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close this book A training manual in conducting a workshop in the design, construction, operation, maintenance and repair of hydrams
close this folder Session 3: Water measurement techniques (3 hours)
View the document Handout 3A: Using a Weir
View the document Handout 3B: Using a Weir - diagram
View the document Handout 3C: Weir table
View the document Handout 3C: Weir table - metric
View the document Handout 3D: The float method of measurement

Session 3: Water measurement techniques (3 hours)

Total Time: 3 hours

OBJECTIVE:

By the end of the session, the trainees will be able to accurately measure the flow rate of moving bodies of water using a weir, a bucket and watch, or the float method.

OVERVIEW:

It is important during this session that the trainees gain experience in estimating flow rates and develop skills in measuring flow rates. Three methods of measurement shall be presented: 1) the weir method, 2) the bucket and stop watch method, and 3) the float method. Each method will entail "hands on" work, constructing a weir, channeling the stream, placing stakes in the stream, etc. The findings from these three methods will be compared.

MATERIALS:

Handouts 3A - 3D

 

lumber, nails, approximately 3' of pipe with a sufficient diameter for the expected flow, sheet metal (optional), bottle with cork, or float. Have a set of materials for each team.

TOOLS:

watch with a seconds function, bucket of known capacity, saw, level, tape measure, hammer, pick or mattock, tin snips (optional), have one set of tools for each team.

*TRAINERS NOTE:

1)Since the purpose of the activity is to learn to measure, not build, pre-construction of site levels, weirs is recommended.

 

2) The weir table is provided in both English and metric units;

 

3) the float method has limited applicability. Decide whether or not to spend time conducting the field activity.

 

4) Identity site for field activity ahead of time, ensuring enough locations for small groups or pairs to work independently; stake out distances if necessary.

 

PROCEDURES

NOTES

1.

Discuss the need for water measurement in hydram systems:

 
 

- amount of water delivered

 
 

- amount of water into ram

 

2.

State objectives for the session.

 

3.

Ask participants to approximate amount of water needed for:

 
 

irrigating an average garden

 
 

domestic use

 
 

potable water

 

4.

For each, ask participants to compute amount of water needed to enter the ram given H=10', h=30'.

This problem links and review Session 2.

5.

Distribute the handouts and make a transition to the task of measuring water available.

 

6.

Describe the weir and what it is used for.

 

7.

Describe how to build and install a weir.

A desk top model would work well for this and could substitute for the real exercise if time and facilities aren't available.

8.

Explain how to use the weir table.

 

9.

Go over the example in the handout and make certain everyone feels comfortable with their ability to use the weir table.

 

10.

Describe how to use the bucket and watch method.

Use discretion as to how much detail to go into as this method is used on flows that would be considered infinite with a ram installation. ( float method)

11.

Describe the float method of measurement .

 

12.

Explain steps in determining cross-sectional area of a stream.

 

13.

Explain procedures in determining the velocity of the stream.

 
 

PROCEDURES NOTES

 

14.

Go over the example in the handout.

 

15.

With the trainees, go over the sequence of events involved in the remainder of this session and how much time is left.

 

16.

Divide the trainees into groups of three or four, giving each group an even level of total skills.

 

17.

Proceed to the creek or stream.

 

18.

Locate a section along the creek or stream where the flow is consistent and there is sufficient room for all the groups to work within sight of each other.

 

19.

Have each group select a site which they feel will be easily developed.

 

20.

Have each trainee make--a guess as to flow rate of the creek or stream they are measuring.

 

21.

Note estimates of flow rate.

 

22.

Calculate flow rate by the float method

Use only if time allows and the water source is appropriate.

23.

Select appropriate section of the stream or creek and determine cross-sectional area.

 

24.

Place two stakes in stream at appropriate spots and distance from each other.

 

25.

Place float in mid-stream and measure time it takes for float to travel from one stake to another.

 

26.

Repeat measurement several times and average the flow rate.

 

27.

Note differences between original estimates and measurements of flow rates.

 

28.

From the measurements made, have each group decide on the size of their weir notch.

 

29.

The trainees next construct their weirs and install them in the creek, making certain that the weirs are well supported and sealed against leakage around the bottom and sides.

It may be a good idea to have each group build their weir out of different materials so that the construction techniques can be compared.

30.

