Finding Navigable Paths through Tidal Flats with Synthetic Aperture Radar

Round 1

Reviewer 1 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

The ms have been improved to a certain extent. I am not sure whether the organisation of the paper is ok. General structure of a paper is introcution-study area-methods-results-discussion-conclusions. 

Author Response

Thank you for reviewing the manuscript. The methods section had been split due to the need for subsubsection headings. To follow a more standard paper structure, as suggested, we have adjusted section headings to more clearly delineate the methods sections: "Methods – SAR Data", "Methods – Detecting tidal channel paths", and "Methods – Data Analysis".

Reviewer 2 Report (Previous Reviewer 2)

Comments and Suggestions for Authors

No comments in this 2nd revision.

Author Response

Thank you for reviewing the manuscript.

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

Very nice. I left comments in the marked-up PDF where I think you could perhaps improve the paper a bit. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

I included some grammar/re-wording in the marked-up PDF, too. Those could be considered suggestions, as it was quite clear to read/understand.

Author Response

Thank you for your detailed review of the manuscript. We implemented the majority of your grammatical suggestions and mentioned Bragg waves in the discussion. The method was primarily reliant on VV intensity values and beyond masking does not exploit the additional polarimetric information. This is work we are currently exploring.

Reviewer 4 Report (New Reviewer)

Comments and Suggestions for Authors

Reviewer report – “Finding navigable paths through tidal flats with synthetic aperture radar”

By Ruaridh Alexander Clark, Ciara Norah McGrath, Astrid Alexandra Werkmeister, Christopher John Lowe, Gwilym Gibbons, Malcolm Macdonald.

Areas identified as tidal flats are where sediments from the tidal flow are deposited and occasionally consolidated.

The coastal environments are dynamic, and conventional mapping and monitoring methods do not offer enough temporal resolution for accurately identifying sandbars and channels. 

This article presents an innovative method for detecting a path through tidal channels using satellite imagery, namely the identification of waterways in all conditions, with Synthetic Aperture Radar images susceptible to variations due to meteorological and tidal flow conditions. Tidal channels vary in Synthetic Aperture Radar presentation, and tidal flats presentation is also found to be influenced by different conditions, and the most important ones have been presented by the authors. Specifically, waterways in any condition are identified using Synthetic Aperture Radar images, which can vary depending on tidal flow and meteorological circumstances. 

The most influential factor is wind, with strong winds causing an inversion of both the way tidal planes and tidal channels are present in SAR images. The proposed method uses the paths of the previous channels as a reference to calibrate the images and reliably identify the latest path.

When weather conditions are considered, the final algorithm produces minimum errors. On the contrary hand, neglecting these modifications causes a large rise in errors. This capacity was used to support Nith Inshore Rescue in attending calls from their base in Glencaple, UK, and the insights gained from a year of tidal channel monitoring have shown how periods of high river flow often precede significant changes in the channel course.

The article deserves publication with the following observation namely it would have been interesting to present the correlation graphs with a proposal of approximation curves (Regression analysis with transformations) in addition to the partial Pearson's correlation of SAR intensity variations for both tidal channel and tidal flat sand/mud with variables such as wind speed, time at high tide, tidal heights, and SAR incidence angle.

The article is very well written. The used references are relevant to the topic and are recent.

The paper should be published because it contains material that the journal's readers will find of interest. This analysis is a complex and laborious analysis, and the conclusions are pertinent.

Best wishes,

The reviewer

Author Response

Thank you for reviewing the manuscript. We appreciate the feedback and your suggestions around expanding on the correlation results. Given the density of figures and results already present in the current manuscript we have decided not to include additional analysis. But we are looking to incorporate these suggestions into are ongoing work in this area.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

the paper entitled"Finding navigable paths through tidal channels with synthetic aperture radar" discussed the tidal channel and sand flat variations using SAR images for 2021. 

this article detailed the information on tidal channels, which naturally change with the river flow and tidal effect. however, the wind and rainfall also vary in the sand flats.

from the paper, I feel not much new invention was made. only the variations in a year detailed. however, during a year, there are many natural phenomenon occurs, by which the tidal channel will change. 

due to the wind effect, only dry sand will move, which may change the tidal channel. and due to rain sand will move with the rain water.

