Evaluation of the Discharge Coefficient of Piano Key Weir under the Unsteady - Gradual Varied Flow with Increasing Discharge

Document Type : Research Article


1 M.Sc. Student, Water Eng. and Hydraulic Structures, Department of Civil Engineering, Islamic Azad University, Isfahan (Khorasgan), Iran.

2 Department of Civil Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, Iran


Introduction: Weirs or spillways are one of the most important types of hydraulic structures that are divided into linear and nonlinear weir categories based on the shape of the plan. The most important types of nonlinear wires are labyrinth wires and piano key weirs that increase the discharge of flow by increasing the length of the spillway crown. These types of weirs are usually made outside the dam body. The most important advantages of these weirs is high flow coefficient, speed in construction and economic efficiency. These weirs are one of the most suitable options for dam weir implementation in free-flow mode. Floods, snow melting, performance changes of hydraulic structures and many other unsteady flow factors are observed abundantly in nature. Therefore, in this study, the discharge coefficient of piano key weirs under the unsteady-gradual varied flow with increasing discharge has been evaluated. Also, in this research, the calibration of discharge coefficient due to changes in the weir height and the effect of weir height changes on the discharge coefficient and other effective hydraulic parameters in the upstream of the spillway under the unsteady-gradual varied flow with increasing discharge has been investigated.

Methodology: The experiments were carried out in a rectangular channel of Islamic Azad University, Isfahan University (Khorasgan) with a length of 10 meters and a width of 0.6 meters. The height of the channel was the same and unchanged along the way, as well as its floor was made of galvanized sheets and its walls were made of tempered glass and completely sealed. Considering the relationships and previous studies and dimensional analysis, the effect of effective parameters on flow discharge coefficient was investigated. In the current research, three types of A-type rectangular piano key weir with variable heights of 10, 15 and 20 cm were used. Other geometrical parameters were constant and experiments done under the unsteady-gradual varied flow with increasing discharge in the flow range of 30 to 50 Lit/s. The experiments has been studied under the range of discharge changes of 5,3 and 1 liter per second and range of time changes of 5,10 and 15 seconds. Therefore, according to the said contents, 27 different modes have been analyzed in this thesis. Flow depth was reported by laser altimeter sensors at the top of the channel. PLC device was used to measure discharge and water head at different times. A magnetic flow meter is attached to the PLC device, which measures the discharge in liters per second and the height of water from the channel floor by sensors installed in millimeters.

Results and Discussion: Discharge coefficient under the unsteady-gradual varied flow with increasing discharge was obtained by deriving from the relationship provided by Machiels et al. Considering that the dc/dt value is not known, the relationships of past researchers for the flow discharge coefficient of type A rectangular piano key weir presented. And comparing the percentage of error in each relationship was done. Finally, it was concluded that the relationship presented by Javaheri and Kabiri-Samani had a lower error percentage. And it was derived from this relationship relative to time, until determine the dc/dt value. Then, according to the experimental data and weir specifications of the discharge coefficient under the unsteady-gradual varied flow with increasing discharge, it was obtained. The results showed that with increasing the weir height, the slope of the discharge charts and the height of the water decreased and the slope of the head-discharge diagrams increased. With increasing the height of the weir, the discharge coefficient increased and in contrast to the froud number and the ratio of water height to the height of the weir decreased. As a result, the height of the weir is directly related to the discharge coefficient and is in contrast to the froud number and the height of the water at the inlet and upstream of the weir. According to the results obtained in the unsteady-gradual varied flow with increasing discharge that led to the calibration of the flow, it was found that the calibrated discharge coefficient (average) in three weirs with heights of 10, 15 and 20 cm is a number between 1.693 and 3.776.

Conclusion: By comparing three type A piano key weirs, it was found that the weir with a height of 20 cm is more efficient than the weir with heights of 10 and 15 cm according to the higher discharge coefficient. It can also be stated that the higher height weir is much more applicable in floods and high flow discharges and better diverts the flow. In fact, all the results indicate that the height of the weir plays an important role in the amount of the flow coefficient of the piano key weir. And by increasing it, the efficiency of the flow discharge can be increased.


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