Water velocity

How fast is the water flowing down the stream? This can vary depending on the amount of water in the stream at any one point in time. It can be affected by drought or heavy rainfall events. You should be able to assess a base ‘normal’.

This Rivers and Us resource is in a downloadable PDF format. Use it as a guide to the scientific methodology for measuring water velocity¹. It also contains discussion and reflection questions.

• Freshwater Monitoring – Water Flow – Velocity

Transcript

VOICEOVER

The current velocity is how fast the water is flowing in a stream, and it’s an important aspect of the overall habitat² assessment.

The current velocity will change depending on the volume³ of water in the stream. After heavy rainfall events, the stream will run faster as the volume of water is larger at that time. You should be able to assess a base normal by regular monitoring and recording of weather⁴ conditions⁵.

The measure of current velocity is distance over time.

In this method, we are measuring surface velocity.

LYN ROGERS

OK, so what we’re doing is we’re measuring the flow of the water through the stream – how fast it’s going – because that makes a difference to the life that’s in the stream and actually to the clarity of the water too.

An orange is an important piece of scientific equipment here, because it’s got the perfect density⁶ for floating at the speed we want it to — you know, it just hangs in the water and it floats down nicely.

VOICEOVER

Start by measuring out a 10 metre⁷ length of the stream reach. It needs to be reasonably straight and free from obstacles and similar width all the way.

A few metres upstream of the beginning of the tape, release an orange or similar object that will float but not harm the environment if you can’t retrieve it. As the orange passes the start of the tape, start the stopwatch and stop when it passes the end of the tape. Don’t forget to catch the orange or object. Repeat this three times and record your data⁸.

And as always, when you come to monitor the flow of your stream again, make sure to use the same testing protocol.

Acknowledgements

Lynnette Rogers
The Fairfield Project
Hannah, Jess and Sam, Waikato Diocesan School for Girls