Stream Gauging

Stream Gauging: Necessity; Selection of gauging sites; Methods of discharge measurement; Area-Velocity method; Venturi flume; Chemical method; weir method; Measurement of velocity; Floats Surface float, Sub–surface float or Double float, Twin float, Velocity rod or Rod float; Pitot tube; Current meter; Working of current meter; rating of current meter; Measurement of area of flow; Measurement of width - Pivot point method; Measurement of depth Sounding rod, Echo- sounder.

1. 15CE5164 Advanced Water Resources Engineering © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 1

2. Stream Gauging: It is a technique used to measure the discharge, or the volume of water moving through a channel per unit time, of a stream. The height of water in the stream channel, known as a stage or gage height, can be used to determine the dischage in a stream. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 2

3. Streamflow representing the runoff phase of the hydrological cycle is the most important basic data for hydrologic studies. Streamflow is the only part of hydrological cycle that can be measured accurately. Runoff from a catchment can be determined by measuring the discharge of the stream draining it. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 3 Stream Gauging:

4. Necessity of Discharge measurement: By measuring the discharge in river the irrigation water can be distributed uniformly. By measuring flow in rivers flood warning can be issued. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 4

5. By measuring flow in canal quantity of loses can be known. The quantity of water flowing in the river helps for future research. Necessity of Discharge measurement:

6. Site selection for stream gauging: 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 vegetal growth, boulders or other obstructions like bridge piers, etc. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 6

7. Site selection for stream gauging: There should be no larger overflow section at flood stage. To ensure good consistency between stage and discharge there should be a good control section far downstream of the gauging site. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 7

8. Site selection for stream gauging: 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 8

9. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 9

10. Velocity-Area Methods: Discharge is the product of cross-sectional area and velocity of water; Q = v * A where Q = discharge [m3/s], v = velocity [m/s], and A = cross-section of flow [m2]. Velocity of flow at a cross-section varies, it is not enough to measure the velocity at a single point. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 10

11. Velocity-Area Methods: 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 11

12. Velocity-Area Methods: © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 12

13. Velocity-Area Methods: The area of the individual portion can be easily calculated if the bed profile and stage are known. The velocity may be measured by a conventional method or by an advanced procedure. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 13

14. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 14 If the physical and hydraulic conditions at the site permit, a fixed, undeformable structure may be constructed to measure river flow. A number of hydraulic structures are used to measure flows in field conditions Discharge Measurement Using Artificial Structures:

15. Weirs: They are used to control upstream water level or for measuring discharge or for both. They produce a critical relationship between stage and discharge by obstructing channel flow. Weirs have a defined cross-section and hence the computation of discharge is simple © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 15

16. Weirs: 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 16

17. Weirs: 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 17

18. Weirs: © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 18

19. 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 19

20. Flumes: 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 20

21. Flumes: © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 21

22. 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 22

23. 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; © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 23

24. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 24 Dilution technique:

25. 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 25 Dilution technique:

26. 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 26 Dilution technique:

27. 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. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 27 Dilution technique:

28. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 28 Dilution technique:

29. At the section X2, where the tracer is completely mixed literally, the flow is sampled continuously. From the conservation of mass in which Q is nearly constant through sampling period © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 29 Dilution technique:

30. The common tracers used are – Salt solutions. (b) Radio active tracer are detected by its scintillation detectors. (c) Fluorescent dyes with flourometers . © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 30 Dilution technique:

31. Advantage of dilution method- They condensed in closed conduits, such as penstocks, Sewers pipe lines, where current-meter measurements are difficult, and they are fast and accurate. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 31 Dilution technique:

32. Disadvantages- Expensive for measuring large stream. Special equipments required for the measurements of concentration. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 32 Dilution technique:

33. Velocity Measurement by Floats: A float is a distinguishable article that floats on the water surface, such as a wooden log, a plastic bottle partly filled with water, or branch of a tree. Surface or near-surface floats are used for streamflow measurement which are wooden cylindrical rods of nearly 0.5 m length. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 33

34. Velocity Measurement by Floats: They are shaped such that they float nearly vertically with one third of the length protruding above the water surface. The floats are painted in bright colours for easy identification in muddy or turbulent water. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 34

35. Velocity Measurement by Floats: For a float measurement, two cross-sections sufficiently far apart on a straight reach of river are selected. The upstream and downstream cross sections should be sufficiently far apart for accurate assessment of float travel time (3 to 5 times the width of the river or a minimum of 20 secs travel time). © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 35

36. Velocity Measurement by Floats: Both the cross sections should be clearly marked by placing markers so that the exact time when the float crosses the cross-section can be identified. The upstream channel cross section should be divided into a number (preferably an odd number) of equal segments as practically feasible in which the floats will be placed. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 36

37. Velocity Measurement by Floats: An observer each is positioned at upstream and downstream ends of the reach such that they are visible to each other. The downstream observer acts as timekeeper and carries a stop watch. Floats may be tossed from a bridge or cableway; if there is no such facility than they can be thrown in the water from the river bank. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 37

38. Velocity Measurement by Floats: When the float crosses the upstream cross-section, the upstream observer gives a signal to the downstream observer who notes the time taken by the float to cover the distance. The velocity of the float is equal to the distance between the two cross-sections divided by the time taken by the float to cover this distance. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 38

39. Velocity Measurement by Floats: The mean velocity in the vertical is determined as the float velocity multiplied by a coefficient which varies between 0.80 and 0.85. This coefficient depends on the shape of the velocity profile of the river and the depth of immersion of the float. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 39

