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Why we're researching gully erosion in the Gisborne area
In 2023, cyclones Hale and Gabrielle caused widespread damage in Tairāwhiti and Wairoa. The government set up a Ministerial Inquiry into Land Use in response. The panel studied the impact of storm damage caused by woody debris and sediment. It recommended ways to reduce the effects of future storms.
In response to the inquiry, Te Uru Rākau – New Zealand Forest Service commissioned an inventory of gully distribution in Tairāwhiti.
Ministerial Inquiry into Land Use – Ministry for the Environment
The damage gullies can cause
Gullies are deep, unstable, eroding channels. They form at the head, side or floor of valleys where no well-defined channels exist. Gullies eat away at productive hill country if untreated. They create large amounts of sediment that washes into streams and rivers.
During severe storms, such as cyclones, gullies contribute to land damage. They destabilise the earth and trigger landslides. Landslides lead to soil and woody debris clogging waterways and causing floods.
Factors that make Tairāwhiti prone to gully erosion
The region has:
- steep terrain
- high rates of tectonic activity (for example, frequent earthquakes and uplift)
- weak geology dominated by fractured and crushed mudstone
- a mixed climate with warm-dry periods, heavy rainfall, and cyclones
- large areas where native forest cover has been cleared.
Results of the 2024 inventory of gully distribution in Tairāwhiti
The 2024 inventory shows the extent of gully erosion in the region. It complements 3 past inventories by Dr Mike Marden, from 1957, 1997, and 2017. The inventory includes an assessment of past erosion control treatments. It also looks at what might work in the future.
We created a spatial dataset as part of the inventory. To do that, we mapped gullies using aerial photos and specialist software. The dataset shows:
- there are 1,449 active gullies covering 2,279 hectares in the region
- 220 gullies have formed since the last inventory in 2017
- 75% of the active gully area is made up of small gullies less than 10 hectares each.
There is a high chance of stabilising small gullies. Once gullies reach 10 ha in size, it's unlikely they can be stabilised.
Next steps
The inventory outlines next steps for treating gully erosion and further analysis.
We're now working on a spatial dataset of gully watersheds. A watershed is the land upslope of the gully. It channels surface water into the gully. We will publish the watershed dataset and the gully dataset in mid-2025.
