Modelling ocean wave conditions at a shallow coast under scarce data availability – A case study at the western coast of the Mekong Delta, Vietnam
Abstract. Against the background of the rising sea level and land subsidence, protecting the progressively eroding coast along the Vietnam Mekong Delta becomes of tremendous importance. Within the presented work, design conditions for breakwaters were derived from offshore climate reanalysis data (ERA5), which were transferred to the nearshore by two numerical approaches, i.e. SwanOne and Delft3D, for different average and extreme wave and weather conditions. Within this process, design wave heights and periods at the nearshore could be determined for 10- to 100-year recurrence intervals. Both models thereby showed sufficient accuracy according to measurements in the field. Limitations must be made regarding the available spatio-temporal resolution, where reanalysis data showed a lack of short but high peak values compared to the observed measurements. Both numerical approaches showed their capabilities, where SwanOne offers a simple and fast calculation method, while it lacks of continuous effects like wind-generated swell or bottom friction. The Delft3D software on the other hand provides a more complete representation, not only of wave but also current dynamics, while it requires a much broader amount of input parameters and more complex boundary conditions. Within this study, the advantages and disadvantages of both models could be demonstrated, whereas for the final calculation of nearshore wave characteristics, only SwanOne was applicable based on the input parameters extracted from statistical analysis of long term ERA5 data.
Moritz Zemann et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1447', Anonymous Referee #1, 02 Feb 2023
- AC1: 'Reply on RC1', Moritz Zemann, 01 Apr 2023
RC2: 'Comment on egusphere-2022-1447', Anonymous Referee #2, 06 Feb 2023
- AC2: 'Reply on RC2', Moritz Zemann, 01 Apr 2023
Moritz Zemann et al.
Moritz Zemann et al.
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Comments and Suggestions for Authors
The manuscript of the research article entitled "Modelling ocean wave conditions at a shallow coast under scarce data availability-A case study at the western coast of the Mekong Delta, Vietnam" submitted as egusphere-2022-1477.
The main aim of this study is to determine extreme wave conditions to support the dimensioning of nearshore breakwaters.Therefore statistically analized long-term offshore data (wave & wind) from a climate reanalysis (ERA5) are verified by onsite measurements. Within the framework of a field campaign in July 2019, wave data were measured offshore and nearshore over a two week period. Within a modell approach, the third-generation wave model SWAN (Simulating Waves Nearshore) was applied to simulate wave propagation from offshore to nearshore using the 1D linear approach featured by SwanOne and the 2D spatial approach featured by Delft3D-WAVE. ERA5 data during the measurement campaign were thereby used as model input to validated with the on-site measurements. Afterwards statistical long-term data for average and extreme conditions from 40 year ERA5 time series were used as input to determine the site-specific design conditions. Besides, a comparison of SwanOne and Delft3D was performed for a eight day lasting storm event in 2000, to assess their overall applicability, reliability, and limitations under different conditions.
The authors have investigated an interesting topic in the Mekong Delta Coast, and indeed, the results are promising and valuable for design of detached breakwaters along the coast under scare data availability. The literature review and background are good, and the methodology, results, and conclusions are overall comprehensive and useful for coastal engineering activities in the delta.
Comment #1: The coastal bathymetry in the western coast of the MD is very shallow and characterised by gradually increasing slopes ranging only between 1:600 up to 1:1200; Since profile of selected transect 2, shallow but continuously increasing is much different from transect 1 where the sudden seafloor increase at the edge of the shelf (see Figure 2) so that some explanations or clarifications should be presented on SwanOne/ Delft3D computation resulting wave transmission from offshore to nearshore at location of existing detached breakwater installed (about 100-300m). Those clarifications will be more useful for engineering designs.
Comment #2: Continuing above mentioned, in section from line 300-324 and figure 6, there are different wave parameter computed and measured in transect 1 rather than that in transect 2 should be explaned.
Comment #3: Reconsider explanation on differences in Hs and Ts between transect 1 and transect 2 (section from line 372 – 380 and figure 7) it is possible mainly due to different bathymetry profiles but not different models (Swan/ Delft3D)?
Comment #4: Regarding term of “return levels of the maximum individual wave height“ presented in line 180 and figure 3, an clarification should be made for that term is based on what (design criteria or wave parameter specification); In case a new design criteria should be developed as max wave height level, the figure 3 should be refined tecnically for not ”example“, otherwise that figure should be removed to avoid reading confusion.
Comment #5: Minor comments: