Stream gauging ppt

According to syllabus of Savitribai Phule Pune university

1. 1.3 Stream Gauging Prepared by Pr a deep Kumawat Assistant Professor Civil Engg. Department Late G. N. S apkal COE

2. Stream Gauging The most important phase of hydrological cycle is runoff. Stream gauging It is the a technique used to measure the discharge, or the volume of water per unit time, moving through a channel or  stream . Stream flow representing the runoff phase of the hydrologic cycle is the most important basic data for hydrologic studies. It was seen in the previous chapters that precipitation, evaporation, transpiration, infiltration and evapo - transpiration are all difficult to measure exactly and the presently adopted methods have severe limitations. Interestingly , Run-off or stream flow is the only part of the hydrologic cycle that can be measured accurately .

3. Necessity of Discharge measurement: B y m e asuring the discha r g e i n ri v er the ir r i g a t i on water can be distributed uniformly . By measuring flow in rivers flood warning can be issued . By measuring flow in canal quantity of loses can be known. The quantity of water flowing in the river helps for future research.

4. Selection of G auge site The bed and banks of the stream should be firm and stable so as to ensure consistency of area- discharge relationship . The bed and banks should be free from vegetable growth, boulders or other obstructions like bridge piers etc. There should be no larger overflow section at flood stage . At all stages, the total flow is confined to a single channel . To ensure good consistency between stage and discharge there should be a good control section far downstream of the gauging site . The section should be straight and uniform for a length of about 10 to 20 times the width of the stream. The stream gauging station should be easily accessible . There should be no eddied or abnormality in the flow.

5. Stage Measurement Staff Gauge Suspended Weight Wire Gauge Automatic Stage Recorders Bubble Gauge

6. Staff Gauge The simplest of stage measurements are made by n oting the el e v a tion of the water surface in contact with a fixed graduated staff. The staff is made of a durable material with a low coefficient of expansion with r es p ect t o bot h t e m pe r a tu r e and moisture. It is fixed rigidly to a structure, such as an abutment, pier, wall etc. The staff may be vertical or inclined with clearly and accurately graduated permanent markings.

7. The markings are distinctive, easy to read from a distance and are similar to those on a surveying staff. Sometimes, it may not be possible to read the entire range of water-surface elevations of a stream by a single gauge and In such cases the gauge is built in sections at different locations. Such gauges are called sectional gauges . When installing sectional gauges, care must be taken to provide an overlap between various gauges and to refer all the secti o n s t o the sam e c om m on datum .

8. Suspended Weight Wire Gauge It is a gauge used to measure the water- surface elevation from above the surface such as from a bridge or similar structure. In this, a weight is lowered by a reel to touch the water surface. A mechanical counter measures the rotation of the wheel which is proportional to the length of the wire paid out. The operating range of this kind of gauge is about 25 m.

9. Suspended Weight Wire Gauge

10. Automatic Stage Recorders The staff gauge and wire gauge described earlier are manual gauges. While they are simple and inexpensive, they have to be read at frequent intervals to define the variation of stage with time accurately . Automatic stage recorders overcome this basic objection of manual staff gauges and find considerable use in stream-flow measurement practice .

11. Float type Gauge Recorder Th e flo a t - o p e r a t e d s t a g e recorder is the most common type of automatic stage recorder in use. In this, a float operating in a stilling well is balanced by means of a counterweight over the pulley of a recorder. Displacement of the float due to the rising or lowering of the water-surface elevation causes an angular displacement of the pulley and hence of the input shaft of the recorder.

12. Mechanical linkages convert this angular displacement to the linear displacement of a pen to record over a drum driven by clockwork. The pen traverse is continuous with automatic reversing when it reaches the full width of the chart. A clockwork mechanism runs the recorder for a day, week or fortnight and provides a continuous plot of stage vs time. A good instrument will have a large-size float and least friction . Improvements over this basic analogue model consists of models that give digital signals recorded on a storage device or transmit directly onto a central data-processing centre . To protect the float from debris and to reduce the water surface wave effects on the recording, stilling wells are provided in all float-type stage recorder installations .

13. Bubble Gauge In this gauge, compressed air or gas is made to bleed out at a very small rate through an outlet placed at the bottom of the river. A pressure gauge measures the gas pressure which in turn is equal to the water column above the outlet. A small change in the water-surface elevation is felt as a change in pressure from the present value at the pressure gauge and this in turn is adjusted by a servo-mechanism to bring the gas to bleed at the original rate under the new head. The pressure gauge reads the new water depth which is transmitted to a recorder .

14. The stage data The stage data is often presented in the form of a plot of stage against chronological time known as stage hydrograph. In addition to its use in the determination of stream discharge, stage data itself is of importance in design of hydraulic structures, flood warning and flood-protection works. Reliable long-term stage data corresponding to peak floods can be analyzed statistically to estimate the design peak river stages for use in the design of hydraulic structures, such as bridges, weirs, etc . Historic flood stages are important in the indirect estimation of corresponding flood discharges. In view of these diverse uses, the river stage forms an important hydrologic parameter chosen for regular observation and recording .

15. Fig. Stage Hydrograph

16. STREAM FLOW MEASUREMENT Stream flow measurement techniques can be broadly classified into two categories as: Direct determination Area-velocity methods Moving Boat Methods Dilution techniques Indirect determination Hydraulic, structures, such as weirs, flumes and gated structures Slope-area method .

17. 1) Direct Measurement Methods 1.1 Area Velocity Method Approximate stream velocities can be determined by floats . The most commonly used instrument in hydrometry to measure the velocity at a point in the flow cross- section is the current meter . It consists essentially of a rotating element which rotates due to the reaction of the stream current with an angular velocity proportional to the stream velocity . Based on accuracy required, width of the stream is divided into a number of vertical portions. In each portions, velocity is measured at one or more points along the depth to get a representative velocity.

