# Hydrosystems Hydrulics open channel flow iit nptel _12_

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Hydraulics                                                                                     Prof. B.S. Thandaveswara

4.2 Discharge measurement by Velocity Area Method
(Chitale, 1974)

This method comprises measuring the mean velocity V and the flow area 'A' and

computing the discharge Q from the continuity equation. The site which satisfies the

requirements such as straightness, stability, uniformity of cross-section is chosen for

discharge measurement. The requirements of the site are dealt with in detail in

standards of the ISI 1192, (1959). The discharge measurement site is then marked by

aligning the observation cross-section normal to the flow direction.

The cross-section is demarcated by means of masonry or concrete pillars on both the

banks, two on each side 30 m apart.

ISI 1192, (1959), "Velocity area methods for measurement of flow of water in open

channels, Bureau of Indian Standards".

4.2.1 Segmentation
The interval at which the depth of water is measured along the cross-section for

channels with different widths is given in Table.

Description of Channel  Number of Observation          Maximum width of
(m)                     verticals                      segments (m)
Width less than 15                15                            1.50
Width between 15 and 90           15                             6.0
Width between 90 and
15                              15.0
150
Width greater than 150            25                               -

The intervals specified are also such that not more than 10 percent and preferably not

more than 4 percent variations in the discharge between two adjacent segments occur.

The discharge through any segment is also not allowed to be more than 10 percent of

the total discharge.

For measurement of velocity, the maximum spacing between adjacent verticals is so

maintained that the mean velocity does not differ by more than 20 percent with respect

to the lower value of the two velocity measurements. In no case less than five velocity

verticals are permitted.

Hydraulics                                                                                         Prof. B.S. Thandaveswara

In case of canals allowing the variation of ± 2 percent in discharge and adopting as 15

verticals as a standard of comparison a lesser number of verticals 15 is adopted. The

verticals for depth and velocity measurements are kept the same according to Table

shown below.

Widths of segments for measurements of depths and velocities in canals

Approximate surface       Number of verticals for
Channel capacity m3/s
width (m)              depth and velocity
(a) Above 85                Above 35                      11
(b) Between 15-85          Between 15 - 35                    9
(c) Between 0 - 15          Between 0 - 15                    5

Method of marking segments varies according to the method of discharge observation.

Pivot point method is common, the details of which are available in the ISI : 1192-1959.

Angular, Stadia method and method of linear measurement are also used for locating

depth and velocity verticals under special circumstances.

4.2.2 Measurement of Depth

When velocities and depths are smaller and width up to 0.9 m, observations can be

difficult, sounding rods of wood and bamboo are used. When depths are in excess of

about 4.6 m or current is too swift to permit the use of sounding rod, hand line is used

for depth measurement. But when the depths are large and velocities are high even the

hand-lines cannot be used. Under such circumstances a cable line is lowered by means

of a crane. Echo sounders of indicator as well as recorder type are being used for depth

measurements.

4.2.3 Measurement of Velocity

For the measurement of velocity the current meters are most commonly used. IS: 3910

- 1966 gives specifications for cup type current meter and IS: 3918 - 1966 gives the

code of practice for use of this type of current meter.

To obtain a mean velocity in a vertical, velocity distribution observations can be made at

a number of points along the vertical. This is done when results are required to be

accurate, or for purpose of calibration. In two-point method the velocity observations are

Hydraulics                                                                                        Prof. B.S. Thandaveswara

made at 0.2 and 0.8 depth below the surface while in one point method observations is

made at 0.6 depth below the surface. Both the two-point and one point methods are in

common use in India, though sub-surface method comprising making velocity

observations just below the surface is also used during floods when other methods are

not feasible.

In high floods at times, even surface measurement of velocity by current meter may not

be possible, float measurements are then used using surface floats, double floats or

special types of floats (IS 3911 - 1966). Velocity rods (IS 4858 - 1968) are also used

generally for velocity observations in canals. Details of the method are given in IS: 1192

(1959).

