If Whanganui people were to sue the river for breaking and entering during the June flood it might well reply: "Serves you right for squatting on my floodplain."

With those pithy words, Whanganui Science Forum co-convenor Ella Grant ended the forum's first 2016 talk.

The talk - by Associate Professor Ian Fuller and Professor Mark Macklin, both linked to Massey University - was about the June floods and future floods that are likely to be bigger. It was followed by questions from the 100-strong audience.

One person asked whether "puny interventions" like stopbanks and dredging did anything to reduce flood risk.


Probably not in a big flood, Dr Fuller said, though interventions could work well for small ones.

"One problem we face in New Zealand is thinking floods are controllable and manageable - of course, as we have seen, we can't manage them."

The answer was to give rivers more space - to keep off the floodplain as much as possible. And if buildings and roads had to be there, to make them resilient.

Dr Fuller said houses could be two-storey, on piles with utilities underneath, to stay above floodwater. Or people could move out altogether, and society should help pay their costs.

Culverts could impede the flow of water and make it back up. Dr Fuller recommended "daylighting" streams rather than running them through pipes.

"Culverts work for small floods, not big ones."

The Whanganui River's main water quality problem is the sediment that makes it a murky brown and clogs the river channel. Reducing that by re-vegetating eroding hills would take agreement and participation by everyone in the river's huge catchment.

Dr Fuller said more water streamed into the river, and faster, off denuded hills than forested ones. Tree cover helped hold on to soil in small floods, but in big ones even native bush did not prevent slips and gullying on Whanganui's soft rocks and soils.

"This terrain really falls to bits when you add water."

Mr Macklin, from Wales, said climate change was having an impact on flooding there, with waves as high as a six-storey building hitting the seafront at Aberystwyth recently.

"We thought it couldn't get any worse, but it did."

New Zealand was lucky to be "stuck in a coldish ocean" and less affected. It had time to plan for change and research was urgently needed.

Some began in Whanganui this week, with Massey student Erica Malloy taking samples of flood sediment to track where it came from.

The June flood had 4775cu m of water sliding past Te Rewa at Parikino every second. It was the biggest on record for the Whanganui - and probably the second biggest on record for the North Island.

But the speakers said "on record" did not mean much, because records only go back about 60 years.

The flood was caused by 150mm-200mm of rain falling over 48 hours and unusually close to the coast. The Matarawa, Awarua, Tutaeika and Kaikokopu streams and Churton's Creek all flooded in the city.

On some coastal hillsides more than 10 per cent of the topsoil slipped away. Most of it did not get as far as the river, Dr Fuller said.

A lot of sediment did get to the river though, while a survey of the river mouth at Castlecliff showed the flood has scoured it out and made it deeper.

It was hard to say whether floods were getting bigger, Professor Macklin said. He is looking for evidence that goes back hundreds of years.

Records do show floods can happen in any month of the year. And the June flood could be linked to the Pacific Decadal Oscillation - an unusually sudden shift to warmer weather and to more rain coming from the west.