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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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Presentation Transcript
measurement approaches
Measurement Approaches
  • Direct methods (stream gaging)
    • Current-meter method
    • Floats
    • Tracer-dilution technique
    • Ultrasonic method
    • Electromagnetic method
measurement approaches4
Measurement Approaches
  • Hydraulic Devices
    • Weirs and notches
    • Orifices
    • Flumes
  • Indirect Techniques
    • Slope-area method
    • Rating Curve
definition of terms
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
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 velocity8
Measuring Velocity
  • Pitot Tubes: Suitable only for clean water
  • Floats: Suitable for straight channel, V = L/T
  • .
mean flow velocity estimation

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
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
Storage Hysterisis
  • In natural rivers, the H-Q relationship in general appears to be a loop, rather than single-valued.
simple rating
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
Use of Manning Equation
  • Stage is also related to the outflow via a relationship such as Manning's equation
measurement of stream flow

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