|The field experiments provide valuable information to validate a numerical model, and this is in my opinion, the main contribution of this work. However, once the model is calibrated and authors find a good approach to reproduce the effect of LW, the model could be used to explore a bit more this interesting topic. For example, by simulating higher magnitude floods. I would expect to see a decrease in the effect of LW during much higher flows, for example…|
The main conclusion is that the best fit between observed and simulated hydrographs is met when the roughness is increased in the entire simulated reach (V1), instead of just at sites where wood pieces are present (V2); and, thus modifying the calculation mesh would be a better solution (V3). This is of course somehow expected, as roughness is area- or volume- dependent, just modifying the roughness value in a few mesh cells would not result in significant changes in the flow. But, what about the use of local energy loss (as internal conditions) instead of varying the friction? Authors just mention this in the discussion (pages 13 and 14, few lines about the work by Keys et al., 2018).This would be a much faster way to simulate the effects of an obstacle to the flow, without modifying the mesh or changing the roughness values along the entire reach. In addition, other approaches could be used, as empirical equations recently proposed to estimate the backwater rise upstream from large wood accumulations. I would recommend the authors to check recent works and, at least, discuss these alternatives. What are the main advantages of using the proposed approaches? If compared with previous approaches (e.g., Keys et al., 2018 or Hafs et al., 2014)? Please, discuss.
Although there are not field measurements available, the numerical model provides detailed information on flow depth and velocity. It could be useful to show some of these results and the comparison between the three scenarios (V1, V2 and V3) to better understand the effect of the roughness or elements added.
Abstract: Lines 24 and 25: remove one point.
Page 2, Line 14: authors may consider also:
Elosegi, A., Díez, J.R., Flores, L., Molinero, J., 2016. Pools, channel form, and sediment storage in wood-restored streams: Potential effects on downstream reservoirs. Geomorphology 279, 1–11. https://doi.org/10.1016/j.geomorph.2016.01.007
Gurnell, a. M., Sweet, R., 1998. The distribution of large woody debris accumulations and pools in relation to woodland stream management in a small, low-gradient stream. Earth Surf. Process. Landforms 23, 1101–1121. https://doi.org/10.1002/(SICI)1096-9837(199812)23:12<1101::AID-ESP935>3.0.CO;2-O
Page 2, lines 16-17: although is cited later, authors may consider here also:
Grabowski, R.C., Gurnell, A.M., Burgess‐Gamble, L., England, J., Holland, D., Klaar, M.J., Morrissey, I., Uttley, C., Wharton, G., 2019. The current state of the use of large wood in river restoration and management. Water Environ. J. wej.12465. https://doi.org/10.1111/wej.12465
Page 2, line 24: examples of reference about large wood transport during floods can be find here:
Comiti, F., Lucía, A., Rickenmann, D., 2016. Large wood recruitment and transport during large floods : a review. Geomorphology, 23–39. https://doi.org/10.1016/j.geomorph.2016.06.016
Lucía, A., Schwientek, M., Eberle, J., Zarfl, C., 2017. Morpholocigal changes and large wood transport in two steep torrents during a severe flash flood in Braunsbach, Germany 2016. Sci. Total Environ. 20, 17920.
Mazzorana, B., Ruiz-Villanueva, V., Marchi, L., Cavalli, M., Gems, B., Gschnitzer, T., Mao, L., Iroumé, A., Valdebenito, G., 2018. Assessing and mitigating large wood-related hazards in mountain streams: recent approaches. J. Flood Risk Manag. https://doi.org/10.1111/jfr3.12316
Ruiz-Villanueva, V., Bodoque, J.M., Díez-Herrero, A., Bladé, E., 2014. Large wood transport as significant influence on flood risk in a mountain village. Nat. Hazards 74, 967–987. https://doi.org/10.1007/s11069-014-1222-4
Ruiz-Villanueva, V., Badoux, A., Rickenmann, D., Böckli, M., Schläfli, S., Steeb, N., Stoffel, M., Rickli, C., 2018. Impacts of a large flood along a mountain river basin: the importance of channel widening and estimating the large wood budget in the upper Emme River (Switzerland). Earth Surf. Dyn. 1–42. https://doi.org/10.5194/esurf-2018-44
Wohl, E.E., Bledsoe, B.P., Fausch, K.D., Kramer, N., Bestgen, K.R., Gooseff, M.N., 2016. Management of Large Wood in Streams: An Overview and Proposed Framework for Hazard Evaluation. Am. Water Resour. Assoc. 1482. https://doi.org/10.1017/CBO9781107415324.004
Page 4: here you may also consider:
Manga, M., Kirchner, J.W., 2000. Stress partitioning in streams by large woody debris 36, 2373–2379.
