Rainfall-induced landslide hazard mapping, Wellington

Background

Landslides can be divided into four groups for the purposes of determining the probability of a landslide occurring at a site. The four groups are rainfall-induced landslides, earthquake-induced landslides, endogenetic landslides (no external trigger) and pre-existing landslides. Landslides may have a wide range of underlying causes but there are two dominant triggers of movement – rainfall and earthquakes. About 90% of all landslides are triggered by rainfall and different rainfall patterns produce different types of landslides. 

The case study

Wellington City experiences multiple rainfall-induced landslides every year, resulting in significant damages and clean up costs. Three separate landslide datasets were used to assess and calculate the landslide hazard in Wellington. The landslide datasets were sourced from a national probabilistic rainfall-induced landslide hazard model (PRILHM), the Wellington City Council (WCC) and the Earthquake Commission (EQC). Collectively the datasets provided a relatively complete picture of rainfall-induced landslides in Wellington City allowing an assessment of landslide hazard to be made. 

Tools associated with the Wellington landslide case study 

Tool Name Tool Reference Key Figure / Table

Council Policy and Plan Auditing Tool

+ spreadsheet example

 [Tool 1.5]
Provides the basis for undertaking a 'health-check' audit of existing planning provisions to take account of climate change effects and to identify gaps and needs for additional planning instruments. 
Sensitivity Matrix Tool for Assessing Vulnerability of Urban Environments to Climate Change  [Tool 1.6]
Appendix: Wellington sensitivity matrix. Rainfall-induced landslides were identified as a significant vulnerability. 
Modelling present-day and future landslide potential  
[Tool 2.3.3]

Figure 2.3: Relationship between slope angle, rainfall and number of landslides per km2.

A series of rainfall events can be analysed for each geology/vegetation pair to estimate how landslide areal-frequency varies with rainfall intensity and to establish uncertainty bounds. Figure 2.3 shows the resulting correlation for greywacke with a woody vegetation cover, and provides a tool for determining landslide probabilities. 

Mapping the landslide hazard  
[Tool 2.3.4]

 Section 3: Results from the Wellington case study.

Relative and absolute landslide hazard maps for the road network, domestic dwellings and open space in Wellington City for different rainfall index bands. 

References

Dellow, G.D.; McSaveney, M.J.; Stirling, M.W.; Berryman, K.R., 2005: A probabilistic landslide hazard model for New Zealand. p. 24 IN: Pettinga, J.R.; Wandres, A.M. (eds) Geological Society of New Zealand 50th annual conference, 28 November to 1 December 2005. Kaikoura : programme & abstracts. [s.l.]: Geological Society of New Zealand. Geological Society of New Zealand miscellaneous publication 119A. 

Map of Wellington City Council reported slips and the location of NIWA Virtual Climate Station sites (from Tool 2.3.2).