A logarithmic plot of the submerged flow data is shown in Figure 2.12. Each data point in Figure 2.12 can have a line drawn at a slope of n, = 1.55, which can be extended to where it intercepts the abscissa at h, - h, = 1.0; then, the corresponding value of discharge can be read on the ordinate, which is listed as Q,,=, in Table 2.5. The value of QA,,=,,oI can also be solved analytically because a straight line on logarithmic paper is a power function having the simple form:

or,

.

.

QS QA,-l.O =

(h, - h$'

(2.11)

(2.12)

Where Q,,=, has a different value for each value of the submergence, S.

Using the term Q A , = , o, implies that h, - h, = 1 .O by definition, so that Equation reduces to:

CS(1 .o)"'

(-k7a"* Q**-,.o = = C,(-l0gs)pfs

I (2.13)'

24

100.0

0

Figure 2.12. Logarithmic plot of submerged flow data for the example open channel constriction.

25

Again, this is a power function where Q,,=,,, can be plotted against (-logs1 on logarithmic paper to yield a straight line relationship. The submerged flow data in Table 2.5 plotted in Figure 2.13. Note that the straight line has a negative slope (-&I and that C, is the value of (I,,=, , when (-logs) is equal to 1 .O. For this particular data, the submerged flow equation is:

0.367(hU - h&’.= 0, =

(-Iogs)’.3’1 (2.14)

By setting the free flow discharge equation equal to the submerged flow discharge equation (Equation 2.7 and 2.1 4), the transition submergence, S,, can be determined:

0.367(hU - hJ’”’ 0.74h:= =

( - 1 0 g s ) ~ ~ ~ ~

0.74(-/0g9’.~~ = 0.367(1 -s)‘.55

(2.15)

(2.16)

The value of S in this relationship is S, provided the coefficient and exponents have been accurately determined. Again, small errors will dramatically affect the determination of S,.

0.74(-10gSJ’”~ = 0.3667(1 -SJ’.” (2.17)

E iatio 2.17 is solved by trial-and-error to determine S,, which in this case is 0.82. Thus, free flow exists when S < 0.82 and submerged flow exists when the submergence is greater than 82 percent..

A final free flow and submerged flow discharge rating is plotted on logarithmic paper in Figure 2.14. Also, Table 2.6 is a typical free flow rating based on Equation 2.7 for the example open channel constriction. The submerged flow discharge can be calculated using the reduction factors in Table 2.7 to multiply by the free flow rating

calculating the ratio Q,/Q,,

. c corresponding to the measured value of h,. This reduction factor is obtained by

r

- 0, - 0.367(hU - h31”5 1 - - Qf ( -10gs)~ .~~ 0.74h:”

26

(2.18)

.. e

! . .

I .

I Figure 2.13.

200.0

100.0

50.0

40.0

30 O

20.c

10 (

5.1

4.1

3 (

2.1

1 . I

-- log s ‘ I

C,= 0.369

Logarithmic plot for determining the submerged flow coefficient, Cs, and the submerged flow exponent, ns, for the example open channel constriction.

27

, -

I * ~

11 IJ ---lid. in m e l e r s

0

0.7

0.1

Figtire 2.14

$. in meters

Free flow and submerged flow rating for the example open channel constriction.

28

TABLE 2.6. Free flow discharge rating for example open channel constriction.

.,

.

.

.

29

TABLE 2.7. Submerged f low reduction factors for the example open channel constriction.

s QJQ, 0.82 1 .ooo

II 0.85 I 0.9902

0.86 0.9856

II 0.87 I 0.9801

0.89

II 0.88 I 0.9735

0.9657

0.90 0.9564

30

0.9455

0.9325

0.91 70

0.8984

0.8757

0.96 0.8472

0.97 1 0.8101 II

which is also eaual to

I .

(2.19)

For example, if h, and h, are measured and found to be 1.430 and 1.337, the first step would be to compute the submergence, S,

1337 1.430

S = = 0.935 (2.20)

Thus the submerged flow condition exists in the example open channel constriction. From Table 2.6, the value of 0, is 1.228 m3/s for h, = 1.430. Then, entering Table 2.7, the value of QJQ, can be found by interpolating halfway between S = 0.93 and S = 0.94. Thus QJQ, = 0.9077. Consequently, Q, can be determined from:

Qs

Qf

Q, = Of - = 1.288(0.9077) - 1.169 msls (2.21)

Finally, it should be noted that all of the preceding graphical solutions to the calibration of open channel constrictions can also be obtained numerically through logarithmic transformations and linear regression. The graphical solutions have the advantage of being more didactic; however, for experienced persons the numerical solution is usually more convenient. It is always useful to plot the results, either by hand or using computer software, to reduce the possibility of errors or inclusion of erroneous data values. Obvious errors tend to be more apparent in plots than in numerical calibration results.

