Observation and Modeling of the Equilibrium Slope Response of a High-Energy Meso-Macrotidal Sandy Beach
Mark Davidson
Round 1
Reviewer 1 Report
I really enjoyed this paper. It was very clear and concise. I have included all my comments and suggestions on the attached manuscript.
Comments for author File: 
Comments.pdf
Author Response
Reviewer #1
Reviewer’s comments
Our response
Change in the text
I really enjoyed this paper. It was very clear and concise. I have included all my comments and suggestions on the attached manuscript.
We thank Reviewer#1 for his constructive comments and support for publication. You will see that all the minor comments on the annotated manuscript have been carefully considered.
This is a key part of the papers message. It might be work fragmenting the clauses a little more to improve variability. Perhaps better clarity and flow could be achieved by using more shorter sentences here?
Agreed, this has been rewritten into : From 0 to 2 m above mean sea level, which is located under the berm crest, slope response at the storm time scale is observed. Beach slope steepens under low energy waves, with the equilibrium model explaining up to 40% of the observed beach slope variability. In contrast, from 2.5 to 4 m above mean sea level, which is above the berm crest, beach slope steepens under high-energy waves. With this region of the beach profile, response time scale is increasingly seasonal upwards, with the model explaining up to 65% of the observed beach slope variability.
This reference might be worth considering as slope and grain size are closely related.
Thank you for suggesting this relevant reference, which is now cited in the revised manuscript in both the introduction and discussion sections.
I agree completely but would it be worth saying how beach slope impacts on beach safety through the generation of dangerous 'shorebreak' conditions in the case of steep shorefaces? This might be useful for less familiar readers?
We thank the reviewer for this comment. This is now clarified in the revised manuscript : For instance, beach slope largely impacts wave breaking type and intensity, from spilling to surging, through plunging […]. Beach slope is thus important for e.g. beach safety and lifeguarding as plunging and dumping waves at the shoreline (shore-break waves) at steep beaches can cause severe spine injuries to those caught in the impact zone […].
This reference might be worth considering.
Agreed. This work is now cited.
I have a feeling that Nathaniel Plant published on this?
We did not find such reference.
Could it be made clearer what the different colours represent here?
Done
Figure 2 : It would be quite good to show the gray zones in Fig 5 on this profile plot too? And Perhaps state datum ?
Done
Interesting plot!
Thank you !
Figure 5: Is the figure produced by just analysing one profile? Is is the same for all profiles? Perhaps expand on this? Or perhaps it's based on a longshore mean profile?
Thank you for this comment. It was not clear from the former text and we now expand on this in the revised manuscript in the : For each date and each 1-m elevation section, beach slopes computed at each 20-m spaced cross-shore transects were then averaged alongshore in order to investigate the equilibrium response of beach slope for different parts of the beach profile.
This seems almost like a standing wave response. Low corratlation at the nodes and inverse behaviour either side.
We agree, this is discussed in the Discussion Section.
This is a somewhat surprising result to me. I might have expected this region inconsistently activated by the surfzone processes in a temporal sense and therefore decoupled with wave forcing. Is this the storm run-up zone would you say? If not, what are the key processes shaping this zone?
This is an interesting comment. We had discussed an explanation for this, but based on this comment we further develop this explanation in the discussion section of the revised manuscript : This may be seen as a surprising result, which will be discuss later, as this region of the beach profile is only rarely activated by the surfzone processes and could assumed de-coupled with wave forcing. And later on : This is because during winter storm wave events, surf zone and swash zone processes smooth out the berm, resulting in a featureless concave winter profile (Figure 3), and finally : This explains why the best equilibrium response is found in the higher part of the profile well above the highest astronomical tide level around 4 m AMSL.
Reviewer 2 Report
The manuscript is well presented, and the structure is easy to follow.
The equilibrium profile model interests several professionals related to coastal work. This manuscript can be recommended for publication.
Even though I will provide some recommendations to the authors, from the reader's point o view:
- Including wave and wind roses facilitate viewing their effects on the coast.
- Some variables of the model need a broader explanation of their meaning and their physical influence on the beach morphology, such as erosion velocity and memory decay. Please elaborate on them.
- Given that the manuscript is short enough, please explain how the model was run in more detail, i.e. are the free parameters changed simultaneously? Is there any impact on the results if the parameters are changed in a specific order? Which parameter are the results more sensitive to? And so on.
- Please elaborate on the low R2 values obtained, the authors recognize the model fails in some parts, but no explanation is given.
- The criteria by which the beach profile is segmented need to be clearly stated. How may the results improve for a different segmentation?
