Landslides

Slide in siltstone in the upper Leader River triggered by the 2016 Kaikōura earthquake

Slides, often called landslides or slips, are the downward movement of rock, debris or earth along a slip plane or surface. Slides can be slow-moving or rapid, and are usually triggered by earthquakes or heavy rainfall. They can be made worse by leaking water pipes, or by earthworks at the bottom of a slope.

The loess (wind-blown silt) that blankets the Port Hills and Horomaka/Banks Peninsula is particularly susceptible to sliding. These slides usually occur after heavy rain, although several slides on the Port Hills were triggered by the 2011 Christchurch earthquake. Many slides also occurred in the soft mudstones of North Waitaha during the 2016 Kaikōura earthquake.

Slides can damage buildings and infrastructure on and at the bottom of the slide and, if fast-moving, can threaten lives.

Damage to a house in Rapaki, Lyttelton Harbour, from several boulders after the 2011 Christchurch earthquake

Rockfalls, or boulder rolls, are when a large rock or group of rocks fall or bounce down a slope, usually triggered by an earthquake.

Individual rocks in a rockfall can bounce or roll a long way along a path and can be difficult to predict.

The 2011 Christchurch earthquake caused rockfalls from the volcanic rock outcrops in the Port Hills that damaged property and led to the loss of lives. Many unstable rocks were removed from the Port Hills after the earthquake, but a high risk of further rockfalls during aftershocks remains in some areas.

Rockfalls are also a hazard in the limestone rocks of the Kaikōura Peninsula and South Waitaha/Waitaki.

Cliff collapse at Redcliffs during the 2011 Christchurch earthquake

Cliff collapse is usually triggered by an earthquake. It involves loss of land from the top of the cliff (cliff top recession) and deposition of the material at the base of the cliff (debris inundation).

The 2011 Christchurch earthquake caused cliff collapses that damaged houses at both the top and bottom of cliffs around the Port Hills and led to the loss of lives.  

Debris avalanche covering State Highway 1 and the rail line near Kaikōura, triggered by the 2016 Kaikōura earthquake

Debris avalanches are landslides made up of soil and rock fragments. They are usually triggered by earthquakes and happen very quickly, destroying everything in their path.

Rock avalanches are very large landslides made up of dry rock that can travel a long way from their source. They are very destructive but usually occur in remote mountainous areas.

Hundreds of debris avalanches were mapped in northeastern Te Waipounamu/South Island after the 2016 Kaikōura earthquake, many of them covering State Highway 1 and the Main Trunk rail line, closing the road and rail for over a year.

There are many mapped rock avalanches throughout Kā Tiritiri o te Moana/Southern Alps. Falling Mountain, on the main divide, was triggered by the magnitude 7 1929 Arthur’s Pass earthquake where material travelled 5 km down the Otehake River.

Debris flow at Rosy Morn on the Kaikōura coast engulfing a house and covering State Highway 1 and the rail line north after Cyclone Gita, 2018 (Photo source: GeoNet)

Debris flows are slurry-like mixtures of water, rock, soil and vegetation that flow rapidly down stream or gully channels and out onto the surrounding land, creating debris fans.

They are triggered by intense rain and can be very dangerous because of their speed and the amount of debris they carry. Because they flow down stream channels, they often damage bridges and roads, particularly in mountainous areas.

Debris flows in Puaha Stream near Little River in 1923 and in Kowhai Stream at Blandswood in 1975 led to the loss of lives.

Rock dislodged from hills during the 2016 Kaikōura earthquake provided material for several damaging debris flows along the Kaikōura coast during the intense rain of Cyclone Gita in February 2018. These debris flows inundated some houses and covered State Highway 1 and the rail line in debris, closing them again only shortly after they had reopened after the earthquake.

 Watch a video of a debris flow in Terrible Gully during Ex-Cyclone Gita.

Hapuku landslide dam and lake created by a rock avalanche (originating from the top left of the photo) during the 2016 Kaikōura earthquake

Landslides can dam rivers that flow through steep terrain, causing water to back up and create a lake. The lakes can breach or overtop the dam in the following hours, days or weeks.

The 2016 Kaikōura earthquake triggered many landslides that dammed rivers in the North Waitaha hill country and Kaikōura ranges. The largest of these was the Hapuku rock avalanche, where 17-21 million cubic metres of rock formed an 80m-high dam across the Hapuku River, behind which a lake quickly formed. The dam and lake level were monitored to detect a dam breach and potential flood down the river, which could affect the State Highway 1 bridge, 9km downstream. The dam partially breached during heavy rain three times in 2017 and 2018. A small lake now remains but no longer poses a significant risk to downstream infrastructure.

River and geotechnical engineers look for and monitor landslide dams after an earthquake. It can take time to locate them all and assess the risk they pose. If a river stops flowing, or the water looks dirty after an earthquake, there could be a landslide dam further upstream and you should stay away from the river.