Erosion type and severity

Present and potential erosion are significant factors for land management and the environmental and economic sustainability of large tracts of New Zealand’s landscape, particularly hill country and steepland.  

The LUC erosion classification is descriptive and incorporates form and process, rather than a geotechnical, process-based classification.

Thirteen erosion types and one deposition category are recognised (see Table 7 in the LUC Handbook). These are grouped into four major categories:

1. Surface erosion involves the movement of a thin layer of particles across the ground by water, wind or gravity.
2. Mass movement erosion includes a wide range of erosion types where material moves down slope as a more-or-less coherent mass under the influence of gravity.
3. Fluvial erosion involves the removal of material by channelised running-water.
4. Deposition is a special category concerning the accumulation of waterborne material across large areas (e.g. silt deposition after flooding).

Tracking (farm and forestry tracks) represent an 'unofficial' but important 5^th category. It is often overlooked that tracks can be a significant 'point source' of water, sediment and nutrient input to watercourses through a combination of bare surfaces, high stock and vehicle use, and runoff/discharge.

Full definitions for each erosion type are provided in Appendix 2 of the LUC Handbook.

In addition to erosion type, erosion severity is assessed following the six-part ranking system originally set out in the LUC Survey Handbook.  The assessment approach varies depending on erosion type (see pages 24-43 of the LUC Handbook.

An example of an erosion code might be 2Ss 1G 1Sh which reads as moderate soil slip, slight gully and slight sheet erosion.