Method to evaluate large wood behavior in terms of convection equation associated with sediment erosion and deposition
Abstract. This paper proposes a method for describing large wood behavior in terms of the convection equation and the storage equation, which is associated with active sediment erosion and deposition. Recent flood hazards are characterized by numerous amounts of sediment and large wood supplied from upstream mountainous areas, which often exacerbate flood disasters in downstream areas. Previous studies proposed methods to simulate large wood behavior by tracking the motion of individual wood pieces in the flood flow using the Lagrangian method. This study aims to propose a method to simulate the behavior of large amounts of large wood in the flow field with active sediment transportation by employing the convection equation and the storage equation with sediment erosion and deposition to simulate the behavior of numerous numbers of large wood pieces. The proposed method is applied to simulate the flood flow with numerous amounts of sediment and large wood in the Akatani river flood disaster, 2017, where the production and transport processes of sediment and large wood from the basin during heavy rainfall events are employed as upstream boundary conditions for the flood flow analysis. The 2-D flood flow computations indicate that the flood flow can be significantly affected by the large wood deposition at the bridge in terms of water level, velocity distribution, and sediment deposition. The comparison between the field and simulation results addressing large wood deposition in the field demonstrates that the simulation method proposed in this study can reproduce large wood behavior successfully. Overall, since the proposed method makes it possible to simulate the behavior of a numerous number of large wood, it can be applied to the management of hazards, such as the Akatani River.
Daisuke Harada and Shinji Egashira
Status: final response (author comments only)
RC1: 'Comment on esurf-2022-52', Anonymous Referee #1, 07 Nov 2022
- AC1: 'Reply on RC1', Daisuke Harada, 20 Dec 2022
RC2: 'Comment on esurf-2022-52', Anonymous Referee #2, 13 Nov 2022
- AC2: 'Reply on RC2', Daisuke Harada, 20 Dec 2022
Daisuke Harada and Shinji Egashira
Daisuke Harada and Shinji Egashira
Viewed (geographical distribution)
This study attempts to model a very challenging problem of wood accumulation and flood hazard in extreme events , using a devastating 2017 event in Japan as the case study. The model builds on some previous work to define the boundary conditions and describe the quantities and size distributions of the sediment and wood accumulations that occurred during this event. As such I think it is an important attempt to offer a practical model for dealing with highly uncertain but incredibly damaging events that exceed the design conditions for which bridges and channels were designed. Ultimately the effort falls short in a few different areas in my opinion and the assumptions should be better justified and tested as part of this effort.
11 – ‘aims to propose’ could be just ‘proposes’
21 – not clear to me in the abstract what type of model you are proposing. Is it a wood budget model or is it a flood level (hydraulic) model, or both?
27 – should use past tense to describe 2017 event.
48 – I would clarify that these are modelling Lagrangian motion rather than tracking. There has been other research attempting to physically track the motion of wood, but that is not what is being reviewed, so you should be clear that this is limited to numerical models. Note as well that there are other wood budgeting efforts that don’t use Lagrangian approaches, but do look at trapping efficiencies and the like. I’m thinking of the Benda et al papers on stochastic forcing of wood budgets in watersheds. It would be good to see this new contribution placed in that sort of context in the literature review.
52 – do you have an estimate for how many? At what point is the Lagrangian method infeasible?
93 – is that a good assumption? I have no information other than what I can imagine such as wood and sediment accumulating in deltas or alluvial fan locations, but what about wood moving into the floodplain and racking on floodplain trees while the sediment is more or less in equilibrium? I would like to see a bit of a review on how sediment deposits in watersheds. Wood can travel long distances in floods, much longer than sediment. How do you resolve the different virtual velocities of these two components?
118- I wonder if this function could be applied to more than just bridges.
147 – figure 6 is not effective. I have a hard time understanding how to see the lateral distribution effect on Figure 6.
159 – sentence beginning with “As for the water discharge…” is not clear. The authors need to explain in more detail. Clarify what is methods and what the results. A discussion point of comparing with other vaguely cited reports is also presented very quickly. What exactly is presented in other reports. And what is the discharge?
175 – the section on boundary conditions is critical for the success of the model but is presented very quickly, citing mainly another paper Harada et al 2022 (Entrainment of bed sediment composed of very fine material in ESPL). It is possible that the authors are citing Harada and Egashira 2022, which has a more relevant title (Methods to analyse flood flow with a huge amount of sediment and driftwood), but this publication is in JSCE in Japanese and so will not be accessible to most readers. I think more detail is needed on these upstream boundary conditions given that they are models in themselves rather than measured time series. The authors tend to say things like “The occurrence of landslide, debris flow, and large wood transport induced by the landslide on the hill slope are evaluated based on the method of Yamazaki et al. (2019),” which seems to assume that we are all familiar with these methods. I at least am not and will need more detail on the basic steps, even if further details may be in the 2019 citation. In another sentence the authors say “in which a section that includes the upstream confluence and excludes the downstream confluence point is designated as the unit channel, and the sediment and large wood runoff for the entire basin is predicted by allocating the unit channels in series and parallel.” I’m sorry but this is too vague.
182 – a lot of calibration in hydraulics is done with the roughness coefficient alone. It would be worth commenting on whether you could calibrate your model with just n. Is 0.03 justified or is it just because it is a typical value?
180 – earlier you said the average slope was 1/70. Which is it or over what reaches do these averages apply?
191 – I might have missed this, but where are the cases described? I see some description in the Discussion (lines 215), but this should have been clearly presented in the methods.
230 – This sentence seems to be the justification for assuming that wood and sediment behave the same in terms of erosion and deposition. I found this assumption questionable given the different properties of the wood and sediment, with the key difference being the density relative to water. Wood travels much faster and at the water surface, interacting more with the banks rather than the sedimentary bedforms, at least at high flow conditions. Figure 10, for example shows that wood deposits far from the main channels, with only minor correlation with the bridges. I don’t think that the correlation between the observed and computed figures is strong enough to say that the assumption is correct. The wood tends to accumulate far from the thalweg, but the bridges are the highest modelled accumulation points because of the equations used and assuming values of 1.0 with respect to the amount of wood trapped.
Figure 5 – repetition of colors and different units are not clear at all. What does 3.5km-2 mean vs 3.5 km-1? Are these repetitions at the same location? Why are two needed? The color scheme for the size distribution is not helpful and overall the figure quality is poor – location diagram is a google earth photo with pins dropped at the approximate locations.
Figure 6 – wood length distribution legend not clear. What areas are used? What does inside the valley mean? Can these be put on a map somewhere?
Figure 8 – best to reiterate what the difference is between cases 1, 2 and 3 in caption
Figure 10 – legend should say ‘wood pieces’ instead of ‘woods’
Figure 11 – figure quality poor due to positions of the axis labels and the low resolution of the graphic. Hopefully vector versions of these plots will be provided.
Benda, L. E., & Cundy, T. W. (1990). Predicting deposition of debris flows in mountain channels. Canadian Geotechnical Journal, 27(4), 409-417. doi:10.1139/t90-057
Benda, L. E., & Sias, J. C. (2003). A quantitative framework for evaluating the mass balance of in-stream organic debris. Forest Ecology and Management, 172(1), 1-16.
Martin, D. J., & Benda, L. E. (2001). Patterns of instream wood recruitment and transport at the watershed scale. Transactions of the American Fisheries Society, 130(5), 940-958.