Landslides, landslips or just "slips"; whatever you call them, New Zealand has lots of them!
Regional landslide studies
We do regional landslip susceptibility mapping, which is useful for Local Authority land use planning and risk avoidance. Other users include logging companies and high country stations.
We collect data on landslides in the Region using aerial and satellite images, on-the-ground observations and historical records, to create a landslide inventory. We then analyse the landslide distribution and compare that to terrain variables such as slope, geology, aspect, faulting, ground cover etc. This statistical analysis produces a map that highlights the areas, assets and infrastructure that are a prone to landslides.
Have you had a landslip?
Landslips are common in New Zealand. When it rains hard or the ground shakes, landslides happen. Fortunately, the mechanisms involved are well understood, so we can identify and minimise landslip risk by analysing a slope’s stability and then designing structures or other methods to prevent, or at least minimise the chance of a landslide.
Typically a field study comprises an analysis of the slope’s shape and the material properties. We test the mechanical properties of the soils and drill or dig a small number of test holes. We then do a slope stability analysis and produce a risk-based plan for further action.
Want to know more?
Rotational landslides are common in deep clay soils, fill material and other soft soil types. Usually triggered by excess rainfall, but other factors include: removal of vegetation, poor stormwater control (point loading of water onto a slope), modification of a slope by excavation and badly built or decayed retaining walls.1 of 9
Translational landslides are similar to rotational landslides, but are usually controlled by layers within the soil or occur where soil is placed on top of inclined bedrock. They are common in deep clay soils, fill material and other soft soil types. Usually triggered by excess rainfall, but other factors include: removal of vegetation, poor stormwater control (point loading of water onto a slope), modification of a slope by excavation and badly built or decayed retaining walls.2 of 9
A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass. Usually occurs as a result of earthquake ground shaking.3 of 9
Rockfalls happen when rocks and boulders become detached from steep slopes or cliffs. Separation occurs along fractures and bedding planes. Movement is by free-fall, bouncing, and rolling, which means that high velocity and long runout distances at the base of the slope are common. Can be triggered by earthquake, rainfall, accidentally by humans, by wildlife or sometimes even by the wind!4 of 9
Toppling failures are the forward rotation of a unit about a pivotal point. The resulting rockfall can be high volume and the debris can reach long distances. Usually triggered by earthquakes.5 of 9
Lateral slides or commonly called “lateral spreads” usually occur on very gentle slopes or flat terrain. The failure is caused by liquefaction, the process whereby saturated, loose, cohesion-less sediments (usually sands and silts) are transformed from a solid into a liquefied state. Usually triggered by rapid ground motion during an earthquake. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may fracture and extend and may then subside, translate, rotate, disintegrate, or liquefy and flow. Extensive lateral spreading occurred around the Avon River during the Christchurch Earthquake.6 of 9
A debris flow (or debris avalanche) is a rapid mass movement of loose soil, rock, organic matter and water mobilised as a slurry that flows downslope, often at great speed. Commonly caused by intense surface-water flow, due to heavy rainfall that erodes and mobilises loose soil or rock on steep slopes. Debris flows also commonly originate from other types of landslides such as translational or rotational slips.7 of 9
With an earth flow the slope material liquefies and runs out, often forming a bowl or depression at the head. The flow itself is elongate and usually occurs in fine-grained materials or clay on moderate slopes and under saturated conditions. However, dry flows of sandy material are also possible.8 of 9
Creep is the slow, steady, downward movement of soil, often not noticed until damage has already happened. Movement is caused by shear stress causing permanent deformation of the soils. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges. Evidence of creep is usually a good indicator that a slope is likely to experience other, more catastrophic landslide types, such as translational, rotational, or debris flows.9 of 9
A landslide is a gravity driven process that carries soil, rock or other debris down a slope. New Zealanders often use the word 'landslip' or ‘slip’ for a landslide.
Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the soil and rock that compose the slope. Additional weight can be applied by buildings, rainfall infiltration or placed fill. Weakening of the slope materials may be a result of weathering, rainfall, or excavation. When the effect of loading overcomes the strength of the soil or rock, the slope will fail. Landslides can be initiated in slopes already on the verge of movement by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors. The most common causes of landslides in NZ are: Heavy rainfall, earthquakes, poorly built or decayed retaining walls, uncontrolled stormwater, and clearing of vegetation (for farming or residential development) that previously supported the slope.
Some landslides such as rockfalls and debris flows can move at very high velocities, and others are almost imperceptibly slow.
Generally, the owner of the land is responsible for repairing a slip on their land. If your neighbours land has moved and caused damage to your land, they have no obligation to repair it, unless they caused the slip by their own negligence.
Having said that, once a landslip risk is identified, landowners do have a general duty of care to act reasonably to prevent or mitigate damage to a neighbour’s property due to a hazard on their land once they are aware of it. So, if you have concerns about a neighbouring property, talk to them first, then get professional advice and inform them in writing, if appropriate.
EQC cover damage caused by all natural disaster events-not just earthquakes! If your house, driveway or land within 8m of your house is damaged, you should lodge a claim with ECQ as soon as possible. You may be able to make a claim with your Private Insurance Company too. You can find out more about what EQC cover here.
A retaining wall lower than 1.5m high does not need a Building Consent. However, if that retaining wall supports a surcharge such as a slope, building or a driveway it does need a Building Consent issued by a TLA.
