Determination of velocity and discharge using floatsTheory If a flow meter is not available or a rough estimateis adequate you can measure flow by using a float The float can be any buoyantobject such as an orange or a partially filled plastic water bottle. Its needsto be heavy enough so that about an inch of it is below the water line. Measure offat least 50 feet along the bank of a straight section of stream if foiblestring a rope across each end of the 50-foot lengthDischargeThe amount of water passing apoint on the stream channel during a given time is a function of velocity andcross-sectional area of the flowing water. Q= AVwhere Q is stream discharge (volume/time), A is cross-sectional area, and V isflow velocity.Velocity The process involved in float method ofmeasuring velocity is by observing the time for a floating body to traverse aknown length and noting it position in channel. The floating body may bespecially designed surface float, subsurface float, or any selected piece ofdrift floating with the current. V=d/t1.
Estimatecross-section area stream one of these ends using total stream width andaverage 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 toreach the downstream tape (If the float moves too fast for an accuratemeasurement measure off 75 or 100 feet instead of 50) Repeat the measurementtwo more times for a total of three measurements.3. Calculate the velocity as distance traveled divided by theaverage amount of the it took thefloat to travel the distance roped off is 60feet and the orange took an average of 100 seconds to get there thevelocity is 0.6ftlsec 60 f =0.
6ft/sec 100sec 4: Correct for the surface versus mid-depth velocity by multiplying thesurface velocityby 0.85. 0.6×0.
85=0.51ft/sec 5: Calculate the discharge in cubic feet per second (cfs) by multiplyingvelocity (ft/sec) by thecross-sectional area (ft2) of the stream. 0.51ft/sec x 10.73 ft2 =5.47 cfs Using ofstaff gauge A staff gage ismeasuring instrument like tapemeasure used to provide a visual indication of depth .Streamgages are the most common and useful measure and aretherefore emphasized here.
However, you also can put a staff gage in alake to monitor changes in lake water level.