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 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.
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.
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.