Rockfall and cliff collapse in the Port Hills

Rockfalls are abrupt, downward movements of rock or swarms of rocks that detach from steep slopes or cliffs. The falling material usually strikes the lower slope causing bouncing. The falling mass may break on impact, may begin rolling on steeper slopes, and may continue until the terrain flattens. Rolling boulders may be focussed down gullies or may fan out over a wider area if the slopes are not dissected by gullies.

Falling rock material and falling boulders can be life-threatening. Falls can damage property beneath the fall-line of large rocks. Boulders can bounce or roll great distances and damage structures or kill people in buildings, vehicles or outdoors. Rockfall can also cause blockages of critical roads.

Where does rockfall occur?

Rockfall is common on steep or vertical slopes. The volume of material in a fall can vary substantially, from individual rocks or clumps of soil to massive blocks hundreds of cubic metres in size.

Rocks and boulders can travel very rapidly, either falling freely, or bouncing and rolling. The rolling velocity (or speed) depends on how steep the slope is, and the nature of the surface of the slope. Vegetation, for example, would affect the speed of the boulder.

Rockfall triggers

Rockfall can be caused by earthquake shaking, weathering effects (such as the freeze/thaw cycle), and human activities such as excavation during road building.

Rockfall – management and mitigation

People’s safety is the number one priority in any decisions around mitigating and managing slope stability hazards.

Mitigating rockfall

Treatment at source to reduce the rockfall hazard may include removal by scaling, blasting, containing loose rock behind rock mesh or other slope covers, stabilisation by use of rock bolts or other similar types of anchoring.

On the slope, engineered earthen bunds and/or catch fences or training walls can be used to prevent rolling or bouncing to areas lower on the slope or to direct boulders down gullies. Protective covers over roadways may be an option in very steep terrain.

In addition to these measures, warning signs are commonly used to advise against stopping, parking or walking under slopes believed to be susceptible to rockfall. You will see signs like this in areas of the Port Hills.

Managing rockfall

The Crown red-zoning identified properties where rockfall (and cliff collapse) presents a risk to life. Areas identified as having a life risk from rockfall and cliff collapse are subject to CERA Section 45 notices, meaning access is prohibited due to the risks these hazards pose.

To manage rockfall risk, the Christchurch Replacement District Plan contains two Rockfall Hazard Management Areas, based on likely risk. The proposed areas would see some development activities restricted. While both areas have restrictive controls proposed for activities such as new building, subdivision and earthworks, Rockfall Hazard Management Area 1 has been identified as an area at greater risk and the rules proposed reflect that. For example, new buildings and structures in Area 1 will become a non-complying activity meaning that any development has to pass a very high test regarding safety and other considerations before it can be allowed to occur.

If you live in an area affected by rockfall, your property may be in Area 1 or Area 2, or partly in both. These District Plan initiatives do not affect your existing use rights. This means you can continue to maintain your existing home, for example, but will require resource consents for additions or new buildings that go beyond your existing use rights.

Property owners can build a rock protection structure to protect individual homes on private land. If you are looking to build a rockfall protection structure, we recommend speaking to our consenting team to understand the requirements for your specific site. You can contact the duty planner on 03 941 8999.

Residents with a red zone offer from the Crown could apply to the Council for assistance in funding a private rockfall protection structure. These funding applications have now closed.

Technical Guidelines for Rockfall Protection Structures

In November 2016, The Ministry of Business, Innovation and Employment (MBIE) released technical guidelines for rockfall protection structures, which replace the former Council guidelines. You can find the MBIE guidelines here.(external link)

Cliff collapse as a result of the earthquakes has affected sea cliffs at Whitewash Head and other headlands around Banks Peninsula, and old (abandoned) sea cliffs at Redcliffs, Peacocks Gallop and along Wakefield Avenue in Sumner.

Smaller cliff failures have also been recorded on old cliffs within McCormacks Bay and in some former quarries. Historically these cliff areas have been relatively stable, with only isolated local failures involving relatively small volumes of material.

The 4 September 2010 Earthquake had very little impact on these cliffs, but on 22 February and 13 June 2011 widespread cliff collapse resulted from the extremely high levels of earthquake shaking generated by active faults directly below the Port Hills.

A cliff collapse begins with the detachment of soil and/or rock from the cliff, most often along a pre-existing fracture or weakness. The material subsequently falls, and may break up in flight or on impact. The collapse material will normally accumulate as a debris wedge or talus slope at the base of the cliff but falling rock impacting on the debris may travel significant distances as ‘fly rock’.

Considerable loss (recession) of the cliff top is associated with these events. Additional cracking of the cliff top behind the collapse zone is common.

Rocks falling from a cliff move very rapidly with a combination of free-fall, bouncing and rolling of detached soil, rock, and boulders.

Cliff collapse triggers

The main cause of cliff collapse is earthquake shaking. Other processes, like the undercutting of slope by natural processes such as sea or river erosion, can trigger events.

Material falling as a result of cliff collapse can be life-threatening. Falling debris can damage property beneath the fall zone, and boulders that bounce or roll can damage structures or kill or injure people in houses, on roads or in open spaces.

Collapse of the ground behind the cliff top can also be life threatening. Cracking can damage buildings and indicate that further, more complete, collapse is likely.

Can cliff collapse be predicted?

Indicators of imminent cliff collapse include ongoing cracking of the cliff top and sometimes the presence of overhanging rock or fractured rock in the cliff face. Very strong earthquake shaking that may trigger collapse cannot be predicted reliably.

Management and mitigation of cliff collapse

People’s safety is the number one priority in any decisions around mitigating and managing slope stability hazards.

Managing cliff collapse

Engineering solutions to manage the risk from cliff collapse are possible, but tend to be very expensive, primarily due to the amount of land which has to be stabilised.

The Crown Red-Zoning identified properties where cliff collapse (and rockfall) presented a risk to life and provided an opportunity for residents to retreat.

Areas identified as having a life risk from cliff collapse and rockfall are subject to CERA Section 45 notices, meaning access is prohibited due to the risks these hazards pose.

To manage cliff collapse risk, the Christchurch Replacement District Plan contains two Cliff Collapse Hazard Management Areas, based on likely risk. The proposed areas would see some development activities prohibited or require resource consent approval. While both areas have restrictive controls proposed for activities such as new building, subdivision and earthworks, Cliff Collapse Hazard Management Area 1 has been identified as an area at greater risk and the rules proposed reflect that. For example, some new buildings and structures in Area 1 will become a non-complying activity meaning that any development has to pass a very high test regarding safety and other considerations before it can be allowed to occur.

If you live in an area affected by cliff collapse, your property may be in Area 1 or Area 2, or partly in both. These District Plan provisions do not affect your existing use rights. This means you can continue to maintain your existing home, for example, but will require resource consents for additions or new buildings that go beyond your existing use rights.

Mitigating cliff collapse

Large cliffs are very difficult to stabilise, even when large amounts of money are spent. Rock bolts or other similar types of anchoring can be used to stabilise smaller cliffs, and scaling of loose or unstable rock can lessen the hazard but cannot remove the hazard.

Rock curtains (also referred to as attenuators) or other slope covers, protective shelters over roadways, removal of rocks or realignment of key lifelines can be used as other mitigation methods.

In addition to these measures, warning signs are commonly used to advise against stopping, parking or walking under hazardous cliffs. You will see signs like this in areas of the Port Hills.

The options for cliff tops are most commonly limited to avoidance of at risk areas. Cliff edge stabilisation may be attempted, but is very expensive and must be considered in relation to the stability of the whole cliff not just an individual site.