Determinationof velocity and discharge using floatsTheory if a flow meter isnot available or a rough estimate is adequate you can measure flow by using afloat.
The float can be any buoyant object such as an orange or a partiallyfilled plastic water bottle. Its needs to be heavy enough so that about an inchof it is below the water line. Measure off at least 50 feet along the bank of a straightsection of stream if foible string a rope across each end of the 50-foot lengthDischargeThe amount of water passing a point on the stream channel during a given timeis a function of velocity and cross-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 the float methodof measuring velocity is by observing the time for a floating body to traversea known length and noting its position in the 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 (feet) x Average depth(feet) = area (ft2) 2.
Free the float at the upstream site Using a stopwatchrecord the time it takes to reach the downstream tape (If the float moves toofast for correct measurement measure off 75 or 100 feet instead of 50) restate themeasurement two more times for a total of three measurements.3. Calculate the velocity as distance traveled divided bythe average amount of the it tookthe float to travel the distance roped off is 50feet and the orange took an average of 100 seconds to get there thevelocity is 0.5ftlsec 50 f =0.5ft/sec 100sec 4: Correct for the surfaceversus mid-depth velocity by multiplying the surface velocityby 0.
85=0.43ft/sec 5: Calculate the discharge in cubic feet per second (cfs) by multiplyingvelocity (ft/sec) by thecross-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 andare therefore emphasized here. However, you also can put a staff gage in alake to monitor changes in lake water level.