More concerns raised over beach movement

SUPPLIED/Nelson City Council

TIME AND TIDE: An aerial photo of Tahunanui Beach, showing erosion since 1948.

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The shifting sands of Tahunanui Beach have city councillors scratching their head again.

They decided last week to enforce a "managed retreat" plan for erosion, with particular emphasis on the western beach, or Back Beach, by the Blind Channel.

Three years ago, the Nelson City Council spent almost $3 million on erosion prevention works at the Rocks Rd end of the beach.

It has now decided to take action to spend what could be millions of dollars on harnessing the tide, if it reaches the roller skating rink tucked behind the beach.

In the meantime, the council plans to adapt to natural forces as much as possible.

Councillor Rachel Reese told last week's community services committee meeting that it was evident from discussions with the Tahunanui Business Association that its members were "very concerned" about what was happening.

The beach was an asset with "intrinsic economic value" to Nelson, Ms Reese said.

Council parks and facilities manager Paul McArthur said it was always anticipated that if the beach retreated to the back of the skating rink, the council would consider the area to be at risk.

Councillor Mike Cotton said it would cost millions of dollars to control erosion of the nature of that affecting the beach's western flank.

"Eventually the beach will run parallel to Rocks Rd. We have to keep an open mind on protecting what is a main asset of Nelson."

Nelson geologist Mike Johnston told The Nelson Mail that current erosion and drift patterns were more likely to result in the beach swinging at a 45-degree angle to the road.

Councillor Gail Collingwood said the high degree of public "ownership" in the Back Beach meant the council was obliged to keep a close watch on what was happening to it.

The committee agreed to make room in its 2009-19 draft community plan to continue the implementation of the current "managed retreat" policy for the western beach. This will make funding available for profile mapping, the relocation of carparks, signs and fences, and further low-cost fencing and planting in suitable areas.

Further funding would also be considered for a further technical report and update on erosion. Mr McArthur said the council would be prudent in getting more information from technical experts.

Dr Johnston said the Blind Channel was moving east, but every so often it switched to the west, as it did in the 1870s.

He said Tasman Bay was infilling at a rapid rate in geological terms, and monitoring movement around Tahunanui Beach was useful to see if the rate of shift to the east accelerated.

However, he cautioned against stepping in with strong preventive measures.

"If substantial work was done to protect the Back Beach and the channel was shifted west, that would release a huge amount of sand. No one's thought about what would happen then."

Dr Johnston said this would leave a similar volume of sand to deal with to that which formed the front beach.

"The council needs to closely monitor things, and if it looks like it's eroding faster to the east, it may be time to think about some action."

- © Fairfax NZ News