Water Flow Instruments
Water flow instruments range from small acoustic doppler current profiling devices or small prop type meters for measuring very low flows in small streams, up to NIWA's flood-gauging POEM.
NIWA uses, and can usually supply several different types of water flow measuring instrument:
Acoustic Doppler instruments - These are becoming increasingly popular and some variants, for example the Acoustic Doppler Current Profiler (ADCP) may ultimately supercede the older mechanical open channel meters that have, over the past few decades, become NIWA's standard instrument for gauging open channel waterways in New Zealand and the Pacific Islands. Others are more suitable for monitoring water flow in pipes.
Propellor current meters - these are the older mechanical meters and still the standard in New Zealand. These have been used to gauge rivers for decades, and despite being mechanical, are reasonably robust and reliable but, unlike the ADCP, need to be calibrated annually. Most have two or three different propellors to extend the range from low to higher flows. Unlike the acoustic doppler devices and POEM, propellor current meters can't measure turbulence.
POEM (Pressure Operated Electronic Meter) - The POEM is used for gauging flooded rivers or measuring flows from the spillways at hydroelectric schemes. Read about how a POEM was used to gauge the flow of water entering the Lake Pukaki canal on the Waitaki hydroelectric scheme POEM has no moving parts and, in addition to measuring flow up to 10m/s, can simultaneously provide useful information about turbulence.
Flow Stations - These are usually a permanent structure located at a carefully selected, or constructed, stable site. They measure water level and convert this to water flow. The conversion is usually done with a rating derived from gauging the cross section of the open channel beside the station, or using the relationship associated with a weir constructed downstream.
Read about NIWA's current meter calibration service and flow stations in this article.