After the weirs are constructed, readings should be taken periodically while the water is seeking its new level and while flow rates are being interrupted by the other weir installations. Once the readings become consistent, they should be considered reliable.

 

31.

Using the weirs as partial dams, in stall the short lengths of pipe and seal around them in the same manner that the weirs were sealed.

 

32.

With all the water flowing through the pipe and into the bucket, time how long it takes to fill the bucket. Again readings should not be considered reliable until they are consistent.

 

33.

At this point, review what has been done thus far in the session.

 

34.

Back at the classroom, list the readings from each group and discuss the reasons for the variations. If different materials were used for the weirs, discuss the advantages and disadvantages of each.-

Point out need to measure seasonal variations of water flow

35.

Ask participants which method they would use, given resources at their site.

 

Handout 3A: Using a Weir

A weir may be defined as an overflow structure built across an open channel, usually to measure the rate of flow of water. Weirs are acceptable measuring devices because, for a weir of a specific size and shape (installed under proper conditions) only one depth of water can exist in the upstream pool for a given discharge. The discharge rates are determined by measuring the vertical distance from the crest of the overflow portion of the weir to the water surface in the pool upstream from the crest, and referring to tables which apply to the size and shape of the weir. For standard tables to apply, the weir must have a regular shape, definite dimensions, and be set in a bulkhead and pool of adequate size so the system performs in a standard manner.

Whenever the flow from a creek is too great to be measured in a bucket and yet is small enough to be dammed by a board, the weir method of measurement should be used.

Determine the dimensions to be used for the weir notch. The width of this notch is related to the measurement of the flow rate by the height of the water in the pool formed behind the weir. This height is measured in inches and by using a weir table, the inches can be conversed to gallons per minute. A number of notches of different widths and height can accommodate a stream's flow. A rule of thumb is to make the width of the notch 3 times the height.

From your estimate of the flow of the stream, Look at the weir table and guesstimate what size notch will accommodate your flow. Keep in mind that the whole stream must pass over the notch end that the pool formed behind the weir should become deep enough for you to easily get a decent height measurement, i.e., 2½" vis a vis 1/16". Example: you estimate the stream is flowing at 150 gal/mint If you made a notch 12" wide and 4" high, at full flow this weir would read approximately 290 gal/min. (4"-- 23.936 gal/mint x 12" = 286.89 gal/min). This weir would fit your stream if an actual weir reading of 2½" water height were obtained, it would indicate a flow rate of 11.818 gal/min/inch of notch or 141.8 gal/min (11.818 x 12") for the stream.

Once you have determined the dimension of the notch, cut the notch in the board and place the weir board in the stream making certain that it is kept level and seal off the stream completely. Support it with stakes and large rocks.

Measure 2 feet upstream from the weir board and drive a stake. Using a level, put a mark on the stake even with the top of the weir board. Next, measure down from this mark to the water level, subtract this measurement from the depth of your notch and that will give you the height of the water level above the bottom of the weir notch.

Using the weir table attached, locate the integer on the left hand column and the fraction on the top column. Where these two rows intersect is the amount of gallons per minute flowing past the weir for every inch of width. Next multiply this figure by the width and this gives you the total flow of the creek.

Example:

Water is flowing through a creek three feet wide and about 3 inches deep. It looks like about 30 gallons per minute. After looking at the weir table we decide that a notch 6" wide and 2" deep would probably work. After cutting the notch in a 4 foot 1x6 piece of lumber, the weir board was placed in the stream. Two feet upstream a stake is driven in the water in front of the notch. A level is used to place a mark on the stake level with the top of the weir board. The water level is then measured to be ½" down from this mark.

We now know by subtracting this measurement from the depth of the notch that the water level is 1¼" above the bottom of the notch. Now looking at the weir table we find 1 on the left hand column and ½ on the top row. These two rows meet at 5.46. We multiply this by the width of the notch (6") to find that the flow rate was 32.76 gallons per minute.

