Hydraulic Structures for Discharge Measurement

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# Hydraulic Structures for Discharge Measurement - PowerPoint PPT Presentation

Hydraulic Structures for Discharge Measurement. Flow Measurement. Measurement Approaches. Direct methods (stream gaging) Current-meter method Floats Tracer-dilution technique Ultrasonic method Electromagnetic method. Measurement Approaches. Hydraulic Devices Weirs and notches Orifices

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### Flow Measurement

Measurement Approaches
• Direct methods (stream gaging)
• Current-meter method
• Floats
• Tracer-dilution technique
• Ultrasonic method
• Electromagnetic method
Measurement Approaches
• Hydraulic Devices
• Weirs and notches
• Orifices
• Flumes
• Indirect Techniques
• Slope-area method
• Rating Curve
Definition of Terms
• Stage: Height of water surface above a fixed datum
• Rating: Relationship between stream stage and discharge
• Control: Feature that results in a one-to-one relationship between stage and discharge
Measuring Velocity
• Current meters
• Cup-type (Price and pygmy)
• Propeller type
• Inductance

V = a + bN

where V = flow velocity;

a = starting velocity to overcome mechanical friction;

b = equipment calibration constant;

N = revolutions/sec

Measuring Velocity
• Pitot Tubes: Suitable only for clean water
• Floats: Suitable for straight channel, V = L/T
• .

Depth < 0.6m

; 0.6 water depth

from the water surface

=

V

V

0.6d

+

V

V

0.2d

0.8d

=

£

£

V

0.6m

Depth

2m

2

+

+

V

2V

V

0.2d

0.6d

0.8d

=

³

V

Depth

2m

4

Mean Flow Velocity Estimation
• Mean Velocity
Stage-Discharge Ratings
• If it is not possible to install a weir or flume (large channel), need to develop a stage-discharge rating
• 3 basic types
• Simple stage-discharge
• Control produces one-to-one rating
• Slope-stage-discharge
• Velocity-stage-discharge
Storage Hysterisis
• In natural rivers, the H-Q relationship in general appears to be a loop, rather than single-valued.
Simple Rating
• In general, ratings in natural channels follow a logarithmic shape (power function)
• Rating has an offset at the “point of zero flow”
• The point of zero flow can be determined graphically
Use of Manning Equation
• Stage is also related to the outflow via a relationship such as Manning's equation

By Manning formula:

1

1

æ

ö

h

2

2

L

=

=

Q

K

S

K

ç

÷

f

è

L

ø

where

1

2

=

´

K

(K

K

)

1

2

2

1

3

=

=

K

A

R

,

i

1,

2

i

i

i

n

i

¹

K

K

if flow cross

-

section is not

uniform, minor loss should

1

2

be subtracted from h

L.

Measurement of Stream Flow

(d) Area-Slope Method