Environmental Engineering Reference
In-Depth Information
Total energy line
H
h
1
h
2
L
Figure 4.5. Broad crested weir
(Paudel, 2009).
The discharge and velocity coefficient for a round nosed, rectangular
broad crested weir are (Bos, 1989):
0
.
1
H
1
L
=
+
C
d
0
.
93
with 0
.
08
< H
1
/L <
0
.
7
(4.27)
H
1
h
1
u
C
v
=
(4.28)
where:
u
=
1.5 for a rectangular cross section.
For most earthen irrigation canals the Froude number is less than 0.5
and the velocity coefficient
C
v
is less than 1.05, and the discharge equation
for a broad crested weir becomes:
CBh
2
/
3
1
Q
=
(4.29)
where:
Q
=
discharge in m
3
/s
C
=
coefficient
=
1
.
705
C
d
C
v
and ranges from 1.585 to 1.79
B
=
width of the weir (m)
H
1
=
upstream energy head above the crest (m)
h
1
=
upstream water depth above the crest (m)
h
2
=
downstream water depth above the crest (m)
A simple sharp-crested weir or notch (see Figure 4.6) has a horizontal
edge over the full canal width and a two-dimensional flow without side
contraction (suppressed weir). The flow is 'modular' or 'free' when the
downstream water level is below the crest. Sharp-crested weirs are not
very common in irrigation canals.
3
2
gh
1
.
5
C
d
B
2
=
Q
(4.30)
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