Report reviews city's biggest deluge

The June 20-21 flood was not only Whanganui's biggest but the second-biggest ever recorded in the North Island, a report released today says.

The report was presented to both Whanganui District Council and Horizons Regional Council yesterday, and was prepared by the Whanganui Flood Management Review Group, co-chaired by district council chief executive Kym Fell and Horizons chief executive Michael McCartney.

The review group said it took some time to figure out exactly how big the flood was.

Horizons' new river manager Ramon Strong said the usual measuring place was at Te Rewa, 50km upriver from town, but in the 36 hours when 131mm of rain fell last June more than usual fell near the coast.

River flow cannot be measured in the urban area because it is affected by tides, so the water added from sidestreams near the coast had to be calculated instead.

Peak flow at the Whanganui City Bridge was 5150m3 of water per second (cumecs), making it the second biggest flood recorded in the North Island, after one of 6370 cumecs in the Mohaka River in 1938.

That volume makes it a once-in-130-years event, with a 0.77 per cent chance of happening in any year. The likelihood of that amount of rain falling in coastal Whanganui in 48 hours is one in 100, or a 1 per cent likelihood in any year.

The flood has caused flood height predictions for Whanganui to be increased.

The Anzac Pde stopbanks were overtopped by 350-500mm of water. They were designed to hold a 30-year flood, which has a 3.33 per cent likelihood of happening in any year, with freeboard for wave slop. They're still said to hold a 30-year flood, but without freeboard.

The Balgownie stopbank was barely tested, Mr Strong said.

"If we had had storm surge and high tide it certainly would have tested Balgownie. As it was, it barely touched Balgownie stopbanks. They're designed to cope with much higher flood events."

They are still suitable for a 200-year flood, which has 0.5 per cent likelihood of happening in any year, but with less freeboard. Taupo Quay, unprotected, has a 5 per cent chance of flooding in any year. Putiki is also unprotected.

Climate change will make the situation worse, with sea level rise and warmer air holding more moisture to fall as rain.

Despite the appearance of a large sandbar under the Dublin St Bridge, the riverbed has just as much space for future floods as before. A survey of cross-sections of the riverbed, from Kaiwhaiki to the mouth, showed it had slightly more capacity overall than before the flood. Any sandbar is likely to be washed downriver by floods.

The mouth at Castlecliff is deeper, having been scoured by the flood peak happening at the same time as an outgoing tide.

Dredging would be little use. It would be expensive, short-term and reversed by the next flood.

"In my experience it's best not to try and fight natural processes like that," Mr Strong said.

A better option could be preventing flooding by work upstream in the catchment, Whanganui Mayor Annette Main said.

The Whanganui River Settlement's contestable funding for river health could help.

Sidestreams in the urban area flooded houses last June.

The urban area of the Matarawa Stream is likely to be added to Horizons' Matarawa flood-protection scheme.

Awarua Stream has been investigated, with a report due. Putiki's Ngatarua Stream is next to be looked at. The Tutaeika and Kaikokopu streams and Zoo and Churton creeks will be waiting longer.

Ponding behind the Balgownie stopbank from the Westbourne-Gonville catchment is to be addressed by a $6 million Whanganui District Council fix.

Mr Strong said it was difficult to draw the line between district and regional council responsibilities for flooding, but this one had been deemed an urban stormwater matter.

Horizons hasn't made any recommendations for increased flood protection - yet.

It will do so when it has plans and costs, Mr Strong said.

"We wish the community to know that we will provide guidance as requested and consider protecting people's safety and livelihoods a priority."

Costs are likely to be higher for people who benefit most directly and consultation is to begin early next year.

In the past Horizons has had mixed messages about Whanganui's appetite for flood protection. Horizons' planned 200-year stopbanks for Taupo Quay, Putiki and Anzac Pde didn't happen after a majority of responses to a postal survey said they were unwanted.

The 200-year stopbanks designed for Anzac Pde would have been 1.1m higher than the ones constructed.

The district council, in consultation with Anzac Pde residents, was the only submitter asking for more flood protection to be considered in the Horizons annual plan this year. The review group said stopbanks were only a protection from smaller floods - when they were overtopped they also kept water where it wasn't wanted.

Some have advocated moving houses out of the flood-prone parts of Anzac Pde.

Horizons councillor Rod Pearce said that was unlikely because the community would have to pay to move all the flood-prone houses in the region.

"It's huge for the whole country. We couldn't do it."

People at Waitotara were used to being flooded every few years; they hosed out their houses and moved back in.

Ms Main said it worried her that people were rebuilding and purchasing in Anzac Pde.

"There have to be programmes in place to minimise damage to people's houses."

There had to be some middle ground between no protection and total protection, district council infrastructure manager Mark Hughes said.

"Doing nothing is unacceptable. Protection against any event that might happen is not practical."

Mr Pearce said there were no deaths or injuries in the flood, which meant the Horizons warning system was good.

The report is up on both councils' websites. People with questions for Horizons staff can also call 0508 800 800.