then multiplying the cross-sectional area of the stream by the velocity to obtain the rate
of flow. The formula used in this determination is:
Q = .85 AV;
Q = quantity of water in cubic feet per second (ft3/sec)
A = the cross-sectional area of the stream in square feet, and
V = the velocity in feet per second (ft/sec).
The flow in ft3/sec may be converted to gallons per minute (gpm) by multiplying by
(1 ft3 = 7.48 gal x 60 sec = 448.8 gpm).
A commonly used formula is a = 6.4 x A x V. The value 6.4 is a multiplication of the
factor .85 by the conversion factor of 7.48 gal per ft3. The procedure is as follows:
(a) Mark off a measured course where the stream is uniform in depth,
width, and grade. If possible, a straight segment of the stream should be used. Drop a
cork, small stick, or other Iight object into the center of the stream and record the time in
seconds required to float between two points. Divide the distance traveled (in feet) by
the time (in seconds) to obtain the velocity. Since the velocity at the surface of a stream
is greater than the average velocity, the factor .85 used in the formula converts the
surface velocity to average velocity. The deeper the stream, the velocity approximation
is more accurate.
(b) Determine the cross-sectional area of the stream by measuring the
depth at several points in order to construct a profile (see figure 5-4). The area of the
cross section may then be computed using any of the common geometric formulas for
Figure 5-4. Determining cross-sectional areas.