​ ​ ​ ​ ​
Contact

Search

Landslide

Regional Landslide Inundation Study

The new regional Landslide Debris Inundation (LDI) report provides a comprehensive assessment of landslide hazard risks across the West Coast, as well as projected scenarios for earthquake and rainfall-induced landsliding, which will be exacerbated by climate change. The model uses an innovative approach that determines the likely flux or flow of debris though an area over time, to give an estimate of risk. Find the report here.

Ohau Point Landslides, 2016 Kaikoura Earthquake. Image source GeoNet.

Overview

The West Coast’s rugged, seismically active landscape and high rainfall make it one of New Zealand’s most landslide-prone regions. Landslides can occur suddenly and endanger life, cause severe disruption, block roads, isolate communities, damage property, and create secondary hazards such as landslide dams and flooding.

Buller River Landslide 1929 Buller/ Murchison Earthquake. Photo by Simon Nathan.

What causes landslides?

Heavy rainfall: Prolonged or intense rain saturates hillsides and riverbanks, making steep slopes prone to slipping and debris flows. Rainfall is the most frequent landslide trigger across the region.

Earthquakes: While the Alpine Fault is a major feature, West Coast history shows that many large landslides have resulted from earthquakes on the region’s numerous smaller faults. For example, the 1929 Murchison earthquake (White Creek Fault) and 1929 Arthur’s Pass earthquake (Poulter Fault) triggered widespread slope failures. With a high number of local faults, earthquakes can strike unexpectedly and trigger numerous landslides at once.

Other triggers: Under-cutting by rivers, erosion, vegetation loss, the loss of glacial ice, and land modification from human activities all contribute to slope instability in parts of the West Coast.

Where are landslides most likely?

  • Steep hills and mountain slopes
  • River valleys and below cliffs
  • Sites near historic landslide scars
  • Areas close to mapped active faults, especially those with repeated past activity

Types of landslides

  • Debris flows: Rapid torrents of mud, rocks, and water, often in steep gullies after heavy rain.
  • Rockfalls and rock avalanches: Collapse of cliffs or steep slopes.
  • Soil and earth slips: Ranging from slow movement to abrupt failures following earthquakes or rainstorms.

Hapuku Landslide Dam 2016 Kaikoura Earthquake. Sourced from GeoNet.

Landslide dams

Sometimes, a landslide falls into a river and blocks its flow. This creates a natural dam and causes water to build up behind it, making a lake. These dams are often weak and can burst suddenly or break down slowly, sending floodwaters and debris downstream.

One well-known example is the Poerua River landslide near Harihari in 1999. The landslide created a dam and temporary lake that failed around a week later resulting in a large flood. This was managed well with pre-emptive evacuation resulting in no casualties.

Dam outburst debris 5km downstream of Ram Creek landslide dam (Nash et al, 2008).

The 1968 Inangahua earthquake caused the Ram Creek landslide dam which was stable for 13 years. In April 1981 during a large rainstorm the dam breached in a complete and catastrophic failure. The resultant dam break flood caused substantial damage of roading bridges and farmland up to 6.5km downstream.

After a landslide dam event is noted, geologists will assess the risk of failure and work with emergency managers to determine a risk-based course of action. Landslide dam warning signs may include a river that suddenly slows or stops, becomes unusually dirty, a new pond or lake forming upstream, and rapid changes in river water levels. To Get Ready for landslides click here

1999 Mt Adams landslide and aggradation. 1990 (left) and 2023 (right).

River aggradation

Big landslides can dump large amounts of soil, gravel, and rock into rivers. This extra sediment makes the riverbed rise or “aggrade.” When the riverbed gets higher, the river may change direction, or flood places that were safe before.

After the Mt Adams landslide in 1999, the Poerua River’s bed built up quickly. The river changed course and washed away farmland and some buildings. Other rivers on the West Coast, like the Waiho at Franz Josef, also experience aggradation. This happens when glaciers melt and heavy rain washes more sediment into the river, causing ongoing problems for homes and roads nearby.

What to do

  • Be alert during and after heavy rainfall or earthquakes (even small ones).
  • Report landslide warning signs or sudden changes in rivers to council or emergency services.
  • Have a household emergency plan if you live or travel near hills or river valleys.
  • Monitor your property for warning signs.
  • If a landslide is damaging or threatens your property, please contact the Natural Hazards Commission (Toka Tū Ake) for advice and to make a claim. Visit their web page or phone 0800 DAMAGE (326 243). Please note that the Regional Council does not manage claims or repairs for damage to private property. For all property insurance matters, get in touch with your insurer or the Natural Hazards Commission directly.

Further information

Our Natural Hazard Portal displays the probability that landslide debris will cause damage to buildings beyond economic repair across the region.

  • Check with your district council, some properties may have restrictions on earthworks due to being in an erosion or landslide prone area.
  • If you require more information, please email our Natural Hazards team on naturalhazards@wcrc.govt.nz
Share this

Share this page

Sharing...