Flood forecasting model a first for Canterbury

A newly developed flood forecasting system – successfully trialled by Environment Canterbury during the May 2010 floods in South Canterbury’s Temuka district – has been commissioned for full installation this month.

“For the first time we were able to predict the timing and quantity of flooding in the Temuka River during the May floods with a good degree of confidence,” said Ken Taylor, Environment Canterbury’s Director of Investigations and Monitoring.

“The MetService provided an early warning of heavy rain and when the forecast was input into the new flood forecasting system it predicted flood flows in the Temuka River nearly two days before flooding occurred.”

The flood forecasting model has been developed by Environment Canterbury in conjunction with international consultancy DHI Group.

“Until now there has been no reliable way of predicting the size and timing of a flood following a heavy rainfall event in short foothill catchments such as the Temuka,” said Ken Taylor.

Flood events in Canterbury’s foothill catchments can occur within a few hours of heavy rainfall. This may not give enough time for councils and civil defence to give adequate warning, putting local communities, stock and property at significant risk.

“The results from running the flood forecasting system during the May 2010 floods were very encouraging and gave Environment Canterbury staff confidence the model would greatly assist their flood prediction capabilities for the Temuka catchment.

“The flood forecasting system provided an initial forecasted peak flow of 530 cumecs, which was within six percent of the actual peak flow of 564 cumecs. In addition the timing of the actual flood peak was within half an hour of the forecast.”

The flood forecasting system is now being installed at Environment Canterbury’s Timaru offices and will be operational during August.

“The flood forecasting system developed by Environment Canterbury and DHI Group is a sophisticated system which uses inputs of forecast and real-time rainfall data, and provides a prediction of river flow against time,” say Ken Taylor.

“Development has involved a lot of work to refine and calibrate the model to allow for the variability in natural systems and provide forecasts of river flows in Canterbury’s variable foothill river catchments.”

The flood forecasting system updates its prediction every hour or so and provides a flow prediction on a rolling basis. The forecast rainfall – which is estimated for points in the Temuka catchment – is provided by the MetService one to two days in advance.

“It is Environment Canterbury’s job to monitor local rivers, make predictions on likely flooding and – in liaison with police, civil defence and district councils – as well as warning famers and communities so they can take action to protect life, property and stock.

“Getting a flood forecast wrong is always a major risk. If a flood is not forecast, or turns out to be significantly worse than forecast, it can have very serious impacts including loss of life and property.

“If we are overly cautious and predict floods that do not occur, however, people may not take future warnings seriously,” said Ken Taylor.

Development of the flood forecasting system has taken six months and has involved river engineers and hydrologists from Environment Canterbury working with technical experts from DHI Group.

“We had specific criteria for a suitable flood forecasting system, and also reviewed the track records of suppliers and looked at the recommendations and experience of other users,” said Ken Taylor.

“We required a continuous real-time system that could be developed and run using Environment Canterbury’s local expertise while tapping into the best of international knowledge.

“The system we are installing has achieved this and will be run on a day-to-day basis by Environment Canterbury staff with ongoing support from DHI Group,” said Ken Taylor.

The system will run continuously and provide automatic alarms of forecasted flood events to Environment Canterbury’s river engineering staff.

Environment Canterbury is continuing to develop and refine the river flow forecasting system and has plans for implementation in other catchments around Canterbury.