Flood flows on South Island's Buller River in July were the largest in New Zealand in almost 100 years, the National Institute of Water and Atmospheric Research (NIWA) announced on Thursday. Flow rates on the river during the flooding on the West Coast earlier this month were more than 10 times the usual rates, scientists found.
As the bridges were being closed, the measurements showed a flow of 7,640 cubic meters per second, the largest direct measurement of river flow ever completed in New Zealand.
NIWA environmental monitoring technician Mike O'Driscoll used a radar gun from the Westport and Orowaiti bridges to measure the speed of the Buller River at its surface on July 17. These measurements, coupled with surveys of the riverbed level carried out from a jetboat after the flood, provided the data for flood flow calculations at the bridges.
The monitoring station is part of NIWA's national network of benchmark hydrometric monitoring stations as well as being a key flood forecasting site for the West Coast Regional Council. Because of its importance, the site has three independent monitoring systems in place - one sensor system to provide rapid updates during floods, plus primary and back-up sensors to record data for future analysis. Niwa hydrodynamics scientist Richard Measures said records showed even greater flooding of the Buller River in 1926.