Community Board opts for 'affordable' refurbished stopbanks

September 16th, 2011
[by David Armstrong]

The Community Board has settled on its submission to the consultation process on Motueka's flood protection scheme, saying it believes the community prefers the middle-ground stopbank refurbishment option.

The submission, which follows public meetings and presentations last month reported here, was endorsed by the board at its September meeting on Tuesday, as long as equal emphasis was placed on the need for the work to be "staged and affordable" for struggling ratepayers.

The refurbishment preference - the widening and raising of the existing stopbank between Peach Island and SH60 - was also qualified by concerns raised by the submission's author David Ogilvie and others about some of the engineering modelling of future scenarios.

First, the board questioned whether the consultants and council are being too extreme in their calculations, which are based on a 2090 climate change allowance which is much higher than rainfall projections and any historic flows.

Their belief is that the annual average river flows are in fact slightly but significantly decreasing, based on some data over the past 10 years. This they attributed to changes in land use away from tobacco to crops which absorb more rainfall water, less sheep farming and more forestry in hill country, and more dwellings requiring rain water for domestic and irrigation purposes.

The board's submission also noted that the river's capacity is increasing due to improved river maintenance and natural river degradation, and that gravel extraction would have an even greater effect.

They also asked if the structural vulnerability of the existing stopbanks have been exaggerated. "Much has been made of the ... strong likelihood of it collapsing in a 24-hour flood event" (due to saturation through the bank rather than over-topping). They said the 1983 flood had waters along the stopbank for 24 hours, while the 1990 flood (not as high as 1983) washed the stopbanks for two to three days, seemingly without any structural damage.

However the board did acknowledge that "there is an obvious urgency to manage the river, maintain the stopbanks, and keep clear the secondary channels and various open drains". It was "disturbing", they wrote, that a one-in-50 years flood would not be contained within the existing stopbanks in places, so "this is an urgent project" for the council.

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