HEL - Manawatu Whanganui Region

The February 2004 storm that struck the Manawatu, Rangitikei, Wanganui and Tararua Districts caused erosion in hill country, and flooding, sedimentation and stream course changes in the lowlands.  Unsustainable land use of hill country was a major factor contributing to this damage and is now one of four major issues identified by Horizons Regional Council in their draft Horizons One Plan.

After the 2004 storm the Council is currently examining options to reduce hill country erosion risk and one of the first tasks is to (better) define and identify land that is highly erodible.  This will enable the Council to direct both regulatory and non-regulatory effort towards those parts of the region where land use and/or land management changes may be required.  At the farm scale this will include the application of rules for vegetation clearance and land disturbance and the encouragement of sustainable management practices through a Whole farm Planning process as currently proposed under the Sustainable Land Use Initiative. 

This work undertaken by Landcare Research for Horizons Regional Council provides guidelines for HEL recognition.  The types of erosion are described, criteria for assessing erosion severity are listed and a table of Land Use Capability units that meet the Horizons criteria for HEL are presented.

Methodology

Definitions of types of erosion, criteria for deciding erosion severity and the selection of highly erodible LUC units were derived by reference to four main sources (Eyles 1985; Fletcher 1987; Fletcher et al 1994 and Jessen et al 1999) and from the experience of the authors in erosion mapping and erosion process research.

The regional scale map of HEL was derived as follows:

  • A slope threshold was defined for each LUC unit
  • All pixels in a 15m pixel DEM above the threshold defined by the pixel’s LUC was initially assigned to landslide risk (land cover is not considered)
  • The map of initial landslide risk was converted to a hillslope basis by using an aspect-based filter with a 25% risk rule (2ha minimum mapping unit)
  • All land considered at risk was examined to see if it could deliver sediment to a water course or not.  Land was considered capable of delivering sediment if it was possible to traverse down DEM streamlines until a water course was reached without encountering two consecutive pixels of low slope (ie: 5 degrees).  Earthflow prone LUC units were mapped as either of moderate or severe risk.

Outputs

This work provides Horizons Regional Council with the following:

  • Definitions of types of erosion
  • Criteria and guidelines for assessing erosion severity
  • A table of LUC units with a potential for severe erosion or moderate erosion where sediment enters a watercourse
  • A regional scale map of HEL using NZLRI data and a 15m pixel DEM
    • with forest excluded
    • land identified as capable or not capable of delivering sediment to streams
    • on a per hillslope basis
    • including riparian areas severely eroded in February 2004 storm
    • areas of earthflow and slump identified by erosion severity.

 

Associated Models

Highly Erodible Land Model (HEL)

The Highly Erodible Land (HEL) system identifies land highly susceptible to mass-movement erosion using a combination of the erosion terrains derived from the NZLRI database, a 15‑metre digital elevation model (DEM) to determine topography, and land cover mapping.

References

Mike Page, James Shepherd, John Dymond, Murray Jessen (2005): Defining Highly Erodible Land for Horizons Regional Council. Prepared for Horizons Regional Council, Landcare Research Contract Report LC0506/050

Dymond JR, Ausseil A-G, Shepherd JD, Buettner L 2006. Validation of a region-wide landslide risk model. Geomorphology 74: 70–79.