MEASURING FLUMES
By
CH. VENKATARAMAIAH
CUTTHROAT FLUME
Flow measurement structures
1) Weirs
2) Drops
3) V notches
4) Cutthroat flumes
5) Rectangular throat flumes
6) Standing wave flumes
7) Parshall flumes
General
1. Cutthroat flume consist of converging section and
diverging section
2. It has flat bottom and vertical walls.
3. Angle of convergence is 3:1 and angle of divergence is
6:1
4. Convergence part consist of 1/3 length of flume
5. Divergence part consist of 2/3 length of flume
6. Ratio of upstream depth of flow to length of flume
should be equal to or less than 0.4 for accuracy. Single
reading of upstream depth of flow to be taken at a
distance of 2L/9 from the neck for free flow condition.
D/S reading at a distance of 5L/9 from the neck is also
required for submerged flow condition.
Design Procedure
1. Data:
Max. discharge
=
Q in cumec
Max. D/S flow depth
=
hb in m
Head loss
=
f in m
Max U/S flow depth
=
ha = hb + f in m
Length of flume
=
L in m
Throat width
=
W in m
Design for free flow condition
1. Read value of transition submergence (St) in percentage from
figure 3.
2. Work out ratio (hb / ha ) x 100 (percent). If it is more than St, the
flow is free
3.
Qf
=
free flow discharge in cumec
Cf
=
free flow coefficient
hf
=
upstream flow depth in m
nf
=
free flow exponent
Kf
=
free flow flume length coefficient
Wf
=
throat width in m.
Qf
=
Cf(ha)nf
Cf
=
Kf W1.025
If value of Qf as calculated is equal to or more than the required
discharge, the result is OK. Otherwise, the variable parameters have to
be changed till the required result is obtained.
Design for submerged flow condition
The flume is designed for the submerged flow condition using the
following equation:
Qs = Cs (ha – hb)nf
(- log St)ns
Where
Qs
=
submerged flow discharge in cumec
ha
=
upstream flow depth in m
hb
=
downstream flow depth in m
Cs
=
submerged flow coefficient = Ks. W1.025
nf
=
free flow exponent
ns
=
submerged flow exponent
St
=
transition submergence
Ks
=
submerged flow flume length coefficient
St, nf , ns and Ks can be read from the graph in figure 3. The discharge Qs
under submerged condition can be calculated for any combination of
values of ha and hb using the above equation.
STANDING WAVE FLUME
(1)Discharge
Q = 2/3 (2/3.g)1/2 x Cf x Bt x H3/2
=1.075 Cf Bt H3/2
Bt = throat width in m
H = d1 – Z + v2/ 15.2
d1 = depth of canal flow
z = height of hump over canal bed
v = velocity of canal flow.
Cf = coefficient of friction
Value of Cf
Discharge
0.97
0.05 to 0.3 cumec
0.98
0.3 to 1.5 cumec
0.99
1.5 to 15 cumec
1.00
> 15
cumec
For satisfactory functioning of standing wave flume, the ratio D2/ D1 should not be less than
0.50
D1 = U/S depth of flow over sill of throat
D2 = D/S depth of flow over sill of throat
The gauge (stilling) well for measurement of depth of flow in the canal should be located in
the straight portion of the canal at a distance of 4 times the maximum head over the sill of
the flume on the upstream side, measured along the axis of canal.
(2) Height of hump
It is required to provide a hump in the canal to maintain proportionality between the rate of
change of depth of flow over sill of the throat and the rate of change depth of flow in the canal.
Where
Z = height of hump
d1 = depth of flow in the canal
D1 = u/s depth of flow over the sill of throat
m = any particular fraction of discharge
x = approach channel index (varies from 1.5 to 2)
Discharge equation of the approach channel is given by:
Q = C 1 d1 x
Discharges Q1,Q1',Q1'',Q1''', etc are worked out for the flow of depths of d1,d1', d1'',d1''', etc
respectively and the value of x in the equation is estimated by least square method by
considering these sets of d1 and corresponding Q.
x =
Where M is the no. of sets.
∑ log Q . Log d1 - (∑ log Q ) ( ∑ log d1 )
M
∑ ( log d1)2
( ∑ log d1)2
M
(3) Head loss:
The head loss consists of the losses in:
(i) Approach transition
(ii) Exit transition
(iii) Friction in structure
(iv) Hydraulic jump
Loss in transitions depends on the amount of fluming and its
gradualness. The friction loss is usually small. The loss in hydraulic
jump is given by the equation:
HL = (d2 – d1)2
4 d1 d2
Where d1 = depth of flow before jump
d2 = depth of flow after jump
(4) Approach transition :
The radius of walls of the bell mouth entrance should be
3.6 H 1.5 starting from the throat. If ‘H’ is less than 0.30m, the
radius may be 2H.
Curvature (formed from the throat) should continue till it
subtends an angle of 600, from where, it should be continued
tangentially to meet the side of the channel.
The bed convergence should begin on the same cross section
as the side convergence. The radius of curvature of hump in
the bed should be:
L12 + Z2
rh =
2Z
Where rh = radius of curvature of hump
L1 = length between the junction of side wall with the
bed of U/S channel and U/S end of throat along the
axis of channel.
Z = height of hump.
(5) Throat:
Sides of throat should be vertical and length should be 2.5 H.
Width of throat should be calculated from the discharge
formula in sheet 2.
(6) Downstream glacis
i.
The length of downstream glacis should be equal to 4H.
ii. Side walls should be of same length.
iii. Slope of the glacis is usually 1 in 20 or flatter.
iv. Divergence of side walls to be 1 in 10 or flatter, so as to make
the width at the toe of the glacis equal to or less than the
downstream canal bed width.
THANK YOU