31

2.5 Rating Canal Outlets

2 .5 .1 General Situation

.

.

There are numerous small structures used to convey water f rom a canal into a tertiary (watercourse1 channel. Generally, thesesmall canal outlets wil l convey 10-250 Ips (0.3 - 8.0 cfs). For this f low range, portable f low measuring flumes are usually the most convenient device for measuring discharge when developing a rating. Either a flat-bottomed trapezoidal flume, Parshall flume, or Cutthroat flume can be temporarily installed downstream o f the small canal outlet structure. Another possibility is to install a temporary trapezoidal broad-crested weir. Although all o f these devices have the disadvantage of increasing the water surface elevation upstream of the f low measuring device, this does not affect the accuracy of the discharge rating for the canal outlet structure, except for not taking into account the channel losses between the outlet structure and the f low measuring device.

Another common method for measuring the discharge in the tertiary irrigation channel is to use a pygmy current meter mounted on a wading rod. The accuracy of this method is not as good as using standard f low measuring flumes, but is sufficient i f the equipment has been properly maintained.

2 .5 .2 Tvoes of Small Canal Outlets

There are t w o basic philosophies used for canal outlets. First o f all, many irrigation systems have manually-operated gates that are used to control the discharge through the canal outlet. Secondly, many irrigation systems have been purposely designed without gates on the canal outlets; the discharge through the outlet is a function o f the water surface elevation, h,, in the canal i f free f low exists, or a function o f the difference in water surface elevation between the canal and tertiary channel, h, - h,, for submerged f low. For the systems without gates, each outlet is to receive a proportion of the available f low in the canal, both during times of water scarcity and water surplus. The types of small canal outlets that will be discussed below are:

1. Flume Outlets; 2. Fixed-Orifice Outlets; 3. Open pipe Outlets; and 4. Gated Orifice Outlets.

The discharge through flume, fixed-Orifice, and open pipe outlets is a function of the water level in the canal for free f low conditions, or the difference in water surface elevations upstream and downstream of the outlet for submerged f low conditions. Thus, the design discharge will only be satisfied when the design water

32

surface elevations occur. Only the gated orifice outlet provides a mechanism for accurately controlling the discharge.

2.5.3 Flume Outlets

A flume outlet is an open channel constriction that can operate with either free flow or submerged flow conditions. A very simple open channel constriction is illustrated in Figure 2.15. A commonly used flume outlet is illustrated in Figure 2.15, which is frequently seen in the Indian subcontinent. For any open channel constriction, the free flow rating is given by Equation 2.22 which is identical to

.

. Equation 2.4:

0, = C, h: (2.22)

where Q, is the free flow discharge, h, is a flow depth measured upstream of the flume throat and using the flume floor or sill as the zero reference datum, C, is the free f low coefficient, and n, is the free flow exponent.

The free flow, or modular flow, discharge equation for the flume outlet illustrated in Figure 2.15 is sometimes written in the literature as:

q = KB,G3p (2.23)

where q = Q,, 6, is the throat width W, and G = h,. Therefore, Equation 2.23 can be rewritten as:

0, = W h i p (2.24)

Equation 2.24 was derived from the theoretical equation for a rectangular long-throat flume, which is expressed as follows:

. 0, = C,,C,.(2/3)- W hip (2.25)

The published values of the coefficient K in Equation 2.23 with throat width are given in Table 2.8 below. The equation defining K is as follows:

33

1 : .

.

L

, . - . - . . , , , . . : . . . . . . . R -m \ SECnON A T

.:-s

" 8 ,

Figure 2.15.

J

-1. I

1 . i I

I 1

I Typical flume outlet frequently used in the Indian Subcontinent.

