Research measures extent of gully erosion in Tairāwhiti

Research lead by Te Uru Rākau – New Zealand Forest Service (TUR-NZFS) has mapped the change and severity of gullies caused by erosion in the Gisborne District following Cyclone Gabrielle.  The mapping is based on post-cyclone Gabrielle aerial imagery and complements three previous inventories from 1957, 1997 and 2017.

The report gives spatial analysis results, current assessments on gully activity, provides historical and current causal factors, and makes observations for next steps and further analysis. 

Key findings in the research include:

• There are currently 1,449 active gullies, comprising 2,279 ha, identified in the Gisborne District.
• 220 gullies, with a composite area of 114 ha, were formed since the last assessment in 2017.
• The regional distribution of active gullies has changed little since treatments to remediate gully erosion began.
• 97% of the active gullies are within size and range where treatment will restore stability and reduce the supply of sediment generated.
• 75% (1,730 ha) of the active gully area comprise gullies which are individually <10 ha which have a high probability of being effectively stabilised if treated now.
• Only gullies >10 ha, which comprise around 24% of the land (548 ha) show limited or no signs of stabilising. 

Debbie Ward, Director Forestry Insights at TUR-NZFS, says monitoring Tairāwhiti’s gully erosion is important, as, unchecked, a gully could continue to expand and affect surrounding hill country and risk creating significant amounts of sediment that could be washed into streams and rivers in a weather event.

“The report will help inform government policy and land management decisions on future land use around gullies,” Ms Ward says. “This includes supporting identifying options for a Land Overlay 3B, (an extension of the current land overlay framework in the Tairāwhiti Resource Management Plan), where the worst eroding land across the region will need to transition out of plantation forestry and/or pastoral farming and into permanent vegetation cover.”

The report was in response to a recommendation by the Ministerial Inquiry into Land Use (MILU) following the widespread damage by cyclones Hale and Gabrielle in 2023 to mitigate the effects of future storms events. 

Click here to read the report

Source & image credit: Te Uru Rākau – New Zealand Forest Service