Geoscience Reference
In-Depth Information
dissipation takes place due to nappe impact on the underlying water cushion
and hydraulic jump.
3
At larger flow rates and for relatively steep chute, skimming flow occurs.
The flow skims over the step edges with formation of recirculating vortices
between the main stream and the step corners. The water flows down in a
coherent stream where external edges determine a pseudo-bottom defined
by the straight line that connects the edges of each step. Most energy is
dissipated in maintaining the recirculation in the step cavities.
1
Oxygen transfer by air entrainment occurs mainly from behind the trail-
ing edge of the stepped structures due to flow separation.
4
Abundant dis-
solved oxygen is stored with breaking of the air bubbles, and this would
give the good habitat condition at the downstream part of the structure.
Hydraulic jump makes the air entrainment more active. Occurrence interval
of the air entrainment increases when the flow becomes supercritical with
Froude number larger than unity.
The eciency of the oxygen transfer
E
is used for representing the
eciency of the air entrainment
5
;
E
=(
C
d
−
C
u
)
/
(
C
s
−
C
u
)
,
(1)
where
C
d
and
C
u
are the dissolved oxygen measured at downstream and
upstream point, respectively, and
C
s
is the saturated dissolved oxygen.
Since the oxygen transfer is affected by the water temperature,
E
is sub-
stituted by
E
20
6
;
ln(1
−
E
T
)
20)
2
,
E
20
)
=1
.
0+
α
(
T
−
20) +
β
(
T
−
(2)
ln(1
−
where
E
T
and
E
20
are the oxygen transfer eciencies at temperature
T
◦
C
and the reference temperature 20
◦
C, respectively.
α
and
β
are constants as
α
=0
.
02103
◦
C
−
1
,
β
=8
.
621
10
−
5
◦
C
−
2
.
Eciencies of the oxygen transfer are estimated by measuring the dis-
solved oxygen and the total head at the upstream and downstream point
of the structures, respectively. Figure 2 shows the example for measuring
points of the drop structure.
All the data are measured 5 m upstream and 10 m downstream of the
structures for considering the data consistency.
×
3. Experimental Tests
To investigate the air entrainment by the flow characteristics and to esti-
mate the relationships between the air entrainment and the hydraulic