34

T --'

B, or W

0.20 to 0.29 f t

0.30 to 0.39 f t

0.40 ft, or more

K = C,,CJ2/3)-

K = CJB, CdC"

2.90 0.940

2.95 0.955

3.00 0.970 -

(2.26)

And,

K = C,/W = C,/B, (2.27)

thus, the value of C,C, can be calculated for each value of K, which is listed above. A field study in the Punjab (Saleem 1991) resulted in K = 2.63 for B, = 0.21ft, and K = 2.75 for B, = 0.24 ft.

The submerged flow rating for a flume outlet would be represented by Equation 2.28, which is identical to Equation 2.10:

C,(h, - ha"' 0, =

(-logs)"* (2.28)

where Q, is the submerged flow discharge, h, is a flow depth measured downstream of the flume throat but using the throat floor or sill as the zero reference datum, S is the submergence h,/h,,, C, is the submerged flow coefficient, and n, is the submerged flow exponent.

For the flume outlet shown in Figure 2.1 5, there are submerged (non-modular) flow rating curves. Howver , these curves were developed many decades ago, before Equation 2.28 was available. Thus, the curves are similar to the original submerged flow rating curves developed by Parshall for the Parshall Flume. Either the original data needs to be analyzed again using Equation 2.28, or new submerged flow ratings should be developed in the laboratory.

35

.

An important consideration in using flume outlets is that the discharge is a theoretically a function of either h, or h, - h, to the exponent 3/2, yet the exponent may actually be somewhat greater than 3/2. Thus, the discharge increases rapidly with increasing flow depths in the canal. Consequently, this type of outlet is best used in the vicinity of the downstream end of the canal, so that if the canal discharge increases (e.g. as a result of rainfall) then the discharge through the flume outlets will rapidly increase.

2.5.4 Fixed-Orifice Outlets

The primary advantage of a fixed-orifice outlet, compared with a flume outlet, is that the discharge is a function of the canal water level to the exponent 1/2 (rather than 3/2). Thus, the discharge increases with increasing water levels in the canal, but not nearly as rapidly as compared with flume outlets. Consequently, fixed-orifice outlets are used in the upper and middle reaches of a canal, whereas flume outlets would be used in the lower reaches.

For a fixed-orifice outlet, the free flow discharge rating would be given by Equation 2.29:

where A is a constant. Thus, Equation 2.29 can be rewritten as:

The submerged flow discharge rating for a fixed-orifice outlet is given by Equation 2.31:

where A is constant. Equation 2.31 can be rewritten as: . Q, = K , d m d

(2.31)

(2.321

36

.

.

A typical fixed orifice outlet commonly used in the Indian subcontinent, particularly in the Punjab, is shown in Figure 2.16. This was originally introduced by E.S. Crump in 1922 as the Adjustable Proportional Module (APM). Later, when the sill level was lowered from 0.6 depth for the APM to 0.9 depth, in order to convey more sediment load through the outlet, the name of the outlet structure was changed to Adjustable Orifice Semi-Module (AOSM). Then, the discharge equation was also modified to:

0, = K B , Y ~ ~ (2.33)

where Q, is the submerged flow discharge, K is a coefficient having a value of 7.3, B, is the width of the orifice (standard widths of 0.20, 0.25, 0.32, 0.40, 0.50 and 0.63 f t have been adopted), Y is the height of the orifice and H, is the difference in elevation between the canal water level and the crown of the orifice (Figure 1.

A value of K = 7.3 in Equation 2.33 corresponds to C,C, = 0.909 in Equation 2.31. The results of field calibrations of a few of these structures in the Punjab are shown in Table 2.9. A detailed field calibration in the Punjab (Saleem 1991) resulted in K = 7.59.

Since the fixed-orifice outlet is really a free flume outlet with a roof block, it would be expected that there should be some relationships between the open channel constriction and the orifice constriction. In fact, the free-orifice rating and the submerged orifice rating can be superimposed upon the free flow and submerged flow ratings for the flume. Figure 2.17 is an example of a flume having a throat width, B,, of 0 .24 and an orifice height, Y, of 0.541 ft.

2.5.5 ODen PiDe Outlets

An open pipe outlet is the simplest of all outlet structures in terms of fabrication and installation, but the most complex hydraulically. Some typical examples are shown in Figure 2.18. The discharge will, in most cases, be a function of the water depth in the canal, h,,, to the exponent 1/2, or the square root of the difference in water surface elevation between the canal and tertiary channel (Figure 2.1 8 ) .

