41
Theory
For Modular Flow (weir operates undrowned, downstream water level low)
2
3
0
2
1
d
m
H
g
C
b
Q
=
therefore:
2
3
0
2
1
m
d
H
g
b
Q
C
=
where:
Q
m
= Modular volume flowrate
(m
3
.s
-1
)
= Volume/time (using volumetric tank)
b
= Breadth of weir
(m)
g
= Gravitational constant
(9.81ms
-2
)
H
0
= Total head upstream of weir crest
(m)
(
)
2
0
2
0
0
2
0
2
0
0
2
0
0
b
y
g
2
Q
y
A
g
2
Q
y
g
2
V
y
+
=
+
=
+
=
y
0
= Upstream depth of flow above weir
(m
2
)
Q
0
= Upstream flow rate = Q
m
for modular flow
(m³/s)
h
= vertical height of weir
(m)
C
d
= Modular coefficient of discharge
(Dimensionless)
When the flow is modular the upstream head is not affected by changes in the
downstream head. A single measurement of upstream head can therefore be taken to
determine the volume flowrate over the weir.
