The process involved in the float method of measuring velocity is by observing the time for a floating body to traverse a known length and noting its position in the channel. The floating body may be specially designed surface float, subsurface float, or any selected piece of drift floating with the current.
The surface float moves with the same velocity as the surface of the water. A subsurface float consists of a submerged float attached to a surface float by an adjustable line and it measures directly the mean velocity. In the absence of a fabricated type of float, a distinct floating body such as a water lily may be used as float during observation.
The float method is used especially in the measurement of flood discharge where excessive velocities, depths and floating debris prohibit the use of other methods. The following are favorable conditions for float measurements:
The river channel must be straight with more or less uniform cross-section along the entire reach.
The length of the reach must be from 50 m to 100 m.
There must be steady flow along the entire reach.
The stream channel should have no under-water surface obstructions that would disburb the flow of water, and consequently the stream velocity.
The following procedure outlines the methods to follow in making discharge measurements:
Reconnaissance is conducted along a segment of the stream to find a convenient site of observation.
When a favorable site is selected, the selected length of reach is measured, say 50 meters, marking the ends with a stake.
A range line through each stake is constructed. Both range lines are made perpendicular to the axis of the stream by using the 3–4–5 triangle method of erecting perpendiculars, or by the compass or other means.
The width of the stream at the upper and lower ends are measured with a tape if the stream is narrow, and by the triangulation method if the stream is wide.
Tag lines, each spanning the stream at the upper end and at the lower end are laid. Similar tags are placed at the first, mid and third quarter points (see illustration on page 71).
The depths are measured at the upper and lower end tag lines.
Members of the survey team do the following:
One observer at the upper end with a signal flag;
One member of the team acts as a recorder;
One observer at the lower end with a stop watch;
One observer further upstream to release the float; and
One member of the team to retrieve the float.
The chief of the survey team signals for the beginning of observations. The float is then released further upstream. Once the float passes the upper end tag line, upper end observer drips down his flag. The lower end observer receives the signal by taking note of the time with the stop watch.
When the float passes at the lower end tag line, the lower end observer again takes note of the time the float has travelled and then recorded. The float is retrieved.
The above procedure is being repeated at the first, mid and third quarter points.
Based on the data gathered, stream velocity and the rate of discharge can be computed:
 | ||
| Where: | v | = Stream velocity |
| d | = distance of float travelled (m) | |
| Q | = rate of discharge | |
| A | = [Width of the river (m)] × [average depth of water (m)] | |
| t | = time (minutes) | |
Example: A wooden float travelled 50 m distance for 30 minutes. Considering that the river is 20 m wide with 1.0 m average depth of water. Calculate the water velocity and rate of discharge of the river:
Solution:
| v | = |  |
| Q | = | (1.66 m/min) (20 m) (1.0 m) |
| = | 33.20 cu.m/min. |
Illustration: Measuring Stream Velocity by Floats and Determination of Discharge
