Properties of Water - Freshwater Ecology: Concepts and Environmental Applications - page 22

METHOD 2.1.
Methods Used to Measure Water Velocity

The simplest way to measure water velocity on a large scale is to place an
object that barely floats into moving water and measure the amount of
time the object takes to move a known distance. For example, an orange
is often used because it floats just at the surface of the water and is a bright
visible color. Experiments in my limnology classes confirm that apples
move at the same velocity as oranges.
For flows in water 5 cm deep, propellers are often used to estimate

velocity. The more rapidly the water is moving, the more rapidly a pro-
peller spins. Given an electronic method to count the revolutions per unit
time of a propeller and suitable calibration constants, water velocity can
be estimated. Electromagnetic flow meters measure the electrical current
that is induced when a conductor is moved through a magnetic field. Be-
cause water is a conductor, a flow meter can be constructed to create a
magnetic field, and the electrical current increases proportionally as water
velocity increases.
For smaller scale (several centimeters or less) measurements of water
velocity, other methods are more useful. Very small particles or dye can be
suspended in flow, and the movement can be timed along a known dis-
tance. Particle movement can be measured by photographing the moving

B

99% of open
channel velocity

A

Water velocity

Solid surface

Zero
velocity

Open channel
velocity

FIGURE 2.7 The concept of a flow boundary layer. (A) Arrows represent the velocity and
direction of water flow. Inside the flow boundary layer, flow is approximately laminar and
slows near the surface; outside the layer, turbulence increases. (B) The outer region of the flow
boundary layer is where velocity is 99% of that in the open channel. Very close to the solid
surface, water velocity approaches zero (modified from Vogel, 1994).