22-10-11-Flow Measurement in Open Channels

7/28/2019 22-10-11-Flow Measurement in Open Channels

1/45

22-10-2011 WALAMTARI, HYDERABAD 1

FLOW MEASUREMENTIN OPEN CHANNELS

B.LAKSHMANARAO, B.Tech., M.B.A., F.I.E,Joint Secretary(Tech) to Govt. andChief Engineer, Srisailam Project, HyderabadCell: 9177908811E-mail: [email protected]


2/45

Water measurement structures

The principal objective of measuring irrigation water is to permitefficient distribution and application. By measuring the flow ofwater, a farmer knows how much water is applied during eachirrigation.

In irrigation schemes where water costs are charged to the farmer,water measurement provides a basis for estimating water charges.

The most commonly used water measuring structures are weirs andflumes. In these structures, the water depth is read on a scalewhich is part of the structure. Using this reading, the flow-rate isthen computed from standard formulas or obtained from standardtables prepared specially for the structure.

22-10-2011 2WALAMTARI, HYDERABAD


3/45



4/45

COMPUTATION OF VELOCITY

CHEZYS FORMULA:

V = C (RS )0.5

MANNINGS FORMULA

V = 1/n* R 2/3 * S 1/2

HAZENWILLIAMS FORMULA:

V = 0.85*C1*R0.63*S0.54



5/45

Mannings Equation



6/45

TRAPEZOIDAL WEIR



7/45

TRAPEZOIDAL FLUME



8/45

d = the liquid depth

b = the bottom width of the channel

P = the wetted length measured along thesloped side

m = the angle of the sloped side from vertical.

The side slope also often specified ashoriz:vert = z:1.



9/45

TRIANGULAR WEIR



10/45

TRIANGULAR FLUME



11/45

B ,the surface width of the liquid in ft for U.S.

& m for S.I. units

, the sloped length of the triangle side inft

for U.S. & m for S.I. units

y, the liquid depth measured from the vertex

of the triangle in ft for U.S. & m for S.I. units

z. the side slope, typically specified in the

form: horiz:vert = z:1.



12/45

FLOAT VELOCITY ESTIMATION



13/45

Release the float at the upstream site. Using astopwatch, record the time it takes to reachthe downstream tape. (If the float moves too

fast for an accurate measurement, measureoff 25 or 30 m instead of 15m). Repeat themeasurement two more times for a total ofthree measurements. The average velocities

should be multiplies with the coefficientsshown on next slide.



14/45

Average Depth-cm

30.5 -

61.0 -

91.5 -

122.0 -

152.5 -

Coefficient

0.66

0.68

0.70

0.72

0.74



15/45

PARSHALL FLUME

The Parshall flume consists of a metal or

concrete channel structure with three main

sections: (1) a converging section at the

upstream end, leading to (2) a constricted or

throat section and (3) a diverging section at

the downstream end.



16/45



17/45

CUT THROAT FLUME



18/45

CUT THROAT FLUME

The cut-throat flume is similar to the Parshall

flume, but has no throat section, only

converging and diverging sections . Unlike the

Parshall flume, the cut-throat flume has a flatbottom. Because it is easier to construct and

install, the cut-throat flume is often preferred

to the Parshall flume.



19/45

CANAL SECTION IN EMBANKMENT



20/45

FLUMES AND WEIRS

1

Weir

Flume

Y1 Y2 Headloss = Y2 - Y1

Head

Headloss



21/45

21

WeirCrest

Free Flow

Note: Air poc ket behind c rest

WeirCrest Note: Downs tream water

level above crest

Submerged

22-10-2011 WALAMTARI, HYDERABAD


22/45

22

90H

1

4 H

L

H

L

TRIANGULAR (90 V-NOTCH) TRAPEZOIDAL (CIPOLLETTI) RECTANGULAR

Types of Weirs



23/45

23

RBC Flumes



24/45



25/45

25

6:1 downstream ramp

6:1 downstream truncated ramp

FLOW

FLOW

RBC Flume - Ramp Recommendation



26/45

26

4-in Diameter Pipe

Install flush to concrete on both ends and flush to the bottom of the canal.

RBC Flume - Flush Pipe



27/45

27

WINFLUME

Albert J. Clemmens

Tony L. WahlMarinus G. Bos

John A. Replogle



28/45

28

Upstream of the

weir, the flow

velocity is

decreased so that

the Froude

number is less

than 0.5

Flow is divided by a

fixed partition

board22-10-2011 WALAMTARI, HYDERABAD


29/45

29

Advantages:

Provided that critical flow occurs in the throat, a

rating table can be calculated with an error of less than2% in the listed discharge. The calculation can be madefor any combination of a prismatic throat and anarbitrarily shaped approach channel.

The throat, perpendicular to the direction of flow, canbe shaped in such a way that the complete range ofdischarges can be measured accurately.

The required head loss over the weir or flume is

minimal to ensure a unique relationship between theupstream sill-referenced head, h1, and the discharge,Q.



30/45

30

Advantages (continued):

This head-loss requirement can be estimated with

sufficient accuracy for any of these structures placed in anarbitrary channel.

Because of their gradual converging transition, these

structures have little problem with floating debris.

Field observations and laboratory tests have shown that

these structures can be designed to pass sediment

transported by channels with subcritical flow. However,

sedimentation can be a problem when sediment loads areexcessively high or when the flume causes a significant

velocity reduction in the approach channel.



31/45

31

Advantages (continued):

Provided that the throat is horizontal in the direction of flow, a

rating table can be computed using post-constructiondimensions. Thus, an accurate rating table can be producedeven if the flume is not constructed to the designeddimensions. The throat may also be reshaped according tochanging site conditions, and a new rating table can be

computed using the modified dimensions.Under similar hydraulic and other boundary conditions these

are usually the most economical of all structures for accuratelymeasuring open channel flows, provided that conditions are

such that a weir or flume is feasible



32/45

32

Control

Sectionhorizontal

H1 h1

3

1

L

weir

0.07 H1/ L 0.7



33/45

33

Approach

channel

An approach channel that is necessary forthe development of uniform and symmetricflow conditions and the establishment of astable water surface whose elevation can be

determined accurately



34/45

34

A converging transition section in which the subcritical approach flow accelerates

smoothly toward the throat with no discontinuities or flow separation. The

transition may consist of plane surfaces or may be rounded

Converging transition



35/45

35

A diverging transition in which the velocity of the supercritical flow

exiting the throat section is reduced and energy is dissipated or

partially recovered

Diverging transition



36/45

36

Weir crest Tailwater channel

16

Recommended transitions22-10-2011 WALAMTARI, HYDERABAD


37/45

37

WINFLUME WINDOW SHOWING THE ACCEPTANCE OF DESIGN



38/45



39/45



40/45



41/45



42/45

42

Cipolletti Weir



43/45



44/45



45/45

45

THANK YOU

Documents

22-10-11-Flow Measurement in Open Channels