Environmental Engineering Reference
In-Depth Information
h
1
Downstream water level below crest
h
1
h
2
Figure 4.9. Flumes with a
raised crest (Paudel, 2010).
Downstream water level above crest
distinct downstream water levels (Figure 4.9). The downstream water level
is always below the crest and the downstream water level is higher than
the crest; in this case
h
2
/
h
1
should be smaller than 0.8 for modular flow.
Undershot or underflow structures
Flow under a gate, through (short) pipes or fixed plate orifices are exam-
ples of an undershot flow. In undershot flows two flow types can be
distinguished (see Figure 4.10):
•
free flow: the opening is relatively small (
h
1
/a >
2) and the contrac-
tion of the streamlines is significant in the vertical direction. The
downstream water level (
h
3
) will not affect the flow and a hydraulic
jump will occur downstream of the vena contracta. The discharge
depends upon the gate opening, the upstream water level and the
contraction;
•
submerged flow: the downstream water level influences the flow and
the discharge depends upon the upstream and downstream water level
and the gate opening.
The discharge through an orifice with free flow is given by Bos (1989):
C
d
C
v
C
c
A
2
gh
=
Q
(4.35)
where:
C
d
=
discharge coefficient
C
v
=
coefficient for velocity head
C
e
=
contraction coefficient, area of the vena contracta/area orifice
opening
B
=
width of the orifice (m)
B
*
a
(m
2
)
A
=
area of the orifice opening
=
h
=
upstream head measured from the centre of the opening to the
water surface.
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