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

charlie-peterson@sccd.org

Spokane County

Conservation District

210 North Havana Street

Spokane, WA 99202

(509) 535-7274

http://www.sccd.org

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