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.

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 (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.