Section 3. HYDRAULIC DESIGN A. Weirs and Orifices A.1. WEIRS (original) (raw)

River Weirs – Good Practice Guide Guide -Section A

This guide is for all parties who have an interest in the construction, operation and maintenance, refurbishment or demolition of river weirs. Its aim is to make them aware of the wide range of issues that are relevant in planning and implementing such works, and by doing so, to ensure that mistakes are avoided and potential benefits are maximised.

Weir velocity formulation for sharp-crested rectangular weirs

Discharge in open channels can be measured by sharp-crested rectangular weirs. Generally, measured head over the weir crest is substituted into an empirical formula derived from energy considerations to calculate the discharge. Assumptions made on the derivation are taken into account by defining a discharge coefficient that fits into the experimental data. In this study, a physical quantity, the average velocity over the weir section defined as 'weir velocity' is directly formulated as function of weir geometry and head over the weir. Weir velocity plotted against the weir head has a universal behavior for constant weir width to channel width ratio independent of the weir size. This unique behavior is described in terms of weir parameters to calculate the discharge without involving a discharge coefficient. Combining weir velocity data for variable weir widths provides a basis for direct formulation of discharge. The weir velocity exhibits simpler functional dependency on weir parameters in contrast to the discharge coefficient.

Development of rectangular broad-crested weirs for flow characteristics and discharge measurement

KSCE Journal of Civil Engineering, 2014

One of the important problems which designers and engineers are faced with is proper measurement of water flow rates in open channels and rivers. To solve this problem, the water discharge to be precisely measured in water conveyance systems such as off takes and distribution ditches. The common procedure to meet this requirement is achieved by using water-measuring structures such as flumes, orifices, weirs and current meters. Among the mentioned structure, engineers prefers the weirs because of their simple structure and economical features. The broad-crested rectangular weirs are very common which are widely used in water ways and canals. In this study, the effect of steeping in the upstream side of rectangular broad-crested weirs over discharge coefficient and flow characteristics is investigated. Five weirs with 15, 30, 45, 60 and 90 weir angles were developed and flow discharge coefficient, negative velocity over the weir crest edge and water surface profiles along the weir crest evaluated in laboratory hydraulic flume. The obtained results showed by decreasing the slope of the upstream side, discharge efficiency and the discharge capacity of the weir are increased subsequently. Consideration of experimental data it is obvious that by using the proposed weir with slope of 15 degree in comparison with standard weir, the efficiency is increased up to 19.17%. It is noteworthy that by reducing the slope, return current on the crest is not observed.

Discharge Relations for Rectangular Broad-Crested Weirs

Tarım Bilimleri Dergisi, 2011

Compound broad-crested weirs have a small inner rectangular section for measuring low flows and then, they broaden to a wide rectangular section at higher flow depths. It can be used as a water measurement device in irrigation canals. This paper presents data that will be of use in the design of hydraulic structures for flow control and measurement. A series of laboratory experiments were performed in order to investigate the effects of width of the lower weir crest and step height of broad-crested weirs with rectangular compound cross section on the values of the discharge coefficient (C d) and the approach velocity coefficient. For this purpose, 15 different broad-crested weir models with rectangular compound cross sections for a wide range of discharges were tested. The sillreferenced heads at the approach channel and at the tail water channel were measured in each experiment. The dependence of the discharge coefficient and approach velocity coefficient on model parameters was investigated. Results show that a discontinuity occurs in head-discharge ratings because the section width suddenly changes shape, experiencing a break in slope when the flow enters the outer section. Values of C d obtained from the experiments on compound broad-crested weirs are lower than those of a broad crested weir with a rectangular cross section because of it's contraction effects.

