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–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– InductanceV = a + bN

where V = flow velocity; a = starting velocity to overcome

mechanical friction; b = equipment calibration constant; N = revolutions/sec

Current Meters

Measuring Velocity

• Pitot Tubes: Suitable only for clean water

• Floats: Suitable for straight channel, V = L/T .

Velocity Distributions

Vertical Velocity Distribution

Mean Flow Velocity Estimation

• Mean Velocity

Depth < 0.6m 0.6dVV ; 0.6 water depth from the water surface

2mDepth0.6m 2

0.8dV0.2dVV

2mDepth 4

0.8dV0.6d2V0.2dVV

Mean-section Method

qx V x V x1

2

dx dx1

2

Wx

qx V xWx 1 Wx1

2

dx

Mid-section Method

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

Stream Gauges

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

21

3249.1

fh SARn

Q

Measurement of Stream Flow

(d) Area-Slope Method

By Manning formula:

where

K1 K 2 if flow cross -section is not uniform, minor loss should be subtracted from h L.

21

Lh K2

1

fS K QL

21,i ,32

iRiAin

1iK

21

)2K 1(KK