title: Landslide planning guidance: reducing landslide risk through land-use planning
Creators: name: de Vilder, Saskia J.
name: Kelly, Scott D.
name: Buxton, Robert B.
name: Allan, Sylvia
name: Glassey, Phil J.
Publisher: name: GNS Science
Year: 2024
Subjects: classificationCode: 1.5
subject: FOS: Earth and related environmental sciences

description: Much of Aotearoa New Zealand is hilly or mountainous. Our land is often composed of weak or highly fractured rocks, and many of the soils that overlie these rocks are only marginally stable and regularly compromised by rainfall and earthquakes. Consequently, landslides are an existing or potential hazard in many parts of the country. Risks from landslides to people and infrastructure, and the costs to the Aotearoa New Zealand economy, are substantial (Bruce 2022; Page 2015). There have been more than 1500 recorded fatalities from landslides in Aotearoa New Zealand since 1760, with 743 fatalities as the result of natural events and 765 fatalities due to human activity, such as cut slopes, mining or land clearance (Bruce 2022). More fatalities have occurred from landslides than from earthquakes (501), volcanic activity (179) and tsunami (1) combined over the past 160 years. The largest natural landslide fatality events were due to four large hydrothermally induced landslides (231 fatalities), followed by two major lahars (162 fatalities), rainfall-induced landslides (157 fatalities), earthquake-induced landslides (63 fatalities) and landslides with no direct causes (59 fatalities) (Bruce 2022). It is estimated that landslides cost Aotearoa New Zealand at least \$250 million (M) per annum, with an average of \$300M per annum (Page 2015). Individual rain-induced landslide events range from large events costing up to ~\$350M (e.g. Cyclone Bola) to small rainfall events of ~\$3.5M (Page 2015). However, the costs of landslides are often combined with other natural hazard events, most often storms, floods or earthquakes. After the 2010/11 Canterbury earthquakes, owners of 475 houses impacted by landslides, or at risk from landslides and rockfall, were offered buy-outs, with losses estimated to be around \$330M. The 1979 Abbotsford landslide on a Dunedin hillside resulted in the loss of 69 houses and the evacuation of over 600 people (Page 2015). It is the largest individual landslide to have occurred in an urban area in Aotearoa New Zealand, and the cost of the damage was estimated at \$10–13M (Hancox 2008). These costs can be reduced ahead of events. Estimates of savings from mitigation measures implemented prior to natural hazard events range from \$ 4 to \$ 11 for every \$ 1 spent (NIBS Multi-Hazard Mitigation Council 2019). The 2016 MW 7.8 Kaikōura Earthquake generated thousands of landslides and hundreds of landslide dams, resulting in damaged hillslopes that are now susceptible to failure during rainstorms and ground shaking. When mobilised, the landslide debris creates new hazards, including further landslides and dams, increased sedimentation into rivers, rapid aggradation and formation of alluvial fans and floodplains and increased river channel instability. These consequences can persist for decades, requiring active management by the impacted communities and stakeholders. The cost of restoring State Highway 1 (SH 1) and the railway line after the Kaikōura Earthquake was in the order of \$ 2 billion (B), excluding costs related to post-event disruption. These examples illustrate the destructive potential of landslides. While not all landslides result in such costly consequences, they cumulatively have potential to cause damage, disruption and loss of life. Under climate-change scenarios, the risk from landslides is likely to increase due to changing patterns in rainfall, snow and sea-level rise. As this guidance was nearing completion, three significant weather events occurred: Cyclone Hale, 10–11 January 2023. Auckland Anniversary Weekend storm, 27 January 2023. Cyclone Gabrielle, 11–14 February 2023. Several regions declared a state of emergency, with this escalating to a state of national emergency. These events resulted in considerable landslide and flood damage across the north and eastern parts of the North Island, estimated to be between \$9B and \$14.5B (Treasury 2023), with about \$1.5B attributed to landslide (McMillian et al. 2023). While three events in quick succession may be unprecedented, many other cyclone events have occurred in past years. Such events demonstrate the contribution of landslides to the cascading hazard of debris-laden flood flows and associated impacts. Hence, landslide hazard and risk must be considered and managed beyond the built urban environment to reduce downstream and cascading, as well as cumulative, impacts. These damaging events emphasise the importance of considering not only likelihood but also consequence. They highlight the impact of landslides on buildings, infrastructure, agriculture and livelihoods and reinforce the need to revise land use in upstream catchments, as well as that the effects of climate change must be considered. It is hoped that these events will be a catalyst in changing land-use planning and practise, utilising tools such as this guidance, so that future impacts of landslides will be reduced.