Detailed map of landslides in the Pukekohe area, southern Auckland

Author:

Dougal B Townsend, David J A Barrell, Luke H Easterbrook-Clarke, GNS Science

Source:

GNS Science, Earth Sciences New Zealand

Publication date:

2024

Topics:

Environment ,People

Executive summary

This report documents a map of landslide features within a rectangular footprint that encompasses the wider Pukekohe area in southern Auckland. The landslide information comes from government-funded geological mapping datasets compiled by GNS Science in recent years. Under contract to Auckland Council, the landslide data were converted into a format suitable for upload to the New Zealand Landslide Database (NZLD) operated by Auckland Council. The project work comprised checking and refinement of a GNS Science archival (unpublished) digital detailed map of landform interpretations (geomorphology) that includes landslide information, extraction of the landslide component of the geomorphological dataset and making it compatible with the NZLD data structure, delivery of a report documenting the landslide dataset and data transfer to Auckland Council and upload to the NZLD.

The GNS Science Pukekohe detailed landslide map dataset is in a geographic information system (GIS) format and is provided as polygons and points. The polygon component shows the extent of each landslide. An accompanying point provides information (attributes) for each of these landslides, including interpreted characteristics of the landslide movement.

The detailed geomorphological map from which the mapped landslide polygons were extracted was compiled in a GIS on-screen by drawing expert interpretations over a high-resolution topographical basemap generated from LiDAR data (airborne laser-scanning of the ground). The mapping has an assigned resolution of 1:10,000-scale. The GNS Science geomorphological map-data structure differs from that of the NZLD. Selected attribute information from the geomorphological dataset was translated into the NZLD format and supplemented by information specific to the NZLD. The project brief required attributes for six specific information categories (fields), noting that data could be added to other NZLD fields where information was readily available and easily incorporated. As a result, most of the landslide features have attributes in up to 20 fields.

The Pukekohe detailed landslide map dataset is an inventory of landslides identified from evidence of land surface form (geomorphology). A total of 921 landslides have been mapped, most of which are in rugged hill country in the east of the map area. Individual landslides range in size from as little as ~200 m² to as much as ~500,000 m².

Interactive discussions between GNS Science, Auckland Council and Beca (NZLD facilitators) identified some refinements for the NZLD to improve its utility for documenting prehistoric landslide features and to supplement its existing utility for accommodating information on more recent landslides, for example, where movement was observed or can be readily identified from damage characteristics. Particularly useful is the addition of an information category that indicates the level of confidence that a landform feature is of landslide origin, rather than relating to some other landscape-modifying process (e.g. other types of erosion).

GNS Science, 2024/101

GNS Science, December 2024