The Coefficient of Broad-Crested Weir in Natural Channels

Weirs have been designed and used extensively in hydraulic structures to control the flow depth and discharge. To estimate the discharge over the weir, a weir coefficient is required. The discharge coefficient can be obtained experimentally as a function of the dimensionless total head of the approaching flow or as a function of various parameters. However, a universally acceptable discharge coefficient does not exist. Beside the man-made weirs that have been constructed by hydraulic engineers, some natural channel characteristics and slopes can be simulated as broad-crested weirs that are clearly noticeable in examining water surface profiles. The study area is Centralia watershed in Central Florida, which contains several cascades of step like waterfalls that can be selected for transecting. Cross-section data from 5 transects and detailed hydraulic data was obtained using the Digital terrain model (DTM) and the Triangular Irregular Network (TIN). The Hydrologic Engineering Center-River Analysis System (HEC-RAS) step-backwater technique was used to calculate water surface profiles for natural channels that are likely used for storm water drainage. The results show that these channels have chains of steps that dissipate the momentum of falling water in steep areas and maintain a steady rate of flow. The weir coefficients were determined by comparing the steady state discharges to the equivalent weir discharges. The results indicate that the weir coefficient can be expressed as a function of weir height and channel cross-section area. The relationship between this function and the weir coefficient showed a high correlation with R 2 = 0.991. The results of this study can be used to estimate the discharge in similar reaches in any water surface profile.

FLOW CHARACTERISTICS OF A TRIANGULAR SHARP CRESTED WEIR

Triangular weir is a simple form of weir best matched for low discharge and is free from aeration difficulties. It is mostly used in various branches of engineering like hydraulics, environmental, chemical and irrigation for the purpose of discharge measurement. Earlier studies conducted on triangular weir indicate that the discharge coefficient related to head or head to weir height ratio covering a limited range of head and vertex angles. Further, no generalized equation proposed to compute either discharge coefficient or discharge for any head and vertex angle. In this study, a total of 160 experimental runs were taken for five weir vertex angles (from 30◦ to 90◦) at apex elevations of16,18,and 20cm. Using the general formula for triangular weir dimensionless discharge and dimensionless head has been defined that helps in merging all the data points of five angles to one single curve. A generalized equation between dimensionless discharge and dimensionless head has been obtained. The maximum error obtained in the discharge computed from this equation is ±7%. This equation also validates the data of previous study (Wahaj, 1999).

study of sharp crested triangular weirs

Triangular weir is a simple form of weir best suited for low discharge and is free from aeration difficulties. It is mostly used in various branches of engineering like hydraulics, environmental, chemical and irrigation for the purpose of discharge measurement. Earlier studies conducted on triangular weir indicate that the discharge coefficient related to head or head to weir height ratio covering a limited range of head and vertex angles. Further, no generalized equation proposed to compute either discharge coefficient or discharge for any head and vertex angle. In this study, a total of 65 experimental runs were taken for five weir vertex angles (from 30 • to 90 • ) at apex elevation of 20cm. Using the general formula for triangular weir dimensionless discharge and dimensionless head has been defined that helps in merging all the data points of five angles to one single curve. A generalized equation between dimensionless discharge and dimensionless head has been obtained. The maximum error obtained in the discharge computed from this equation is ±5%. This equation also validates the data of previous study (Wahaj, 1999).

Hydraulics CHARACTERISTICS OF FLOW OVER CONTRACTED CLEAR OVERFALL WEIRS WITH BOTTOM-PIPES

JES. Journal of Engineering Sciences, 2014

Fayoumi Standard Weirs have been modified by adding one, two or three pipes, to convey much water to downstream direction. Measurements of discharge with these structures are complicated due to existence of the openings. In this research, an experimental study is carried out to investigate the effect of installing pipes through a piped clear over fall weir and bottom-pipes. Twelve weir models with different heights are tested in horizontal laboratory flume of 17 m length, 0.3 m width, and 0.5 m depth. Weir height is changed two times. Contraction of weirs is changed two times. For each one, the diameter of opening is changed three times. The downstream depth is changed to cover all the concerned flow regimes of flow rate for a submerged pipe and a free weir. The downstream water depth begins with the upper tangent of the pipe and ends with the crest level of the weir. The contraction and the un-contraction of the weir are discussed. The effect of downstream water depth is taken into consideration. The results of flow over weir with opening are compared with those of weir without opening having the same dimensions. It is found that there is a large difference between them. Multiple regression equations based on energy principal and dimensional analysis theory are developed for computing discharge over clear over fall weir with bottom pipes. Also, values of discharge coefficients were estimated for combined flow over the weir and through the pipe. Then the total discharge can be computed by multiplying the discharge coefficient by the summation of theoretical weir discharge plus theoretical orifice discharge. Equations for computing the discharge of combination are awarded.