Review of the paper: ‘Compound flood impact forecasting: Integrating fluvial and flash flood impact assessments into a unified system’ by Josias Ritter and colleagues.

This paper proposes to combine the simulations of two impact forecasting methods (fluvial and flash floods, i.e.  EFAS RRA respectively ReAFFIRM) for a recent catastrophic episode of compound flooding: the DANA event of September 2019 in Southeast Spain

The strong points of this publication are that the authors can explain how to attain a significant improvement in missed severe flood damages in this case: the false negatives (misses) in 15 heavy hit municipalities have been reduced to zero through the combination of the two methods (Fig. 9).

Also, of particular interest is figure 3, where it is obvious how important real-time adjusting of OPERA-radar in this case was.

In my opinion this publication merits in any case a publication, mainly because a clear description of the proposed procedure.

The paper is well written, the structure is clear, the authors largely mention the necessary and useful references.

All the same, I have some suggestions how to improve the paper.

Abstract:

Lines 7ff:

…, this paper proposes the integration of different flood type-specific approaches into one compound flood impact forecast. This possibility has been explored by combining the simulations of two impact forecasting methods (representing fluvial and flash floods) for a recent catastrophic episode of compound flooding:

Suggestion:

…, this paper proposes the integration of two flood type-specific approaches (representing fluvial and flash floods) into one compound flood impact forecast. For this scope a ‘unified system’ was developed by combining the simulations of two impact forecasting methods: One based on the European Flood Awareness System (EFAS), the other on flash flood hazard nowcasts obtained with the European Rainfall-Induced Hazard Assessment (ERICHA) system. This possibility has been explored by combining the simulations for a recent catastrophic episode of compound flooding:…

I think it’s important to mention already in the abstract on what existing products your proposed impact forecast is based.

Lines 10-11:

the DANA event of September 2019 in Southeast Spain.

Suggestion:

the DANA (Depresión Aislada en Niveles Altos, Cut-off Low) event of September 2019 in Southeast Spain.

For the non-Hispanic reader, it may not be clear what DANA means.

Line 14:

Although the compound impact estimates were less accurate at municipal level, they corresponded significantly better to the observed impacts ...

Suggestion:

Although the compound impact estimates were less accurate at municipal level, they corresponded much better to the observed impacts...

It’s only one case, thus ‘significantly’ may not be adequate, I would say: MUCH better

Introduction:

Line 85:

has been taken as an opportunity to explore the possibility of such an integrated system.

Suggestion:

has been taken as an opportunity to explore the possible advantages and drawbacks of such an integrated system.

I think to make a system more complex may always also imply some disadvantages/drawbacks.

2 The DANA event of September 2019 in Southeast Spain:

Perhaps you can explain a little bit more in detail what a DANA-event is:

see e.g. Ferreira, 2021; Garcia-Ayllon, S.; Radke, J., 2021; Giménez-García et al., 2021; Giménez-García et al., 2021.

3 Methods employed for assessing the compound flood impacts

Figure 2:

Concerning the legend: I suppose the colors are representing the RETURN PERIOD! If this is the case, please write it!

This is a very nice figure, but I don’t see how the reader can compare the return period at a gauging station with a peak flow runoff in m3/s. Perhaps you can calculate a return period of the runoff gauges, if not, perhaps you can indicate the rank of the runoff, or a similar metric which is in a way comparable to a return period.

5 Conclusion

Perhaps you should mention in the conclusion that this is a case study and that is important to try this method also for other extreme large events as for example the event of 01.09.2021 over Castilla to explore better the advantages and drawbacks of the proposed product.

References:

Ferreira, Rosana N. 2021. Cut-Off Lows and Extreme Precipitation in Eastern Spain: Current and Future Climate, Atmosphere 12, no. 7: 835. https://doi.org/10.3390/atmos12070835

Garcia-Ayllon, S.; Radke, J., 2021. Geostatistical Analysis of the Spatial Correlation between Territorial Anthropization and Flooding Vulnerability: Application to the DANA Phenomenon in a Mediterranean Watershed. Appl. Sci. 2021, 11, 809. https://doi.org/10.3390/app11020809

Giménez-García, R., Ruiz-Álvarez, V., and García-Marín, R., 2021. Chronicle of a forecast flood: exposure and vulnerability on the southeast coast of Spain, Nat. Hazards Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/nhess-2021-134

Martín-Vide, J., Moreno-García, M.C., López-Bustins, J.A., 2021. Synoptic causes of torrential rainfall in south-eastern Spain (1941–2017). Geographical Research Letters 47, 143–162. doi:10.18172/cig.4696

Review of the paper hess-2021-387 by Ritter et al. submitted for publication in Hydrology and Earth System Sciences

Title: Compound flood impact forecasting: Integrating fluvial and flash

flood impact assessments into a unified system

The paper shows how an early warning system should account for the contribution of both fluvial and pluvial flooding when predicting the impacts of flood events. This is of crucial importance when Civil Protections and first responders operationally use these systems for Decicion Making before and during an emergency.

The manuscript represents a interesting contribution to scientific progress within the scope of Hydrology and Earth System Sciences by providing new concepts and methods                    

The scientific approach is well documented and apply an innovatieve methodology that can easily be replied and validated in other sites across Europe.

The results are well discussed and are based on previous works well documented and referenced.                   

The overall approach and methdology applied here and the conclusions of the work are innovatives and the topic is suitable for publication in Hydrology and Earth System Sciences.

Questions and discussion

Uncertaities in the estimation process.

1. How to account for uncertainies deriving also from the integration of flood-type specific forecasts?

Even if it is not the purpose of this paper could you provide some ideas on how to account for the uncertainties resulting from the combination of the two separate approaches (EFAS RRA and ReAFFIRM)?

2. Which are the most relevant sources of uncertainty that can be associated to the ReAFFIRM methodology? 

Maybe it is reported in previous papers from the same author but a short discussion should be also reported here.

3. How the uncertainties in the flood estimation can be translated into uncertainties in impact estimation? I think this process is different from flood and flash-flood processes. 

A short discussion related to this issue shoud be added.

Review comments :

Compound flood impact forecasting : Integrating fluvial and flash flood impact assessments into a unified system

This manuscript addresses the assessment of socio-economic flood impacts in the specific case of a compound flood which combines a flash flood and a fluvial flood. This event, the DANA event, occurred in Southeast Spain in September 2019. The introduction presents a brief state of the art of flood forecasting methods, and methods developed to transform the hazard forecasts into expected socio-economic impacts. The case study event and the methods used to assess the impacts of this compound flood are then presented. The central part of the manuscript deals with the estimation of the socio-economic impacts of the compound flood and its evaluation by comparing the estimated flood impacts to the reported impacts.

The subject is suitable to HESS.  I agree with RC1 and RC2 that the manuscript is interesting and original and deserves a publication in HESS. I have therefore several minor concerns about the proposed methods and their evaluation:

- The methods employed in the manuscript are intended for civil protection and emergency services. It would be interesting that the authors recall the needs of these authorities in that field for the two flood types addressed in the manuscript : fluvial floods and flash-floods.

- The reading of the manuscript would be easier if the authors presented how these methods can be used in an operational and “real-world” context according to the type of flood.

- It is not clear if the assessment of flood impacts is performed from forecasts, simulations or observations.

- It would have interesting to study the sensitivity of the obtained to forecast uncertainties, especially for flash-floods where these uncertainties are often very large.

- As noticed by the authors, a fluvial flood and a flash flood display very different dynamics which could result in different applications conditions. How the authors deal with this point to estimate of a compound flood ?