Please see attached pdf.

Review of “Long term hazard of pyroclastic density currents at Vesuvius….,” by Dellino et al.

Reviewer:  Greg Valentine

This is a very interesting manuscript, and the authors are to be lauded for their effort to integrate deposit data with a theoretical model to produce probabilistic maps of “impact parameters” (a.k.a. hazard parameters) from pyroclastic currents at Vesuvius.  This is a great problem of societal importance.

I am recommending major revision.  The goal of my review is to help strengthen the paper, so even if comments seem difficult, I hope the authors will consider them in that spirit.

I have made many editorial suggestions throughout the manuscript, as can be seen on the pdf mark-up I have uploaded.  The authors should consider that many, probably most, readers will not be familiar with the details of hazards work at Vesuvius.  The authors could do a better job of setting up the context of their work, for example by including a figure at the beginning that shows the current red zone.  They could then clearly discuss the issue that the red zone map does not have any details of the impact parameters, i.e. one is either “in” or “out.”  Also an issue with the red zone approach is the assumption of a certain (or narrow range of) eruption scenarios, which does not take advantage of the range of behaviors that the volcano has exhibited, as recorded in the deposits.  This sort of set up would help then when we come back around to the discussion and conclusions at the end.  Also, it would help if some terms were defined when they are first used (see notes in ms).

Here are my major comments (Note that the most important comments are related to the Appendix model):

Line 65 – is the value of 34% a media value?  What is the uncertainty around that?

Figure 1 – see my comments.  Improvements are needed.

In general there seems to be a sort of narrow approach taken to interpreting the deposits, meaning that it is stated – as fact, with no consideration of other possibilities and very little observational motivation – that all massive deposits are sourced the impact zone process, and that each current’s deposits reflect the bipartite model.  I would like to see the authors discuss a broader range of possibilities for the origins of these deposits.  I have made some notes on the ms that I hope are of use.

Line 115 and Figure 2 – what is the basis for the location and size of the zone of impact for the Mercato eruption?  This is sort of presented as a fact, but what is the evidence?  What was the topography of the volcano like at that time?  Also, Fig 2 needs some “help” to make it easier for the reader.

Line 230 – here and other places layers are interpreted to be part of a single current, but no evidence is presented to convince the reader that this is the case.  It would really help strengthen the paper if some summary of the evidence used to determine whether a sequence of layers are from a single current.

Line 310 – there needs to be more information about how the hazard maps are produced.  If I understand correctly, at each point where there are field data, the model was used to compute a pdf of the impact parameters.  The 84^th percentile value at that point was then used, along with the values at other points and from other eruptions, to make contours of individual impact parameters (such as dynamic pressure).  If this is correct, then this should be said in a simple and straightforward way (I don’t mind if the authors use the exact words above, if they make sense).  Was there any sort of integrative method such as Monte Carlo used to sample across the uncertain parameters?  It is critical that the process used to generate the maps, and the associated assumptions, be totally clear and transparent, if the authors want to maximize their use in decision making.

APPENDIX A (MODEL) COMMENTS:

The assumptions and simplifications behind the theoretical approach need to be explicitly and completely stated.  For example, the concentration and velocity profile models are based on data/models that were originally obtained/tested for steady, uniform flows of water and particles in gently sloping flumes.  These conditions are a departure from pyroclastic currents (especially surges) with time-dependent sources, non-isothermal and (sometimes) compressible flow, and complex topography.  I do not mean to say that the approach is not useful, but please be clear about the simplifications involved and how they might affect the results.

Equation A12, which is a significant part of the theoretical logic chain, is only appropriate for particles settling through a stationary column of fluid.  I.e., not for particles settling for a flow, where the duration also is important in determining final layer thickness.  This is something that cannot be glossed over.  The authors need to present a compelling argument about why this simplified model is ok to use for the application at hand.  Note that the time factor is subsequently, and separately, used to compute the duration of a flow via the aggradation rate.  While this makes sense, it seems to make A12 to be an internally contradictory part of the overall model.

Equation A17 - If I understand this correctly, there is no heat transfer between particles and gas phases.  In other words, the decline in temperature with distance is soley due to the dilution due to entrained air, i.e., a mixing model.  Is this correct?  If so this needs to be clearly stated as a simiplification, along with some description of the implications of the simplification compared to a model that transfers heat between particles and gas.