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 streem one of these ends

using total stream width and average depth.

Total width (feet) x Average depth

(feet) = 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 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 nothing more

than a long ruler placed semi-permanently in a

stream or lake and used to

read water 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.