Most of the complexities involving the hydraulic of "Culverts" pertains also to open pipe outlets. First of all, there is the question of free orifice flow (inlet control) or submerged orifice flow. Secondly, the geometry of the inlet has a pronounced effect on the inlet flow. Thirdly, there is the question of pipe slope; the design may have called for zero slope, but the installation during construction may result in a slope which will have a highly significant effect as illustrated in iab le 2.10. Fourthly, the length of the pipe outlet will also affect the discharge being conveyed to the tertiary channel. Thus, there are many parameters to be evaluated in designing an open pipe outlet.

37

SECTION A A

I I

Figure 2.16. Typical fixed-orifice outlet used in the Indian Subcontinent, particularly in the Punjab.

38

TABLE 2.9.

R.D. Outlet Type

3117401il O F

3 1361911 A P M

313976lL A P M

31377214 A P M

4448/R A P M

44561L A P M

72821L A P M

77561L A P M

82741R A P M

11662iL A P M

13589lL A P M

143141R PIPE

5545rL PIPE

18016lL APM

204641R PIPE

21400,L PIPE

21725lL A P M

23500iR PIPE

23600 PIPE

2503OlL A P M

263421R PIPE

263521L PIPE

Field calibration of some fixed-orifice outlets in the Punjab

Authorized Discharge

C I S Bt D Y f t f t

.. 0.87 - -

0.60 0.20 1.32

0.61 0.20 0.25

1.79 0.20 2.17

0.61 0.20 0.25

0.42 0.20 0.20

1.04 0.20 0.44

0.40 0.20 0.20

0.46 0.20 0.20

0.72 0.20 0.32

0.86 0.20 0.41

0.28 0.21 --

0.64 0.50 .- 3.37 0.32 1.00

0.84 0.20 0.36

0.29 0.25 --

1.04 0.20 0.56

0.07 0.17 ~-

0.49 0.33 --

1.34 0.20 0.62

0.19 0.29 - -

0.16 0.21 - -

Measured Discharge

ha hb S a c t s ft f t %

1.21 1.20 92

Current Meter- >

0.44 0.39 88

0.51 0.49 96

0.48 0.36 74

0.29 0.16 57

0.68 0.32 47

0.60 0.28 46

0.42 0.36 86

0.76 0.35 47

0.54 0.39 73

0.27 0.18 66

0.85 0.60 71

1.22 0.99 79

0.38 0.22 57

0.31 0.22 72

0.42 0.16 38

0.17 0.10 12

0.22 0.12 54

0.39 0.17 44

0.18 0.1 1 61

0.20 0.11 57

1.50

12.00

0.55

0.60

0.70

0.31

1.46

1.16

0.50

1.79

0.93

0.26

2.05

4.03

0.50

0.35

0.60

0.12

0.18

0.52

0.13

0.15

Notes: 1. Calibration performed on the Niaz Beg Distributary Canal. 11 to 16 March. 1987 2. The watercourse discharge was ineasures wing Cutthroat flumes. 3 . O.F. ~ "Open flume": APM - "Adjustable Proportionel Module". 4. "Bt 0" is Bt f a r O.F. and APMs. arid ig D ldinmclerl lor pipes.

. 39

Calibration

B1 D Y (low hu f t hd 11 Cd (1

11 type cfs

0.29 -- Free 2.37 0.39 1.30

0.92 - - Free 3.00 ~~ 2.15 0.89

0.25 0.75 Free 1.22 -~ 0.44 0.55

0.20 0.83 Free 3.72 ~- 0.29 0.60

0.22 0.60 Free 1.20 ~~ 0.68 0.70

0.23 0.29 Subm 1.25 0.12 0.54 0.31

0.82 1.46 0.20 0.80 Free 2.30

0.20 0.80 Free 1.71 0.74 1.16

0.20 0.20 Free 1.55 1.20 0.50

0.54 0.91 Free 2.30 0.90 1.79

0.28 0.58 Free 1.41 ~~ 0.61

0.21 -- Subm 1.39 0.93

0.50 ~- Frse 2.75 0.79

0.32 1.04 Free 3.80 .- 0.78

0.20 0.39 Subm 0.87 0.84 0.13

0.29 ~~ Subm 1.28 0.55 0.29

0.22 0.52 Free 0.63 0.64

0.17 -~ Subm 0.88 0.73 0.70

0.33 ~. Subm 0.47 0.18

0.20 .- Subm 0.78 0.53

0.29 -- Free 0.10 0.77 0.16

0.21 ~~ Subm 0.29 0.78 0.25

8

1c

or

1 .o

hu - hd

Submerged Orifice c ,, Free Orifice v

(--- Free f low

0.1

10

Q f

Q s

or

I .o

1.1

Figure 2.17. Discharge rating curves for a flume fixed orifice (Y = 0.541

40

ft). outlet 0.24ft) with a

.

