Improving the accuracy of Acoustic Doppler Current Profiler measurements in small channels

Andrew Willsman - NZ NIWA , 22 August, 2008

Since 2002 the national hydrometric monitoring agency in New Zealand (NIWA) has
been using small Acoustic Doppler Current Profilers (ADCPs) to measure flow in open
channels. ADCPs measure thousands of water velocities and depths, so can potentially
increase the accuracy of flow measurements compared to the mechanical current meter
method. These benefits are recognized by most hydrographers in NIWA and many use
ADCPs when they are available.

Some hydrographers are reluctant to use an ADCP in a small channel due to the
complexity of the ADCP instrumentation and the requirements to setup and use a
continuous loop to tow the ADCP across the transect. Another apparent barrier is the
extra time involved to complete an ADCP measurement compared to a single velocity
point current meter gauging. These apparent barriers were considered and we have
adopted practices, training, and simple technology to encourage all staff to use ADCPs in
small channels.

One solution was the development of a lightweight tagline traveller to tow the ADCP
across the measurement transect. This device enables smooth, speed adjustable transect
float towing which helps the hydrographer to regularly achieve the NIWA standards for
ADCP flow measurement. Towing the ADCP with the traveller rather than with a
continuous loop means many small channels may be measured in less than 30 minutes.
Results from two channels will be presented illustrating flow measurement accuracy and
time efficiency gains using the Teledyne RDInstruments StreamPro ADCP with the
traveller. A comparison is also presented of low flow measurements using this
methodology against a calibrated V notch weir to illustrate how it extends the
StreamPro’s low flow capability.