Wanganui 'lucky' thanks to time, tide

If the tide had arrived a few hours later last Tuesday night, flooding would have been much worse in Wanganui.

As it was, floodwaters breached part of the stopbanks in the city's Wanganui East suburb, forcing the evacuation of more than 100 people from homes along Anzac Pde.

But Allan Cook, Horizons Regional Council operations manager, told the Chronicle the city was "lucky" not have suffered greater impact.

"Had the flood peak arrived in the city six hours later, floodwater levels at Kowhai Park would have been about 250mm higher on account of coinciding with a high tide," Mr Cook said.

"While further sandbagging could have been done, it's very questionable whether such work would have held," he said.

He said the 2km length of stopbanks had weathered the flood "pretty well" considering Horizons staff had identified a significant variability in the height of the banks late last year.

That report showed the stopbanks could have been breached in the event of a 20-year flood.

Mr Cook said it was only "judicious" sandbagging and other temporary measures that essentially contained the floodwaters to a smaller area upstream and downstream of the Dublin St Bridge.

He said while a more thorough assessment had still to be done, the only area of real concern was a 25m-long section of the stopbanks that had slumped and cracked.

"Now water levels have receded, we will be undertaking a detailed engineering inspection of the stopbank and we may identify other problems."

Repairs to any slumping or cracking would be carried out as soon as the area dried out and could be completed within two weeks.

But Mr Cook said remedial work would be no more than was required to return the banks to the condition they were in before last week's event.

"More substantial improvements in terms of both height and structural integrity of the stopbanks that are clearly required will be undertaken during the upgrade scheduled for February next year. The banks will continue to protect from only moderate floods in the interim," he said.

Horizons has budgeted $1.045million in the 2013-14 year to upgrade the stopbanks to provide protection from a 50-year flood.

Mr Cook said the stopbanks already in place further downstream in the city's industrial area of Balgownie were never under pressure.

He said because the high tide did not coincide with the peak of the floodwater in the river, water levels barely reached the toe of the stopbanks and were even short of the barrier near Q-West boat yard.

"Of course, those stopbanks are designed for the much larger 200-year flood," he said.