Sharp-crested Weir Research Articles

Lateral flow through the sharp crested side rectangular weirs in open channels

A side rectangular weir is a flow diversion structure provided in the side wall of a channel to divert water from main channel to a branch channel. They are widely used in hydraulics, environmental and irrigation engineering for controlling and directing flow of water in irrigation–drainage systems, waste water channels, drainage canal systems, diverting excess water into relief channels for flood-protection works, and as storm overflows from urban sewage systems. The present study is aimed at to compile the past observations on coefficient of discharge for side rectangular sharp crested weir, supplement them with new experimental results and reanalysing resulting data bases with a view towards seeing if better prediction are possible. Therefore, a new model for determination of coefficient of discharge of side rectangular sharp crested weir is developed. It is observed that approach Froude number is an important parameter for coefficient of discharge. The ratio of the crest height of side weir to length of the side weir, ratio of width of the main cannel to length of side weir and ratio of the upstream depth in the channel to length of side weir also affect the coefficient of discharge. The performance of the present model is based on the coefficient of correlation of the linear regression line between predicted values from the present model and desired output (R), mean absolute percentage error (MAPE), average percentage error (APE), root means square error (RMSE) etc. The qualitative performance of the present predictor indicates that it has highest R (0.865) and lowest MAPE (4.8936), RMSE (0.0337), APE (−0.346) as compared to other existing predictors. Flow profile and flow pattern are also observed in the vicinity of side weir in the main channel.

Improvement of extreme learning machine using self-adaptive evolutionary algorithm for estimating discharge capacity of sharp-crested weirs located on the end of circular channels

Weirs are used in different shapes such as rectangular, triangular and circular to control and measure the flow in open channels. To design a weir properly, determining its discharge coefficient is very important. In this study, the discharge coefficient of sharp-crested weirs located on circular channels is modeled by the self-adaptive optimization of extreme learning algorithm using the differential evolutionary algorithm (SaDE-ELM). Also, Monte Carlo simulations (MCs) are used to study the compatibilities of SaDE-ELM models. However, the k-fold cross validation method (k=5) is used to investigate the abilities of the used numerical models. According to the input parameters, four models of SaDE-ELM are introduced. Study of the numerical results shows that the superior model simulates the discharge coefficient as a function of the Froude number (Fr) and the ratio of the circular channel diameter to the weir crest height (D/P) and a relationship is provided for the superior model. The values of mean absolute relative error (MARE), root mean square error (RMSE) and correlation coefficient (R2) for the superior model are calculated 0.184, 0.002 and 0.997, respectively. However, the maximum error value for this model is less than 3%. Also, the results of the uncertainty analysis show that the superior model has an underestimated performance which its 95% prediction error interval is simulated between 0.000314 and −0.000314. In other words, the width of uncertainty band for the superior model is calculated equal to −0.000314.

Performance of Flow over a Weir with Sloped Upstream Face

Measurement of flow in open channel is almost based on weirs. A relatively smallest height of water generated upstream weirs is sometimes important issue, especially when the banks of channel are not enough. Upstream crest slope to the crested edge of the weir reduces loss and due to that less head need to pass a discharge. The effect of the sloped upstream face is investigated experimentally by testing four different weir heights and four slopes for each weir height. The measured surface flow profile shows that the shape of profile curvatures are effaced by the length of upstream sloped face and then the performance of weir compared with sharp crested weir, the maximum increase in the value of discharge coefficient is 20% compared with the sharp crested weir of the same height. The discharge coefficient decreases with increase of relative value of the head to the weir height (H/P) and increase with the value of face length to the head (L/H) and with face length to the depth of water over crest (L/h). The water surface profiles are predicted by HEC-RAS and then compared with measured profiles. The comparison shows a reasonable acceptance. Dimensionless relations are suggested for each weir height to predicting the discharge coefficient with acceptable coefficient of determination R 2 .

A comparative study of predicting oxygen transfer in weirs and water jets in cross-flows

A water jet in a cross-flow drives a substantial content of oxygen into a water body through air ingression. An experimental study was conducted on the oxygen-transfer capacity of a water jet in a cross-flow in which the major influential parameters of the water jet and the receiving water were systematically varied. The mechanism of air entrainment and the hydrodynamic behaviour of the generated two-phase air–water flow reveal major similarities between a low-drop sharp-crested weir aeration system and a jet in a cross-flow. Notwithstanding the cross-flow as the main source of distinction, similar to weir systems, equations were developed to predict the oxygen-transfer efficiency of a water jet in a cross-flow. Amendments were applied to account for the cross-flow characteristics as well as experimental conditions. The results indicated that cross-flow characteristics such as depth and velocity should be included in predictive equations as they influence aeration efficiency. However, minor modifications of independent parameters were promising in some cases. Furthermore, an equation was developed for the estimation of oxygen-transfer efficiency using dimensional analysis.