Note: If the retaining wall's purpose is to stop a landslip from happening, then by definition it is supporting a slope (surcharge), so it will need to be designed by a Chartered Engineer and Consented.
The short answer to this is, yes, you should! If there is evidence of rockfall then there is a high likelihood of rockfall in the future and in the event of an earthquake this may be very significant. Tree planting and other rockfall defences may be a cost effective way to reduce the risk.
The term "erosion" is usually used to describe slow, on-going removal of material from a slope, beach front or a river bank. Whereas a landslide is much more rapid and occurs as a result of an event such as heavy rainfall. Slow, on-going erosion can lead to landslides. EQC does not insure against erosion.
No. Whilst this is a common technique around NZ, this has actually lead to a number of significant damaging landslip events. Tyres will decay in sunlight over time and leave the artificially steepened slope completely unsupported.
A landslide has happened on my property
I'm worried a landslip will happen
If a landslide happens on your property, your first concern should be your own safety and the safety of others at the site.
- In the event of a landslip where lives are in danger, evacuate, warn other people, then dial 111.
- Stay away until authorities give the all-clear. Landslides are often active for hours, or days after the initial slip.
- Check for injured and trapped people, without entering the landslide area.
- Report broken utilities (water, gas, electricity) to the appropriate companies.
- Be aware of any changes to the ground after a major rainstorm or earthquake. Specifically, look for ground cracks or ground bulging.
- Put barriers in place if there’s any risk of people getting too close (danger tape, if you have it!)
- Safely, take photographs of the damage and lodge a claim with EQC by phone (0800 326 243) or online here.
Preventing further damage
When it's safe to do so you can try to prevent any further damage:
- Move any heavy loads (cars, firewood piles, etc) away from the area above the landslip if you can
- Divert water flowing towards the slip.
- You could use sandbags or dig shallow trenches to intercept rainwater runoff
- Divert damaged pipes or extend these to below the landslip
- Drape plastic sheeting over the slip from the crest to stop rain soaking into the slip or eroding the surface.
Once these are in place, contact a local engineering consultancy to provide advice on the next steps. Ask for a Suitably Qualified Person such as an Engineering Geologist or Geotechnical Engineer.
Other things you can do yourself
When the landslip is safe:
- Remove any loose material from the headscarp area by hand (the top of the landslip)
- If ground cracks are present above the landslip these can be filled with clay (usually from the debris)
- Take care when removing debris that you do not destabilise any more ground (particularly if using a digger)
- Replant the exposed ground with native seedlings to help stabilise the ground and prevent erosion
IMPORTANT: Retaining walls designed to stop any further landslip motion should be designed by a Chartered Engineer and a Building Consent granted by a District Council before construction begins.
It's not possible to accurately predict landslides, but there are ways to identify and reduce landslide risk at your property:
Have a look around and identify the risks:
- Look up:
- Are there cliffs above your property? If so you may be at risk of rockfall.
- Is there steep ground (particularly fill slopes, gullies and exposed soils) above your property? If so, you may be at risk from debris flows.
- Are there old or damaged retaining walls above your property? Talk to your neighbours if you are concerned about these.
- Look down:
- Is your property supported by fit-for-purpose retaining walls?
- Does your stormwater discharge onto an unprotected slope? Poorly managed stormwater is a very common cause of landslips in NZ.
- Is the slope below your property artificially steepened, or modified? This can cause instability if not done sensibly
- Is there a creek or river below your property that could cause erosion of the slope?
- Look around:
- Look for damaged or poorly built retaining walls around your property. Failed retaining structures are a very common cause of landslips in NZ.
- Look for blocked or poorly maintained drainage ditches or stormwater pipes. Point-loading of water onto a steep slope is a very common cause of landslides in NZ.
- Look for evidence of creep (this can eventually lead to more catastrophic failure):
- Cracks or bulges in the ground.
- Deformed fences or buildings.
- Settled foundations.
- Cracks in steep driveways.
- Check with your local Council to find out if there is a history of landslips in the area.
- Check the GNS landslide database for local historical landslip information
Reducing the risk of landslips
There are two broad categories for risk reduction: Reduce the likelihood of the event happening, or reduce the consequences if it does occur.
Ideally, you'll prevent landslips from happening by:
- Ensuring that retaining structures are fit-for-purpose.
- Maintaining or installing effective drainage to avoid point-loading of storm-water onto a slope.
- Replanting cleared slopes with native vegetation (even just throwing collected flax seeds will help). Coastal area planting info is here. Forest replanting info is here.
- Constructing retaining walls (expensive option, but sometimes the best option). Get a Chartered Engineer for this.
- Avoid making steep cuts into a slope
- Avoid creating large fill slopes when excavating
- Avoid placing heavy loads onto a slope (vehicles, firewood piles, water tanks, etc.)
- Protect river or stream banks from erosion
Or, you can reduce the consequences by:
Rock fall from above:
- Plant fast growing trees between the rockfall source area and your property to act as a barrier to rockfall.
- Construct engineered rockfall barriers, or "catch fences".
- Construct earth bunds
Debris flows from above:
- Construct deflection fences to channel debris away from your house.
- Construct diversion ditches to channel debris away from your house.
If you're still concerned about the risk of landslips at your property, make an evacuation plan or consider moving to a safer location.
IMPORTANT: Retaining walls designed to prevent landslips should be designed by a Chartered Engineer and a Building Consent granted by a District Council before construction begins.