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
Top Related