2-5
RECTANGULAR CHANNELS
Flow in rectangular channels is calculated by the following:
• Determine with the .2, .4, .8 method as described on Page 1-3. For channel widths of six feet or
larger, use the .2, .6, .8 method as described on Page 2-6 for rivers and streams. Velocity units
must be in ft/sec.
• Calculate the cross-sectional area in ft
2
by:
[(Depth of Flow) in. ÷ 12] x [(Channel Width) in. ÷ 12]
• Calculate flow by:
x (Cross-sectional Area)
The result should be a flow rate in ft
3
/sec (CFS). You can convert this to other flow units with the flow
unit conversion multipliers in Table III on page 2-7.
Example:
What is the flow in a channel 24 inches wide with a 10-inch deep flow?
Solution:
• Velocity measured at
.2 = 1.5 ft/sec
.4 = 1.7 ft/sec
.8 = 1.8 ft/sec
• (1.5 + 1.8) ÷ 2 = 1.65 ft/sec
• = (1.65 + 1.7) ÷ 2 = 1.67 ft/sec
• From Table II on Page 2-5, 10 in = 0.83 ft
• Area = 0.83 ft x 2 ft = 1.66 ft
2
• Flow = 1.67 ft
2
/sec x 1.66 ft = 2.77 ft
3
/sec
From Table III on Page 2-7.
.64632 x 2.77ft
3
/sec = 1.7903 MGD
Table II Inch to Feet Conversion
IN 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
FT 0.04 0.08 0.13 0.17 0.21 0.25 0.29 0.33
IN 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00
FT 0.37 0.42 0.46 0.50 0.54 0.58 0.62 0.67
IN 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00
FT 0.71 0.75 0.79 0.83 0.87 0.92 0.96 1.00
U
U
U
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