Cost-Benefit Analysis as a Basis for Risk-Based Rockfall Protection Forest Management

Mountain forests fulfill an important protective effect being the reduction of risk due to natural hazards. Knowing the value of this service is required to efficiently allocate financial resources in protection forest and risk management. In this chapter, we evaluate the protective effect of forests against rockfall at local and regional scale using a risk-based approach. We present a method to quantify rockfall risk under current forest conditions for a case study region along the Gotthard highway (Switzerland). Rockfall runout zones and relative requencies were determined based on the energy line principle and occurrence frequencies were estimated based on inventory data. We quantified the protective effect of the current forest using a statistical approach and calculated the potential risk without forest. The risk reduction provided by the forest varies between 23 and 60% or 400 and 4500 CHF/(year.ha−1). In a second step, we evaluated a single protection forest complex calculating its Net Present Value (NPV) for a time frame of 100 years based on the risk reduction and compared it to technical protection measures. The NPV of the current forest is positive, whereas protection measure variants including rockfall nets have a highly negative NPV. The results evidence the efficient riskreduction of rockfall protection forests. The presented methods allow for a differentiated procedure for protection forest planning at local and regional scale. A simple risk approach requiring a manageable data set enables practitioners to prioritize forest management. A more detailed economic analysis of protection forest efficiency finally facilitates the planning of protection forest measures at local scale

Quantifying the effect of forests on frequency and intensity of rockfalls

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 power-law 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

Cost-Benefit Analysis as a Basis for Risk-Based Rockfall Protection Forest Management

Mountain forests fulfill an important protective effect being the reduction of risk due to natural hazards. Knowing the value of this service is required to efficiently allocate financial resources in protection forest and risk management. In this chapter, we evaluate the protective effect of forests against rockfall at local and regional scale using a risk-based approach. We present a method to quantify rockfall risk under current forest conditions for a case study region along the Gotthard highway (Switzerland). Rockfall runout zones and relative frequencies were determined based on the energy line principle and occurrence frequencies were estimated based on inventory data. We quantified the protective effect of the current forest using a statistical approach and calculated the potential risk without forest. The risk reduction provided by the forest varies between 23 and 60% or 400 and 4500 CHF/(year.ha−1). In a second step, we evaluated a single protection forest complex calculating its Net Present Value (NPV) for a time frame of 100 years based on the risk reduction and compared it to technical protection measures. The NPV of the current forest is positive, whereas protection measure variants including rockfall nets have a highly negative NPV. The results evidence the efficient risk reduction of rockfall protection forests. The presented methods allow for a differentiated procedure for protection forest planning at local and regional scale. A simple risk approach requiring a manageable data set enables practitioners to prioritize forest management. A more detailed economic analysis of protection forest efficiency finally facilitates the planning of protection forest measures at local scale

Impacts of soil erosion

Lorren, Luuk et al.-- 11 páginas, 1 tabla, 42 referencias.-- Volumen II: Taskgroups on Soil Erosion.-- La serie completa consta de seis volúmenes, en total 872 páginas.-- [email protected] Definition of soil functions, soil quality and quality targets The identification of soil functions, properties and processes which are affected by soil erosion is needed to evaluate the impacts of erosion on the soil system. Definition of soil loss tolerance according to soil types and environmental characteristics. 3.2 Development of criteria and indicators to assess soil sustainable use and soil protection measures What are the impacts of soil erosion on soil functioning and soil quality? How does soil erosion affect environment health and security? The efficiency of soil protection and conservation measures must be evaluated by measuring the reduction of the soil erosion impacts. 3.3 Development of criteria and indicators to assess off-site impacts What are the impacts of soil erosion in down slope or downstream areas, i.e. the off-site effects? 3.4 Development of studies of the economic impact of soil erosion. Review and extract conclusion of existing studies. Development of specific studies on the social, health and economic impact of erosion.Peer reviewe

Numerical modeling using an elastoplastic-adhesive discrete element code for simulating hillslope debris flows and calibration against field experiments

