Determination of velocity and discharge using floats
If a flow meter is not available or a rough estimate
is adequate you can measure flow by using a float The float can be any buoyant
object such as an orange or a partially filled plastic water bottle. Its needs
to be heavy enough so that about an inch of it is below the water line.
at least 50 feet along the bank of a straight section of stream if foible
string a rope across each end of the 50-foot length
The amount of water passing a
point on the stream channel during a given time is a function of velocity and
cross-sectional area of the flowing water.
where Q is stream discharge (volume/time), A is cross-sectional area, and V is
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.
1. Estimate cross-section area streem one of these ends
using total stream width and average depth.
Total width (feet) x Average depth
(feet) = area (ft2)
2. Release the
float at the upstream site Using a stopwatch record the time it takes to
reach the downstream tape (If the float moves too fast for an accurate
measurement measure off 75 or 100 feet instead of 50) Repeat the measurement
two more times for a total of three measurements.
3. Calculate the velocity as distance traveled divided by the
average amount of the it took the
float to travel the distance roped off is 50feet and the orange took an average of 100 seconds to get there the
velocity is 0.5ftlsec
4: Correct for the surface versus mid-depth velocity by multiplying the
Calculate the discharge in cubic feet per second (cfs) by multiplying
(ft/sec) by the
cross-sectional area (ft2) of the stream.
0.43ft/sec x 10.73 ft2 =4.62 cfs
A staff gage is nothing more
than a long ruler placed semi-permanently in a
stream or lake and used to
read water depth. Stream gages are the most
common and useful measure and are
therefore emphasized here. However, you
also can put a staff gage in a
lake to monitor changes in lake water level.