Council carpark made sea erosion worse at Cobden – NIWA scientist

5:09 pm on 13 May 2020

A chunk of Jellyman Park may have to be sacrificed to protect Cobden from the sea - and coastal erosion made worse by a council car park development.

West Coast, South Island.

A section of the West Coast. File image. Photo: RNZ / Tracy Neal

That is the finding of a report to the West Coast Regional Council by National Institute of Water and Atmospheric Research (NIWA) coastal scientist Mike Allis.

Dr Allis was asked to advise on the risks and possible solutions to the coastal erosion at Cobden, after a joint meeting with the Grey District Council last August.

In 2017, Dr Allis recommended the removal of the Grey District Council's new beachfront car park, with tarseal and kerbing, warning that it had been built too close to the sea, and was making the erosion worse.

But his advice was ignored.

Instead, the council upgraded the car park and strengthened the rock protection along the seaward face.

"This has continued to cause interference to the beach sediment transport process, and worsened the downdrift erosion impact to the north," Dr Allis said.

The car park was wiped out by subsequent storms.

The regional council says district council assets now at risk from the sea include the Cobden breakwater, Jellyman Park itself, and the old Greymouth capped landfill next to it.

Private property was also in jeopardy, the council has warned.

"Sea water has overtopped the car park, existing beach foreshore areas and roads and has flowed into Jellyman Park and on to private property in recent times."

Several storms had affected Cobden beach and the seaside infrastructure since his 2017 assessment, Dr Allis noted.

"Most notably, in early 2018, the Jellyman Park car park and adjacent Cobden landfill were severely damaged by large waves during ex-tropical Cyclone Fehi, which uncapped the dumpsite resulting in ... (thousands of tonnes of) rubbish being scattered over the beach."

A rock seawall was then built to protect the dumpsite and appeared robust and well-designed, Dr Allis said.

But its construction and tie-in to the "poorly-located" car park had reinforced its exposed position, interfered with beach processes, and increased the rate of shoreline erosion to the north.

"This has exacerbated the storm impacts over a 100m stretch of the vegetated berm, closed Hill Quay and continues to compromise the integrity of the remaining berm from Bright Street to Kettle Street," Dr Allis said.

"Private properties, Jellyman Park and council assets will be exposed to overwashing flows if a large storm erodes the rest of the car park embankment and temporary bund along Hill Quay."

Managing the risk to the Cobden community would mean building a new berm to act as a buffer - but the car park in front of Jellyman Park and what remained of its embankment would have to go.

"This is no great loss, as the coastal car park is now redundant with the new landfill car park (nearer the tiphead) better serviced for freedom campers and sightseeing."

But the best option for locating the new rock revetments (seawall and supporting rock/gravel berm) would be to set it 50m back, and that would mean the partial loss of Jellyman Park, Dr Allis said.

"The inland alignment would provide the best protection but be the most invasive to the Cobden community."

The second option, setting the buffer 25m back, was an attempt to balance encroachment on to Jellyman Park but would not be as successful over the medium term and would see large storms erode the beach, he said.

The NIWA scientist said in the long-term it may become too expensive for the council to continue fending off the sea.

"Long-term sea-level rise will escalate the coastal hazard impacts from the sea and the river/lagoon on the coastal infrastructure and the communities behind them. These will require increased investment to rebuild and upgrade over this time as the frequency or magnitude of wave impact increases with sea-level rise," Dr Allis said.

"Ultimately if sea-level rise trajectories over the latter part of this century are in the middle to upper range of current projections a continued protection strategy for existing infrastructure and some properties may not be a feasible or affordable pathway."

The West Coast Regional Council will discuss the report at its meeting tomorrow.

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