OPINION: The proposed Geotech seawall for a section of coast by Mullet Rd (Cosy Nook) is an urgent and well-overdue coastal erosion repair project, but there seem to be some glaring commonsense shortcomings in the proposed scheme.
The two streams either side of the sea wall are not to have complete rock protection as previously indicated by the Southland District Council engineer.
This means outer eroding areas both sides of the seawall are not fully covered by new armour rocks at the base.
This will not be a cost saving, even in the short term for the council or the taxpayer, as erosion in these outer unprotected areas will accelerate, regardless of the sea wall or even because of it. We, as Southlanders, are getting only a part solution. Do we want to secure this road long-term?
It's known the sea wall design absorbs wave shock but will accelerate erosion on either side and very possibly threaten the sea wall itself.
The sea finds weak points as any child playing with sand on a beach knows .
The council has pondered a solution for the cliff disappearance over a number of years but it risks having to do more repair unnecessarily quickly.
While the costs would increase with extra rock placement extending to the base of all eroding areas, it would seem prudent for SDC with this scheme to attempt a complete fix now.
Southland District Council roading asset management engineer Hartley Hare replies: The proposed design is to address the erosion that is immediately threatening approximately 75m of Mullet Rd.
It has been designed specifically to dissipate wave energy whilst minimising the effect on any adjacent areas of Mullet Bay, and has made allowance for future sea level rise.
Whilst there is erosion occurring either side of the location of the proposed scheme, the road is not under immediate threat in these areas, and expenditure on erosion protection can be scheduled for future years. When prioritising spending, the district council must assess what are the most urgent needs of the network and what expenditure can be scheduled for future years.
The coastal erosion problem will be ongoing at Mullet Bay and the district council expects future work will be required.
The need for further work will be managed by monitoring of the site on a regular basis and by undertaking maintenance of the proposed rock wall as and when required to ensure its integrity.
As this project will be jointly funded by the district council and NZ Transport Agency, the project must qualify for funding under the agency's criteria. This process takes account of the whole of life cost, and compares the cost of a project against the cost of the minimum amount of maintenance that can be undertaken to maintain an asset.
Outside the 75m of Mullet Rd under immediate threat, there is a very minimal amount of maintenance cost expected in the immediate future, so the cost of erosion protection cannot be justified.
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