We can't hold back the tide, says ECan

Residents fear the barrier beach at Washdyke which shelters the lagoon will be washed away by storms and sweeping tides.

They say high seas have been pounding the shores, breaking over the beach, and are concerned about erosion and the effect it may eventually have on the lagoon's natural habitat.

One resident who didn't want to be named said she had contacted the Timaru District Council and Environment Canterbury but felt her concerns had not been addressed.

She feared coastal erosion would leave the lagoon exposed to the ocean.

Some land around the lagoon is managed by the Conservation Department as a conservation reserve while other parts are freehold titles.

ECan's coastal resource scientist Justin Cope said the whole stretch of coast between Washdyke Lagoon and the Rangitata River mouth was in a state of long-term erosion.

Waves breaking the beach barrier or "coastal overtopping" was a common occurrence.

"It's the overtopping which causes the coastal erosion via a process known a `rollover'.

"Beach gravel and sand is pushed over the top of the low barrier beach by storm waves and deposited on the landward side.

"This is an irreversible process."

ECan monitors erosion rates while ensuring coastal activities such as building and subdivisions do not exacerbate coastal erosion.

However, its hands were tied when it came to altering the course of nature, Mr Cope said.

"ECan monitoring has shown that over the past 30 years, coastal erosion rates between Washdyke Lagoon and the Opihi river have averaged between one and two metres per year."

He said stopbanks were there to primarily stop seawater flooding property during coastal storms.

"The role of stopbanks is not to halt erosion.

"It's been shown that they temporarily slow down coastal erosion but inevitably the erosion overtakes them and they have to be moved further inland."

ECan measures the Washdyke coastline twice yearly.

Surveys have been carried out for the past 30 years, Mr Cope said.

Erosion at the Washdyke Lagoon has been recorded at about 1.2 metres a year, while further north at Seadown, the rate has been greater, averaging 1.9 metres.

- © Fairfax NZ News