(A) OPEN PIPE OUTLET FROM AN EARTHEN CANAL

I

.

Figure 2.18 Typical examples of open pipe outlets.

41

.

When developing the field discharge rating for an open pipe outlet, the first question is whether free orifice flow (Equation 2.29) or submerged orifice flow (Equation 2.31 I is occurring. Then, a single discharge measurement combined with the measured value of h, (or h, and h,, whichever is appropriate) will allow the coefficient of discharge, C,, to be calculated in either Equation 2.29 or Equation 2.31, depending on the flow condition. This calculated value of Cd based upon field measurements will be affected by all of the parameters described in the paragraph above.

An example will illustrate the variability in C, for open pipe outlets. Table 2.10 lists the results of field calibrations on twenty-one open pipe outlets along the Warsak Gravity Canal in the North West Frontier Province of Pakistan. All of these open pipe outlets were designed using the equation:

where Q is the discharge, K is a coefficient equal to 5, A is the cross-sectional area of the open pipe outlet, and H is the depth of water in the canal over the invert of the pipe inlet. A value of K = 5 corresponds to a value of 0.62 for C, in Equation 2.29, which is a reasonable value for design purposes. In Table 2.10, Cd ranges from 0.32 to 0.99, which is plus or minus 50 percent from the design value of C,. This is a rather dramatic variation from the design value, but not surprising when the complexities of culvert hydraulics are recognized.

The lesson to be learned from Table 2.10 is the absolute importance of field calibrating pipe outlet structures. Otherwise, the error in discharge may very easily be 50 percent. Only irrigation projects having too much water can tolerate such high levels of error. Also, even with vastly improved designed procedures, the expected difference between the design discharge and actual field discharge will be at least 10 percent, and often 20 percent. Consequently, in the near future, the necessity for field calibration of each open pipe outlet cannot be avoided.

.

42

TABLE 2.10. Example of field calibrations of open pipe outlets.

Second Third Mean Value

Q hu Cd ha hb a hu Cd Cd

Outlet Sane. Pipe First Location Disch Diameter MeaS"reme"t Measurement Measurement

R.D. a Saw. Meas. ha hb a hu Cd ha hb f t ft cfs ft fr ft cfs ft ft ft cfs f t cfs in I"

1.44 2.96 0.53 0.53 80755lL 1.08 6 6 0.50 0.20 1.26 2.33 0.52 0.54 0.21 1.45 2.88 0.54 -

6.12 2.81 0.58 0.58 80850/L 4.61 12 12 1.03 0.62 4.67 2.17 0.50 1.24 0.76 6.69 2.72 0.64 -

8147OlL 0.20 4 4 0.28 0.22 0.40 1.92 0.41 0.19 0.10 0.19 2.14 0.18 0.32 0.20 0.55 2.26 0.52 0.52

- 0.51 3.38 0.78 0.78 83020R 0.27 3 3 0.29 0.12 0.46 2.37 0.76 0.31 0.11 0.52 2.66 0.81

- Closed .. 0.48 83600/L 0.12 3 3 0.20 0.30 0.49 2.91 0.73 0.23 0.10 0.31 3.30 0.24 . 83800/L 0.98 4 6 0.60 0.30 1.16 3.33 0.40 0.70 0.42 1.38 2.33 0.64 0.66 0.36 1.38 1.83 0.64 0.66

84650/L 2.13 6 6 0.66 0.33 1.38 1.83 0.64 0.82 0.30 2.06 3.16 0.73 0.75 0.27 1.75 3.12 0.63 0.64

85340/L 1.30 6 6 0.79 0.48 1.90 2.83 0.71 0.81 0.40 3.16 3.12 0.70 0.81 0.40 3.16 3.12 0.70 0.70

85632/L 2.52 7 9 1.04 0.50 3.20 1.29 0.80 1.16 0.56 3.90 2.62 0.68 1.12 0.51 3.66 2.32 0.68 0.68

865OOlL 0.92 6 9 0.80 0.40 2.00 1.38 0.48 1.09 0.52 3.48 1.96 0.68 1.11 0.47 0.54 2.09 0.69 0.68

