|In my previous round of comments, I recommended:|
“Please present more details on the governing equations, variables, shoreline identification, transport schemes, diffusion schemes, etc."
Unfortunately, I feel that I must ask for that again. In particular, I am still a bit unclear on the calculation of the shoreline angle, and I would like to see some more of the numeric/algorithmic details. For example, in response to my previous comment (i.e., comment #10), the authors mentioned: “the shoreline angle so the angle may be 0, 22.5, 45, 67.5 or 90 degrees”.
This is certainly better than 0, 45, 90 degree increments (like I thought was the case previously), but I think the method could still be improved to be better than 0, 22.5, 45, 67.5 or 90 degrees. If you look three grid points on either side, then would this give you 15 degree increments? And, if you look four grid points on either side, then would this give you 10 degree increments? I’m guessing that 4 grid points on either side (i.e., 10 degree increments) would probably be sufficient. If you have any mathematical details to present on this, then I would greatly appreciate seeing more of the detail.
It appears the modeled shorelines on Figures 14 and 15 are fairly “staircased” or jagged. Can you please comment on whether this issue of limited angular resolution is contributing to that jaggedness. Overall, CEM seems to have a much smoother representation of the shoreline than CEM2D.
Previously, I also asked a question about “how the longshore transport redistributed in the cross-shore”. The authors mention a “steepest descent method” (which means something else to me, i.e., an optimization method). I think more algorithmic details are still needed on this. It is clear qualitatively what they are doing in Figure 6, but it’s not clear quantitatively what the method is doing, and thus I recommend more details, equations, etc. In the context of beach erosion models, I would maybe call this an “avalanching scheme” after Roelvink et al., (2009) “Modelling storm impacts on beaches, dunes and barrier islands” – Coastal Engineering.
Validation: I agree with the other reviewers comment that “Presenting a new model requires validation” and that “we need validation/proof that CEM2D mimics the natural landscape evolution.” I also agree with authors’ approach to try to validate against existing models, in the absence of a better validation test case. Shorelines with high-angle wave instabilities are quite a rare natural phenomenon, and it’s even less likely that good data exists on them. I wish I could offer any suggestion to resolve this, but I really don’t have a good solution. I’m guessing that I would leave it up to the editors to mediate.
The biggest problem that I’m still struggling with, in terms of the model validation, is why the results in CEM and CEM2D in Figures 10 and 11 look so drastically different, to me. Yes, they both capture some degree of the high wave angle instability features, but I am not entirely satisfied with the authors response in comment #13. To me, if the validation of CEM2D hinges on is ability to reproduce results similar to CEM, then there is perhaps still some work to be done on this front, because they still seem to be producing very different behaviors.