Experimental study of discharge coefficient and trapping ratio in mesh-panel bottom rack for sediment and non-sediment flow and supercritical approaching conditions

Bottom racks have been widely used in dam-less projects as a means to divert some part of the river flow. Mesh-panel bottom rack is a new form of bottom rack which is introduced in this paper and consists of longitudinal bars along the river over transverse bars perpendicular to the river flow. Deformation of mesh-panel racks under static and dynamic loads is less than other kinds of bottom intakes. In the present study, firstly, physically-based relationships were derived for rack discharge coefficients in sediment and non-sediment flows. To do this, three mesh-panel racks were built, and the effects of hydraulic characteristics of approaching flow, rack geometry (slope, length, bar shapes), and sediment characteristics on discharge coefficient were investigated, and, also, a sensitivity analysis was taken. The data was obtained from a series of tests carried out in a laboratory flume equipped with inclined bottom rack and a sediment chamber at the upstream. For each run, diverted and remaining discharges were measured by two separate fully-aerated sharp crested rectangular weirs. Secondly, trap ratio of mesh-panel rack which was defined as the weight of passed sediment through the mesh panel to the total input amount of sediment at upstream, were experimentally studied and discussed. Finally, a physically-based dimensionless relation for estimating trap ratio was presented, and sensitivity analysis was done.

Flow discharge computation over compound sharp-crested side weirs

Side weirs are generally rectangular sharp-crested in form, but for accurate flow measurement and management in a wide range of flow discharges, compound side weirs perform better. Such a commonly used one is rectangular compound side weir, which has a small rectangular weir in lower section for measuring low flow discharges, and a wide rectangular section alongside for measuring higher flows. Determination of discharge coefficient for such a compound side weir is difficult and needs to be calibrated using a large set of laboratory tests. In this paper, based on the practical design method of May et al., a new approach was proposed for predicting flow discharge over compound side weirs. This new approach has been evaluated against experimental data in subcritical flow conditions, and has shown very good agreement with the experimental data, with the mean overall and absolute relative errors being 1.6 and 7.8%, respectively. Because the proposed method is independent of discharge coefficient for overflow rate computation, it will provide a reliable and convenient tool for flow measurement of compound side weirs and their water surface profiles.

Testing the Outflow Process over a Triangular Labyrinth Weir

AbstractIn this paper, the dimensionless stage-discharge relation for a sharp-crested triangular labyrinth weir, determined in a previous study, is initially tested by some experimental runs carrie...

Experimental investigation of discharge coefficient over novel kind of sharp-crested V-notch weir

Thin-plate weirs are the simplest and least expensive devices, which frequently have been employed in many water projects. In this research, a unique type of Sharp-Crested V-notch weir, entitled SCVW, has been introduced. The hydraulic characteristics of the present weir were investigated theoretically and experimentally under free aerated and non-aerated flow conditions in an open channel for large physical models. To investigate the variations of the discharge coefficient of SCVW versus weir height and vertex angle, a comprehensive laboratory experiments were conducted by measuring the discharge and the water head over the crest of weir. Possibility of different formulations for the head- discharge relationship of SCVW was examined and suitable analytical equation was proposed. The computed discharge using the suggested equation was within 0–10% of the observed ones. According to the experimental observations, the SCVW showed better performance in comparison with normal wire.

Experimental and numerical evaluation of discharge capacity of sharp-crested triangular plan form weirs

Weirs are those structures used extensively for flood passage, flow measurement, flow deviation and controlling the water level in dams, rivers and open channels. In this study, the numerical model was simulated using flow-3D software and the results from experimental and numerical model were compared. Investigations have shown that the results of RNG turbulence model are better compared to k-e and LES models. Then, to model the turbulence, and to determine the location of free-surface profile, RNG model and VOF method were used respectively. Experimental results show proper coordination with estimated results of numerical model. For similar low values of h/p, the performance of 30 degree sharp crested triangular plan form weir for discharging of overflow was more suitable than other sharp-crested triangular plan form weirs.

