Hydrosystems Hydrulics open channel flow iit nptel _12_

Document Sample
Hydrosystems Hydrulics open channel flow iit nptel  _12_ Powered By Docstoc
					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
                     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.

Indian Institute of Technology Madras
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

                made using wading or suspension rods. However, when wading observations are found

                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


                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

Indian Institute of Technology Madras
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


                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:

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

                of 52.4 m0.5 s-1.

Indian Institute of Technology Madras
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).

Indian Institute of Technology Madras
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.


                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.

Indian Institute of Technology Madras

Shared By: