The land least likely to survive future flooding

EYEWATERING metrics in a Manaaki Whenua Landcare Research report on Cyclone Gabrielle’s impact reveal the different impacts pasture, exotic plantation and native forestry planting had on land resilience in Hawke’s Bay and Tairāwhiti .

Just released, the rapid assessment report uses satellite imagery of land before and after the event, determining that the regions experienced 300,000 landslides, with an average landslide containing 1000t of soil.

The resulting 300 million tonnes of soil lost is estimated to be worth $1.5 billion. The impact was most severe on land with pasture and exotic forest plantations.

The report found a 90% reduced likelihood of land collapsing when covered in woody vegetation.

But the outcome depended on the region being studied and type of vegetation.

In southern Hawke’s Bay, the likelihood of land collapse was reduced by 90% for indigenous forested area, and 80% for land covered with exotic forest.

In northern Hawke’s Bay exotic forestry proved less effective, with a 60% reduction in collapse, against indigenous reducing risk by 90%.

Further north in the Tairāwhiti region, however, the ability of exotic forestry to protect the land was even less at only 50% effective.

Across northern Hawke’s Bay and Tairāwhiti, the report says, forestry management practices such as non-thinning of trees, thin soils, and multiple forestry rotations all contributed to a failure to hold land in place.

In the Ngātapa-upper Wairoa catchment area, the likelihood of landslides in harvest forest was determined to be twice as likely as any other land cover.