In New Zealand, landslide data are collected by many different agencies. The databases are populated with data intended to meet the needs of the collecting agency, and therefore, the data structures tend to be different and incompatible, making transfer and comparison of data difficult. Conceptual work on developing a centralised and consistent database began at GNS Science in 2002 (Dellow et al. 2003; Glassey et al. 2002).
This current landside database was initially developed through the GeoNet project (www.geonet.org.nz), New Zealand’s geological hazard monitoring system. The NZLD is a source of landslide information for New Zealand, and it currently contains 22,575 records (Fig. 1). The New Zealand Landslide Database is now available online at http://data.gns.cri.nz/landslides.
The landslide database combines a number of existing landslide datasets held by GNS Science. Data held by other agencies will be added as it becomes available. The landslide database is comprised of two groups of data: (a) historical landslide data and (b) data collected from on-going landslide mapping, monitoring and research. The main data sources are listed in Table 1 and outlined briefly below. The number of records derived from each of the data sources is listed in Table 2.
Table 1 Summary of data sources for the New Zealand Landslide Database
Table 2 Numbers of records from each of the pre-existing datasets that were combined to form the New Zealand Landslide Database
Large landslide dataset
A GIS-based landslide dataset was first developed by GNS Science in the 1990’s using data from a large landslide mapping project. The large landslide mapping project began in 1993 with the objective to catalogue all landslides in New Zealand with volumes greater than 105 m3. Attributes include area, volume, landslide type, activity, causal factors and environmental effects. Landslides were plotted on 1:50,000 topographic maps (NZMS 260 map series), digitised and stored in a spatial database (Glassey et al. 2002). The mapping comprised landslide features plotted by hand onto 1:50,000 NZMS260 topographic maps from air-photo interpretation. Initially, this was restricted to large landslides, but subsequently, some smaller landslides, down to ∼10,000 m2, were recorded on the topographic sheets. The spatial data plotted on the maps includes landslide features (outline “scar”, head scarp, source area and debris) and some other geomorphic features associated with landslides and other erosion processes such as hummocky ground, gully erosion, depressions, landslide-dammed lakes. The landslide spatial data in this dataset has an accuracy of ±200 m. Very few records contain information about the age or timing of the landslides (some large historical earthquake-induced landslides were mapped as part of this dataset).
Earthquake-induced landslide dataset
The earthquake-induced landslide datasets contain the locations of landslides associated with major earthquakes in New Zealand, at a scale of 1:250,000 (Hancox et al. 1997, 2002). It was developed independently from the Landslide GIS, using different attributes to describe landslides. The Earthquake-induced Landslide maps were prepared using information compiled from a review of New Zealand literature on earthquake-induced landslides and analysis of aerial photos and topographic maps. These landslide distributions, together with other types of environmental effects (e.g. liquefaction and ground cracking), were used by Hancox et al. (2002) to estimate Modified Mercalli (MM) shaking intensities.
These earthquake-induced landslides were initially captured as point coordinate data only, rather than their spatial extent (polygons), although landslide source and deposit areas have been mapped for more recent events, since the 2003 M
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7.2 Fiordland Earthquake (Hancox et al. 2003, 2014, 2015). The dataset also includes areas of liquefaction and other ground damage sites, shaking intensity contours (isoseismals) and the epicentres of earthquakes. Only data relevant to landslides have been transferred to the new database. The spatial data in this dataset has frequently been retrieved from historical written accounts. This means that, particularly for many of the smaller landslides, the location is poorly constrained (±2500 m). However, because the landslides are attributed to specific earthquakes, the information on when the landslides occurred is well constrained, usually to within a few minutes of the earthquake.
QMAP landslides
QMAP is the 1:250,000 scale Geological Map of New Zealand (http://www.gns.cri.nz), and it originally used the large landslide dataset to form a layer of landslides. The 2nd edition Geological Map of New Zealand was completed in 2012. Additional landslides, not contained in the large landslide dataset, were mapped as part of the revision and compilation of the QMAP series (http://data.gns.cri.nz/geology/) using aerial photo interpretation. Only landslide deposits and headscarps are included in the maps, and only substantial landslide deposits, thicker than 5 m and covering an area greater than 500 × 500 m, are shown (Rattenbury and Heron 1997). The QMAP landslides comprise landslide boundaries including landslide source, deposit and total landslide area (usually referred to as the landslide scar, Cruden and Varnes 1996). It also contains linear features representing head scarps and ground cracks. Mapping was done at 1: 50,000 scale; however, the data are presented at 1: 250,000 scale with some generalisation and simplification with consequent loss of mapped landslide detail (extent/size/characteristics) (Rattenbury and Heron 1997). No information on the date or dates of landslide movement are included in this dataset.
