With the weather events battering our region, many people can now add “amateur hydrologist” to their CV.
But how exactly do the professionals measure those river flows that have been capturing our attention, and how vulnerable are those systems to the very waters that they are designed to measure?
Responsibility and measuring sites are shared between the Tasman District Council and Earth Science New Zealand – formerly NIWA.
TDC team leader – environmental data, Matt McLarin, says there are several tools used to measure the water level, which is “the raw parameter actually measured in the river” to inform the flow rates.
One method, seen at Woodstock near the bridge, is a stilling tower on the riverbank. A platform hangs over the water and is connected to a vertical pipe that leads down into the water. A float within the pipe moves up and down as the water levels change, and its position is measured.
Like most other methods, the data pinpointing changing water levels is then run through a formula to establish flow.
Matt says that although stilling is one of the most accurate methods, “it is an expensive exercise to construct, and does also leave your measurement infrastructure right in the path of the flooded river”.
He says more modern systems, like that used at Motueka’s Woodman’s Bend since 2000, include “gas bubblers” which measure the back pressure of compressed air pushed down a small tube into the water.
Another more recent addition are the self-contained radar systems that are mounted to bridges or structures above the river surface. Tapawera’s bridge has housed one since 2022.
Earth Science New Zealand environmental monitoring technicians, Hamish Sutton and Fraser Goodmanson, were at the Woodstock site earlier this month, checking the equipment and recalculating the data.
The stilling tower had fared okay in the recent storm aside from some erosion at its base, but the gas bubbler, which acted as a back-up system, had not survived the 11 July flood.
The pair were also doing further tests with a “Surfbee”, which resembles a remote-controlled body board. The device is equipped with sensors that use sound waves to establish river depth and flow.
This data is used to check and recalibrate the “real time” rates, which are far less unreliable in floods.
“The gauging that you get in a big flow are nowhere near the quality you get in a normal flow,” Hamish says.
Fraser explains that to get reliable data of extremes, they need strong data of normal river flows to compare it to.
“The assumptions are based on 70 or 80 years of data and confirmations of those,” he says.
There is also the issue of changing river beds that might “scour out in one section and build up in another”, and trees that are washed away from the riverbanks.
“Trees provide roughness that slow the water down,” Fraser explains.
Matt says that as more information is collected over the years, the understanding and classifications of a “big flood” can also change.
“So in short, description of flood size is a fluid proposition!”
