Detailed map of landslides in the Silverdale area, northern Auckland

Author:

Dougal B Townsend, David J A Barrell, Luke H Easterbrook-Clarke, K J Bland, K E Jones, J M Lee, Earth Sciences New Zealand

Source:

Earth Sciences New Zealand, GNS Science

Publication date:

2025

Topics:

Environment ,People

Executive summary

This report documents a map of landslide features within a 30 km by 40 km rectilinear footprint that encompasses the wider Silverdale area in northern Auckland. The landslide information reported here was compiled in conjunction with government-funded geological mapping datasets being produced by GNS Science (now a business unit of Earth Sciences New Zealand as of 1 July 2025). A contract with Auckland Council provided co-funding to aid creation of a digital detailed map of landform interpretations (geomorphology) with emphasis on landslide information. The detailed geomorphological map dataset provides an information base from which Earth Sciences New Zealand will produce generalised geological and geomorphological maps of the wider Silverdale area for publication. The contract helped support the mapping of the landslide component of the geomorphological dataset in a format compatible with the New Zealand Open Landslides Database (NZOLD) overseen by Auckland Council, the delivery of a report documenting the landslide dataset and the transfer of data to Auckland Council and upload to the NZOLD.

The Earth Sciences New Zealand Silverdale detailed landslide map dataset is in a GIS format and 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, which includes the landslide polygons, is being 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) collected in 2016. The mapping has an assigned resolution of 1:10,000 scale. The Earth Sciences New Zealand geomorphological map data structure differs from that of the NZOLD. Selected attribute information from the geomorphological dataset was translated into the NZOLD format and supplemented by information specific to the NZOLD. The project brief required attributes for six specific information categories (fields), noting that data could be added to other NZOLD fields where information was readily available and easily incorporated. As a result, most of the landslide features have attributes in up to 21 fields.

During the project, two amendments were made to the scope of work. The first enabled additional assessment of landslides within built-up areas and preparation of brief descriptions for specific landslides, including the evidence used in landslide recognition and interpretation. The second provided enhanced information on where landslides have had their diagnostic natural terrain modified, and to some extent obscured, by human actions such as earthworks. This information, conveyed via attributes in the landslide point and polygon datasets and as a separate ‘landslide modifications’ polygon dataset, is intended to better quantify the nature and extent of landslide modification.

The Silverdale detailed landslide map dataset is an inventory of landslides identified from evidence of land surface form (geomorphology). The dataset documents the locations and extents of previous landslide movements that have left diagnostic landform features in the landscape. In built-up areas, where possible, the dataset includes original landslide extents where these have subsequently been partially or completely obscured by land modification and development. The dataset does not imply anything about past, present or future land stability in relation to the mapped landslides.

In total, 7310 landslides have been mapped. The smaller landslides are dispersed throughout the map area, but there are concentrations of larger landslides in the north, west and east. Individual landslides range in size from as little as ~700 m² to as much as ~767,000 m². The Silverdale detailed landslide map dataset represents landslides as mapped, interpreted and provided to Auckland Council in late March 2025. The completion in coming years of the Silverdale geomorphological record map as part of the Earth Sciences New Zealand Urban Geological Mapping Project will include placing the landslide extents into context with other geomorphological and geological components of the mapping. This may result in some refinements to the landslide interpretations presented here.

GNS Science, 2025/01

Earth Sciences New Zealand, October 2025