This paper presents a discrete-element-based elastoplastic-adhesive model which is adapted and tested for producing hillslope debris flows. The numerical model produces three phases of particle contacts: elastic, plastic and adhesive. A parametric study was conducted investigating the effect of model parameters and inclination angle on flow height, velocity and pressure, in order to define the most sensitive parameters to calibrate. The model capabilities of simulating different types of cohesive granular flows were tested with different ranges of flow velocities and heights. The basic model parameters, the microscopic basal friction (ϕb) and ratio between stiffness parameters √k1/k2, were calibrated using field experiments of hillslope debris flows impacting a pressure-measuring sensor. Simulations of 50 m3 of material were carried out on a channelized surface that is 41 m long and 8 m wide. The calibration process was based on measurements of flow height, flow velocity and the pressure applied to a sensor. Results of the numerical model matched those of the field data in terms of pressure and flow velocity well while less agreement was observed for flow height. Those discrepancies in results were due in part to the deposition of material in the field test, which is not reproducible in the model. Results of best-fit model parameters against selected experimental tests suggested that a link might exist between the model parameters and the initial conditions of the tested granular material (bulk density and water and fine contents). The good performance of the model against the full-scale field experiments encourages further investigation by conducting lab-scale experiments with detailed variation in water and fine content to better understand their link to the model's parameters

Quantifying the long-term recovery of the protective effect of forests against rockfall after stand-replacing disturbances

Introduction: Increasing disturbances may significantly impact the long-term protective effect of forests against natural hazards. Quantifying the temporal development of the protective effect of forests is thus crucial for finding optimal management strategies. Methods: In this study, we analyzed the long-term recovery of the protective effect of the secondary stands of spruce (Picea abies), fir (Abies alba), and beech (Fagus sylvatica) forests against rockfall after stand-replacing disturbances based on data of the Swiss National Forest Inventory (NFI). We therefore derived the age of the inventoried forest stands of those tree species based on a growth parametrization and quantified their energy dissipation capacity in rockfall processes as a function of stand age. We then analyzed the development of their protective factor for varying rockfall dispositions. Results: Generally, it takes between 50 and 200 years to regain the maximum possible protective effect, depending from the site conditions and the rockfall disposition. This implies that the recovery of the protective effect after a severe disturbance may require more time than the decay of the protective effect from disturbance legacies, resulting in a long lasting gap of the provided protection. Discussion: The here presented approach can serve as a basis to estimate the general range of recovery of the protective effect of beech, fir and spruce forests against rockfall provided by forest stands. Future research should analyse the effects of environmental and forest conditions as well as varying disturbance intensities and legacies to enable the assessment of specific trajectories of the short- and long-term recovery of the protective effect

Temporal trends in the protective capacity of burnt beech forests (Fagus sylvatica L.) against rockfall

Beech (Fagus sylvatica L.) forests covering relief-rich terrain often provide direct protection from rockfall for humans and their property. However, the efficacy in protecting against such hazards may abruptly and substantially change after disturbances such as fires, windthrows, avalanches and insect outbreaks. To date, there is little known about the mid-term evolution of the protective capacity in fire-injured beech stands. We selected 34 beech stands in the Southern European Alps that had burnt in different intensity fires over the last 40 years. We inventoried all living and dead trees in each stand and subsequently applied the rockfall model Rockfor.net to assess the protective capacity of fire-injured forests against falling rocks with volumes of 0.05, 0.2, and 1 m3. We tested forested slopes with mean gradients of 27°, 30°, and 35° and lengths of 75 and 150 m. Burnt beech forests hit by low-severity fires have nearly the same protective capacity as unburnt forests, because only thin fire-injured trees die while intermediate-sized and large-diameter trees mostly survive. However, the protective capacity of moderate- to high-severity burns is significantly reduced, especially between 10 and 30 years after the fire. In those cases, silvicultural or technical measures may be necessary. Besides the installation of rockfall nets or dams, small-scale felling of dying trees and the placement of stems at an oblique angle to the slope can mitigate the reduction in protection provided by the forest