Hygelund, B., Manga, M., 2003. Field measurements of drag coefficients for model large woody debris. Geomorphology 51, 175–185. https://doi.org/10.1016/S0169-555X(02)00335-5
Page 4: Line 9: about backwater rise, please consider:
Schalko, I., Lageder, C., Schmocker, L., Weitbrecht, V., Boes, R.M., 2019. Laboratory Flume Experiments on the Formation of Spanwise Large Wood Accumulations: Part I: Effect on backwater rise. Water Resour. Res. 0–3. https://doi.org/10.1029/2019WR024789
Page 4, line 25: replace were by where
Page 5, line 7: the bed material is not visible in Figure 2, please provide a description about the grain size, if data is not available.
Figure 2: the LW pieces are not so visible in the pictures, maybe you can add arrows or try to highlight them. Some of them seems to be outside the channel…or hanging above…
Page 5, line 13: what about the effect of these boulders and channel spanning steps on the flow? How were the boulders and steps reproduced by the model? Also as roughness elements?
Page 6, line 3: why this model produces a higher goodness-of-fit compared to others? Which other models have been compared with this one? The reasons given in the text to choose the model are not fully related to the conditions in the study reach (dike breaching…). In my opinion, there is no need to justify the choice of this model here, but the advantages and limitations of the model, and a brief comparison with other available tools should be discussed in the discussion.
Page 6, line 25: if logs were placed lengthwise, I guess the effect would be much lower than in the case of perpendicular logs. Moreover, the height between the log and the river bed is important. If logs were standing above the channel bed (hanging from the river banks), water is flowing under and above the LW piece. This effect could not be reproduced by adding discrete obstacles in the calculation mesh, without porosity…please, discuss this issue more in detail. This is also the main reason why Keys and Hafs et al used a different approach…
A deeper discussion about the representation of the actual shape of LW pieces is also missing in the discussion. Although just cited in the introduction, it could be useful to discuss this topic and consider the work by:
Allen, J.B., Smith, D.L., 2012. Characterizing the Impact of Geometric Simplification on Large Woody Debris Using CFD. Int. J. Hydraul. Eng. 1, 1–14. https://doi.org/10.5923/j.ijhe.20120102.01
Page 7, line 9: please, provide some more details about the interpolation methods used here (although you provided the reference, it could be very useful for the readers to know what you did)
Page 10, line 30 and elsewhere: remove references from the results section
Page 12, line 15: however, the area affected by the presence of LW has not been analysed here, and V1 assumes an increase in the roughness along the entire reach, right?
Page 12, lines 20-28: I am not sure why the roughness was not adjusted along the floodplain during the calibration without LW. This is also important in terms of the storage of water and the resulted hydrograph downstream, as described in the results…I would suggest providing results with the calibrated roughness in the floodplain only. And, how many values of roughness values have been used? Did you map homogeneous roughness units? Or just assumed one single value for the entire domain? Please, provide details and justify. A map with roughness values could help, and a discussion about that would be beneficial.
Page 12, line 30 and following lines: this part of the discussion is very interesting, maybe a table summarizing these studies and their main findings together with the results from this work could be very useful for the readers.
Figure 3: the evaluation and comparison between simulations and observations is missing in the workflow.
Figure 4: some improvements could be useful. Please, add a scale, flow direction, and also arrows showing the LW pieces could be useful. Actually, it could be better to show the roughness maps instead of just the calculation mesh.
Figure 5: adding inflow and outflow (in parenthesis together with the corresponding weir) could be helpful.