Search for right experts on Esplanade

By Debbie Porteous

Photo / Mark Stevenson
Photo / Mark Stevenson

A nationwide search is on for appropriately skilled people to provide advice on fixing the Esplanade in Dunedin and retaining sand at St Clair Beach in the long term.

An invitation to consultants to lodge expressions of interest by Monday for the two stages of work at St Clair was sent by the Dunedin City Council to seven firms yesterday.

Council roading maintenance engineer Peter Standring said the council was not looking for consultants to come back with answers, but for them to convince the council they were the right person or company to provide the advice.

At this stage, it appeared it was a change in environment, not a problem with the sea wall structure, which caused the sand reduction issues that caused large sections of the Esplanade to drop away over the past 12 days, he said.

The total costs of remediating the wall and Esplanade damage and creating a long-term solution for retaining sand on the beach would fall to the council and ratepayers, although that was still subject to further investigation.

A University of Otago geomorphologist says the sea wall itself is the cause of the loss of sand on the beach.

Senior geography lecturer Dr Wayne Stephenson, who specialises in coastal geomorphology, coastal management and Quaternary coastal environments, said he was bothered that people might think the loss of the beach was "some natural processes".

"It isn't. It is actually a consequence of the wall."

He said it was well known in the engineering and geomorphology fields that vertical sea walls actually caused the loss of beaches.

A normal healthy beach dissipated wave energy through breaking and shoaling, but if the waves were hitting a wall, their energy could not be dissipated so had to be deflected.

As that happened, the waves scoured the beach and carried sand offshore.

It became a vicious cycle, because the subsequent loss of beach reduced wave dissipation and exposed the wall to even more wave energy.

While the best solution might be the most radical - tearing down the wall and re-establishing the natural beach environment (he noted the mean high water mark along the rest of the beach towards Lawyers Head had not changed in 120 years, meaning there had been no real erosion there), it was an unlikely option, given the community perceptions of erosion and expectations that the wall would protect land-based assets behind it.

The wall was there and, although it was not perfect, the community had to understand any wall, by the very nature of it being a wall, would require ongoing maintenance, which ratepayers would be required to pay for.

"It's a case of we've got it, so we will have to maintain it in perpetuity, and that's a thought that makes me, as a ratepayer, very nervous too."

Other options were to build a bigger and better wall to cope with the increased wave energy it will have to deal with - something the current design seemed not to have factored in; fix the current problems and expect that intermittently similar problems will happen; and/or replenish and retain the sand in front of the wall to deflect wave energy from it.

No solution would be cheap.

"Now we have a wall we are going to have to live with it."

The cordon around undamaged areas of the wall was removed yesterday and declared safe for the public to use after test holes and ground-penetrating radar established the stability of the area.

Sheet-piling to stop water getting under the sea wall at the northern end was expected to be completed within a week.

- Otago Daily Times

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