Review Reports - Representing Small Shallow Water Estuary Hydrodynamics to Uncover Litter Transport Patterns

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study is clear, and the choice of models are plausible, but the scope and conclusion might be a bit misleading, the reviewer’s major comments focus on this perspective.

The authors have presented a lot of studies on hydrodynamics and flow characteristics, but not much on the modeling of litter transport. Different particles, and marine litters have different behaviors in flow due to their density, size, geometry etc. and the particle transport may not be linearly correlated to water level and velocity. Without a sufficient description of the marine litters’ properties and transport modelling, we can’t make conclusions on the transport patterns purely based on the hydrodynamic behaviors. So some of the conclusions might not be supported and need to be modified.
This is a nice site with shallow water, however, also quite a lot stratification takes place. So what about the vertical hydrodynamic behavior impact on the litters? Any insights here.
What about wave actions and winds and other marine environmental factors, any estimation of their contribution. The conclusion is that the tide seems to be dominant, but any insights about the proportional of the others?
Follow up with comment 1), since the focus is on the flow and not transport, the discussion should be that this study will benefit future studies on transport patterns rather give a conclusion what is important for transport as if the study has already shown the prediction of litters transport pattern.

Author Response

Reviewer 1

The study is clear, and the choice of models are plausible, but the scope and conclusion might be a bit misleading, the reviewer’s major comments focus on this perspective.

We want to thank the Reviewer for their suggestions and comments that help us to greatly improve the manuscript. Please, find below the answers to your concerns.

The authors have presented a lot of studies on hydrodynamics and flow characteristics, but not much on the modeling of litter transport. Different particles, and marine litters have different behaviors in flow due to their density, size, geometry etc. and the particle transport may not be linearly correlated to water level and velocity. Without a sufficient description of the marine litters’ properties and transport modelling, we can’t make conclusions on the transport patterns purely based on the hydrodynamic behaviors. So some of the conclusions might not be supported and need to be modified.

We fully agree with the Reviewer comment. We are aware that depending on marine litter characteristics, they will present different behaviour. This can only be assured with lagrangian models that define the properties of the particles according with litter properties. However, lagrangian models also present some limitations, and the implementation of a lagrangian model is not the main objective of this manuscript. The presented work is the first step performed to understand the hydrodynamic conditions of the Ave Estuary, providing key information to the MAELSTROM consortium to design and implement a marine litter removal technology and uncover the regions prone to litter accumulation due to the velocity patterns. So, you are right. As the Reviewer already mentioned, this manuscript, it is more linked with hydrodynamics and flow characteristics. Following the Reviewer suggestion, we adapted the entire manuscript to avoid misunderstandings. Particularly the Conclusions were carefully analysed and summarized. We also added a new Discussion section in which we concentrate the discussion of marine litter, and included a paragraph enhancing the limitations of lagrangian models in the absence of key data, and the utility of hydrodynamic models for marine litter patterns understanding (pages 24-25, lines 617-698).

This is a nice site with shallow water, however, also quite a lot stratification takes place. So what about the vertical hydrodynamic behavior impact on the litters? Any insights here.

Thanks for your comment. Since it is a stratified estuary mostly of the time, as already mentioned in a previous work of the authors (Iglesias et al, 2024), the main transport is performed on the horizontal layers. So, no vertical movements of the litter are expected in this estuary. The litter will be transported in its equilibrium depth regarding water density, upstream and downstream the estuarine region, mainly depending on the river flow and the tides. However, this is a hypothesis considering the estuarine patterns that we already measured, and unfortunately, we cannot confirm it. We do not have litter transport data on the vertical column that could give some insights about the quantity of litter transported in the entire water column. Some monitoring activities regarding floating litter were performed to assure the effectiveness of the litter removal technology installed in this estuary. Unfortunately, the monitoring activities were dedicated to count the number of litter items passing on a specific transect before and after the installation of the removal technology, without following them to understand the regions of accumulation. The data available that give us some insights about the accumulation hotspots are the values obtained during the clean-up activities and included in the manuscript (page 25, lines 666-672)

Reference:

I. Iglesias, F.A. Buschman, G. Simone, F. Amorim, A. Bio, L. R. Vieira, H. Boisgontier, L. Zaggia, V. Moschino, F. Madricardo, I. Sousa-Pinto, S. C. Antunes (2024): Hydrodynamics of a highly stratified small estuary and the influence of nearby river plumes. Estuarine, Coastal and Shelf Science, 304: 108843. doi: 10.1016/j.ecss.2024.108843

What about wave actions and winds and other marine environmental factors, any estimation of their contribution. The conclusion is that the tide seems to be dominant, but any insights about the proportional of the others?

Due to the configuration of the estuary, waves and wind have a small effect on the estuarine dynamics, and this is why those two parameters were neglected from the hydrodynamic numerical model simulations. The Ave estuary is a really narrow (maximum width around 100 m) and short (less than 2 km) water body. Its orientation regarding the predominant winds (northern winds) makes that these winds cross the estuarine region without affecting too much the water lenses. In addition, on the northern margin can be found the urban area that, due to the roughness with the buildings, will diminish the wind strength near the surface. Oceanic waves did not have also great effect on hydrodynamic conditions, and so on litter transport, due to the configuration of the breakwaters at the mouth. Waves generated by ships passing by should have a strong effect that the oceanic waves. However, since this estuary is not navigable, only small fishing ships and some nautical activities (canoeing, paddle surf) are present in the estuary, without great effect on litter transport.

Among that, in a previous work we demonstrated the influence of neighbour river plumes (the Douro River plume) on the Ave Estuary dynamics (Iglesias et al, 2024). However, these events were present in very specific conditions, when the Douro River has enough river flow to generate a plume and the Ave River has low river flow to allow the interaction.

Considering this, tide and river flow are the predominant drivers for hydrodynamic patterns on this estuary. The manuscript was edited accordingly to clarify this issue (page 8, lines 295-307).

Reference:

I. Iglesias, F.A. Buschman, G. Simone, F. Amorim, A. Bio, L. R. Vieira, H. Boisgontier, L. Zaggia, V. Moschino, F. Madricardo, I. Sousa-Pinto, S. C. Antunes (2024): Hydrodynamics of a highly stratified small estuary and the influence of nearby river plumes. Estuarine, Coastal and Shelf Science, 304: 108843. doi: 10.1016/j.ecss.2024.108843

Follow up with comment 1), since the focus is on the flow and not transport, the discussion should be that this study will benefit future studies on transport patterns rather give a conclusion what is important for transport as if the study has already shown the prediction of litters transport pattern.

We agree with the Reviewer as previously stated. Changes in the manuscript were performed following the Reviewer’s suggestions and a new Discussion section was included in the new version of the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript is a hydrodynamic modelling study of the Ave Estuary in Portugal. The study appears to be solid. The hydrodynamic model is well developed and validated and appears to produce sensible results. This link to plastic pollution is somewhat tenuous. After reading through the information about the model, its development and application, the final conclusion about litter transport patterns appear to be that litter pools in areas of low flow. I guess I was expecting more about litter transport patterns.

The manuscript also appears to be a part of a larger MAELSTROM project and this estuary has been hydrodynamically modelled before.

https://www.sciencedirect.com/science/article/pii/S0272771424002312#sec3

Is this the same model?

Some of the figures appear to be raw data from validation results. Though the authors provide an interpretation of the figures, the results are difficult to visualize. The results should be synthesized further so that the point they make is clearer to the reader.

The link to litter transport patterns should be elaborated on. Though the introduction addresses the litter issues, the discussion does not properly link back the findings of this study to the issues in the introduction. How does your work related to other similar studies? What aspects of this study are important enough to to advance the knowledge is litter transport? What is new and hasn't been published in the literature before?

I am not really sure what to recommend. As a stand alone hydrodynamic study, it is well conducted. Though, it is unclear if this work has already been published. The link to litter transport is tenuous and should be more clearly defined and linked throughout the manuscript. The figures should be refined and synthesized further.

Perhaps a major revision is appropriate.

Author Response

Reviewer 2:

We want to thank the Reviewer for their suggestions and comments that help us to greatly improve the manuscript.

Regarding the tenuous link with plastic pollution and plastic patterns, we fully agree with the Reviewer, since the presented work is more related with hydrodynamics and flow characteristics. All the manuscript was edited to avoid this expectation about marine litter transport. As already explained to Reviewer 1, marine litter transport can only be assured with a lagrangian model that define the properties of the particles according with the litter properties, which is not the main objective of this manuscript. So, a new Discussion section was included in where we delineated several hypotheses for marine litter transportation in this estuarine region.

The manuscript also appears to be a part of a larger MAELSTROM project and this estuary has been hydrodynamically modelled before.

https://www.sciencedirect.com/science/article/pii/S0272771424002312#sec3

Is this the same model?

The manuscript referred by the Reviewer (Iglesias et al., 2024) analysed the hydrodynamic behaviour of the Ave Estuary by means of the analysis of in situ data obtained during several sampling campaigns performed during the MAELSTROM project. No hydrodynamic numerical model was implemented in Iglesias et al. (2024).

The numerical model that was presented in Iglesias et al (2024) but also in this manuscript is the hydrological model that provide both studies with river flow data at the estuary region. And yes, this is the same model in both manuscripts, but in Iglesias et al. (2024) we had a shorter simulation. In the submitted manuscript, a larger, precise and more detailed simulation was performed.

Reference:

I. Iglesias, F.A. Buschman, G. Simone, F. Amorim, A. Bio, L. R. Vieira, H. Boisgontier, L. Zaggia, V. Moschino, F. Madricardo, I. Sousa-Pinto, S. C. Antunes (2024): Hydrodynamics of a highly stratified small estuary and the influence of nearby river plumes. Estuarine, Coastal and Shelf Science, 304: 108843. doi: 10.1016/j.ecss.2024.108843

Following the Reviewer suggestion, new explanations and better description of the validation figures were included in the new version of the manuscript in order to make them clearer to the reader.

We agree with the Reviewer concerns. Following the Reviewer suggestions, a new Discussion section was included in the new version of the manuscript.

Perhaps a major revision is appropriate.

We assure that this work has not been published before, nor submitted to another journal. Following the Reviewer’s suggestions, the link with litter transport was improved, as well as the description of the figures, and we expect that the new version of the manuscript clarified the Reviewer’s concerns.

Reviewer 3 Report

Comments and Suggestions for Authors

The hydrodynamic conditions in estuaries are complex due to multiple forcings. It is important to investigate the estuarine hydrodynamics for compound flood management and litter transport insights. This paper detailed the survey programme and hydrological/hydrodynamic modelling in a small estuary and showed results for model validation and the extreme river flood impact. The reviewer has the following comments that the authors might consider addressing.

Title: The results in this paper did not provide sufficient information to support “forecast litter transport patterns”.
Line 239: Any justification of using a 2D rather than a 3D model in an estuary considering the strong stratification?
Line 260: The hydrodynamic model does not include storm surges / storm tides. Any reason of not including storm surges? Will storm surges affect the results?
Figure 3: Where are the locations Escudeiros, Brancelhe, and Parada? These locations and the Ave River hydrological basin should be clearly shown in a map. An introduction of the basin is also missing, e.g., catchment area and characteristics.
Table 6: Why are there no error metrics for VS1, V1?
Figure 7 & 8: It seems the model performance for velocity (validation) can be further improved. Little information is given for the calibration of velocities. It is good to see the comparison of velocities during the calibration stage.
Litter transport: The discussion about litter transport is spread across the results section. It is recommended to include this in a discussion section.
Section 5: The conclusion is unnecessarily long. It is recommended to summarise the most important findings in concise paragraphs.

Author Response

Reviewer 3:

We want to thank the Reviewer for their suggestions and comments that help us to greatly improve the manuscript. Please, find below the answers to your concerns.

Title: The results in this paper did not provide sufficient information to support “forecast litter transport patterns”.

Agree. The title was modified accordingly. Now states “Representing small shallow water estuary hydrodynamics to uncover litter transport patterns”

Line 239: Any justification of using a 2D rather than a 3D model in an estuary considering the strong stratification?

We agree with the Reviewer that a 3D model is needed if we want to represent the stratification of an estuary, the salt wedge penetration, or the effects of oceanic and riverine drivers. However, the main objective of this work was to provide accurate hydrodynamic solutions simplifying the numerical model complexity, and understand the effects of this simplification on the model results. Despite considering a 2DH model, the obtained results were quite satisfactory and useful for the MAELSTROM consortium, giving key information for the installation of a marine removal technology on the Ave Estuary.

Line 260: The hydrodynamic model does not include storm surges / storm tides. Any reason of not including storm surges? Will storm surges affect the results?

