Quantifying the effect of forests on frequency and intensity of rockfalls

Abstract

Abstract. Forests serve as a natural means of protection against small rockfalls. Due to their barrier effect, they reduce the intensity and the propagation probability of falling rocks and thus reduce the occurrence frequency of a rockfall event for a given element at risk. However, despite established knowledge on the protective effect of forests, they are generally neglected in quantitative rockfall risk analyses. Their inclusion in quantitative rockfall risk assessment would, however, be necessary to express their efficiency in monetary terms and to allow comparison of forests with other protective measures, such as nets and dams. The goal of this study is to quantify the effect of forests on the occurrence frequency and intensity of rockfalls. We therefore defined an onset frequency of blocks based on a powerlaw magnitude-frequency distribution and determined their propagation probabilities on a virtual slope based on rockfall simulations. Simulations were run for different forest and non-forest scenarios under varying forest stand and terrain conditions. We analysed rockfall frequencies and intensities at five different distances from the release area. Based on two multivariate statistical prediction models, we investigated which of the terrain and forest characteristics predominantly drive the role of forest in reducing rockfall occurrence frequency and intensity and whether they are able to predict the effect of forest on rockfall risk. The rockfall occurrence frequency below forested slopes is reduced between approximately 10 and 90 % compared to non-forested slope conditions; whereas rockfall intensity is reduced by 10 to 70 %. This reduction increases with increasing slope length and decreases with decreasing tree density, tree diameter and increasing rock volume, as well as in cases of clustered or gappy forest structures. The statistical prediction models reveal that the cumulative basal area of trees, block volume and horizontal forest structure represent key variables for the prediction of the protective effect of forests. In order to validate these results, models have to be tested on real slopes with a wide variation of terrain and forest conditions.

The influence of abiotic and biotic disturbances on the protective effect of alpine forests against avalanches and rockfalls

“…plazovi Along with snow and topography characteristics, forest stand structure is one of the main factors influencing the occurrence of avalanches (Bebi et al, 2001;Holtmeier and Broll;. The most relevant forest stand characteristics in terms of the hazard component of risk (onset probability, propagation probability and intensity) for avalanches are canopy cover, species composition, surface roughness, tree size, stem density, canopy gap size and diameter at breast height (DBH) distribution (Meyer-Grass and Schneebeli, 1992, after Rammig et al, 2006Frehner et al, 2005, after Berger et al, 2013Berretti et al, 2006, after Berger et al, 2013Gauquelin et al, 2006, after Berger et al, 2013Bebi et al, 2009;Moos et al, 2017). Forest has the most important mitigation role in release areas, where it serves the functions of stabilizing the snow pack and intercepting precipitation, while in avalanche tracks the effect of forest is limited to the lateral spreading and slowing down of smaller events (< 100 m 3 ) (Teich et al, 2012).…”

“…Moreover, forested slopes also decrease the bounce height (by 33 %) and velocity (by 26 %) of rocks. The most relevant forest characteristics in terms of the hazard component of risk (onset probability, propagation probabil-ity and intensity) are tree density, canopy gap length, diameter at breast height distribution, species composition, presence of trees in the release area, length of the forested part of the slope and surface roughness Stokes et al, 2005;Frehner et al, 2005, after Berger et al, 2013Berretti et al, 2006, after Berger et al, 2013Stokes et al, 2006;Gauquelin et al, 2006, after Berger et al, 2013Brang et al, 2006;Bebi et al, 2009;Radtke et al, 2013;Dupire et al, 2016;Moos et al, 2017;Moos et al, 2018).…”

“…The required stem density and mean stem diameter can be calculated based on the mean diameter of falling rocks, mean kinetic energy of the rocks, maximum length of the stopping zone and tree species Stokes et al, 2005;Brang et al, 2006). A high basal area should be maintained at the foot of the release area (Radtke et al, 2014;Dupire et al, 2016;Moos et al, 2017). With trees that have DBH ≥ 15 cm, basal area is recommended to be ≥ 25 m 2 /ha in the rockfall propagation area and ≥ 20 m 2 /ha in the rockfall deposit area (Bebi et al, 2009;Berger et al, 2013).…”

“…They occupy steep slopes at high elevations and play a crucial role in stabilizing slopes, thus providing protection for people, settlements and infrastructure (e.g. Schönenberger, 2000;Brang et al, 2001;Brang et al, 2006;Sakals et al, 2006;Moos et al, 2017). Namely, forests can act as a protective barrier, absorbing and dissipating the kinetic energy of gravitational processes and reducing their onset probability (likelihood of initiation of a process), propagation probability (probability of spatial occurrence) and intensity (size and velocity) (e.g.…”

“…Wehrli et al, 2006). The protective effect of forest is related to the ability of a forest stand to withstand disturbance without being altered (resistance), and to its adaptive and regenerative capacity (resilience) (Moos et al, 2017). The resilience and resistance of forest stands to natural disturbances, and consequently their capacity to protect against natural hazards, is strongly related to stand parameters that describe the structure of the forest (e.g.…”