Estimated highly erodible land in the North Island

  • Image, Estimated highly erodible land in the North Island.

    Some areas of New Zealand’s North Island are classified as highly erodible land. They have steep slopes and are at high risk of mass soil movement because they lack woody vegetation cover with deep roots to hold the soil in place. This can lead to soil erosion. It is important to identify areas of land at risk of severe erosion to inform land-use decisions and help prioritise soil conservation work.

    We classified Estimated highly erodible land in the North Island as a national indicator.

    Key findings

     Trend not assessed

    Of the estimated 2,688,000ha of land classified as potentially highly erodible land in the North Island, 31 percent (840,000ha) was at risk of severe erosion in 2012.

    • Of the 840,000ha of highly erodible land at risk in the North Island, 61 percent (515,000ha) had a high risk of landslides.
    • Manawatu-Wanganui had the largest area of highly erodible land at risk (255,000ha) of all North Island regions.
    • The Gisborne, Hawke’s Bay, and Northland regions also each had more than 100,000ha of highly erodible land at risk of severe erosion.

    Figure 1

    Note: Landslide erosion is the shallow (approximately 1m) and sudden failure of soil slopes during storm rainfall. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides over decadal time scales. Earthflow erosion is the slow downward movement (approximately 1m/year) of wet soil slopes towards waterways.

    Figure 2

    Estimated highly erodible land in the North Island, 2012 – interactive map

    Figure 3

    Note: Landslide erosion is the shallow (approximately 1m) and sudden failure of soil slopes during storm rainfall. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides over decadal time scales. Earthflow erosion is the slow downward movement (approximately 1m/year) of wet soil slopes towards waterways.

    Definition and methodology

    New Zealand experiences high rates of soil erosion. In the North Island, this is mostly due to the historical clearance of forest for agriculture (see also Estimated long-term soil erosion). In contrast, erosion in the South Island is mostly due to natural processes, primarily high rainfall and steep mountain slopes.

    Highly erodible land comprises land at risk of landsliding, gullying, or earthflow erosion if it does not have protective woody vegetation (Dymond et al, 2006). Landsliding occurs on steep slopes where the soils do not have the support of tree roots.

    Gullying and earthflow erosion can occur on all slopes, irrespective of steepness, but the land is only considered at risk if it does not have woody vegetation.

    Landslide erosion is the shallow (approximately 1m) and sudden failure of soil slopes during storm rainfall. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides, over decadal time scales. Earthflow erosion is the slow downward movement (approximately 1m/year) of wet soil slopes towards waterways.

    Data quality

    We classified Estimated highly erodible land in the North Island as a national indicator.

    Relevance

    relevance-direct This national indicator is a direct measure of the ‘Status of and susceptibility to erosion’ topic.

    Accuracy

    accuracy-high The accuracy of the data source is of high quality.

    See Data quality information for more detail.

    References

    Dymond, J, Ausseil, A-G, Shepherd, J, & Buettner, L (2006). Validation of region-wide model of landslide susceptibility in the Manawatu-Wanganui region of New Zealand. Geomorphology, 74, 70–79.

     

    Published 21 October 2015

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