18. Velocity Measurement by Floats A floating object on the surface of a stream when timed can yield the surface velocity by the relation : Vs = S/t where , S = distance travelled in time t Average velocity, Va = 0.85 Vs Discha r g e is th e p r od u ct o f c r os s - sectiona l a r ea and velocity of water, Q = Va * A where, Q = discharge [ m3/s] and A = cross-section of flow [m2 ] This method of measuring velocities while primitive still finds applications in special circumstances, such as (i) a small stream in flood , ( ii) small stream with a rapidly changing water surface, and (iii) preliminary or exploratory surveys.

19. While any floating object can be used, normally specially made leak proof and easily identifiable floats are used. A simple float moving on the stream surface is called a surface float. It is easy to use and the mean velocity is obtained by multiplying the observed surface velocity by a reduction coefficient . However, surface floats are affected by surface winds. To get the average velocity in the vertical directly, special floats in which part of the body is underwater are used. Rod float in which a cylindrical rod is weighed so that it can float vertically, belongs to this category.

20. 1.2 Moving Boat Method The measurement of velocity is an important aspect of many direct stream-flow measurement techniques. A mechanical device, called current meter, consisting essentially of a rotating element is probably the most commonly used instrument for accurate determination of the stream-velocity field . The present day cup-type instrument and the electrical make-and-break mechanism were invented by Henry in 1868. There are two main types of current meters. Vertical-axis meters Horizontal-axis meters

21. 1. Vertical-Axis Meters These instruments consist of a series of conical cups mounted around a vertical axis. The cups rotate in a horizontal plane and a earn attached to the vertical axial spindle records generated signals proportional to the revolutions of the cup assembly . The Price current meter and Gurley current meter are typical instruments under this category. The normal range of velocities is from 0.15 to 4.0 m/s. The accuracy of these instruments is about 1.50% at the threshold value and improves to about 0.30% at speeds in excess of 1.0 mls . Vertical-axis instruments have the disadvantage that they cannot be used in situations where there are appreciable vertical components of velocities. For example, the instrument shows positive velocity when it is lifted vertically in still water.

22. 2. Horizontal-Axis Meters These meters consist of a propeller mounted at the end of horizontal shaft. These come in a wide variety of size with propeller diameters in the range 6 to 12 cm, and can register velocities in the range of 0.15 to 4.0 m/s. Ott, Neyrtec and Watt-type meters are typical instruments under this kind . These meters are fairly rugged and are not affected by oblique flows of as much as 15°. The accuracy of the instrument is about I% at the threshold value and is about 0.25% at a velocity of 0.3 mls and above . The instruments have a provision to count. the number of revolutions is a known interval of time. This is usually accomplished by the making and breaking of an electric circuit, either mechanically or electromagnetically at each revolution of the shaft . The revolutions per second is calculated by counting the number of such signals in a known interval of time, usually about 100 s. Present-day models employ electromagnetic counters with digital signal processing capabilities.

23. Fig. Horizontal-Axis Current Meters

24. 1.3 Dilution Technique Two dilution techniques are : the steady feed method and the instantaneous, point - source time indigenous method For steady feed method, a solution of tracer material with concentration C1 is injected at the constant injection rate QT. The tracer disperses laterally into the flow and tracer concentration distribution is similar to as shown in figure. At some point X2 downstream, where the tracer material is approximately uniformly mixed, the flow is sampled continuously.

25. If the tracer mixer has properties similar to the water, so that there are no density gradients, vertical mixing is very rapid due to turbulence of the flow. Theoretically , complete lateral mixing occurs at X but practically it occurs between 20 to 100 times the channel widths . By instantaneous injection method, a quantity of tracer w, is injected, instantaneously at section X and time t0 . The cloud of tracer disperses laterally and longitudinally as it moves downstream .

26. 2) Indirect determination Methods By Notches, Weirs, Venturiflumes and Spillways If the physical and hydraulic conditions at the site permit, a fixed, un-deformable structure may be constructed to measure river flow. A number of hydraulic structures are used to measure flows in field conditions

27. W e i r s : They are used to control upstream water level or for measuring discharge or for both . Th e y p r oduc e a c r iti c al r el a tion s hi p b e t w een stage and discharge by obstructing channel flow . W ei r s h a v e a d e fine d c r os s -secti o n and hence the computation of discharge is simple. The size and cost of the structures increase as the size of the river increases . Site requirements consist of a reasonably straight approach channel which should be free of excessive sedimentation, weeds and other aquatic growth .

28. W e i r s The structure should be rigid, water-tight, normal to the flow direction, and should be capable of passing high flows without any damage to its body . The stage-discharge relation at the site depends on the geometrical characteristics of such a structure. Q = Cd ∙ L ∙ H^n

29. W e i r s

30. Venturi Flumes : A flume is a flow measuring structure formed by a constriction in a channel . The constriction can be either a narrowing section of the channel or a narrowing section in combination with a hump in the invert . A unique stage-discharge relationship exists independent of the downstream conditions. For a rectangular flume, the discharge of an ideal fluid is expressed as - here, H represents the upstream energy and b is the typical width dimension for the particular cross-section shape of the flume .

31. Flumes: By introducing suitable coefficients, this equation can be generalized to the following form – Where, Cv = coefficient to take in to account the velocity head in the approach channels, Cs = coefficient to take account of the cross-section shape of the flume, Cd = coefficient for energy loss, and h = depth of water, upstream of the flume, measured relative to the invert level of the throat .

32. ADVANCED TECHNIQUES FOR GAUGE MEASUREMENT Electromagnetic method, and Ultrasonic method .