Open channel flows

a. Weirs. Flow in open channels can be measured by weirs constructed in the channel. Certain dimensional relations should be recognized in constructing a weir to obtain the most accurate flow measurements as shown in figure F-9. The weir plate should be a non­corrosive metal about Vi inch thick with the crest l/8 inch wide, and the downstream portion of the plate beveled at 45 degrees, The crest should be smooth, and the plate should be mounted in a vertical plane perpen­dicular to the flow. The channel walls should be

Open channel flows

1.0

Подпись:0.9

0.8

Подпись:0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

U. S. Army Corps of Engineers

Figure F – 7. Effective area factor for partially filled pipe.

Table F-l. Flow (gallons per minute) from Vertical Pipes

ght of Crest H

Nominal

Diameter

of Pipe,

inches

inches

2

3

4

5

6

8

1-1/2

22

43

68

85

110

160

2

26

55

93

120

160

230

3

33

74

130

185

250

385

4

38

88

155

230

320

520

5

44

99

175

270

380

630

6

48

110

190

300

430

730

8

56

125

225

360

510

900

10

62

140

255

400

580

1050

12

69

160

280

440

640

1150

15

78

175

315

500

700

1300

18

85

195

350

540

780

1400

21

93

210

380

595

850

1550

24

100

230

400

640

920

1650

TJ. S. Army Corps of Engineers

Open channel flows

‘J. S. Army Corps of Engineers Figure F-8. Fountain flow measurement.

 

Open channel flows

(Courtesy of Fairbanks Morse. Inc. Pump Division)

 

Figure F-l 1. Plan andelevation of the Parshall measuring flume.

 

smooth and parallel, and extend throughout the region of flow associated with the weir. Complete aeration of the nappe is required for rectangular suppressed weirs. The approach channel should be of uniform sec­tion and of a length at least 15 times the maximum head on the weir. Smooth flow to and over the weir is essential to determination of accurate rates of flow. The head on the weir should be measured with a hook gage located in a stilling box at the side of the ap­proach channel. The communication pipe to the still­ing box should be about 1% inches in diameter and should be flush with the side of the channel. Formulas for calculating the flow over various types of weirs are shown in figure F-10.

b. Parshall flume. Flow in an open channel may also be measured with a Parshall flume (fig. F-ll). The head drop through the flume is measured by two gates (fig. F-ll); but if the depth of water at the lower gage is less than 70 percent of the depth at the upper gage, the flow is termed “free” and the discharge can be de­termined by reading the upstream hook gage alone. The construction and dimensions of a Parshall flume are shown in figure F-ll and table F-2. The free flow discharge of a Parshall flume is given in table F-3.

 

Limiting Dimensions В — ЗА or more Z = ЗЛ or more 4h min. to 10h max.

 

(Courtesy of Fairbanks Morse. Inc., Pump Division)

 

Figure F-9. Rectangular suppressed weir.

 

ppressed,

 

FrancisFRfffettilfioQ -3.33Bh3/2 or the more accurate Rehbock Formula,

 

г rmula.

Q =3.33h*’2 (B – Q.2K)

 

: 13.228 + 0.4359 BA,1/1

 

Q

 

(Courtesy of Fairbanks Morse, Inc, Pump Division)

 

Open channel flowsOpen channel flowsOpen channel flowsOpen channel flows

Table F-2. Dimensions and Capacities, Parshall Flumes.

Width

A

2/э A

В

с

D

Е

F

G

к

N

X

Y

Мах.

Міп.

0’3"

l’G%”

1’04"

1’6"

0’Г

О’ЮА"

Г4"

4’

і’

1"

24"

1"

14"

1.1

0.03

0’6"

2’0iV

Г4Д"

2’0"

1’3%"

1’3%"

2’0"

і’

2’

3"

44"

2"

3"

3.9

0.05

0’9"

2’10%”

I’ll 4"

2’10"

І’З"

1’10%"

2’6"

і’

14′

3"

44"

2"

3"

8.8

0.09

l’O"

4’6"

3’0"

4-4 %"

2’0"

2’94"

З’О"

2′

3′

3"

9"

2"

3"

16.1

0.35

Г6"

4’9"

3’2"

4’ТУа"

2’6"

3’4%"

З’О"

2′

з-

3"

9"

2"

3"

24.6

0.51

2’0"

50"

3’4"

4’10%"

3’0"

ЗИЧ"

З’О"

2′

3′

3"

9"

2"

3"

33.1

0.66

ЗО"

5’6"

3’8"

Б’4%"

4’0"

Б’1%"

З’О"

2′

3′

3"

9"

2"

3"

50.4

0.97

4’0"

6’0"

4’0"

5-10 %"

5’0"

6’44"

З’О"

2′

3′

3"

9"

2"

3"

67.9

1.26

6’0"

6’6"

4’4"

6’4 4"

6’0"

7’6%"

ЗО”

2′

3′

3"

9"

2"

3"

85.6

2.22

6’0"

70"

4’8"

610%"

7‘0"

8’9"

З’О"

2′

3′

3"

9"

2"

3"

103.5

2.63

TO"

TO"

5’0"

T4V."

8’0"

911%"

З’О"

2′

3′

3"

9"

2"

3"

121.4

4.08

8’0"

8’0"

5’4"

7’104"

9’0"

11Т%"

З’О"

2’

3′

3"

9"

2"

3"

139.5

4.62

Throat Free Flow Cuft./Sec.

(Courtesy of Fairbanks Morse, Inc., Pump Division)

Table F-3. Flee-flow Discharge (in cubic feet per second), Parshall Flume.

Open channel flows
Q = 4 W Ha1K2W° °!S

NOTE: Approximate values of flow for heads other than those shown may be found by direct interpolation in the table.

(Courtesy of Fairbanks Morse, Inc., Pump Division)