Simply put, landslides are down-slope movement of rocks and soil. They can happen slowly, over the course of years; or quickly, in the blink of an eye.
It's little wonder that landslides hit the headlines with such frequency. Several factors make the Land of the Long White Cloud more susceptible than other countries - including steep, rugged topography, variable weather patterns and a geologically restless landscape associated with our location on a plate boundary.
Apart from earthquake-induced landslides and human activity to steepen slopes, prolonged or intense rainfall is mostly the trigger for large, sudden downward movement of earth. Things start getting precarious when water saturates the ground faster than it can drain out.
The most serious landslides threaten human life, wreak havoc on local communities, disrupt transport and communications, and damage buildings and infrastructure. Cue a wet winter and the devastating recent events in Nelson, Marlborough and Wellington.
Climate change is projected to result in higher rainfall in many parts of the country. In fact, we are already seeing this increased rainfall. Crown Research Institute GNS Science notes an increased frequency of rainfall-induced landslides since 2000, causing damage to roads, bridges, houses, hill country farms and other property. These are costing the country an average of $250-$300 million each year so it's understandable that nerves are fraying about the prospect of larger, even more damaging slips.
An increase in extreme weather events like the record breaking atmospheric river that descended with such destructiveness in August can also be expected with climate change – upping an already high landslide risk. But there are reasons to be optimistic.
| Omoto Road slip, Greymouth – 1988.
We understand more about landslides
The science of landslides is improving all the time. This is important because mitigating risk starts with better understanding the nature of the hazard. By studying how and where landslides form and move, engineers can better develop measures to limit the impacts and consequences.
Thanks to comprehensive geological and climate mapping done over decades by Crown Research Institutes, Aotearoa’s science and engineering community has a good handle on where the greatest risk lies.
Developing accurate hazard maps is an ongoing process. More data is always coming to light. Recent studies done for Hutt City, Tauranga City and Bay of Plenty, for instance, have mapped new areas susceptible to landslides - adding to the geotechnical body of knowledge of where’s safe, and where’s not. Elsewhere, Auckland Council is documenting the location of existing landslides in a new landslide database to supplement GNS’ national landslide database.
Because the natural environment is constantly in flux, more work is always needed in this space. Using disasters as a real-life laboratory to learn lessons is a technique that engineers and geologists use to better understand these natural events. WSP is currently researching how best to assess slopes and design them to address landslide hazards from earthquakes and follow-on storms, funded by MBIE’s Endeavour Fund with GNS Science. The aim of the project is to learn from the landslides, debris generated and their impacts from the Kaikoura earthquake.
We know to focus development in low-risk zones
Landslides wreak havoc only when we build in harm's way, increasing our risk to landslides. One of the best ways to minimise landslide risk is to avoid building in high-risk areas. Prudent land use planning and development controls by councils can reduce the impact of slips on future land use development.
In a risk-based planning approach, residential and commercial development is prohibited in high-risk zones. Nelson’s famous Tahunanui Slump is one example of an active slip that, over the decades, had been built on. Residential development has been restricted there since 1985.
The geotechnical and engineering community has a key role to play in this area. WSP, for example, recently provided input into Whanganui District Council’s District Plan to develop measures to manage the risk of landslides to development.
We understand the impacts on the built environment
Understanding the impacts of natural hazards, including landslides, on lifelines and their resilience is another important step that authorities can take. We need to (and can do) more to understand the resilience of our built environment. Doing so is an important part of planning for emergency response, and for introducing risk mitigation measures.
Wairarapa’s transport resilience study and Wellington’s transport resilience business case are examples where resilience assessments are helping communities develop and prioritise measures to enhance resilience. For the past 20 years, Wellington City Council has been investing in enhancing road resilience, following on from earlier resilience studies. This is having a positive impact on transport resilience in the city.
We can manage the risk of landslides to existing infrastructure and buildings
Monitoring known hazard areas with the latest in smart sensing tech helps scientists and geotechnical specialists learn more about the physical processes that trigger landslips, and how best to control their movement. WSP Research is doing this with KiwiRail on a slip-prone stretch of rail between Paekākāriki and Plimmerton on Wellington’s Kāpiti Coast.
Existing property and infrastructure in high-risk areas, of course, will require strengthening measures. Or, in a worst-case post-event scenario, may have to be relocated. There’s no two ways about it - this will be costly. But a variety of engineering techniques can be used to reduce the risk to bricks and mortar in hazardous areas.
Slopes can be reshaped, sealed, drained, reinforced or reforested. WSP, for example, has developed measures to reshape, reinforce, vegetate and strengthen slopes posing a risk to buildings at Wellington East Girls College - done as part of the development of the school buildings. We are also implementing measures to stabilise slopes against storm events using anchors, soil nails, netting and fences for the state highway and rail networks.
As with many things, it’s more useful to face the landslip issue in time, rather than seek a remedy after the damage is done and a lot of disruption has been experienced.
We can manage the risk to new developments and infrastructure . . . and this doesn’t need to cost more
Landslides have always been a feature of life here. Clearing landslide with shovels – date unknown. Photo by Tudor Washington Collins, 1898 – 1970.
Early focus on resilience and sustainability will allow us to plan and build our infrastructure in locations and using methods to minimise the resilience impacts of potential landslides. That way we can manage the consequences of landslides at minimal costs.
One thing that recent events have again taught us is that landslides can have a devastating impact on people, property and infrastructure. That’s why it’s so important to approach the development of the built environment sensibly and take advantage of the latest in hazard mitigation and resilience strategies.
A case in point is the Transmission Gully motorway. An early focus on resilience has meant that it was planned in a way that survived and provided access in a recent storm, while the pre-existing State Highway 59 was closed for a few weeks.
With the right blend of avoidance, effective mitigation and resilience, communities can bounce back quicker and reduce the magnitude of the physical and human costs from landslides – even in the face of an evolving, uncertain climate.
It's imperative that engineers, geologists, planners and policy makers work together and implement effective strategies to manage the risk from landslides, climatic and other natural hazards in a holistic manner. Now, more than ever, it’s become an economic and social necessity.