894801L 1.47 6 6 0.86 0.34 2.25 2.17 0.96 0.89 0.38 2.40 2.84 0.90 . 2.41 2.84 0.93 0.93 0.85 2.95 0.73 0.73 910801L 0.47 4 4 0.46 0.15 0.71 2.29 0.69 0.52 0.18 0.89 2.92 0.76 -

0.41 3.58 0.32 0.32 91504lL 0.18 4 4 0.31 0.08 0.34 2.99 0.29 0.36 0.08 0.45 3.50 0.35 -

931OOlL 0.96 5 6 0.63 0.23 1.27 2.83 0.47 0.74 0.28 1.71 3.21 0.60 0.72 0.28 1.62 3.21 0.56 0.56

93500/L 3.41 9 12 1.28 0.58 4.68 2.33 0.48 1.23 0.54 4.40 2.56 0.43 1.24 0.62 4.41 2.55 0.44 0.44

94460/L 1.71 9 9 1.16 1.10 2.60 0.59. 0.89 1.05 0.92 2.70 0.52 0.88 0.99 0.80 2.65 2.90' 0.86 0.87

95630/L 65.02 12 12 1.28 0.72 10.17 2.50 1.02 1.29 0.52 10.43 2.92 0.97 - - 10.62 2.90 0.99 0.99

97003/L 0.53 4 6 0.43 0.31 0.95 1.18' 0.55 0.52 0.46 1.10 0.94' 0.72 0.47 0.21 1.17 0.26' 0.45 0.57

100000/L 0.74 4 6 0.73 0.31 2.25 2.98 0.82 0.74 0.13 2.38 3.43 0.82 - 2.46 3.66 0.82 0.82

100330/L 0.41 4 4 0.39 0.13 0.79 2.39 0.73 0.40 0.12 0.83 3.00 0.69 - - 0.86 3.03 0.71 0.71

102OoOlL 0.85 4 4 0.46 0.20 1.08 2.66 0.94 0.48 0.21 1.17 3.32 0.92 . 1.19 3.36 0.93 0.93

Notes: 1. Calibration was performed on the Warsak Gravity Canal, NWFP. 21 Feb to 5 Mar. 1987. 2. Pipe diameters are Sanctioned ISanc.1 and Measured (Meas.). 3.' = measured (h. - h,l submerged-flow condition.

2.5.6 Gated Orifice Outlet

The canal outlets described above have the discharge controlled by the water level in the canal, or the difference in water level between the canal and tertiary channel. The advantage o f placing a gate at the upstream (inlet) end o f a canal outlet is to al low more precise regulation of the discharge. Thus, if water levels change, the gate opening can also be changed in order to maintain a more constant discharge through the canal outlet. Some typical examples o f gates orifice outlets are illustrated in Figure 2.19 and 2.20. Field calibration of these structures is identical to the procedures described in the section "Rating Orifices", where either Equation 2.29 would be applied i f free orifice f low exists, or Equation 2.31 would be used if submerged orifice f low exists.

44

b N O I E : ALL DIMENSIONS N 1 E 111 METERS

Figure 2.19. Typical example of a circular gated orifice outlet.

45

Figure 2.20.

,

d I /

Typical example of a rectangular gated orifice outlet.

46

.

CHAPTER 3: METHODOLOGY USING A CURRENT METER FOR MEASURING DISCHARGE

3.1 Current Meters for Discharge Measurement

3.1.1 Tvoes o f Current Meters

There are many countries that manufacture good quality current meters. One of the more recent innovations is the electro-magnetic current meter that displays the velocity measurement. The electronic types of current meters will be used much more in the future.

Current meters with a rotating unit that is sensing the water velocity are either vertical-shaft or horizontal-shaft types. The vertical-axis current meter has a rotating cup with a bearing system that is simpler in design, more rugged, and easier to service and maintain than horizontal-shaft (axis1 current meters. Because of the bearing system, the vertical-shaft meters will operate at lower velocities than horizontal-axis current meters. The bearings are well protected from silty water, the bearing adjustment is usually less sensitive, and the calibration at lower velocities where friction plays an important role is more stable (Hagan, 1989).

