Flow through sharp-crested rectangular side orifices under free flow condition in open channels

Side orifices are widely used in irrigation and environmental engineering to spill or divert water from a channel. Flow characteristic of sharp-crested rectangular orifices under free flow condition in open channels is studied in the present paper. Existing discharge equations are checked for their accuracy using the data collected in the present study and available data in the literature and a new discharge equation has been proposed. The coefficient of discharge mainly depends on the approach flow Froude number and ratio of the size of orifice and bed width of the channel. Relationships for the coefficient of discharge, treating the orifice as large and small were developed. The computed discharges using these relationships were within ±5% of the observed ones. Measurement of three-dimensional velocities and visualisation of streamlines in a horizontal plane at the centerline of the orifice indicates that for the flow of low Froude number, almost all the streamlines divert towards the orifice. However, in the case of high Froude number flows, only those streamlines which are close to the side orifice are diverted towards the orifice. The opposite side of the boundary has significant effect on the diverted discharge of low Froude number compared to the flow of high Froude number. Circular orifice has been found to be more efficient compared to the rectangular orifice of the same opening area.

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► Coefficient of discharge depends mainly on Froude number and ratio of size of orifice to width of channel.
► Relationships for coefficient of discharge, treating orifice as large and small were developed.
► Computed discharges using the proposed relationships are within the practical range.
► Discharge through the orifice can be computed treating it as small orifice within range of data studied in this study.
► Circular orifice is found to be more efficient compared to square orifice.