Landslide catalogue
In August 1996, a collation of news media accounts of landslides was started by GNS Science and has continued since. As part of GNS Sciences on-going geohazards monitoring through the GeoNet project (www.geonet.org.nz), monthly landslide summaries are produced of all landslides reported by (1) the media, (2) New Zealand Transport Agency (NZTA), (3) New Zealand Automobile Association (AA) (road closures), and (4) GeoNet landslide response activities. Data are mostly point data, due to the nature of source material (media reports). As a consequence, the accuracy with which the landslides are located spatially is variable, ranging from a ±100 m through to ±25 km. However, the date on which the landslide occurred is routinely recorded and in most cases can be constrained to the day of occurrence. The landslide catalogue requires the date and location of a landslide for every record. If additional data about the landslide (size, type) or its impacts (costs to reinstate roads/infrastructure, deaths/injuries, length of time a road is closed) is available, it will be recorded.
Site/event/project specific landslide studies (GeoNet)
Detailed information on landslides is collected after major storms and earthquakes as part of on-going geohazard event monitoring through the GeoNet project (www.geonet.org.nz), and as part of research and/or commercial projects carried out by GNS Science. A landslide response is undertaken if a landslide/s occurs that causes death or serious injury, has the potential to cause subsequent catastrophic events (such as the breach of a landslide dam), direct damage of greater than NZ$1 million and economic losses of greater than NZ$10 million, threatens public health (such as contaminated water supplies), or provides significant research interest. The purpose of the landslide response is to ensure that potentially important ephemeral information is not lost, and appropriate advice is available to maximise public safety. A GeoNet landslide response will often involve an aerial reconnaissance immediately after the event to document the extent of landsliding (Dellow 2001; McSaveney et al. 2010).
Typically, data are derived from field inspection (for a single landslide) or from satellite or aerial photo imagery when many landslides have been triggered. Local government authorities often contribute to funding the mapping and analysis of landslide distributions from storms affecting their region. Detailed mapping of landslide distributions, usually at scales of <1:10,000, is completed which can then be compared to other GIS datasets to extract other relevant information such as slope, aspect, vegetation, geology and rainfall.
An example of detailed landslide mapping, contained in the database, is illustrated below in Fig. 2. Landslide source areas (scars typically refer to source and debris outline) and debris deposits of different age were delineated on historic aerial photographs and verified in the field. This particular study was undertaken to determine the impact of mass movement erosion on New Zealand’s soil carbon monitoring system (Basher et al. 2011).
Other sources of landslide data
Other sources of landslide information include data collected by network operators (e.g. NZTA, Kiwirail (railway), Vector (gas and electricity)) and data collected by local authorities as part of their obligations under the Resource Management Act (1991) to collect and provide information on hazards, including landslides. Data collected by network operators is useful to understand the general areas affected by landslides, but due to the specific interests of each network operator, the landslides recorded are usually biased towards particular positions in the landscape (e.g. data from NZTA only provides information specifically for state highways affected by landslides). Data are often hard to obtain, and it is often collected in a manner that is not easily transferred to a database (e.g. “truckloads” of dirt removed from a stretch of road) (Page 2015). Data are often held by contractors working on behalf of the asset owners/managers, who change frequently or do not wish to release the true cost of landslides on their networks.
On-going data collection
Data are added to the landslide database on an on-going basis from GeoNet landslide monitoring and reporting, and specific studies. Large event-based datasets, consisting of thousands of landslides, will be loaded into the database (by GNS staff) using a batch-upload routine that requires the data format to conform to a predetermined data structure (to map the correct attributes).
New earthquake-induced landslide datasets that will be imported into the database include the recently completed mapping of historical earthquake-induced landslides triggered by the 1929 M
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7.8 Murchison earthquake (Hancox et al. 2015) and the 1968 M
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7.1 Inangahua earthquake (Hancox et al. 2014) that contain 6108 and 1400 landslide polygons, respectively. Recent earthquake-induced landslide datasets to be imported include the Canterbury Earthquake Sequence (Massey et al. 2014), Eketahuna (Rosser et al. 2014), Wilberforce (Carey and Rosser 2015), Cook Strait and Lake Grassmere (Van Dissen et al. 2013) earthquakes. A landslide inventory (>10,000 landslides) is currently being mapped for the M7.8 Kaikoura earthquake (Kaiser et al. 2017).
Other datasets include Dunedin City landslides (Glassey and Smith Lyttle 2012) and rainfall-induced landslide datasets from storms affecting Hawke’s Bay (Jones et al. 2011) and Kapiti (Page and Rosser 2015).