Sea rise sparks flood risk

Councils have been warned not to build on parts of the Wairarapa coastline after areas at high risk of flooding and erosion have been identified in reports released by Greater Wellington Regional Council.

Senior hazards analyst Iain Dawe said the two reports researched the rate of sea-level rise and the likelihood of coastal flooding in the Wellington region.

The reports show that sea levels are rising at over 2 millimetres per year and the Wellington region is tracking towards 1 metre of sea level rise over the next 100 years.

Areas in Wairarapa which will be affected in future include Lake Onoke, the coast road to Mataikona and along Tora coast, Castlepoint Beach, Riversdale, and low lying land around river mouths such as Whakataki.

During a storm, there is the potential for sea levels to increase temporarily by up to 80cm, an effect known as storm-tide.

Dr Dawe said in the event of a major storm, there will be drainage problems - as seas continue to rise - with the Lower Wairarapa Valley flood scheme.

"The whole scheme relies on drainage through Lake Onoke and as this is very low lying we will start to see longer periods of flooding and ponding following storm events," he said.

"Around river mouths a similar effect will occur whereby flood waters back-up in storm events as water levels at the coast become higher, slowing down the ability of rivers to drain when they are in flood."

There could also be coastal erosion on beaches and changes around river mouths as the shoreline adjusts to rising sea levels.

Dr Dawe said there are already mitigation measures in some places, such as the Castlepoint seawall and recent work on Cape Palliser Rd which faces coastal erosion.

He said these measures can be effective for some time, although the reports may deter building new infrastructure in areas more likely to be affected.

"There are some areas along the Wairarapa Coast which are high risk," he said.

South Wairarapa District Council said infrastructure and services manager Mark Allingham said information presented in the reports was not new, and council was already undertaking preemptive action along the South Wairarapa coast.

This included placing boulders along Cape Palliser Rd to try and prevent erosion.

"It's something we knew about and something we have in our asset management plans," he said.

"Any possibility of change in the future is something that we need to be aware of and mitigate."

The reports - which were undertaken by scientists from Niwa - will be taken into account as regional council prepares their 10-year plan.