Thursday, April 25, 2024

Manaaki Whenua report reveals East Coast landslides data

An assessment of land affected by Cyclone Gabrielle has provided useful data about the extent of landslides in hill country areas of the North Island’s East Coast.

The Ministry for the Environment commissioned Manaaki Whenua Landcare Research to undertake a rapid assessment of land damaged by the cyclone in February 2023.

Manaaki Whenua based its assessment on satellite photos of Tairāwhiti/Gisborne, Hawke’s Bay and Wairarapa coast.

It found that:

  • there were more than 300,000 landslides, with each landslide comprising about 1,000 tonnes of soil, the equivalent weight of 548 single-cab utes;
  • in southern Hawke’s Bay/northern Wairarapa hill country, land under native vegetation was 90% less likely to slide, while land under exotic forest was 80% less likely to slide than hill country pastoral land;
  • in northern Hawke’s Bay, land under native vegetation was 90% less likely to slide, while land under exotic forest was 60% less likely to slide than hill country pastoral land;
  • in Tairāwhiti/Gisborne coastal hill country, land under native vegetation was 50% less likely to slide than hill country pastoral land. Land under exotic forestry and pasture had the same extent of land sliding.  

Understanding the land damage assessment

The Ministry says the assessment indicates that hill country land under native forest was less affected than land under exotic forest. Areas in pasture and harvested pine were the most affected. The assessment doesn’t draw conclusions about the underlying reasons.

It says a more in-depth study is needed before conclusions about the soil stabilisation effectiveness of different types of tree/vegetation cover can be made.

“Pine is often planted on hill country areas that have been in pasture, and is likely to have a history of soil disturbance. In Tairāwhiti/Gisborne, pine was planted in areas with extensive erosion damage in an attempt to stabilise the land. Regenerating native vegetation takes longer than pine to stabilise highly erodible areas.”

The Ministry of Business, Innovation and Employment (MBIE) is funding GNS Science to map landslides at a finer scale. It’s envisaged this will provide more definitive information than provided in the preliminary rapid assessment.

The Manaaki Whenua and GNS Science reports will form part of the scientific advice that informs the Ministry’s severe weather integrated response and its policy advice to the Government.

Read the report.

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