T w o o f the commonly used vertical-axis current meters are the Price Type A Current Meter and the Price Pygmy Current Meter, which is used for shallow f low depths and low velocities. A diagram for the Price Type A Current Meter is shown in Figure 3.1. In addition, there are some rugged, high quality horizontal-axis current meters that give excellent results.

The horizontal-shaft current meters use a propeller. These horizontal-axis rotors disturb the flow less than the vertical-axis cup rotors because of axial symmetry with the f low direction. Also, the horizontal-shaft current meters are less sensitive t o the vertical velocity components. Because of its shape, the horizontal-axis current meter is less susceptible to becoming fouled by small debris and vegetative material moving with the water (Hagan, 1989).

. Some common horizontal-axis current meters are the Ott (German), the Neyrpic (France) and the Hof f (U.S.A.). Some recent models have proven to be both accurate and durable when used in irrigation channels. . *

Electronic (electromagnetic) current meters are now available that contain a Some o f the earlier models

had considerable electronic noise under turbulent f low conditions. Fortunately, present models yield more stable velocity readings and have been used in irrigation channels. Undoubtedly, these instruments will be further improved in the near future.

. sensor wi th the point velocity being digitally displayed.

47

10 I

. . .

3

C XPL A N A TI ON

1 CAP FOR CONTACT CHAMBER 2 CONTACT CHAMBER 3 IN.SULA1ItK UUS!~tlt.IC FOR CONTACT

C SINGLE-CONTACT BINDING POST (UPPER) fi fYNTA--CON1ACT OlN[)ING POST (t.OIMR)

7 SCT SCREV!S E YOKE 9 HOLE FOR HANGER SCRE\Y 10 TAlCPlECE 11 BALANCE W G t I T I 2 SI IAFT 1.3 BUCKET VMEEL HUB 1 4 BUCKET WHEEL I-iUB NUT 15 RAISING NUT 16 PIVOT BEARING 17 PIVOT 10 PIVOT ADJUSTING NUT

20 CJEARING LUG 21 OUCKET \V!lCCL

BINDING POS'l

ti f'LN'lA-GEAR

lo KEEPER s c r w OFR PIVOT ADJUSTING NUT

Source: Don M. Corbett et a!. 1962

I Figure 3.1. Assembly Diagram for a Price Type A Current Meter

48

3.1.2 Care of EauiDment

Accuracy in velocity measurements can only be expected when the equipment is properly assembled, adjusted, and maintained. The current meter should be treated as a delicate instrument that needs meticulous care and protective custody, both when being used and when being transported. The required treatment of a current meter is analogous to a surveyors careful attention with a transit or level.

The current meter necessarily receives a certain amount of hard usage that may result in damage, such as a broken pivot, chipped bearing, or bent shaft that will result in the current meter giving velocity readings that are lower than actual velocities. Observations of velocities near bridge piers and abutments, water depth readings taken a t cross-sections having irregular bed profiles with the current meter attached to the measuring line, and the periodic occurrence of floating debris represent the greatest hazards to the equipment (Corbett and others, 1943).

Damage to current meter equipment during transportation is generally due to careless packing or negligence in protection. A standard case is provided by all manufacturers of current meter equipment, which should always be employed before and after taking a discharge measurement. In particular, the equipment case should always be used when transporting the current meter, even when the distance is relatively short. Transportation of assembled equipment from one location to another is one of the most common sources of damage (Corbett and others, 1943).

In some countries, it is common to see current meter equipment without a case. Also, during transport, this equipment will be placed on the floor of the vehicle. In one case, there was a 30 percent variation in velocity measurements among seven current meters as a result of improper care and protection.

3.1.3 Current Meter Ratinos

Usually, a current meter is calibrated in a towing tank. The current meter is attached to a carriage that travels on rails (tracks) placed on the top of the towing tank. Then, a series of trials are conducted wherein the current meter is towed at different constant velocities. For each trial, the constant velocity of the carriage is recorded, as well as the revolutions per second (rev/s) of the current meter. This data is plotted on rectangular coordinate graph paper to verify that a straight-line

an example of a velocity rating based on the rating equation for one particular current meter:

. . relation exists; then, the equation is determined by regression analysis. Table 3.1 is

. Velocity (m/sl = 0.665 (revls) + 0.009

49

- I -----I--

I I

I I Table 3.1. Velocity Rating for an Example Current Meter

.

L

*

50