Measurement of Stream Discharge by Wading Water Resources Investigations Report 00-4036 By K. M. Nolan and R. R. Shields
Measurement of Stream Discharge by WadingWater Resources Investigations Report 00-4036
By K. M. Nolan and R. R. Shields
Reference materials
Agenda
•The velocity-area concept
•Making the measurement
•Measuring width
•Measuring depth
•Wading rod use
•Measuring velocity
•Site selection
x Water Velocity
Cross section area
Discharge = (Area of water in cross section) x (Water velocity)
THE VELOCITY-AREA METHOD
Discharge of each sub-section = Area x Average Water Velocity
Channel cross section is divided into
numerous sub sections
Width
Depth
Area = Width x Depth
Area of each sub-section determined by directly
measuring width and depth
Water velocity in each sub-section
is estimated using a current meter
to measure water velocity at
selected locations
Total Discharge = ((Area1 x Velocity 1) + (Area2 x Velocity2) + ….. (Arean x Velocityn))
1 2 3
Stream discharge is sum of discharges in
all sub-sections
n
Computing width and depth of individual
subsections
• Example of computing
sub-section width
• Assume the following
measurements applied
• Starting point - 0 feet
• Next point - 1 feet
• Next point - 3 feet
• Next point - 5 feet
0 1 3 5
Meter locations
Recording Width
• Width at point 0 = (1 - 0)/2 = 0.5
• Width at point 1 - (3 - 0)/2 = 1.5
• Width at point 3 = (5 - 1)/2 = 2.0
0 1 3 5
1.5
2
Meter locations (ft.)
Widths
0.5
?
Recording Width and Depth
0 1 3 5
1.5
2
Meter locations
Widths
0.5
?
Depth, in this
example, is
zero, therefore
no flow
Note: See table from Rantz
(1982) for help estimating
velocity when edge of water is
vertical.
Measuring Cross-Section Area
Width
Measuring Cross-Section Area
Depth
• Wading
rod is
marked
every
0.1 foot
Wading Rod Use• Stand beside and downstream of rod
10
8
6
4
2
0
2
6
5
4
3
7
8
0.6
Depth2.6 ft.
Total
Depth
0.8
Depth
0.2 depth
Setting
RodDepth
Scale
Rod set so
meter placed
at 0.6 (2.6 ft)
2.08 ft.
0.52 ft.
1.04 ft.
Wading
Rod Use
(cont.)
Wading Rod Use (cont.) • Must estimate depth when velocity
causes “pile-up” on rod.
• Visually extend water surface to
rod.
Velocity Determination
• USGS generally uses
Price current meters
• AA for large depths
• Pygmy for shallow
depthsStandard AA Meter
Pygmy Meter
See OSW memos 85.07
and 85.14
Meter Depth
range
(ft.)
Recommended
velocity range
(ft/s)
Price AA > 1.5 0.10 - 12
Pygmy 0.3 – 1.5 0.5 - 12
Meter Use
Headset and Stopwatch
• Velocity is determined
by placing meter in
stream and counting
number of revolutions
in a measured amount
of time
Stop Watch and Headset
Measure velocity for at least 40
seconds
• Velocity should be measured for at least 40 seconds
• Evens out short-term velocity fluctuations
Marking Meter• Meter revolutions can
sometimes be counted
manually by marking
one meter cup.
Rating Table
• Velocity can
then be
determined
using a
current-meter
rating table
Partial listing of standard rating #2
Rating Table Equations
• Equations for standard Rating Tables:
For AA meter
• V = 2.2048 R + 0.0178
For pygmy meter
• V = 0.9604 R + 0.0312
R = Revolutions per second
See:
•OSW Memo 99.05
Velocity from Digitizer, Aquacalc,
and DMX• Current meter digitizers
Aquacalcs and DMX
units have equations for
rating table built in.
• These devices provide
direct computation of
velocity
Average Velocity
• The goal is to represent the
average velocity in the
vertical
• Measured at 0.6 the
depth when depths are
shallow
• Measured at 0.2 and
0.8 the depth when
depths are large. These
two velocities are
averaged to represent
average velocity in the
vertical
Typical velocity profile
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5
Velocity, in feet per second
Dis
tan
ce b
elo
w w
ate
r s
urfa
ce i
n
percen
tag
e o
f to
tal
dep
th
10
8
6
4
2
0
2
6
5
4
3
7
8
0.6
Depth2.6 ft.
Total
Depth
0.8
Depth
0.2 depth
Setting
RodDepth
Scale
Rod set so
meter placed
at 0.6 (2.6 ft)
2.08 ft.
0.52 ft.
1.04 ft.
Wading
Rod Use.
Velocity Measurement Methods
Guide to velocity-measurement methodsDepth, in ft. Meter Velocity Method
2.5 ft and above Type AA 0.2 and 0.8
1.5 – 2.5 Type AA 0.6
0.3 – 1.5 Pygmy 0.6
1.5 ft and above Pygmy 0.2 and 0.8
Non Standard Conditions
•Use of 0.6 and 0.2/0.8 methods assume velocity profile is
logarithmic.
•Velocity should decrease closer to bottom due to friction
•If velocity at 0.8 depth is greater than velocity at 0.2 depth
or if velocity at 0.2 depth is twice the velocity at 0.8 depth
then the velocity profile is considered abnormal and the
three-point method must be used.
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5
Velocity, in feet per second
Dis
tan
ce b
elo
w w
ate
r s
urfa
ce i
n
percen
tag
e o
f to
tal
dep
th
Three-Point Method
• Three-point method
computed by
averaging velocity
measured at 0.2 and
0.8 the depth and
averaging that result
with velocity
measured at 0.6 the
depth.
Site Selection
•Reach should be straight
and uniform for a long
enough distance to provide
uniform flow through the
measuring section
•Streambed should be stable
and free of large rocks,
weeds, and protruding
obstructions.Upstream view of excellent
measuring section, Little Blackfoot
River
Assessing Measurement
• Assessing accuracy of measurement
•Semi-quantitative based upon:
1. Cross section uniformity
2. Velocity uniformity
3. Stream bed conditions
4. Etc.
See “Determination of Error in
Individual Discharge
Measurements” by Sauer and
Meyer announced in OSW
memo 93.14
Blackfoot measuring section
Assessing Measurement (cont.)
Measurement
Rating
Percent off from true
discharge
Excellent Within 2
Good Within 5
Fair Within 8
Poor Greater than 8
Selected References• Buchanan, T.J., and Somers, W.P., 1969, Discharge measurements at gaging
stations: USGS Techniques of Water-Resources Investigations, Book 3, Chapter
A8, 65 p. (In revision)
• International Organization for Standardization, 1983, Measurement of liquid
flow in open channels, Handbook 16, 518 p.
• Nolan, K.M. and others, Surface-water field techniques training class, USGS
WRIR 98-4252, (http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods)
• Rantz, S.E., 1982, Measurement and Computation of Streamflow:Volumes I and
II, USGS Water Supply Paper 2175, 631 p.
• V.B.Sauer and R.W. Meyer, Determination of Error in Individual Discharge
Measurements”, USGS Open-file report 92-144
Charlie Peterson
Spokane County
Conservation District
210 North Havana Street
Spokane, WA 99202
(509) 535-7274
http://www.sccd.org
For more information: