Assessing the rockfall protection efficiency of forests at the regional scale
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C. Lanfranconi I G. Sala I P. Frattini I G. B. Crosta I A. Valagussa
Assessing the rockfall protection efficiency of forests at the regional scale
Abstract The aim of this paper is to achieve a quantitative assessment of rockfall protection forest efficiency at regional scale, considering site specific forest, morphological and lithological parameters. At first, a semi-automatic GIS-based method, integrated with a multi-scenario 3D-rockfall model realized by using the simulation code HY-STONE, was used to map protection forests of Regione Lombardia (central Italian Alps). For each different forest type, a rockfall protective efficiency was assessed by using empirical (energy line angle) and modelling (HY-STONE) approaches. The empirical approach shows an increase of the energy line angle value from about 36° for the bare slopes to over 40° in forested slopes, with a value ranging from 37° to 44° for different forests types. The modelling approach is based on a new efficiency index EEI ranging from 0 (minimum efficiency, equal to no forest condition) to 1 (maximum efficiency): the efficiency of different forest types ranges from 0.08 to 0.98 by using average values of the controlling parameters. To modulate the efficiency in each single forest at regional scale, a set of parametric simulations was performed to evaluate the effects of controlling parameters. The parametric simulations show that block volume, slope gradient, DBH (diameter at breast height) and forest density are the most important parameters at controlling the efficiency. These parameters were used within a multiple linear regression function to associate a protection efficiency to each specific protection forest in the regional map. This allows to discriminate quantitatively the individual forests according to their actual efficiency. Most of the protection forest area (46%) shows an efficiency greater than 0.50 and only the 24% of the total covered area shows a value lower than 0.25. Keywords Protection forest . Rockfall . HY-STONE modelling . Efficiency . Regional scale Introduction Forests can play a role at defending structures and infrastructures from rockfalls and avalanches (Jahn 1988; Berger et al. 2002; Dorren et al. 2004; Perret et al. 2004). Since many decades, this role is recognized at regulatory level, as witnessed by the Italian Royal Decree 3267 dating back to 1923, which states that “forests that […] defend lands or buildings from avalanches, the rolling of stones, the fury of the winds […] may, upon request of the provinces, municipalities or other interested institutions and private citizens, be subjected to limitations in their use”. Protection forests can be an efficient tool for stopping rockfalls or avalanches detached from upslope or to decrease the probability of primary (for avalanches) and secondary detachments, as in case of unstable blocks located on a debris slope. The role of forests has been studied in the literature through either empirical (Elkin et al. 2013) or mathematical/numerical modelling approaches (Dorren et
