samani and Magallanez for use in a horizontal channel. The flume is obtained by inserting two semicylinders in a rectangular cross section. Then, using dimensional analysis and the self-similarity theory, the stage-discharge relationship of the flume is theoretically deduced. For determining the two coefficients of the power stage-discharge equation, some experimental runs are carried out using flumes characterized by different values of the contraction ratio (ranging from 0.17 to 0.81) and of the flume slope (ranging from 0.5 to 3.5%). Finally, for a given range of the contraction ratio, the relationships for estimating the two coefficients of the stage-discharge equation are obtained.     

Use of Brink Depth in Discharge Measurement

The behavior of free surface flow at a rectangular free overfall is studied experimentally to obtain a relation between the brink depth and the flow rate. A series of experiments were conducted in a tilting flume with wide range of flow rates covering subcritical, critical, supercritical regimes, and two different roughnesses in order to develop a relationship between the discharge and the brink depth. An equation is proposed to determine the flow rate using the brink depth for a channel of known roughness and bed slope.     

Velocity Distribution and Energy Dissipation along Stepped Chutes Lined with Wedge-Shaped Concrete Blocks

Stepped channels lined with wedge-shaped concrete blocks may constitute a low-cost alternative to provide overtopping protection of embankment dams if the discharge capacity of existing spillways is not adequate or even to be used as the main spillway of newly built embankment dams. This paper addresses the velocity distribution and the energy dissipation, downstream of the inception point, on stepped chutes lined with wedge-shaped concrete blocks. An experimental setup was developed with two flumes designed with a relative scale of 1∶2.5. Air concentration was measured with an optical probe in several cross sections of both flumes. The velocity profiles along chutes lined with wedge-shaped blocks with the upper face sloping downstream were analyzed. The measurements’ accuracy was checked by comparing discharges indicated by a facility flowmeter and obtained by the integration of velocity and air concentration profiles. The effect of the steps-slope in the energy dissipation is studied. Values of the Darcy-Weissbach friction factor are proposed for this type of chute lining, for transition flows, and for skimming flows.     

Effect of Shape on Incipient Motion of Large Solitary Particles

The aim of this study is to investigate the effect of shape and size of solid particles on their initiation of motion in open channel flows. Initial motions of 22 solitary particles having different shapes and sizes were observed in a tilting flume of rectangular cross section. A smooth fixed bed and an obstructing element of smaller height with respect to the particle size was used throughout the experiments. The ratio of the height of the obstructing element to the height of the particle was kept constant at 1/5. By either changing the slope of the tilting flume or the discharge, or both, a range of shear stress values was obtained. Various equations and graphical representations in terms of dimensionless bed shear stress, grain Reynolds number, and the ratio of flow depth to grain diameter were presented to determine the flow conditions corresponding to the initiation of motion of solitary particles of given shapes. The experiments have revealed that critical flow conditions are dependent not only on the particle size and shape but also on the ratio of flow depth to grain diameter.     

Broad-Crested Weirs with Rectangular Compound Cross Sections

A series of laboratory experiments was performed in order to investigate the effects of width of the lower weir crest and step height of broad-crested weirs of rectangular compound cross section on the values of the discharge coefficient, the approach velocity coefficient, and the modular limit. For this purpose, nine different broad-crested weir models with rectangular compound cross sections and a model with a rectangular cross section were tested in a horizontal laboratory flume of 11.0 m length, 0.29 m width, and 0.70 m depth for a wide range of discharges. The compound cross sections were formed by a combination of three sets of step heights and three sets of lower weir crest widths. The sill-referenced heads at the approach channel and at the tailwater channel were measured in each experiment. The dependence of the discharge coefficient, approach velocity coefficient, and modular limit values on model parameters was investigated, and these quantities were compared with those of the broad-crested weir models with a rectangular cross section.