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Trapezoidal Weirs Flow Formula and Calculator

Fluids Engineering and Design
Civil Engineering and Design

Trapezoidal Weirs Flow Formula and Calculator

A trapezoidal weir is essentially a rectangular weir with a triangular weir on either side. (See Fig. 1) If the angle of the sides from the vertical is approximately 14° (i.e., 4 vertical and 1 horizontal), the weir is known as a Cipoletti weir. The discharge from the triangular ends of a Cipoletti weir approximately make up for the contractions that would reduce the flow over a rectangular weir. Therefore, no correction is theoretically necessary. This is not completely accurate, and for this reason, Cipoletti weirs are not used where great accuracy is required. The discharge is

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Trapezoidal Weir
Fig. 1 - Trapezoidal Weir

Eq. 1,
Q = 2/3 Cd b (2g)0.5 H1.5

The average value of the discharge coefficient is 0.63. The discharge from a Cipoletti weir is found by using Eq. 2.

Eq. 2,
Q = 1.86 b H1.5

Where:

H = height, m
b = width of weir bottom, m
Q = Flow, m3/s
g = gravity, 9.81 m/s2
Cd = discharge coefficient (average = 0.63)

Reference:

  • Civil Engineering Reference Manual, Fifteeth Edition

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