Anzac Pde's stopbanks need $1.02 million of work to withstand a 50-year flood, and Horizons Regional Council wants Wanganui's views on that.
Since 2006 Horizons has been responsible for flood protection for Wanganui city. It had a duty to inform residents of flood danger, chief executive Michael McCartney said, and let them decide what to do.
The state of the 2.025km stopbanks caused debate at the council's catchment committee meeting last week.
Chairman Bruce Gordon moved that work be factored into the next long-term plan, 2015 to 2025, subject to discussion with Wanganui District Council and ratepayers. Councillor Greg Cox seconded that. The work would mean the average Wanganui household paying an extra $2.37 to $4.36 over a period of up to six years - depending on how long it took and how it was financed.
It would prevent $10.5 million worth of damage to 57 houses in a 50-year flood. It could also prevent the stopbanks breaching at weak points, which could cause extra damage. A cheaper alternative would be to spend $610,000 to get the stopbanks up to 30-year flood standard.
Both options stop well short of the new stopbanks at Balgownie, built to withstand a 200-year flood.
The cheapest alternative would be to do nothing to the Anzac Pde stopbanks. If they are left as they are, planning solutions could be tried. One expensive option would be to move the 57 houses at an estimated $1.6 million plus the cost of land to put them on and getting services connected.
The council contracted ICE Geo and Civil to investigate the existing stopbanks. It found the sides were too steep and the crest too narrow in places. There were also vulnerable layers of sand and no records of the kind of compaction needed. Some parts might even fail in a 20-year flood. During the 30-year flood in 1990, water came into several houses.
There are three low points, and the downstream end is most vulnerable overall.
Horizons now wants to hear the views of Wanganui district councillors, and will decide how to proceed after that.