The Coastline Evolution Model 2D (CEM2D) V1.1

Leach, Chloe; Coulthard, Tom; Barkwith, Andrew; Parsons, Daniel R.; Manson, Susan

doi:https://doi.org/10.5194/gmd-14-5507-2021

Articles | Volume 14, issue 9

https://doi.org/10.5194/gmd-14-5507-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gmd-14-5507-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 9

Development and technical paper

|

07 Sep 2021

Development and technical paper |

| 07 Sep 2021

The Coastline Evolution Model 2D (CEM2D) V1.1

Chloe Leach, Tom Coulthard, Andrew Barkwith, Daniel R. Parsons, and Susan Manson

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Final revised paper (published on 07 Sep 2021)
Preprint (discussion started on 09 Sep 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review – CEM2D (Leach et al)', Eli D Lazarus, 15 Oct 2019
- AC1: 'Initial, Short Response to Referee Review', Chloe Leach, 01 Nov 2019
RC2: 'GMD-2019-197 REVIEW', Anonymous Referee #2, 18 Dec 2019
AC2: 'Response to Reviewers', Chloe Leach, 26 Jan 2020
AC3: 'Response to Reviewers - Track Changes', Chloe Leach, 26 Jan 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Chloe Leach on behalf of the Authors (26 Jan 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (31 Jan 2020) by Lutz Gross

RR by Eli D Lazarus (11 Feb 2020)

RR by Anonymous Referee #2 (19 Feb 2020)

RR by Anonymous Referee #3 (24 Apr 2020)

Suggestions for revision or reasons for rejection

The authors present CEM2D, an update of the Coastal Evolution Model (CEM) of Ashton & Murray (2006). This update allows the representation of topography in each grid cell, which in turn allows the model to account for processes such as sea-level rise in modeling coastal change. While this is, no doubt, a worth feature to include as a model update, the manuscript presenting CEM2D seems to lack several critical details about the model. Often found myself wanting to know more about the governing equations and solution procedure of the model, but found myself confused about the differences between the CEM and CEM2D. I think these details should be considered and added in a thorough revision of the manuscript. In my opinion, it makes it very difficult to evaluate the model in the absence of key algorithmic details (some particularly important issues are given below)

1. Please present more details on the governing equations, variables, shoreline identification, transport schemes, diffusion schemes, etc. I think my biggest confusion of the model is role of the sediment fraction variable ‘Fi’. In CEM, Fi=1 means a sandy cell, Fi=0 means an ocean cell, and 0<Fi<1 is a shoreline cell. This variable is very important because it allows the shoreline to have a “partial cell representation”. The 0<Fi<1 value of Fi is used determine the location of the shoreline. And as Ashton & Murray (2006) put it “the fractional value within a cell represents the cross-shore extension of the sub-aerial shore” This fractional shoreline cell approach can be seen in Fig 4 of the current manuscript. This approach seem somewhat in contrast to the “full cell” shown in Fig 2 of the current manuscript.

Thus my confusion is: does CEM2D use the variable Fi and a partial cell representation of the shoreline or not? It is very hard to tell from the current manuscript. From what I have read, I’m guessing not, because the wet or dry cells can be simply identified as those that are below or above sea level, respectively. The problem with the “full cell” representation (that is the lack of a partial cell identification) is that the shoreline angle only exist with an angular resolution of 45 degree increments. See figure 2 for example, where the angle of the shoreline on the left half of the figure is either zero or 45 degrees. The shoreline angle on Figure 4 of the current manuscript instead has a much more subtle variation in the shoreline angle. And, of course with longshore transport, the angle of the shoreline relative to the waves is critical to determining the transport (see Eq. 1).

Therefore, in my opinion, the text / figures greatly need to improve to clarify some critical details about the model. For example, Figures 2 and 4 are so similar, yet also seem to present contrasting representations of the shoreline.

2. There is no mention of potential physical / numerical instability of the model, unlike in Ashton & Murray (2006). In CEM, the numerical scheme to shift between a central scheme and an upwind scheme (when a critical angle of 42 degrees is exceeded) is key to the model’s stability. Are similar schemes used in CEM2D? This should be discussed.

3. Like the previous reviewers, I found myself wondering how the longshore transport redistributed in the cross-shore, and thus how the offshore bathymetry changes in response to growing shoreline perturbations? (In CEM this is handled by extrapolating the sub-aqueous shoreface slope away from the shoreline) I assume that this is handled only via the diffusion scheme, but it is not obvious that this is the case in the manuscript.

4. I was quite surprised to see the strong difference between CEM and CEM2D in Figures 10, 11, and 18. Why are they so different if this update is still relatively straightforward? (As an aside, I recommend using the same color scheme to the CEM results (e.g., blue and tan instead of gray, even though there is no gradation due to topography in CEM) as for the CEM2D results. This would greatly improve the readers ability to compare the results.

5. I know that CEM and CEM2D are typically applied in high wave angle, high wave asymmetry conditions (U,A>0.5), but do these models work when in low wave angle, low wave asymmetry conditions (U,A<0.5)? These low wave angle, low wave asymmetry conditions (U,A<0.5) persist for most coastlines, in general. I would like to see the behavior of this model in a more ordinary test case such as the diffusion of a Gaussian shaped shoreline bump to evaluate the differences between CEM and CEM2D, rather than going straight to the cases of flying spits.

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ED: Reconsider after major revisions (07 May 2020) by Lutz Gross

AR by Chloe Leach on behalf of the Authors (17 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Oct 2020) by Lutz Gross

RR by Eli D Lazarus (04 Nov 2020)

RR by Anonymous Referee #3 (14 Nov 2020)

Suggestions for revision or reasons for rejection

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.

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ED: Reconsider after major revisions (16 Nov 2020) by Lutz Gross

AR by Chloe Leach on behalf of the Authors (15 Jan 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (18 Jan 2021) by Lutz Gross

RR by Eli D Lazarus (22 Jan 2021)

ED: Publish as is (22 Jul 2021) by Min-Hui Lo

AR by Chloe Leach on behalf of the Authors (29 Jul 2021) Manuscript

Short summary

Numerical models can be used to understand how coastal systems evolve over time, including likely responses to climate change. However, many existing models are aimed at simulating 10- to 100-year time periods do not represent a vertical dimension and are thus unable to include the effect of sea-level rise. The Coastline Evolution Model 2D (CEM2D) presented in this paper is an advance in this field, with the inclusion of the vertical coastal profile against which the water level can be altered.