The Clarence River in flood.
The Clarence River in flood. Contributed

Clarence reached largest recorded flow ever in NSW

DURING river gauging at Grafton on Tuesday, January 29,  NSW hydrographers recorded the largest flow ever measured in NSW - some 1.45 million megalitres* per day flowing down the Clarence River.

"That's roughly 2.5 times the size of Sydney Harbour," NSW Water Commissioner David Harriss said.

He said the accuracy of flood modelling and warnings issued by the Bureau of Meteorology during the current flooding on the North Coast, especially the Clarence River and the inundation of Grafton and downstream townships, was testimony to the importance of timely river flow information.

"The great work of the Bureau of Meteorology (BoM) and that of the SES during times of flood is supported by an equally dedicated band of professionals from the NSW Office of Water (NOW) who ensure river flow data is available," Mr Harriss said.

Accurate and instantaneous river heights and flow information are an essential part of flood forecasting to improve public awareness and safety.

"NOW hydrographers are responsible for the installation and maintenance of a state-wide moni- toring network that feeds river levels and volumes directly to the BoM for the purpose of flood forecasting."

"The quality and reliability of this information has been greatly enhanced over the past decade with the upgrading of many existing river monitoring stations to enable 'real-time' data streaming via the web."

Mr Harriss said that while the monitoring network was critical in delivering timely information, the accuracy and usefulness of the data in flood forecasting still relied upon hydrographers gauging flows in the field.

"With each flood, the shape and size of river channels can change, which means the accuracy of the height and flow information may also alter."

"In order to revise or recalibrate the information coming from the river monitoring network - NOW hydrographers need to physically venture into the floodwaters and take readings of the water levels and flow velocities."

He said that gauging during major flood events, especially those which set records, was essential to improve future flood forecasting.

He said hydrographers from NOW had been actively gauging the current North Coast floods since January 27 in anticipation of what has turned out to be the largest recorded flood on the Clarence River.

NOW hydrographers will continue to undertake river gauging along many North Coast and inland streams over the coming weeks as the flood flows subside.