Future sea level rise is likely to aggravate existing erosion rates at Kai Iwi. Photo / Bevan Conley
Future sea level rise is likely to aggravate existing erosion rates at Kai Iwi. Photo / Bevan Conley
The Whanganui District Council is urging caution around the cliffs at Kai Iwi Beach, which are retreating at a rate of about 30 to 50 metres a century.
Last week, a slip was reported on the southern side of the beach, with the council telling beachgoers to “keep well clear”.
Council deputy chief executive Lance Kennedy said the entire coastline – both cliffs and beaches – was retreating landwards in a natural process that had been ongoing since the Tasman Sea reached its current elevation about 7500 years ago.
“Reports indicate the Kai Iwi coast has retreated a significant distance over the last few thousand years, at a rate of about 30 to 50 metres per century,” Kennedy said.
“Future sea level rise is likely to aggravate existing erosion rates.”
He said rock revetments – barriers made of large rocks – had been placed along the length of the beach below Sunset Parade from the 1980s to protect against erosion.
They had been repaired and replaced over the years and were “generally successful at holding the line”.
“Our current management approach is to maintain the revetments. However, they will need to be removed or replaced at some point in the future.
Kennedy said that included looking at the long-term management of erosion in the areas from Archer’s Bridge to the Mowhanau embayment.
“All of the options for managing erosion along this coastline require a complex trade-off of environmental impacts, health and safety implications, and costs.”
Whanganui District Council deputy chief executive Lance Kennedy. Photo / Bevan Conley
At the end of 2022, environmental and engineering consultancy Tonkin + Taylor produced a report that reviewed erosion management options for the beach.
They were the removal of the revetments, maintenance of the existing revetments, replacement with an engineered rock revetment or the construction of rock groynes.
Groynes can reduce erosion by retaining enough sand to dissipate wave energy before it reaches the shore.
Kai Iwi Beach Community Group spokesman Chris Dibben said locals had reviewed the options and most picked the groynes as their preference.
No one wanted to remove the existing rock walls.
According to Tonkin + Taylor’s draft report, the estimated cost of installing groynes – which could potentially create “a wide, sandy beach” – would be $10.5 million.
The report said that, while a detailed assessment hadn’t been undertaken, two to three structures around 150 metres long might be required to have any noticeable effect on erosion.
Dibben said the beach was “for all of Whanganui” and he hoped to canvass the thoughts of people from the city as well.
“The amount of people that come here in summer is just phenomenal.
“Our dream is to have two groynes, to collect sand either side of them.
“I know there is a huge cost to them but what is it going to cost to remove all the infrastructure that’s around this area, as well as buying people’s houses out?”
The group had asked the council to reinstate a rock wall in front of the Mowhanau playground.
“We know we are fighting against nature but things can be achieved over a period of time.”
Kennedy said the council was now seeking further technical inputs to support the work in the report.
“We are also investigating a ‘community reference panel’ approach which would involve the community and hapū working together on assessing long-term management options for the coastline to recommend to the council.”
Some sections of the coastal action plan could be implemented, while other areas were still being worked on.
“This staged approach is necessary as the challenges at Castlecliff and Kai Iwi Beach are starkly different.
“Castlecliff is an accreting coastline with issues with windblown sand and vehicles and pedestrians damaging the dunes, while Kai Iwi Beach is a retreating coastline with naturally high erosion rates that are likely to be further exacerbated by climate change.”
Mike Tweed is an assistant news director and multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.