Experimental and numerical evaluation of discharge capacity of sharp-crested triangular plan form weirs

Experimental and numerical evaluation of discharge capacity of sharp-crested triangular plan form weirs

Experimental Studies on Determination of Discharge Capacity of Circular Labyrinth Weirs Located on A Straight Channel

Labyrinth weirs are particularly well suited for spillway rehabilitation where dam safety concerns, freeboard limitations, and a revised and larger probable maximum flow have required replacement or modification of the spillway. Labyrinth weirs with multiple crest elevations can be used in spillway design to confine base flows to a section of the crest and/or satisfy discharge hydrograph requirements. Labyrinth weirs provide higher discharge capacity than conventional weirs, with the ability to pass large flows at comparatively low heads. Over past 50 years many research investigations have considered the hydraulic performance of labyrinth weirs, particularly as dependent on the geometric features. The previous work has improved the design basis for such weirs. In the present study, discharge coefficients were experimentally determined for both sharp crested semi-circular labyrinth weirs and trapezoidal labyrinth weirs of side wall angle ( α =37 0 ). A comprehensive laboratory study including 9 models was conducted to determine the discharge coefficient of the semi-circular labyrinth weirs. It was found that from this experimental study the discharge coefficient of the circular labyrinth weir is higher than that of the linear weir, but lower than that of the trapezoidal weir.

River safe yield with the constraint of a contracted rectangular weir for surface water supply

Aydin et al. introduced the weir velocity concept in 2011 for discharge measurement in contracted rectangular sharp-crested weirs. Gharahjeh et al. further reinforced this formula in 2015 to allow discharge to be computed using only weir velocity and crest width. These newly introduced expressions are tested and demonstrated to determine surface water yield of Tapah River in Sarawak. Aydin and Gharahjeh's formula is found to perform similarly to the improved discharge coefficient developed by French in 1986, but is simpler and easier to use. Exploring Aydin and Gharahjeh's formula through theoretical, experimental, and field approaches, it has been found that the formula performs well in a controlled environment. Discharge computations based on field data were determined to have been formerly underestimated, with the reason as yet undetermined.

Runoff of small rocky headwater catchments: Field observations and hydrological modeling

In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼0.032 km2, average slope ∼320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp-crested weir. Six runoff events were recorded in the period 2011–2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve-Number (SCS-CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape, including peak value and timing, is obtained only by combining the SCS-CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins.

Modeling scour depth downstream of grade-control structures using data driven and empirical approaches

Local scour occurs in the immediate vicinity of structures as a result of impinging on a bed with a high velocity flow. Prediction of scour depth has an important role in control structure management and water resource engineering issues, so a study of new heuristic expressions governing it is necessary. The present study aims to investigate different methods' capabilities to estimate scour depth downstream of grade-control structures using field measurements from the literature. Accordingly, data driven feed forward neural network and gene expression programming techniques were selected for the investigation. Additionally, the optimum data driven based scour depth models were compared with the corresponding physical–empirical based formulas. Three data categories corresponding to (a) scouring downstream of a ski-jump bucket, (b) a sharp-crested weir, and (c) an inclined slope controlled structure (as grade-control structures) were applied as reference patterns for developing and validating the applied models. A sensitivity analysis was also performed to identify the most influential parameters on scouring. The obtained results indicated that the applied methods have promising performance in estimating the scour depth downstream of spillways and control structures. Nevertheless, the applied data driven approaches show higher accuracy than the corresponding traditional formulas.

Propagation of an Air-Water Interface from Pressurized to Free-Surface Flow in a Circular Pipe

AbstractHydraulic transients with the movement of an interface between pressurized flow and free surface flow can be observed in rapid water emptying in the pipes. This study focuses on air cavity intrusion into horizontal and inclined circular pipes. Laboratory experiments were carried out to observe the negative surge during depressurization in a circular pipe. In order to generate the undular bore in a circular pipe, in some cases a sharp-crested weir at the open end of the horizontal pipe was used. Various behaviors of air cavity were observed for a series of weir heights. A numerical model is proposed and applied to the cavity flow in a circular pipe. To reproduce the undular bore in circular pipes (when water depth is greater than pipe radius), this study derived the depth-averaged shallow water equations with the effects of vertical acceleration using the Boussinesq equation. In this study, the continuity and the momentum equations of free surface and pressurized flows with the hydrostatic and the ...

