Key Takeaways:
- Landslides are the deadliest natural hazard in New Zealand, with a long history of causing fatalities and damage.
- Heavy rainfall is a major trigger for landslides, particularly in regions such as the Bay of Plenty, Coromandel, Northland, and Tai Rāwhiti.
- New Zealand’s steep and geologically young landscape is prone to landslides, with many past events leaving visible scars on the landscape.
- Advanced mapping and modeling techniques can help predict landslide risk and identify areas of higher vulnerability.
- Developing detailed and publicly available landslide hazard maps could be a crucial step in reducing the risk of landslides and saving lives.
Introduction to Landslides in New Zealand
The recent tragic events in the Bay of Plenty, where a landslide swept through a holiday park and another struck a home, are a stark reminder of the dangers of landslides in New Zealand. These events occurred at the tail end of a weak La Niña cycle, which brought unusually wet conditions to northern New Zealand. The region received over 200 millimeters of rain in just 24 hours, well above the typical thresholds known to trigger landslides. Landslides are the deadliest natural hazard in New Zealand, responsible for more deaths than earthquakes and volcanic eruptions combined since written records began in 1843.
Understanding Landslide Triggers
Most landslides in New Zealand are triggered by heavy rainfall, which weakens surface soils and the highly weathered rock beneath them, allowing shallow landslides to detach and flow downslope. The risk of landslides is often invisible, accumulating quietly beneath the surface until a sudden collapse occurs. This complex interplay of intrinsic and extrinsic factors, including slope angle, soil and rock strength, and vegetation cover, makes landslides difficult to predict. However, advanced mapping and modeling techniques can help identify areas of higher vulnerability and predict the likelihood of landslides.
The Landscape Tells a Story
The shape of the land itself can tell a story about past landslides. At Mount Maunganui, the surrounding hill slopes are riddled with the scars of past landslides, revealing a landscape that has been repeatedly reshaped by slope failure over time. New high-resolution mapping techniques, such as LiDAR-derived digital elevation models, can strip away vegetation to reveal the bare land surface, showing numerous landslide features across the slopes. Many of these features cluster along coastal cliffs, and two particularly large ancient landslides can be seen directly above the holiday park. These older slips left behind prominent head scarps, indicating where large volumes of material once detached and flowed downslope onto flatter ground below.
Subsurface Evidence and Progressive Slope Collapse
Subsurface evidence reinforces the picture of a landscape shaped by past landslides. A geotechnical investigation carried out in 2000 near the northern end of the campground’s toilet block found a 0.7-meter layer of colluvium, loose debris deposited by earlier landslides and erosion, buried beneath the surface. The site itself sits atop the remnants of past slope failures. The 22 January landslide appears to have initiated in the narrow zone between the two earlier slips, a particularly vulnerable position where neighboring landslides can cause the remaining wedge of land to lose lateral support and become unstable. Over long timescales, this kind of progressive slope collapse is a normal part of landscape evolution, but when it unfolds in populated areas, it can turn an ancient geological process into a human disaster.
From Prediction to Prevention
Predicting how far a landslide will travel and which areas it might inundate is critically important, but it remains an inexact science. Advanced computer models, such as Rapid Mass Movement Simulation (RAMMS), can simulate how landslide material might flow across the landscape, providing estimates of how far future landslides could travel, how deep the debris might be, and which properties could be affected. These models can be used to generate landslide hazard maps, showing areas of higher and lower risk under different rainfall conditions. These maps are not predictions of exactly what will happen, but they provide crucial guidance for land-use planning, emergency management, and public awareness. Developing similarly detailed and widely available maps for landslide hazards would be a logical and potentially life-saving next step in reducing the risk of landslides in New Zealand.
Conclusion and Future Directions
The recent landslides in the Bay of Plenty are a tragic reminder of the dangers of landslides in New Zealand. By understanding the triggers and mechanisms of landslides, and using advanced mapping and modeling techniques, we can better predict and prepare for these events. Developing detailed and publicly available landslide hazard maps could be a crucial step in reducing the risk of landslides and saving lives. As New Zealand continues to grapple with the challenges of natural hazards, it is essential to prioritize landslide research and mitigation efforts, working towards a safer and more resilient future for all.