Dunes key to protecting Christchurch's coastline - report

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A new report has identified the need for sand dune work to hold back future sea level rises in Christchurch's coastal suburbs.

The report, which was commissioned by Christchurch City Council staff, recommends adding a planted sand dune in front of the wall at North New Brighton surf club, which will help absorb energy and delay the need for more costly intervention.

North New Brighton is considered highly susceptible to breaching and overtopping as sea levels rise.

After large, or repeat storms, the dunes require fast recovery to ensure they can continue to protect landward areas.

The report says current dune management work near the New Brighton pier should be continued to provide sufficient buffer to reduce the risk of waves reaching and damaging the wall and landward areas and assets.

A breach of the Southshore spit could change how the lagoon floods and functions, including ecological implications.

Strengthening and rebuilding dunes at Sumner and Taylors Mistake to help delay damage and reduce impacts on surrounding communities has also been recommended.

Brooklands Lagoon was also identified as being vulnerable.

​Taylors Mistake resident Malcolm McClurg, who has lived in the area since 1978 and has served on the Taylors Mistake Surf Life Saving Club committee for 55 years, said the risk has long been recognised.

An architect, he was involved in the surf club rebuild, which opened in February 2021.

“The building itself was established at 12.3m, basically anticipating sea level rise, we have a large concrete seawall out the front which assists with breaking up the waves,” he said.

The main structure uses pre-cast concrete panels and steel portal frames designed to withstand major storms and wave action.

“It’s a high risk zone but the purpose of the building is that it’s a rescue amenity so it is where it has to be,” he said.

The report found that sequences of large and small storms can rapidly erode open coast dunes, sometimes by several metres in a single event and tens of metres over successive major events.

While some areas have wide dunes capable of absorbing impacts, locations with narrow dunes or those modified by structures face greater risk of breaching.

The report warns that as sea levels rise, dunes will be exposed to higher water levels and more frequent wave impacts.

This will increase erosion frequency, reduce recovery time between events, and lead to more frequent overtopping across lowered dunes.

Dune width will shrink and crest heights will lower in many areas, although this process may be slowed by ongoing beach sand supply.

Eventually, the report says, dunes may no longer be able to perform their protective function in some locations, particularly without continued management.

By the time sea level rises 0.6m – which could occur by 2080 – the length of coast considered susceptible to storm-induced erosion and overtopping is expected to more than double.

With 1.2m of sea level rise, large parts of the open coast, and almost all of Sumner and Taylors Mistake beaches are considered susceptible to overtopping.