Numerical modeling of free surface flow in hydraulic structures using Smoothed Particle Hydrodynamics

A meshless particle numerical model based on the Smoothed Particle Hydrodynamics (SPH) is developed and evaluated for two-dimensional simulation of free-surface flows passing through an orifice and over a sharp crested weir. A practical technique is introduced making the model capable of handling the inlet and outlet boundaries. This technique is able to impose known-inflow flux (as in sub-critical open channel flow) at the inlet boundary, while the inlet pressure/depth is automatically adjusted (without imposing any unwanted boundary reflection and pressure fluctuation). Solution stabilizer techniques, such as XSPH and artificial viscosity, are also employed to increase the accuracy and stability of the model, and their effects on the results are evaluated. The developed model proves to be able to reproduce the free surface complexity with good accuracy. A 3-D finite volume mesh-based model is also employed to verify the result of the present SPH model, showing quite compatible pressure and velocity fields.

Water Surface Characteristics of Submerged Rectangular Sharp-Crested Weirs

AbstractLaboratory experiments were carried out to investigate the water surface characteristics of submerged flow over sharp-crested weirs. Interesting observations were made on the water surface profiles near the weir as weir submergence sets in as well as for various stages of submergence. Submerged flow was divided into four regimes: (I) impinging jet, (II) surface jump, (III) surface wave, and (IV) surface jet. It was found that a surface jump turned into a surface wave when the wave trough became level with the tip of the weir. The surface jet regime began as the surface waves faded out. A condition was set to quantify the boundary between the surface wave and surface jet regimes. Based on flow observations, the surface jet regime sets in as the amplitude of the surface wave becomes small (i.e., less than 0.1h, where h is the head). An empirical equation was developed based on the literature data to predict the discharge reduction factor in the generally used discharge equation. This formulation was...

Experimental Investigation of Discharge Capacity of Labyrinth Weirs with and without Nappe Breakers

Labyrinth weirs provide higher discharge capacity than conventional weirs, with the ability to pass large flows at comparatively low heads. Labyrinth weirs are primarily used as spillways for dams where the spillway width is restricted. In recent years, many research investigations have considered the hydraulic performance of labyrinth weirs, particularly as dependent on the geometric features. The previous work has improved the design basis for such weirs. However, their design still requires experimentally derived and generalized performance curves. It is especially important to observe the behavior of the weir nappe to ensure the design provides hydraulic optimization and to account for pressure fluctuations, possible vibrations, resonance effect, noise and flow surging. In the present study, discharge coefficients were experimentally determined for both circular labyrinth weirs and sharp crested trapezoidal labyrinth weirs of varying side wall angle (α). Additional studies were completed with nappe breakers included to reduce the impact of vibration on the labyrinth weirs. In general, the test data indicated that nappe breakers placed on the trapezoidal labyrinth weirs and circular labyrinth weirs reduced the discharge coefficient by up to 4% of the un-amended weir.

Numerical Modeling of Sharp-Crested Triangular Plan Form Weirs Using FLUENT

Weirs are hydraulic structures used for measuring flow rate. Triangular labyrinth weir is a kind of non-linear weirs which are used to control free flow passing over weirs in low width. Accordingly, the present paper attempted to investigate the characteristics of flow in triangular labyrinth weirs through FLUENT Software. In this regard, using GAMBIT Model, geometrical and grid generation conditions of flow was created for flow field solution. Using numerical model of Fluent, boundary conditions and flow field conditions were implemented for the model. Meshing conditions is for about 300 thousands cells with at most about 11% error. In the next test, simulation error is decreased about 30% through minimizing the size of computational cells and flow field solution for 580 meshes and the maximum depth simulation error reaches to 8%. Re-decreasing the cells’ size and the limited number of about 1 million computational cells causes that simulation error approaches to 7%.Through more careful computational cells selection and given to hardware limitations during simulation time, about 580 thousands cells with average size of 20 mm were selected. It is due to the fact that smaller selection leads to the increase of flow field modeling time up to 2 times more than time. Numerical model of FLUENT is highly able to simulate flow field in labyrinth weirs.

Elementary discharge coefficient of a triangular–rectangular sharp-crested side weir in subcritical flow

ABSTRACTSide weirs or lateral weirs are widely employed to divert flows from rivers, channels, sewers and reservoirs or to divide flows between components of water treatment systems. The side weir is a fixed structure installed at one side of a channel for water level control in water treatment plants and irrigation and drainage systems. Hydraulic characteristics of a compound sharp-crested triangular–rectangular side weir under subcritical flow were studied in this paper. The concept of the elementary discharge coefficient that is related to the head to weir height ratio was used to determine the flow through the side weir. The advantage of this method is to calculate the channel water surface profile along the weir. Generalized equations for an elementary discharge coefficient for compound triangular–rectangular side weirs were proposed. Experimental results show the accuracy of the proposed equations.