Parshall flumes - E- courses@VTU

Hydraulics Prof. B.S. Thandaveswara
Parshall flumes
Parshall flumes are devices for the measurement of flow of water in open channels.
They were developed by Parshall in 1922 after whom the devices have been
recognized.
The flume consists of a converging section with a level floor, a throat section with a
downward sloping floor, and a diverging section with an upward sloping floor.
Flow
Indian Institute of Technology Madras
X
P D
M B L G
A
Converging
section
B L G
E Crest
Level floor N
hb
a
Crest
0
Alternate 45
wing wall
Y
b C
Throat
section
Diverging
section
PLAN
Dimensions of Parshall flume (refer to Table)
x
Section along X-X
hb
hc
K
Water surface, Submerged
Water surface, Free flow
In deviation from the general rule long throated flumes, where the upstream head is
measured in the approach channel, Parshall flumes are calibrated against a piezometric
head, ha, measured at a prescribed location in the converging section. The
"downstream" piezometric head hb is measured in the throat. In practice this is also
used in the cut-throat and H flumes.
X

Hydraulics Prof. B.S. Thandaveswara
Parshall flumes were developed in various sizes. Care must be exercised while
constructing the flumes exactly in accordance with structural dimensions (See table) as
the flumes are not hydraulic scale models of each other. Since throat length and throat
bottom slope remain constant for series of flumes while other dimensions are varied,
each of the 22 flumes is an entirely different device. This factor becomes more
important as the size becomes smaller and smaller. For example, it cannot be assumed
that a dimension in the 3.657 m flume will be three times the corresponding dimension
in 1.219 m flume. The discharge can be measured within 3 to 5% if the flume is
constructed properly as per recommended standard dimensions.
On the basis of the throat width, Parshall flumes have been classified into three main
groups.
(i) Very small - 25.4 mm to 76.2 mm.
(ii) Small 152.40 mm to 2438.4 mm.
(iii) Large 3048 mm to 15240 mm.
Dimensions in mm (Refer to the figure above)
b A a B C D E L G H K M N P R X Y Z
VERY SMALL
1" 25.4 363 242 356 93 167 229 76 203 206 19 - 29 - - 8 13 3
2" 50.8 414 276 406 135 214 254 114 254 257 22 - 43 - - 16 25 6
3" 76.2 467 311 457 178 259 457 152 305 309 25 - 57 - - 25 38 13
SMALL
6" 152.4 621 414 610 394 397 610 305 610 - 76 305 114 900 406 51 76 -
9" 228.6 879 587 864 381 575 762 305 457 - 76 305 114 1080 406 51 76 -
1' 304.8 1372 914 1343 610 845 914 610 914 - 76 381 229 1492 508 51 76 -
1'6" 457.2 1448 965 1419 762 1026 914 610 914 - 76 381 229 1676 508 51 76 -
2' 609.6 1524 1016 1495 914 1206 914 610 914 - 76 381 229 1854 508 51 76 -
3' 914.4 1676 1118 1645 1219 1572 914 610 914 - 76 381 229 2222 508 51 76 -
4' 1219.2 1829 1219 1794 1524 1937 914 610 914 - 76 457 229 2711 610 51 76 -
5' 1524.0 1981 1321 1943 1829 2302 914 610 914 - 76 457 229 3080 610 51 76 -
6' 1828.8 2134 1422 2092 2134 2667 914 610 914 - 76 457 229 3442 610 51 76 -
7' 2133.6 2286 1524 2242 2438 3032 914 610 914 - 76 457 229 3810 610 51 76 -
8' 2438.4 2438 1626 2391 2743 3397 914 610 914 - 76 457 229 4172 610 51 76 -
LARGE
10' 3048 - 1829 4267 3658 4756 1219 914 1829 - 152 - 343 - - 305 229 -
12' 3658 - 2032 4877 4470 5607 1524 914 2438 - 152 - 343 - - 305 229 -
15' 4527 - 2337 7620 5588 7620 1829 1219 3048 - 229 - 457 - - 305 229 -
20' 6096 - 2845 7620 7315 9144 2134 1829 3658 - 305 - 686 - - 305 229 -
25' 7620 - 3353 7620 8941 10668 2134 1829 3962 - 305 - 686 - - 305 229 -
30' 9144 - 3861 7925 10566 12313 2134 1829 4267 - 305 - 686 - - 305 229 -
40' 12192 - 4877 8230 13818 15481 2134 1829 4877 - 305 - 686 - - 305 229 -
50' 15240 - 5893 8230 17272 18529 2134 1829 6096 - 305 - 686 - - 305 229 -
Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
Very small flumes
The discharge capacity of the very small flumes ranges from 0.09 l/s to 32 l/s. The
capacity of each flume overlaps that of the next size by about one-half the discharge
range. The flumes must be carefully constructed. The exact dimensions of each flume
are listed in Table. The maximum tolerance on the throat width b equals ± 0.0005 m.
The relatively deep and narrow throat section causes turbulence and makes the hb
gauge difficult to read in the very small flumes. Consequently, an additional gauge hc,
located near the downstream end of the diverging section of the flume is introduced.
Under submerged flow conditions, this gauge may be read instead of the hb gauge. The
hc readings are converted to hb readings by using a graph.
Small flumes
The discharge capacity of the small flumes ranges from 0.0015 m 3 /s to 3.95 m 3 /s. The
capacity of each size of flume considerably overlaps that of the next size. The length of
the side wall of the converging section, A of the small flumes with 304.8 mm upto
2438.4 mm size flume has a throat width 'A' in meter given by
is the throat width in meter.
Large flumes
Indian Institute of Technology Madras
b
A = + 1. 219 in which b
2
The discharge capacity of the large flumes ranges from 0.16 m 3 /s to 93.04 m 3 /s. The
capacity of each size of flume considerably overlaps that of the next size. The axial
length of the converging section is considerably longer than it is in the small flumes to
obtain an adequately smooth flow pattern in the upstream part of the structure. All
flumes must be carefully constructed to the dimensions listed, and careful leveling is
necessary in both longitudinal and transverse directions. When gauge zeroes are
established, they should be set so that the ha - , hb-, and hc - gauges give the depth of
water above the level crest - not the depths above pressure taps.
The Parshall flume is not to be operated above the 0.60 submergence limit, there is no
need to construct the portion downstream of the throat. The truncated Parshall flume
(without diverging section) has the same modular flow. The truncated flume is
sometimes referred to as the "Montana flume". As with the cut - throat flume, the

