Wave Attenuation and Erosion-Risk Reduction for Sustainable Sediment Management at a Marsh-Creation Site in Coastal Louisiana

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study evaluated offshore-to-nearshore wave transformation, erosion-risk reduction, 14 wave runup, and hydrodynamic loading at a representative marsh-creation site in 15 Plaquemines Parish, Louisiana. The topic is very useful for coastal engineering and is within the scope of Sustainability. The manuscript is well-organized, and therefore I would suggest minor revision. Below are my comments for minor revisions:

1. The format of the manuscript should be checked, and the English can be improved.
2. Introduction:

This section should discuss more existing research of this topic and clarify the research objectives more clearly. 

3. Methodology:

More details are needed. For example, what is the physical consideration for describing the effective roughness condition? Which equation was adopted for determining the bottom friction? Did the authors consider the hydrodynamic factors of wave-current interaction at this site? All of these need to be stated more clearly.

4. Results and Discussion:

Many figures are presented but there is a lack of physical explanations. For example, more physical explanations and implications should be discussed for Figure 8.

Author Response

attached

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper applies the SWAN spectral wave model to a marsh-creation site in lower Plaquemines Parish, propagating a 25-year offshore design wave from WIS Station 73136 under bare-bed and marsh-roughened foreshore conditions and three still-water levels. Reductions in dike-toe Hs are translated into 2% exceedance runup and a quasi-hydrostatic loading scale, with Web Soil Survey data used to characterize the local erodibility context. The headline finding is that increased shallow-water roughness reduces toe Hs by 16-27% and loading by 28-46% across the three water levels.

Major comments:

P7, L215-218: The marsh Cf = 0.060 matches the old Bouws-Komen wind-sea default, not a marsh-specific calibration. The SWAN manual recommends Cb = 0.019 m²s⁻³ for the Gulf of Mexico bed, well below the 0.038 used here. Reframe both as scenario bounds, not physical roughness.

P7, L209-214: I'd want engagement with the implicit-versus-explicit SWAN-vegetation literature here. Garzon et al. (2019, MDPI Geosciences) and Suzuki et al.'s SWAN-VEG work both show implicit bottom-friction underestimates marsh dissipation versus the explicit VEGETATION command; frame the present results as a conservative bound.

P7, L233-234: Why use the USDA K factor here? K is the USLE rainfall-erodibility coefficient, not a wave-erosion index; either drop it or substitute critical shear stress.

P6, L176-181: The 25-year Hs at η = 0.00 m has a joint return period much greater than 25 years; waves and surge co-occur here. Note in Section 4.7.

P14, Table 3: Hs² and wave-power columns are the same metric in shallow water.

Minor comments:

P5, L165: Tm-1,0 used before being identified as the energy-mean period; this is defined only at L252-253. P6, L215: Cf has no units stated. Add m²s⁻³ for consistency with the JONSWAP convention. P7, L218-219: Misplaced comma in "By varying only, the bottom-friction coefficient while holding other model inputs constant". P7, L226: Expand USDA NRCS on first use. P8, L271-273: Equation 8 is missing dz in the differential; same in Equation 9 at L275. P10, L329-336: Figure 6 caption references "model point B84" which is never defined in the text. P10, L332: WVHT is the NDBC parameter code. Note in the caption that it equals significant wave height. P12, L398-399: Table 5 shows toe-proxy depths of 2.07, 1.93, and 2.03 m across the three η levels. State whether the extraction tracks the same x,y point or a moving depth contour. P12, L407-408: The phrase "in the following sections" implies several, but only Section 4.4 follows. P16, Ref [9]: "Groves, D.; Panis, T.; Wilson, M. Coastal Master Plan: Planning Tool Overview. Version I 2021, 51." is missing the publisher. P21, Ref [27]: Web Soil Survey citation is a bare URL. Add access date and survey area code. P6, Table 1: "SWAN computational cells 1142 × 598" and "SWAN output grid points 1143 × 599" carry the same information. Merge. P3, L115-116: "approximately 57.08 km" is over-precise for "approximately"; either drop the decimals or the qualifier.

Author Response

attached

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study conducts a numerical investigation into the effects of wave dissipation and erosion mitigation by coastal marsh platforms on containment dikes, aimed at marsh restoration in coastal Louisiana, utilizing the SWAN spectral wave model. The research quantifies reductions in wave height, erosion potential, wave runup, and hydrodynamic loads on dikes, integrating hydrodynamic results with local soil erodibility data for sediment management analysis. The research framework is well-structured, and the quantitative outcomes hold significant practical value. The manuscript is deemed acceptable pending minor revisions.

1. The study employs an enhanced JONSWAP bottom friction coefficient to effectively represent marsh-induced bed roughness, serving as an alternative to explicit vegetation drag formulas. It is essential to delineate the applicable scope of this aggregated roughness approach to prevent any misinterpretation of the friction coefficient as parameters specific to vegetation.
2. The risk of erosion is assessed indirectly through the use of Hs2 and wave power proxies, without the incorporation of sediment transport simulations. It is important to note that these indices are limited in that they represent only the relative tendency for erosion, rather than providing quantitative measures of scour volume or erosion depth.
3. Constant water levels and fixed offshore wave boundaries are employed in this study, with the exclusion of in-domain wind-generated waves, wave-current interactions, and temporal variations in storm surge. It is important to note that this simplification introduces certain limitations. Specifically, the dynamic fluctuations in water levels that occur during actual hurricane events can significantly affect the depth of marsh submergence and the efficiency of wave damping. These factors contribute to moderate uncertainties when modeling transient extreme storm conditions.
4. Enhance the engineering framework for Louisiana's 2023 Coastal Master Plan by proposing preliminary minimum foreshore marsh widths necessary for dike protection under varying water-level scenarios. Additionally, conduct a comparative analysis of the construction and maintenance costs associated with traditional hard dikes versus hybrid "dike + fronting marsh" nature-based structures to augment their practical applicability.

       5. Perform comprehensive English proofreading to correct grammatical errors and address incomplete figure caption

Author Response

attached

Author Response File: Author Response.pdf