Reply to Anonymous Referee #1

First, and regarding the fragmentation algorithm described in section 2.1.3, I suggest a more in detail

explanation of each section of the algorithm. Even though some details on how fragmentation works on Hy-

Stone can be found in Frattini et al. (2012), some parts of the process are not clear. Being the quantification

of fragmentation effect one of the goals of this article I suggest a more technical description of the algorithm,

including an explanation of how the Yashima et al. (1987) criterion is adapted for the fragmentation threshold

and the criteria used to determine the post fragmentation directions (since this may control the spreading of

the model).

Thank you for your comment. In the revised version, we will add a more detailed section regarding

the Hy-Stone fragmentation algorithm and the Yashima et al. (1987) criterion adopted.

Second, in the analysis of Roisan event, where fragmentation played a significant role as detailed in the

manuscript, it would be very interesting to show the volume distribution of the deposited fragments in the

field and compare it with the volumetric distribution obtained by the model during the calibration. If the model

results correspond to the field measurements, the model calibration would be more complete regarding

fragmentation. In this sense, it would also be convenient to explain what initial volumes are simulated to

represent the event and how that initial distribution was determined. These suggestions could be included in

section 3.1 if appropriate.

Thank you for your suggestions. For the revised paper, we will prepare a figure comparing the

volumetric distributions of the in-situ block size distribution (IBSD), the mapped block size

distribution in the deposit (RBSD), and the simulated block size distribution with and without the

algorithm. The figure will be added to the supplementary materials. Additionally, we will included a

brief explanation in section 3.1 about the initial volumes simulated and how they were identified.

Some minor comments:

Pag 5 line 133. For the calculation of the transit relative frequency when considering fragmentation, the total

number of simulated paths include all generated fragments during propagation? When no fragmentation

occurs this calculation looks trivial, but increasing the number of blocks during the propagation may lead to

misuse of the classic approach.

Indeed, the comment is very interesting. In the calculation of the transit frequency, we use the

number of simulated block before fragmentation. This may see counterintuitive because

fragmentation can generate many blocks passing through a cell, and the value of frequency can

potentially be larger than 1 (when the number of blocks passing through a cell is larger than the

number of simulated blocks). This is correct, because this value is multiplied by the onset frequency,

which is calculated without considering the dynamic fragmentation, and therefore it is consistent

with the number of simulated blocks.

Pag 7 line 214: Typo: «Figure 2Figure 2»

Fixed

Pag 12 line 359: Is the sentence «From the results of hazard computation with and without fragmentation

algorithm, which the hazard decreases.» correct? Can’t get the meaning.

The sentence is incorrect. It will be replaced with “The results with and without the fragmentation

algorithm shows that the hazard level decreases when fragmentation is explicitly simulated”

Pag 13 line 391: Possible bullet missing.

Fixed

Reply to Anonymous Referee #2

The manuscript represents a very interesting contribution to the understanding of the effect of forest and fragmentation in probabilistic 3D rockfall modelling. The rockfall model HY-STONE and the applied rockfall hazard methodology are valid and outlined clearly. However, the manuscript still lacks a bit of a clear structure, with in particular a sharp distinction between methods, results, and discussion. Furthermore,

please check the required reference formatting and reference completeness text/list.

In the revised version, we will attempt to make the chapter division more sharp. However, the overall paper is already divided into methods, analysis and results, and discussion.

We will also check the references carefully.

Some comments:

Line 80: please add if block volume & form are considered in Hy-STONE and how the forest/trees are included/considered in the model

Thank you for the comment. In the revised version, we will provide more details on how the tree impact algorithm works. The volume and shape of the blocks are taken into consideration by Hy- Stone. For both case studies, spherical blocks were simulated.

Line 155: HY-STONE is capable of simulating such large volumes?

Yes, Hy-Stone simulates individual blocks and it is able to compute the volumes described in the manuscript.

Line 214: typo “Figure 2Figure2”

Fixed

line 286: typo "Rockfall Hazard"

Fixed

Figures: please add the resolution of the data/simulations

As stated in line 172, we simulated all scenarios using the 1 x 1 m Lidar DTM of Aosta Valley Region. The parameters used with the two algorithms and the characteristics of each scenario are reported in supplementary Table S1, S2, and S3. In the revised version, we will add details about the simulated volumes and the number of simulated blocks.