Environmental Analysis 2012-13
Environmental Analysis 2012-13: Measuring Discharge in a
Creek or River: Little Klickitat River
There are basically two methods of measuring discharge (the amount of water flowing
through a river bed per unit time), measured in the US as ft3 s-1 in Europe as m3 s-1.
1) The average method: Here we average the depth of the creek across its width and then
also average all our velocity measurements. We make the assumption that this will give us an
estimate of the cross-sectional area of the creek and velocity within a reasonable margin of
error.
i) Average (Mean) depth = Sum of depths/No of samples
ii) Average (Mean) velocity = Sum of velocities/No of samples
iii) Cross-sectional area = Average depth x width
iv) Discharge (Q) = Average velocity x area
2) The sum rectangle or velocity area method
(USGS method) USGS site on measuring creek
discharges: We are using a minor adaptation of the
standard method. Here we break the cross-section of
the river into smaller rectangles that we then determine
the individual flows for each rectangle. Finally, we sum
these individual flows.
i) Discharge in a rectangle (Q) = velocity in 1 rectangle
x area of that rectangle
ii) Total discharge (Q) = Sum of all discharges
Current velocity(ft/s) = the reading on the swaffer current meter or similar device
An example is a cross-sectional diagram (USGS).
Site Measurements
To calculate the discharge using both of the methods above we need to take a series of
measurements at the point of interest on the creek/river.
Determining Width and Depth
1) Select a site to measure the rivers width, record it, and divide it into 5-10 equidistant
intervals (no interval should exceed 10 ft).
2) At each interval measure and record the depth.
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Environmental Analysis 2012-13
3) Place your data in the table below and use it to calculate the cross-sectional area of the
stream by both methods.
Determining River Current Velocity
1) At each of the intervals above place the current meter into the stream, with the meter
facing into the current and the operator standing downstream of the current meter. Make sure
that eddies around the operator's legs do not disturb the activity of the current meter.
2) If the depth is less than 60 cm (~24”), read the velocity at 0.4 x D, measured upward from
the stream bed. If the depth is greater than 60 cm, read and record velocities for 0.2 x D and
0.8 x D. The mean velocity is the average of the two velocities.
3) Place your data in the table and use it to calculate the mean velocity or velocity per
rectangle.
Little Klickitat River Discharge
Total river width at point of measurement _________
Distance from river right (ft/in) Distance from river right (ft/in)
where depth was measured where velocity was measured
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Depth (ft)
Vel (ft s-1)
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Environmental Analysis 2012-13
Calculations and Figures
1) Draw the cross section of the river and label the measuring points.
2) Calculate the discharge of the river by both the average and sum rectangle methods.
3) Compile your final data in a table such as the one below.
Little Klickitat River Sample Table
Cross sectional area
(ft2)
Current velocity
(ft s-1)
Discharge
(ft3 s-1)
Method 1
Method 2
Thoughtful question:
The Evergreen State College uses approximately 6.5 million gallons of water per year for all
its operations, how long would we need to stand beside the creek/river to watch 6.5 million
gallons pass by?
To Hand in: (per pair)
A short report comprising of a complete set of your raw data and calculated values for the two
data sets we collected today in ft3/s. Do the calculations by hand (calculator) or using a
spreadsheet program, such as Excel. Present your final data in tabulated form (computer
generated in excel or word) in a form similar to the sample table. To be handed in by
8:45am on Monday 15th Oct outside my office Lab 2 2261.
Handout based on Discharge measurements at gaging stations U.S. Geological Survey,
Techniques of Water-Resources Investigations, Book 3, Chapter A8 by T.J. Buchanan and
W.P. Somers.
Some useful conversion factors for you:
1 inch = 2.54 cm
1 foot = 12 inches
1 cubic foot = 7.48 US gallons
1 cubic meter = 1000 liters
3.7854 liter = 1 US gallons
1 day = 86400 seconds
1 m3 = 35.314 ft3
1 ft = 12 in = 0.3048 m
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Environmental Analysis 2012-13
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