Key Takeaways:
- Geotechnical engineers advised Tauranga City Council 20 years ago to avoid building in landslide "runout" zones without special protection, but the advice was not consistently followed.
- New science has led to smaller runout zone sizes in some neighborhoods, but the area around Mauao was not mapped.
- Auckland Council has made advances in landslide mapping and risk assessment, which could inform development decisions and help direct future development away from high-risk areas.
- Experts recommend avoiding building in runout areas and investing in in-house technical expertise to support decision-making.
- Landslide risk is a significant concern in New Zealand, particularly in areas prone to heavy rainfall and steep slopes.
Introduction to Landslide Risk
The recent landslide at Mauao, Mount Maunganui, which resulted in the loss of several lives, has highlighted the importance of understanding and mitigating landslide risk. Geotechnical engineers had advised Tauranga City Council two decades ago to avoid building in landslide "runout" zones, which are areas at the base of a slope that may be affected by a landslide, unless special protection measures were in place. However, it appears that this advice was not consistently followed, and the area around Mauao was not mapped for landslide risk.
Historical Context of Landslide Risk Assessment
In 2005, a study into over 300 landslides across Tauranga triggered by a big storm recommended that buildings should not be located within runout zones unless special measures were taken, such as the construction of retaining walls or piled foundations. The study suggested that the runout zones should be calculated as 4H:1V, where H is the horizontal distance from the base of the slope and V is the vertical height of the slope. However, new science has since led to the adoption of smaller runout zone sizes in some neighborhoods, with a suggested ratio of 2H:1V. Despite this, the area around Mauao was not included in the mapping, and it is unclear how the runout calculations might apply to this area.
Advances in Landslide Mapping and Risk Assessment
Auckland Council has made significant advances in landslide mapping and risk assessment, which could inform development decisions and help direct future development away from high-risk areas. The council has developed statistical models that can predict landslide susceptibility, which are now being used to inform planning decisions and Land Information Memorandums (LIMs). This means that people can make more informed decisions when choosing where to buy or rent a house, and infrastructure and asset owners can assess and modify their use of existing assets if necessary. While checks on individual sites are still sometimes needed, the new mapping and modeling capabilities provide a more comprehensive understanding of landslide risk.
Expert Recommendations and Concerns
Experts recommend avoiding building in runout areas and investing in in-house technical expertise to support decision-making. Professor Martin Brook, an applied geology expert, notes that understanding landslide runout is crucial, particularly in areas with a history of slips like Mauao. He emphasizes that avoiding building in runout areas is a sensible approach, but acknowledges that there is limited research on runout and that most of New Zealand is under-researched from a geological standpoint. Geotechnical engineer David Buxton highlights the challenges faced by councils in obtaining technical advice and making informed decisions, and suggests that investing in in-house technical expertise could improve the response to landslide risk.
Council Response and Future Directions
Tauranga Mayor Mahé Drysdale has announced an independent review into the facts and events leading up to the landslide, and Auckland Council has shared its expertise and advances in landslide mapping and risk assessment with Tauranga and the Bay of Plenty region. While there are still gaps in the understanding and management of landslide risk, the recent advances and expert recommendations provide a foundation for improving the response to this significant natural hazard. By prioritizing landslide risk assessment and mitigation, councils and communities can work together to reduce the risk of landslides and protect people and property.