Impact of mapping strategies on rockfall frequency-size distributions
Introduction
A rockfall is a type of extremely rapid mass movement characterized by a potentially long travel distance from the release (source) to the deposition area. Local rock properties and slope geometry condition the rockfall behaviour, making it difficult to prepare rockfall inventories with consistent and comprehensive spatial and temporal information. Methods used to collect information on rockfalls depend on the size and complexity of the study area, and on the scope and the resources available for the investigation, and include (i) field mapping, (ii) systematic search of archives, chronicles, newspapers and technical and event reports, (iii) visual inspection and in-situ monitoring of rock cliffs, (iv) interpretation of remote sensing imagery or photogrammetry, and (v) rockfall dating techniques.
In the literature, different statistical methods were proposed to analyse spatial, temporal, and size information of rockfalls. Implicitly or explicitly, all methods require “completeness” and “representativeness” of the rockfall catalogues and series (Corominas et al., 2017a; De Biagi et al., 2017b, De Biagi et al., 2017b; Malamud et al., 2004; Rossi et al., 2010). However, determining the level of completeness or the representativeness of the catalogues and time series is not trivial, and this can jeopardize the significance of the statistical analyses performed on the rockfall records.
Published rockfall statistics are rare, covering mostly small areas (slope scale) and short time periods, and are focused mainly on volume-frequency analysis or for the definition of a “design block” (Agliardi et al., 2009; Brunetti et al., 2009; Corominas et al., 2017a, Corominas et al., 2017b; Crosta et al., 2015; De Biagi et al., 2017b; Dussauge et al., 2003; Dussauge-Peisser et al., 2002; Guzzetti et al., 1994; Lambert and Bourrier, 2013; Lari et al., 2014; Macciotta, 2014; Malamud et al., 2004).
In this work, we analyse 15 catalogues with information on the size of rockfalls obtained using different mapping methods for seven study areas in Austria, Italy, and the USA. We compare the rockfall information, and we discuss the impact of the different mapping methods on the quality, representativeness, and completeness of the size distributions of the rockfalls.
Different statistical approaches are introduced and exploited to estimate frequency-size rock fall statistics. The advantages and limitations of the different approaches are presented and discussed providing a guidance for their proper usage and promoting standard and comparable procedures to analyse frequency-size rock fall statistics for the definition of a design block, which is a fundamental part when it comes to the planning of technical rockfall protection measures.
Section snippets
Definitions
In this work, we define a “primary rockfall” a rock that detaches from a rock wall or rock outcrop by sliding, toppling or falling, and a “secondary rockfall” the remobilization of a previously deposited (rockfall) boulder, typically on a slope. After detachment, a rock either falls along a rock wall or moves along a slope by bouncing and flying along ballistic trajectories, or by rolling and sliding. A rock may fragment into individual rockfall boulders impacting the ground and/or trees. The
Rockfall data sets
We used 15 catalogues listing rockfall volume in Austria, Italy, and the USA, to analyse the impact of the mapping methods on the quality, representativeness, and completeness of the size distribution of the rockfalls. Some of the catalogues are parts of comprehensive rockfall inventories comprising detailed information about other relevant rockfall features. Different mapping methods and strategies were adopted to collect the rockfall data in the different catalogues (Table 1).
Statistical analysis
We first use the
Results
To identify the factors influencing the volume size distribution we considered factors such as (i) lithology and the structural setting (catalogues CUM, CBB, COM, CBT, CH, CYV), (ii) the position of the rockfalls on the slope (catalogues CH4, CH6 and CH7), (iii) topographic factors, including slope geometry, and the presence of lakes and rivers at the bottom of the slope (catalogues CH2 and CH4), and (iv) possible human influences, including e.g., removal of boulders by local habitants.
Discussion and conclusions
We analysed fifteen catalogues listing information on rockfall size in Austria, Italy, and the USA with respect to the impact of mapping strategy i.e., choice of mapping method and data source. Results showed that specific factors have an impact on frequency-size distributions of the rockfalls. These factors should be considered when planning and conducting data collection campaigns i.e., the definition of a “boulder scenario” (or design block). The main results of this work can be summarized
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The Geological Survey of Austria is thanked for supporting S.M. during the paper completion also after leaving the Geological Survey. Special thanks to the National Park Service, CA, for the Yosemite rockfall dataset.
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