Holding back the churning floodwaters

Work is almost finished on fixing a partially undermined section of stopbank on the lower Ruamahanga River at West Pukio Road at Kahutara.

Greater Wellington flood engineer Ranjan Cyril says if this piece of stopbank were to give way in a 1 in 50 year flood event it would flood large parts of valley from Kahutara to Lake Ferry and around Lake Wairarapa.

“It would certainly have the potential to cause the worst flooding seen in this area for decades, much worse than the 2004 flood.”

Mr Cyril says the stopbank has been moved back from the river by 37 metres to give the river more width, increasing the volume of water that can flow during floods, without the threat of overtopping.

The new stopbank is 1.1 kilometres long, with an average height of 2.5 metres and a base width of 20 metres.

It is taking 22,700 cubic metres of river silt to build and all of this was found on river bank within the confines of the stopbanks.

Mr Cyril says it is important to use materials that can be compacted to form a dense, strong structure that is not easily eroded. When shaping is finished the stopbank will be sown in grass to help bind and protect the surface.

Mr Cyril says the cost of the work is $1.3 million and a significant part of that was the purchase of a lifestyle property adjacent to the stopbank. Once the work is finished this property will go on the market to recoup some of the cost.

Less critical parts of the stopbank on either side of the new stopbank will be improved over the next two years.

The work is being paid for half each by Greater Wellington and the Lower Wairarapa Valley Development Scheme – a collective group of landowners that benefit from flood protection works in the area. The cost of the work is spread over three years to ease the burden on scheme members