Detachment model. Figure 3.2 shows a screenshot of the detachment model section of the program. The exposure model describes the frequency of rockfall events from the detachment point. In a given slope, there exist several possible transfer zones in which the rockfall trajectory can proceed. Assuming that the transfer events in the slopes are stochastically independent, the analysis can be performed separately for each zone and the results can be aggregated subsequently. The model considers only rocks with a certain volume. It is assumed that rocks smaller than 0.1 m3 do not endanger the galleries. Rockfall events involving rock volumes larger than 50 m3 are considered to represent events that are too large for the rockfall galleries to be able to offer effective protection. The volume range considered is therefore between 0.1 to 50 m3 which is then divided into seven intervals. Here, since generally more information is available over the lower volume range, this is discretized more finely than the larger volume intervals in which events occur rarely. The representative volume corresponds to the mean between the minimum and maximum volume in the respective interval. For each of the defined volume ranges, the exceedance frequency in terms of 2.5% and 97.5% quantile values needs to be specified. These values are generally estimated through expert judgment and experience or through the use of the power-law model. For the former, it is considered that exceedance frequency in each volume range follows a Lognormal distribution. Following this, the parameters of the detachment model are estimated using both the expert judgement and the power law approaches and relationships between the detached rock volume and the exceedance frequency are obtained and illustrated in graphical representations as shown in Figure 3.2.
Detachment model. In order to estimate the failure probability of the rockfall gallery, the quantity of interest is the description of the biggest event in a given period and generally not the description of the frequency of an event. Further, failure events in the built environment are typically expressed in terms of annual risk with the corresponding probabilities expressed as annual values. From the exceedance probability relationships, the distribution of the annual maximum detached volume is calculated. This forms the basis for calculating the failure probability of the gallery.
