Development of a Rainfall-Induced Landslide Forecast Tool for New Zealand

Landslides kill 2–3 people per annum in New Zealand and cost the country on average NZ$200–300 million dollars per annum. The majority of landslides (90%) in New Zealand are triggered by rainfall and often involve thousands to tens of thousands of landslides being triggered by a single event that can extend over areas up to 20,000 km2. Steep hillslopes (>26°) occupy over 60% of the New Zealand landmass, and much of this (5%) is classified as highly erodible land at risk of severe mass-movement erosion. To reduce the risk associated with landslides it is important to be able to predict where and when they might occur. To this end we are developing a landslide forecast tool for the National GeoHazards Monitoring Centre that will be used to forecast and warn the public of possible damaging rainfall-induced landslide events. We used logistic regression to investigate the influence of landslide triggering variables on landslide occurrence on a dataset of 20 recent and historic landslide-triggering rainfall events. From this we developed relationships to predict the probable spatial distribution of landslides triggered from a given forecast rainfall event.