Handout 3B: Using a Weir - diagram


Using a "Weir" to measure large quantities of water

Handout 3C: Weir table

Height of water above weir notch in inches

 

0

1/8

1/4

3/7

1/2

5/6

3/4

7/8

0

000

.0748

.374

.673

1.047

1.421

1.945

2.394

1

2.992

3.516

4.114

4.787

5.46

6.134

6.882

7.63

2

8.452

9.2

10.098

10.92

11.818

12.716

13.614

14.586

3

15.484

16.531

17.503

18.55

19.523

20.645

21.692

22.814

4

23.936

25.058

26.18

27.377

28.499

29.696

30.967

32.164

5

33.436

34.707

35.979

37.25

38.522

39.868

41/215

42.561

6

43.908

45.329

46.75

48.171

49.518

51.014

52/435

53.931

7

55.352

56.848

58.344

59.915

61.411

62.982

64.552

66.048

8

67.694

69.265

70.836

72.481

74.127

75.772

77.418

79.064

9

80.784

82.504

84.15

85.87

87.591

89.311

91.032

92.827

10

94 547

96.342

98.138

99.933

101.73

103.6

105.393

107.263

Flow rate per inch of weir notch in gal/min.

Handout 3C: Weir table - metric

 

0

3.2

6.35

9.5

12.7

15.9

19.

22.2

0

0

.283

1.415

2.547

3.963

5.379

7.362

9.062

25.4

11.3

13.3

15.573

18.12

20.668

23.219

26.051

28.882

50.8

31.994

34.825

38.225

41.336

44.735

48.135

51.534

55.214

76.2

58.613

62.577

66.256

70.219

73.902

78.150

82.113

83.332

101.6

90.608

94.855

99.102

103.633

107.88

112.412

117.223

121.754

127

126.57

131.380

136.195

141.007

145.822

150.913

156.016

161.111

152.9

166.21

171.589

176.968

182.347

187.446

193.109

198.488

204.151

177.8

209.53

215.193

220.856

226.803

232.466

238.413

244.356

250.019

203.2

256.25

262.197

268.143

274.37

280.601

286.828

293.059

299.29

228.6

305.8

312.312

318.542

325.053

331.568

338.08

344.594

351.388

254

357.899

364.694

371.493

378.288

385.09

392.169

398.956

406.035

Flow rate per millimeter of weir notch in liters/men,

Handout 3D: The float method of measurement

The float method of measurement is a simple procedure for obtaining a rough estimate of the flow of the stream. It will give a ball park figure for looking at the stream's potential. It should not be used for final determination of the hydram system to be used unless the flow rate needed for the ram is such a small percentage of the stream's total flow that what's taken from the stream, for all practical purposes, amounts to a minimal portion of the stream.

The float method is based upon two aspects of the stream: it's cross-sectional area and the velocity of the stream. The cross-sectional area should be determined at some accessible spot in the stream, preferably in the middle of a straight run. Measure the width (w) of the stream. Then, using a stick, measure the depth at equal intervals across the width of the stream (see figure below). Record the depth at each interval and calculate the average depth (d). Now multiply the width (w) by the average depth (d) to get the cross-sectional area (A).


Figure A

Example: The width of a stream, at the point of making depth measurements, is 4 feet. The average depth is 1.1 feet. Therefore, the cross-sectional area (A) is:








square feet

The stream velocity can be determined by choosing a straight stretch of water at least 30 feet long with the sides approximately parallel and the bed unobstructed by rocks, branches or other obstacles. Mark off points along the stream. On a windless day, place something that floats in midstream, upstream of the first marker. A capped bottle partially filled with water works well because it lies with a portion of the bottle submerged and doesn't just ride the surface of the water. Carefully time the number of seconds it takes the float to pass from the first marker to the second. Repeat this process several times and average the results.

Example: The average time for a float to travel between two markers placed 30 feet apart is 30 seconds. The velocity (V) of the float is therefore:

V = 30 feet

30 seconds

V = 1 foot/second

V = 60 feet/minute

The flow rate of the stream can now be calculated by multiplying the cross-sectional area (A) by the stream velocity (V). The usable flow (F) can then be determined by multiplying the stream flow rate by a fraction representing the portion of the stream flow that you can or want to use.

Example: If you will be using 25% of the stream flow, the usable flow (F) is:

F = A x ¼" x .25

F = 4.4 square feet x 60 feet/minute x .25

F = 66 cubic feet per minute

This flow in cubic feet per minute can then be converted to the appropriate units by multiplying by the correct conversion factor:

cubic feet/min x 7.48 = gallons/min

cubic feet/min x 28.3 = liters/min

SOURCE: Micro-Hydro Power, National Center for Appropriate Technology (1979).