Determination be heavy enough so that about an

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 float method of
measuring velocity is by observing the time for a floating body to traverse a
known length and noting it position in 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 (ft) x Average depth (ft)=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 60feet and the orange took an   average of 100 seconds to get there the
velocity is 0.6ftlsec
                   60 f       
=0.6ft/sec  
                  100sec

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

                                           0.6×0.85=0.51ft/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.

           
                            0.51ft/sec x 10.73 ft2 =5.47 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
        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.

 

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