Determination
of velocity and discharge using floats

Theory
            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.  

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Measure off 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

Discharge
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.
                                                                       Q = AV
where Q is stream discharge (volume/time), A is cross-sectional area, and V is
flow velocity.

Velocity
 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.
                                                                   
V=d/t

1.      Estimate
cross-section area stream one of these ends using total stream width and
average depth.
       Total width (feet) x Average depth
(feet) = area (ft2)
        

2.Free 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 correct measurement measure off 75 or 100 feet instead of 50) restate 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
                   50 f       
=0.5ft/sec  
                  100sec

       4: Correct for the surface
versus mid-depth velocity by multiplying the surface 
            velocity
by 0.85.

                                          
0.5×0.85=0.43ft/sec
       5: 
Calculate the discharge in cubic feet per second (cfs) by multiplying
velocity
            (ft/sec) by the
cross-sectional area (ft2) of the stream.

                                       
0.43ft/sec x 10.73 ft2 =4.62 cfs

           Using of staff gauge
           A staff gage is measuring instrument  like tape measure  used to provide a visual  indication of depth. Stream gages are the most

        general and helpful measure and
are therefore emphasized here. However, you
        also can put a staff gage in a
lake to monitor changes in lake water level.

        

 

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