Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

Flow Measuring Structures

Flow measurement structures are required in irrigation canals in order to facilitate the

distribution of water through out the system and to keep account for seepage losses,

etc. However, in the smaller channels the flow measurement structures or devices are

closely associated with local water management practices of an irrigation command.

Several individuals have carried out investigation on flow measurement structures and

have developed discharge rating relationship for them, but it must be noted that both

national and international organisations are responsibility of Technical Committed TC

113 "Measurement of liquid flow in open channels.” A list of standards relating to flow

measurement structures is given in Table 1.

ISO Standards

ISO 1438/1 Thin plate weirs and flumes ISO 4360

ISO 4361

Triangular profile weirs.

Round nosed weirs

ISO 4359 Standing wave flumes for different throat section viz, Rectangular, U-shape, Trapezoidal.

ISO 6417 Compound gauging structures. ISO 3846

ISO 3847

Rectangular broad -crested weirs.

By the brink depth method. ISO4374 Round nose horizontal crest wiers. ISO 4377 Flat V weirs.

ISO 748 Liquid flow measurement in open channel by velocity area method.

ISO 1070 Liquid flow measurement in open channels by the slope area method.

ISO 1100 Established and operation of a gauging station and determination of the stage discharge relations.

ISO 2425 Measurement of flow in tidal channels * International Organisation for Standardization

* On the suggestion of India, in 1954 the technical committee ISO/TC 30 of the ISO took

upon the task of standardization of flow measurements in open channels set up a

separate subcommittee.

Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

Labyrinth Weir as High-Capacity Field Outlet

Irrigation engineers have been forced to adopt new solutions to the engineering

problems in order to ensure that irrigation projects are viable, or to deal with specially

adverse conditions. Updating of existing spate irrigation systems which have been

operating along traditional lines with reasonable success for substantial periods of time

required a novel approach to build the structures involved. The important feature of

these spate irrigation systems is that they are fed from non perennial rivers (normally

dry but occurring flash flood flows when storms occur in the upper catchments).

Diversion from these steep rivers was traditionally made by temporary earth banks

which are often washed away during the flood period.

Figure shows a plan view of a section of canal which includes a high- capacity field

outlet having a cross - regulator immediately down stream. Closure of the cross -

regulator cause backwater in the canal, and the water levels and the extent of the

backwater is determined by the head required over the weir crest to discharge the

necessary flow of water. A short length of weir crest would result in high backwater

levels and a long length of crest would result in a relatively small increase of water

levels due to backwater. The importance of the increase of water levels and of the

extent of backwater requires raising of the canal banks, involving substantial

investment. The labyrinth weir is one such solution. This should be cheaper than a

straight weir having the same length as the developed length of a labyrinth weir. In

Figure a simple two - cycle labyrinth weir has been shown near outlet. The configuration

of labyrinth weir is determined by experiment. With some configurations, there is a

possibility of, the nappes meeting from two of the sloping sides of the labyrinths forming

a jet which may cause scour in the downstream.

A Labyrinth weir is characterized by a broken axis in plan, the total length thus being

compressed in concertina (Small musical instrument resembling an accordion but

having button like keys) form into the space available on site. The purpose of the

Labyrinth weir is to increase the discharge per unit width for a given operation head.

Another advantage of this weir can be raised for the same maximum elevation of water

level, thereby gaining substantial storage capacity.

Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

5.5 m

Note:This can develop submeged turbulent rollers at very low discharges

Simple weir

Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

5.5 m

Simple weir with splash plate

Note:Distributes the flow over a greater surface area

1.2 m

Weir with cascade

1

23

Steps 1 and 2 = 1.83 mSteps 3 = 1.52 m

Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

Note: When a simple weir is used energy dissipation would not be very effective.

Depending on the Tail water level, a hydraulic jump forms. In order to dissipate the

energy possible alternatives are shown above. A simple weir with splash plate will help

in distributing the flow over a great surface and the baffle blocks will assist in break up

of submerged jump. The turbulence level is not reduced by this combination. Further

cascade of splash plate will estimate the submerged jump.

Reference:

Don Richarad and others, “Low Head Dam Safety with Hydraulic Models”, Proceedings

National Hydraulic Engineering Conference, ASCE, 1987, pp-528-533.

5.5 m

Typical Labyrinth weir

Note:Longer length, lower head, reduced over action

Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

PLAN

UPSTREAMCHANNEL DOWNSTREAM

CHANNEL

X

UPSTREAM SURFACEPROFILE

DOWNSTREAM SURFACEPROFILE

SECTION - 'XX'

LABYRINTH WEIR

Hydraulics Prof. B.S. Thandaveswara

Indian Institute of Technology Madras

1:2

Warped masonrywall

Curved masonrywall

0.25

High-capacity field outlet upstream of a cross regulator. Plan view of V high capacity field outlet.Design outflow 5 m /s

Scale 1:1001:3

1:1

1.10 0.90

Curved masonrywall

1:1

4.81

0.2

3.04

0.25

1.3

Dry rubblepitching

Cross-regulator

Stop logs orgates

8.0

3

With the low ratios of head to crest length, the effectiveness of the labyrinth weir

configuration can be measured by a weir equation, such as

= 3 / 2 dQ C g Lh

in which, Cd is the coefficient of discharge, L the developed length of the weir crest and

h the head over the crest. High values of Cd indicate an efficient structure, where as the

head h inevitable reduces as the length L increases.