Rockfall susceptibility assessment of the rock slopes above the Town of Omiš

Postgraduate Study

Marin Sečanj, mag.ing.geol.

Rock falls are a common phenomenon on the steep slopes and road cuts in the Dinarides. They are the result of unfavourable characteristics of the rock mass, weathering in combination with heavy rainfall and anthropogenic factor. One of the most endangered places by rockfalls is the Town of Omiš, located just at the toe of the steep Omiška Dinara Mt slopes (fig 1). Comprehensive geotechnical investigation financed by the Town of Omiš identified 22 potential rockfall source areas for which was necessary to design remedial measures to ensure protection of buildings, infrastructure, and citizens (Arbanas et al., 2019). Identification of rockfall source areas was performed by conventional field investigations based on visual inspection of the slopes, without rockfall susceptibility analysis, which should be the first step in rockfall hazard and risk reduction.

 

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Fig 1 Damaged house in the Omiš old town centre caused by a rockfall.

Rockfall susceptibility is defined as the likelihood of a rockfall occurring in an area based on local terrain conditions (Brabb, 1984). Rockfall susceptibility assessment at the slope scale is based on the analysis of relation between discontinuities and slope morphology (Matasci et al., 2017).

20200515 9Fig 2 SketchUp of most common questions on which rockfall susceptibility needs to answer.

For collection of all relevant discontinuity parameters, necessary to identify areas prone to rock falls, remote sensing techniques were employed to rapidly obtain information from the rock face, including in the field inaccessible areas. Rock mass structural data can be quickly obtained by mapping on the digital surface model, i.e. by remote sensing on a high-resolution point cloud. To generate high-resolution point cloud, field surveying and collection of spatial data, was performed by phase-shift terrestrial laser scanner (TLS) and by unmanned aircraft vehicle (UAV) (fig. 3). Based on TLS and UAV based photogrammetry method, high-resolution 3D digital model of the investigated Omiška Dinara Mt. slopes was created, which covers an area of around 6,5 ha (fig 4.)

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3. Photos of: (a) Terrestrial laser scanning of Omiška Dinara Mt. slopes andi (b) Omiška Dinara Mt. slopes captured from UAV while surveying the area.

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Fig 4 High-resolution 3D point cloud model of Omiška Dinara Mt.slopes above the town of Omiš.

In total 2.637 discontinuity planes were extracted from 3D models of investigated area, using semi-automatically and manual methods in the Split-FX and CloudCompare software. Spatial kinematic analysis were performed for each cell of digital model, based on orientation data of the slope face and discontinuities. With determined possible failure mechanisms and along with calculation of maximal kinematic hazard index, rockfall susceptible areas were identified (fig 5). Results of analysis have shown that location of the high values of MKHI correspond with previously identified potentially hazardous rock blocks and potential rockfall source areas identified by previous field investigations (fig 5).
The approach using MKHI to estimate susceptibility is a useful tool in preliminary rock fall hazard assessment, where areas with higher KHI values enable more objective selection of representative sources for slope stability analysis and rockfall simulations. In a rockfall hazard assessment the Kinematic Hazard Index should be used in combination with the deterministic rock slope stability analyses which are of crucial importance in a risk reduction by efficient design of rock slope protection measures.

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Fig 5 Results of the automated kinematic analysis and MKHI calculation. The higher MKHI, the higher probability that the analysed instability mechanism will take place. Potential hazardous rock blocks identified by previous investigations are depicted to show overlapping with rockfall susceptibility assessment.

 

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Marin Sečanj, mag. ing. geol. is an assistant at Department of Geology and Geological Engineering, Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb. He is a member of Croatian landslide group and he is currently in final phase of completing his doctoral thesis: Quantitative rockfall susceptibility assessment by integrating kinematic and statistical analyses

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