Opening the sandbar that separates the river from the ocean is a method of reducing the volume of water in the river and reducing the impact of flooding in some areas under some conditions.

However, there are many factors that come into play and if the right procedure is not followed it can cause more flooding.

A flood level of three metres at the Nowra bridges is the trigger to open river entrance at Shoalhaven Heads.

However the river water level at the entrance, the ocean level, the tide, swells and operator safety all help determine the most appropriate time to make the breach.

If the ocean level is higher than the river level a breach would likely cause more flooding. That was the case when the trigger point was reached on Wednesday.

The first flood warning was issued at 11pm on Tuesday. It was a prediction of 2.5 metres, which would not trigger an opening.

Shoalhaven City Council acting environmental services manager Isabelle Ghetti was on deck monitoring the levels throughout the night.

“The last update in the night was 3am Wednesday, by 4am we made the call with council staff to open the entrance,” she said.

“However in the early morning it was high tide with big swells. This was about 7.30am. The predicted trigger time was about 9am.

“We went out there and our safety assessment found high tide was too risky for the operator to make the breach.

“Later that morning as the tide was falling, about 10am, the call was made to send out the machinery, and the breach was cut out at 1pm,” she said.

Shoalhaven City Council general manager Russ Pigg said a range of factors played a big part in when the river could be opened.

“Every flood is different, there’s a difference in timing, a difference in rate of river rises in the different part of the catchment. Timing is important because if we don’t get early enough notification and miss the window and get stuck at night time, safety becomes a big issue.

“There are limitations. Opening the river is not going to eliminate flooding, and there are certain conditions where we can’t open it.

“People ask why we didn’t open it the day before. 

“The day before up until lunchtime the river was just normal and there was no information coming out to signal any minor flooding let alone moderate flooding. There was just no trigger, no indication we should get machinery on site the day before.”