Infrastructure-damaging landslides from an extreme rainfall event: case study from Gisborne, New Zealand

Landslides are widespread natural hazards that are responsible for substantial economic and societal damage globally each year. In New Zealand, landslides frequently occur on soil and rock, often triggered by high rainfall and/or seismic activity. This study focuses on the Gisborne district on New Zealand’s North Island. The area is particularly susceptible to landslide hazards due to (1) the region's location on an active plate boundary, (2) steep slopes, (3) relatively young, soft geology, (4) land use change, and (5) extreme rainfall events including landfall of extra-tropical cyclones. The interplay of several of these factors led to a particularly damaging rainfall-induced landsliding event after 4th November 2021, following >200 mm of rain falling in parts of the district over 24 hours. Effects across the region were widespread. Damage to, and evacuation of, residential properties occurred in Gisborne city itself, from shallow rotational slumping and earthflows. The vulnerability of the city’s water supply (via the Te Arai Pipeline) was highlighted by the reactivation of a large complex landslide, that extended to within a few meters of the pipeline. Destruction of the (mothballed) Gisborne-Wairoa railway line occurred near Beach Loop due to reactivation of the Whareongaonga Landslide. Across the Gisborne district, detecting ground deformation related to landslides is vital for identifying and managing areas at risk. Interferometric synthetic aperture radar (InSAR) revealed that many of the landslides that occurred following the November 2021 rainfall event were on slopes that had been actively deforming for several years. Thus, in future, InSAR should prove useful for detecting, mapping and monitoring landslides in the district, and assisting with planning decision-making.