In adopting the float method or the surface velocity method in which current meter is

used, a reduction coefficient is used to change surface velocity into mean velocity in

each vertical. Measurements on Indus River in Sind at Mithankot, Sukur and Kotri

during 1911-1920 (Indus River commission records, "discharge, silt, velocity and

miscellaneous observations", parts I to IV, 1911 - 1920 printed at Commission press

1922, part II, pages 1 to108) showed that reduction coefficient varied between 0.74 and

0.92 when the depth variation was from 2.44 to 13.72 m and surface velocity from 0.19

to 5.09 m/s.

The studies in canal were similarly made by Mysore Engineering Research Station at 32

sites. The mean velocity V of the cross-section was obtained by the current meter

whereas the surface velocity was measured using floats.

The following relationship was obtained

V (m/s) = 0.8529 Vs + 0.0085

A relationship between the surface velocity Vs and the mean velocity V in terms of

Chezy C has been developed and is given by:

Vs
=1 + 2.5 g / C
V
V
The usual assumption made in practice is that s = 0.85 which corresponds to 'C' value
V

of 52.4 m0.5 s-1.

Hydraulics                                                                                         Prof. B.S. Thandaveswara

Theoretical considerations based on the logarithmic velocity distribution law indicate

that the reduction coefficient would be applicable only to a particular stream for which it

has been determined, since it would depend on the relative roughness of the channel,

depth, slope, etc., and hence it would be different for different streams, and for

fluctuating flood stages even in a given stream. It is therefore, recommended by the BIS

that the reduction coefficients should be found out from actual field observations made

by a current meter and only if such determination of the coefficient is not possible during

high flood stages then the reduction coefficient should be extrapolated to the stage from

data collected at lower stages.

4.2.4 Slope-Area Method
In the event of infeasibility of velocity area method due to either rapid rise and fall of

stage or lack of equipment, the slope area method is adopted for rough estimation of

the discharge.

The requirements of the site are mostly similar to those for area velocity method. The

cross-sectional area is measured adopting the procedure as in case of area velocity

method. The velocity formula used is that of Manning, the energy slope for non-uniform

flow . The roughness coefficient value to be used is related to bed material size and

condition of the channel. These recommendations are given in Indian Standards

Institutions IS : 2912 (1964).

4.2.5 Stage-Discharge Relationships

Regular recording of discharges over a period of time is essential for correct estimation

of water resources of river basins and subsequent planning and utilization. Daily

discharge observations over a long period are sometimes not feasible. The estimation

of the discharge is then achieved by using proper stage discharge relation. The method

adopted for the preparation of the stage discharge relationship for the different river

basins as well as the the exhaustive instruction for adopting the method of estimation of

discharge by establishing stage discharge relationship are contained in the Indian

Standard Recommendations IS: 2914(1964).

Hydraulics                                                                                        Prof. B.S. Thandaveswara

4.2.6 Details of Existing Indian Standards
A - Stream Gauging:

1. Printed Standards / Under Print:

(a) Measurement/ Estimation, Analysis and Recording:

IS: 1191      Glossary of terms and symbols
IS: 1192      Velocity area methods
IS: 1193      Notches, wiers and flumes
IS: 1194      Forms for recording measurement
IS: 2912      Slope area method
IS: 2913      Flow in tidal channels
IS: 2914      Stage discharge relation
Instructions for collection of data for
IS: 2915
analysis of errors
IS: 3918      Use of current meter
IS: 6059      Weirs of finite crest width
IS: 6062      Standing wave flume-falls
IS: 6063      Standing wave flume
IS: 6330      End depth method for rectangular channels

(b) Instruments

IS: 3910         Current meters
IS: 3911         Surface floats
IS: 3912         Sounding rods
IS: 4073         Sounding weights
IS: 4080         Vertical staff gauge
IS: 4858         Velocity rods
IS: 6064         Sounding and suspension equipment.

Reference:

Chitale S.V., Discharge Measurement - Technology and Data Analysis, Hydraulics of

Alluvial Streams, Central Board of Irrigation and Power, a Status Report Number 3, New

Delhi, June 1974. Page 13 to 24.