The prodigious rainmaker that hit Canterbury earlier this month saw NIWA field teams out in the elements collecting flood data from bridges, cableways and jetboat gaugings. NIWA's forecasting services team covered the event closely, relaying the dramatic rainfall figures and responding to a torrent of media enquiries.

The weather event was caused by a concentrated flume of water coming down from the north in what’s known as an atmospheric river. It caused a state of emergency, closed roads throughout Canterbury and had Metservice issue its second-ever Red Warning.

Christchurch-based principal environmental monitoring technician Marty Flanagan says NIWA's climate and hydrometric stations in the Canterbury region coped well, with minimal structural damage and data loss.

Because the event was so well forecast, the Christchurch and Tekapo field teams were ready to go when the rain started falling. They went about the region performing observations and flow gauging measurements to capture valuable field data.

"This included flood gauging the Selwyn River at Whitecliffs, Hurunui River at Mandamus, and Hakataramea River, and maintaining the dense network of hydrometric monitoring sites NIWA operates for Christchurch City Council (CCC) which provided real-time rainfall and river level data to help the council's flood response management teams.

"We also responded to a request from Environment Canterbury (ECan) to obtain flood measurements at the Ashburton River at SH1 during the peak of the event."

"The field teams used several different methods to safely obtain flood data in these challenging high-event conditions. The acoustic doppler current profilers and handheld surface velocity radar sensors deployed in these methods provide quality, high-frequency data which will enable a more accurate estimate to quantify these extreme flow records," says Marty.     

The NIWA supercomputer-powered river forecasting system, which predicts river flows as above or below ‘normal’ river flow, also received plenty of airtime in the media. Underpinning the river forecasting system is NIWA's Water Model.

Hydrological scientist Dr Céline Cattoën-Gilbert - who leads the flow forecasting project - says while we’ll have to wait for flood data from ECan to fully evaluate how well the model performed, on first look it appeared to perform well.

"The model showed the worst affected rivers were Ashburton and Hinds, which media reports seem to corroborate. The timing of the flood peak in the model also seemed to match the expectations with the flooding beginning to recede from Monday afternoon."

Learn more about NIWA's River flow forecasting.