Hydraulics Prof. B.S. Thandaveswara
Parsahall flume may be used in both the modular and non-modular flow ranges and the
modes of operation are similar. A second head measurement is again required in the
non-modular range of flows and it is usually taken towards the downstream end of the
throat. Parshall flumes are, however, predominantly used in the modular flow range
(Skogerboe et al,.1967). Recently, Parshall flumes have gone out of favor due to their
construction complexity and likelihood to trap sediment compared to newer flume
designs. Partial flume is not recommended for measurement of submerged flow as long
throated flumes can be designed for 90% of submergence, as the drop in water surface
level required is less in long throated flume particularly modified Broad crested weir
types. If the submergence is not expected in the downstream converging section in the
downstream need not be constructed.
Imprecision of head measurement increases discharge error by 4 to 20 % over the
primary free flow accuracy of 3 to 5 %. The coefficient C and exponent n ranges
between 0.338 to 186.88 in FPS units and 'n' varies from 1.55 to 1.60 in general for the
range of Parshall flumes.
Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
Indian Institute of Technology Madras
4
3
2
1.5
1
0.8
0.6
0.5
0.4
0.3
0.2
0.15
0.10
0.08
0.06
0.05
0.04
0.03
0.02
0.015
0.01
1' to 8' Parshall Flumes
Small Parshall Flumes ( 621 to 2438 mm )
95 90 80 60 0.005 0.01 0.02 0.05 0.10 0.20 0.40
Percentage of submergence
Head Loss H through Flume in metre
Head-loss through Parshall flumes. ( 621 to 2438 mm ) (after Bos)
6'
5'
4'
3'
7'
8'
2'
1'

Hydraulics Prof. B.S. Thandaveswara
Indian Institute of Technology Madras
4
3
2
1.5
1
0.8
0.6
0.5
0.4
0.3
( 10' to 50' Parshall Flumes )
95 90 80 60 0.02 0.05 0.10 0.20 0.50 1
Problem
60
50
40
30
20
15
10
8
6
5
Large Parshall Flumes
( 3048 to 15240 mm )
Percentage of submergence Head Loss H through Flume in metres
Head-loss through Parshall flumes ( 3048 to 15240 mm wide )
50'
15'
12'
10'
40'
30' 25'20'
Design a Parshall flume for 15 m 3 /s, given the bed width of the approach channel being
6 m. The cross section of the channel is rectangular. Comment on the error.