40. Pitot tube: It is used for velocity measurements in order to calculate discharge in laboratory flumes or vey small streams. It is not recommended for rivers because it is very difficult to support it when channel is very wide and deep. The head generated by Pitot tube in open channel is generally very small due to very low velocities thus, discharge cannot be measured accurately. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 40

41. Formula for velocity computation is -  V=(2gh) 1/2 where, h = Water height in tube above surface of water © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 41 Pitot tube:

42. Current Meters: It is used to measure velocity at a point in the flow cross–section. Accurate measurements of the velocity profile of the stream cross section are made by current meters. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 42

43. Working of Current Meters: 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. The angular velocity acquired by the rotor is proportional to the velocity of water. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 43

44. By placing a current meter at a point in a stream and counting the number of revolutions of the rotor during a time interval, the velocity of water at that point is determined. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 44 Working of Current Meters:

45. There are two main types of current meters – Vertical-axis meters. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 45 Current Meters:

46. These instruments consists of a series of conical cups mounted around a vertical axis. The cups rotate in a horizontal plane and a cam attached to the vertical axis spindle records generated signals proportional to the revolutions of the cup assembly. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 46 Vertical - Axis Meters:

47. The normal range of velocities is from 0.15 to 4.0 m/sec. The accuracy of these instruments is about 1.5% at the threshold value and improves to about 0.3% at speeds in excess of 1.0 m/sec. These have the disadvantage that they cannot be used in situations where there are appreciable vertical components of velocities. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 47 Vertical - Axis Meters:

48. (b) Horizontal-axis meters. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 48 Current Meters:

49. It consists 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/sec. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 49 Horizontal - Axis Meters:

50. These meters are fairly rugged and are not affected by oblique flows of as much as 15°. The accuracy of the instrument is about 1% at the threshold value and is about 0.25% at a velocity of 0.3 m/sec and above. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 50 Horizontal - Axis Meters:

51. The number of rotations are measured and correlated to velocity using the formula- v = a + bN where N is the rotation of the propeller (revolutions per sec { rps }). a and b are coefficients determined by calibration in an experimental flume. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 51 Rating of the Current Meter:

52. These constants differ from one current meter to the other as a result of manufacturing variations as well as change with time due to wear and tear. Therefore, each current meter should be recalibrated periodically. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 52 Rating of the Current Meter:

53. Considering the velocity profile with depth, average value of velocity can be obtained at 0.6 of the depth. i.e. V = average velocity is at about 0.6 D. An alternative of using the 0.6 D velocity is to take 0.2 and 0.8 velocities and obtain the averages. The latter method is more accurate but in a shallow cross-section, the velocity at 0.2 D may be difficult to measure so use a single value at 0.6 D. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 53 Rating of the Current Meter:

54. 1 b Propeller Rotation, N Velocity a Surface Velocity Average Vel 0.6 D D Rating of the Current Meter:

55. Measurement of Area of Flow:  Measurement of Width- (Pivot point method) This method is based on the principle of similar triangles.  X and X are two points fixed on the cross-section line on the bank of the channel. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 55

56. From point X a pivot line is erected at right angles to cross-section XX.  The length of the pivot line should be about half the width of the channel or 300 m whichever is more. Other extremity of the pivot line is called a pivot point, marked P in Fig. 15.10. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 56 Measurement of Area of Flow: 

57. At 1/5 length of the pivot line from the pivot point a direction line DD 1  is drawn at right angles to pivot line. The direction line DD 1  is divided in suitable parts of equal length by points d 1 , d 2 , d 3  etc. Let the length of each part be 3 m. Then from similar triangles Pd 1 d 2  and Ps 1  s 2  the length of s 1  s 2  is 5 times the length of d 1 , d 2  (since PX is 5 times PD). © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 57 Measurement of Area of Flow: 

58. The length of each strip s 1 s 2 , s 2 s 3  etc., is 15 m. The points etc., can be located on the cross-section line very accurately with the help of a theodolite placed at P. When the gauge site is to be made permanent the points D, d 1 , d 2  etc., on the direction line, points and on the cross-section line and the pivot point should be constructed with masonry in the form of blocks with a hole in the centre to fix a flag exactly on the point © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 58 Measurement of Area of Flow: 

59. Measurement of Area of Flow:  2.Measurement of depth- (a) Sounding rod It is a wooden rod 5 to 8 cm in diameter with markings on it. A bamboo pole may also be used as a sounding rod. Many times flat iron of 5 cm x 0.6 cm size may also be used for the purpose. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 59

60. Measurement of Area of Flow:  The graduations are generally in tenth of a metre. To prevent sinking of a rod and to achieve accuracy in measurement the rod is provided with a flat base plate. The base plate is in the form of an iron disc 10 to 15 cm in diameter attached to the lower end of the rod. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 60

61. (b) Eco sounder It is based on an electrical principle. It consists in transmitting a sound impulse (by a transmitter) from the surface of water level to the bed of the river. When the sound waves reflect back in the form of an echo they are arrested by a receiver. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 61 Measurement of Area of Flow: 

62. There is an automatic arrangement for plotting the time of transmission and the time of reception. The velocity of sound in water is known (1470 m/sec). From this fact the depth of water is automatically computed. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 62 Measurement of Area of Flow: 

63. This method is of very common use on ships for observing depths. It can also be used successfully on rivers. © 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 63 Measurement of Area of Flow: