CIVIL ENGINEERING FORMULAS

HYDRAULICS AND WATERWORKS FORMULAS 331

Trapezoidal (Cipolletti) Weir

The Cipolletti weir, extensively used for irrigation work, is trapezoidal in
shape. The sides slope outward from the crest at an inclination of 1:4 (horizontal:
vertical). The discharge is

(12.126)

whereb, h, and Qare as defined earlier. The advantage of this type of weir is
that no correction needs to be made for contractions.

Broad-Crested Weir

The discharge of a broad-crested weir is

(12.127)

Values of Cfor broad-crested weirs with rounded upstream corners generally
range from 2.6 to 2.9. For sharp upstream corners, Cgenerally ranges from 2.4
to 2.6. Dam spillways are usually designed to fit the shape of the underside of
a stream flowing over a sharp-crested weir. The coefficient Cfor such a spill-
way varies considerably with variation in the head, as shown in Table 12.3.
Q, b, and hare as defined for rectangular weirs.

QCbh3/2

Q3.367bh3/2

TABLE 12.3 Variations in Head Ratio and

Coefficient of Discharge for Broad-Crested Weirs

Ratio of actual head Coefficient of

to design head discharge

0.20 3.30

0.40 3.50

0.60 3.70

0.80 3.85

1.00 3.98

1.20 4.10

1.40 4.22

TABLE 12.2 Discharge of Triangular Weirs

Notch (vertex) angle Discharge formula*

90° Q0.685h2.45

60° Q1.45h2.47

30° Q2.49h2.48

*his as defined above in the Francis formula.