Rock, debris and earth falls
A rock, debris or earth fall is a fragment of rock (a block), body of debris or earth (here taken to include mud) detached by sliding, toppling, or falling, that falls from a vertical or sub-vertical cliff and proceeds down slope by bouncing and flying along ballistic trajectories or by rolling on talus or debris slopes (adapted from Highland and Bobrowsky, 2008).
Primary reference(s)
Highland, L.M. and P. Bobrowsky, 2008. The Landslide Handbook – A guide to understanding landslides. U.S. Geological Survey Circular 1325.
Annotations
Additional scientific description
Falls are abrupt, downward movements of rock or earth, or both, that detach from steep slopes or cliffs. The falling material usually strikes the lower slope at angles less than the angle of fall, causing bouncing. The falling mass may break on impact, may begin rolling on steeper slopes, and may continue until the terrain flattens (Sassa et al., 2018). Materials from falls sometimes generate flows (HIP Ref) such as avalanches.
Metrics and numeric limits
Very rapid to extremely rapid, free-fall; bouncing and rolling of detached soil, rock, and boulders. The rolling velocity depends on slope steepness (Hungr et al., 2014).
Key relevant UN convention / multilateral treaty
Sendai Framework for Disaster Risk Reduction 2015-2030.
Drivers
Drivers for rockfall include undercutting of slopes by natural processes such as streams and rivers or differential weathering (such as the freeze/thaw cycles and heave and desiccation cycles related to sustained high temperatures), human activities such as excavation during road building and (or) maintenance. Volcanic activities and earthquake shaking or other intense vibration are also drivers for rockfall (Hungr et al., 2014).
Impacts
Falling material 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. Damage to roads and railroads is particularly high: rockfalls can cause deaths in vehicles hit by rocks and can block highways and railroads (Highland and Bobrowsky, 2008).
Multi-hazard context
The figure below summarises common interactions between falls and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.
Multi-hazard diagram
Risk Management
Mitigation measures for rockfall include rock curtains or other slope covers, protective covers over roadways, retaining walls to prevent rolling or bouncing, explosive blasting of hazardous target areas to remove the source (scaling), removal of rocks or other materials from highways and railroads can be used to minimise risk (Sassa et al., 2018).
Rock bolts or other similar types of anchoring used to stabilise cliffs, as well as scaling, can lessen the hazard. The removal of loose boulders can also lessen the hazard. Warning signs are recommended in hazardous areas for awareness. Stopping or parking under hazardous cliffs should be warned against (Highland and Bobrowsky, 2008).
Monitoring
The section and the table below offer an overview of monitoring spreads. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.
| Which institution(s) produce(s) Disaster Risk Data/Information? | Local Authorities; Geological Surveys may produce disaster risk data/information |
| How is the Hazard Observed/Monitored/Forecast? | Slopes or cliffs that falls may detach from can be monitored via satellite imagery, InSAR, and visual observations, among others. |
References
Highland, L.M. and P. Bobrowsky, 2008. The Landslide Handbook – A guide to understanding landslides. U.S. Geological Survey Circular 1325.
Hungr, O., S. Leroueil and L. Picarelli, 2014. The Varnes classification of landslide types, an update. Landslides, 11:167-194.
Sassa, K., F. Guzzetti, H. Yamagishi, Z. Arbanas, N. Casagli, M.J. McSaveney and K. Đặng (eds.), 2018. Landslide Dynamics: ISDR-ICL Landslide Interactive Teaching Tools.
Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L. and R.J. Krizek (eds), Landslides, Analysis and Control. Special report 176: Transportation Research Board, National Academy of Sciences, pp. 11-33.