Quick Dips

Measuring Fast Water

Scientists at the National Institute for Water and Atmospheric Research Ltd (NIWAR) have come up with a new system for measuring water velocity which is not only more efficient than traditional methods, but safer as well.

Traditionally, propeller meters have been used to measure water speed. However, these are difficult to operate at the high water velocities generally found when New Zealand's rivers are in flood, says Dr Richard Ibbitt, leader of NIWAR's Technological Development Section.

A propeller meter is attached to a weighted cable and lowered into a river. The turning of the propeller indicates the water velocity. The new Pressure Operated Electronic Meter (POEM) is used in the same manner but, instead of a propeller, has a Pitot tube commonly used as airspeed indicators in aircraft.

A Pitot tube has two holes in it, one at the end of the tube which faces into the current being measured and the second in the side of the tube. The difference between the pressures measured at each hole gives the water velocity.

The POEM makes measurements much faster than a propeller meter by sensing the pressure electronically. A propeller meter takes around 40 seconds to get one reading, whereas the POEM can make many measurements per second. The new meter can also make continuous measurements as it is lowered, while the propeller meter is lowered to a particular depth and held there. This allows many more point measurements to be made so that river flows can be established more accurately.

Because the POEM is faster, it is safer to use, especially during floods. It also means less time is spent collecting data.

Measurements of water velocity very close to the river bed can also be made using the POEM, as the meter is housed in the weight rather than mounted above it. This means that some of the velocities important to river ecology, which have been difficult to measure in the past, are now easier to measure, says Ibbitt. The meter could be used to gain new information on the finer details of river flow.

"This would throw a lot of light on what is actually happening in the river," says Ibbitt.

Janine Griffin