From Table 1, the flow rate is 1,005 gpm. If the pipe is flowing only partially full, find the ratio of the unfilled portion of pipe to the diameter of the pipe to estimate flow rate in gallons per minute, as shown in Table 2. The measured distance U is 2 inches. The flow rate is 825 gpm. The flow rate in a stream or channel can be measured as shown in Figure 2 by timing a float.
Flow rate is equal to the cross-sectional area times the velocity.
Multiply the average cross-sectional area times the average stream velocity in fps to get the rate of flow in cfs. Estimating the cross-sectional area is the hard part. A simple way to do it is to measure the bottom width of the channel and the top width, then average the two. Multiply this average times the depth of the water. A float traveled 100 feet in 33 seconds.