- The sand size along actual beach profiles is far from homogeneous. It seems the model was run for the same sediment properties for all the profiles. Can the results be improved if this size is changed, i.e. modelling a fall velocity value distribution along the profile?
Author Response
Reviewer #2
Reviewer’s comments
Our response
Change in the text
The manuscript is well presented, and the structure is easy to follow. The equilibrium profile model interests several professionals related to coastal work. This manuscript can be recommended for publication. Even though I will provide some recommendations to the authors, from the reader's point of view.
We thank Reviewer #2 for their constructive comments. Below you will see that have all been carefully considered.
- Including wave and wind roses facilitate viewing their effects on the coast.
We do not think that such roses will provide more insight into coastal response. While wind is important to the coastal dune morphological changes, previous work demonstrate that it has no impact on shoreline response. In addition, previous work (e.g. Castelle et al., 2014 MARGO; Splinter et al., 2014 JGR) shows that Truc Vert beach shoreline change is cross-shore transport dominated. However, according to this comment we now provide more information on the wind climate with a relevant reference in the revised manuscript : Truc Vert can be exposed to severe windstorms, with 10-m hourly wind speed exceeding 30 m/s, causing large coastal dune morphological changes […].
- Some variables of the model need a broader explanation of their meaning and their physical influence on the beach morphology, such as erosion velocity and memory decay. Please elaborate on them.
We now provide more information on the different parameters : where C^± are change rate coefficients for erosion velocity (C^-) and accretion velocity (C^+), which represent efficiency rates determining the rate of beach slope change in response to the wave forcing (…) and Φ the memory decay of the system, in days. This implies that Ω_eq incorporates all past beach state information for the past 2Φ days and constantly evolves in time and maintains a weighted “memory” of antecedent incident wave conditions.
- Given that the manuscript is short enough, please explain how the model was run in more detail, i.e. are the free parameters changed simultaneously? Is there any impact on the results if the parameters are changed in a specific order? Which parameter are the results more sensitive to? And so on.
We use a non-supervised optimization method, namely simulating annealing, which does not use any specific order in the free parameters nor give any sensitivity to one of them, instead, it finds the optimal parameter combination that minimized the model error. We now provide more detail on the simulating annealing algorithm in the revised manuscript : The error minimum was found using a simulated annealing non-linear optimization algorithm […]. Such probabilistic approach can overcome local RMSE minima in this 4-parameter space and find global optimum. Simulating annealing optimization al-ready gave good results with beach equilibrium models […].
- Please elaborate on the low R2 values obtained, the authors recognize the model fails in some parts, but no explanation is given.
We agree that explanation were only briefly provided in the discussion section, we now expand on this in the revised manuscript : Similar observation can be made at the higher (> 4.5 m AMSL) and lower (< 0 m) parts of the profile where poor correlation is systemically found, for different reasons. Above 4.5 m AMSL beach slope is steep (> 0.1) and evolves without any apparent link with incident wave conditions (Figure 6b). In contrast, below 0 m AMSL beach slope is low (< 0.05) and also shows variations which are not always readily related to changes in incident wave energy. Consistent with shoreline response at this elevation (…), this is because this part of the profile is strongly influenced by the inner-bar dynamics. Given that the cross-shore inner-bar dynamics at Truc Vert beach is influenced largely by tide-range variations (…), this is not captured by the equilibrium model.
- The criteria by which the beach profile is segmented need to be clearly stated. How may the results improve for a different segmentation?
This segmentation was based on preliminary work indicating that this segmentation is a good balance between model skill variability and slope noise (which increases with decreasing elevation range). This is now clarified in the revised manuscript : This segmentation was based on preliminary tests showing it best emphasizes the vertical variability of beach slope response.
- The sand size along actual beach profiles is far from homogeneous. It seems the model was run for the same sediment properties for all the profiles. Can the results be improved if this size is changed, i.e. modelling a fall velocity value distribution along the profile?
This is an interesting comment. We agree that beach slope depends on grain size, and that this grain size can vary in both time and space. This is PhD topic of the first author who recently performed devoted field experiments to address this issue. However, there is no current framework to include this in an equilibrium modelling approach. This limitation is now given in the discussion section : We assumed a sediment grain size constant in both time and space. However, large variations (0.2–0.7 mm) in grain size over tens of meters linked with morphological variability have been observed at Truc (…). In addition, grain size has been found elsewhere to largely vary in time in response to changes in wave conditions (…). Including changes in grain size could improve beach slope model skill, but there is currently no clear framework.
