Transcript STREAMFLOW and HYDROGRAPH ANALYSIS

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
– Inductance
V=a+b N
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
V=V
; 0.6 water depth from the water surface
0.6d
+V
V
0.2d
0.8d
0.6m  Depth  2m V =
2
Depth  2m
+ 2V
+V
V
0.2d
0.6d
0.8d
V=
4
Mean-section Method
Vx + Vx +1 d x + d x +1 
q x = 
Wx


 2
 2
Mid-section Method
Wx1 + Wx+1 
q x = Vx
dx


2
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
1.49
Q=
ARh S f
n
2
3
1
2
Measurement of Stream Flow
(d) Area-Slope Method
By Manning formula:
1
L 2

h
Q = K S 2 = K 
f
L
1
where
1
K = (K  K ) 2
1
2
2
1
K = A R 3 , i = 1,2
i n i i
i
K1  K 2 if flow cross -section is not uniform, minor loss should
be subtracted from h L.