Hazard and risk classification of unstable rock slopes

In 2012, NGU published a national system for hazard and risk classification of unstable rock slopes. This system is used in order to prioritize detailed investigations, periodic displacement monitoring, and continuous monitoring at a national level.

Figure: Map of the unstable rock slope Mannen in Møre og Romsdal County, showing the unstable area, the run-out area, and buildings within the run-out zone. 

The classification system is used to provide a hazard and risk assessment for each unstable rock slope. It is now a standard tool for defining the level of risk, and the basis for decisions regarding further actions. An overview of necessary follow-up activities, and the conditions of how and where they will be performed, will be specified in a forthcoming NVE document.

The classification of risk involves two steps; a hazard assessment and a consequence assessment. The hazard level is based upon a series of geomorphological and structural geological criteria (development of back-scarp and flanks, morphological signs of the failure surface, kinematic analysis), as well as signs of activity (displacement rates and acceleration over time, increased rockfall activity and previous rock avalanche events). The result is a hazard score between 0 (very low hazard) and 12 (very high hazard). Due to the use of probabilities for each of the criteria, an uncertainty can be determined for the hazard score. The final score for a given area is a hazard level with minimum, average and maximum values. See Hermanns et al. (2012) for details.

The consequence assessment focuses on potential loss of life associated with a rock avalanche scenario. No other consequences are considered in the analysis. The number of residents in the run-out areas is retrieved from population data from Statistics Norway. In addition, standard values are defined for the number of people in various types of commercial building, similar to the methodology used for the national plan for landslide hazard mapping (Devoli et al, 2011). The minimum, average, and maximum values for the number of people in commercial buildings are used. Information about building types is retrieved from cadastral data.

The risk for each scenario is determined using a log-normal chart, which plots hazard scores against potential loss of life. The chart defines three general risk levels: low, medium and high. In the risk matrix, the risk of each scenario is represented as a point, with uncertainty bars for hazard and consequences axes.
 

NGU report: 

  • NGU Report 2012.029
Example of a risk matrix, with hazard plotted against impact. The diagram is divided into three general risk zones: high in red, medium in pink and low in blue.
Example of a risk matrix, with hazard plotted against impact. The diagram is divided into three general risk zones: high in red, medium in pink and low in blue.