Storm surges are common in the Portuguese coast when the passage of cold fronts and low-pressure systems affect this area, which are more frequent during winter. If a storm surge is presented in this area, it is expected a higher penetration of the tide and higher water levels, affecting the hydrodynamic conditions in this estuary. However, and considering than storm surges are normally accompanied with precipitation, it is expected also an increment of the river flow. And so, it is not clear which driver will have more strength in driving the hydrodynamic patterns inside the estuary. It will depend on the storm surge value (which can reach 1 m at the Portuguese coasts) and the river flow (which maximum values for the Ave estuary was around 500 m3/s). Storm surges simulations will be really interesting to be performed. Unfortunately, we do not have data measured during storm surges conditions that allow us to validate the numerical model.

Figure 3: Where are the locations Escudeiros, Brancelhe, and Parada? These locations and the Ave River hydrological basin should be clearly shown in a map. An introduction of the basin is also missing, e.g., catchment area and characteristics.

Following the Reviewer’s suggestions, a new figure (Figure 3) with the location of the stations and the Ave River hydrological basin was included. The description of the basin was also included in the manuscript (page 3 lines 118-137)

Table 6: Why are there no error metrics for VS1, V1?

Scenario VS1 (Table 2) corresponds with the campaign number 1 (Table 1) that took place between 27/06/2022 and 26/07/2022. During this period only CTD measurements were available. This is why no error metrics could be presented for the velocity at sampling point 1 (V1)

Figure 7 & 8: It seems the model performance for velocity (validation) can be further improved. Little information is given for the calibration of velocities. It is good to see the comparison of velocities during the calibration stage.

Considering the limitations of the velocity measurements, we think that the velocity results are quite satisfactory and we do not think that the sensitivity tests performed during the validation phase will bring additional and useful information to this manuscript. As stated in the manuscript (pages 18-19, lines 479-511), we have the next limitations in the representation and validation of water velocity:

The Ave is a highly stratified estuary, that develops a salt wedge configuration for low river flow conditions. During flood tidal conditions, the two layers (freshwater and saltwater) can present opposite current directions, with the freshwater at the surface flowing downstream, and the saltwater at the bottom flowing upstream. This cannot be fully represented in a 2DH model. During stronger river flows, the salt wedge entrance is restricted to the estuarine mouth and the water column is fully filled by freshwater.
The ADCP measurements have inherent uncertainties due to equipment limitations and sampling protocols. The ADCP, deployed at the bottom, presents a no data region between the bottom and 1.5 m above the bottom, underestimating the bottom velocity. Also, the measurements near the surface region should be neglected due to reflection processes. This corresponds to the ADCP upper cell, around 0.5 m depth. This means that the ADCP is neither able to measure the full behaviour of the salt wedge nor to fully capture the freshwater flow at the surface, as only about two-thirds of the water column can be observed.
There is also an additional uncertainty introduced by using numerically modelled river flow, and also considering that the flow observations were at one profile while the simulated flow is averaged over the grid cell.

Litter transport: The discussion about litter transport is spread across the results section. It is recommended to include this in a discussion section.

Agree. The litter transport discussion was improved and grouped into a new Discussion section.

Section 5: The conclusion is unnecessarily long. It is recommended to summarise the most important findings in concise paragraphs.

Agree. Conclusion section were summarized and focused on the most important results.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have responded to the reviewer's main concerns and have overhauled the narrative and conclusion, so now the results that were presented match the intention and conclusions. With that, the manuscript is considered acceptable in its current form.

Author Response

We want to thank the Reviewer again for their suggestions and comments that help us to greatly improve the manuscript

Reviewer 2 Report

Comments and Suggestions for Authors

Comments from the initial review were sufficiently addressed.

Author Response

We want to thank the Reviewer again for their suggestions and comments that help us to greatly improve the manuscript

Reviewer 3 Report

Comments and Suggestions for Authors

Thanks for the authors to address the raised concerns. Minor revision is still needed (mainly format, grammar and text editing):

Line 130: [34, Error! Reference source not found.].
Line 385: the figure 4 caption is missing.
Line 660: Error! Reference source not found., 57
Line 674: Grammatic error

Author Response

We want to thank the Reviewer again for their suggestions and comments that help us to greatly improve the manuscript. Your minor revisions were considered and improved in the new version of the manuscript