I expect the authors need to give the scientific impact of the paper.

 

Author Response

Thank you for your feedback on the manuscript. To better highlight the scientific impact of the paper, we have clarified the scientific contribution around the variation of tidal flat SAR presentation due to weather and tidal conditions. Examples include:

Abstract: “SAR images are not agnostic to weather and tidal conditions; tidal channels are known to vary in SAR presentation and we find that tidal flats are also influenced by conditions. The most notable influencing factor is the wind, with high winds causing an inversion in both how tidal flats and tidal channels present in SAR images.”

Introduction: “The study of SAR variation contributing to an improved understanding of how SAR imagery can vary over tidal zones depending on weather and tidal conditions, with new insights into tidal flat variation.”

Conclusion: “Tidal channels presentation in SAR images is known to be susceptible to wind speed, but we found that the sand/mud in tidal flats is also influenced by wind speed among other tidal conditions.”

Reviewer 2 Report

Comments and Suggestions for Authors

This paper presents a method for detecting navigable channel paths through tidal channels using a series of synthetic aperture radar - SAR - images. The study area is the Solway Firth on the West Coast of UK, a very dynamic coastal environment. SAR images are used instead of optical images because SAR is not obscured by cloud or fog and the tidal channels are easily recognizable from the surface differences detected in the radar signal. However, SAR images are highly susceptible to varying weather conditions and variations associated with water flow and wet sand/mud: these factors can severely limit safe navigation in images that obscure the boundaries between channel and sand.

Here, the authors present a method that, through a sequence of steps, is able to support routing in variable SAR images by relying on prior knowledge of channel location. The validation of the use of SAR imagery for the generation of navigable channel paths was performed by a series of depth measurements.

As stated by the authors, the main aim of this technique is to assist the rescue services (in this case, the Nith Inshore Rescue) with navigational safety, but the paper does not present how this assistance works effectively in practice. This part should be included in the paper, considering that the technique proposed by the authors can be used in similar environments characterized by strong and abrupt changes in terms of weather conditions and tidal dynamics. This methodology could indeed be a pilot practice in these environments and could be systematically adopted by port security services and coastguards operating in areas with strong tidal fluctuations. I therefore suggest that this part be included in the manuscript to make it more attractive not only to the scientific community, but above all for its possible practical applications in the field of coastal navigation safety.

Author Response

Thank you for your feedback on the manuscript, in particular the suggesting to describe how the assistance to the Nith Inshore Rescue works in practice. In collaboration with their representative and co-author on the paper we have included a section on Nith Inshore Rescue Application:

“Historically, Nith Inshore Rescue would rely on periodic depth measurements along their route from Glencaple out towards the Solway Firth, aided by visual inspection of channel positions at low tide. Their lifeboat is fitted with a Raymarine sonar system and chartplotter; the former provides depth measurements and the latter is able to record previous routes, as well as upload routes in the form of GPS Exchange Format (GPX).

The presented technique is implemented by Nith Inshore Rescue by first acquiring the latest satellite-based SAR imagery at low-tide conditions. A safe passage channel path is generated using the described technique and overlaid on the image, allowing the team to visually review the proposed path before uploading it to the Raymarine chartplotter in a GPX format. The Raymarine system provides a heading towards each waypoint along a route depicting the best navigational channel that when following the route automatically updates upon close approach to provide a heading to the next and subsequent waypoints. To avoid continuous navigation updates, a downsampled set of waypoints are provided with at least 30 times larger spacing than the waypoints in the full channel path. In addition, the sequence of waypoints are visualised on the digital nautical chart to advise on the channel location such that the Nith Inshore Rescue can use this insight in combination with knowledge of the area, current tides and known hazards; picking and choosing which waypoint in the sequence to head to next both for safety and best speed, hence the waypoints are often not followed in a linear sequence from start to end.”

Reviewer 3 Report

Comments and Suggestions for Authors

After read it carefully, I found the ms was well written and this new method would be of significance for tidal flats analysis, particularly in macro-tidal estuaries or coasts. I only have some minor comments.

 

1. Please add the main place names mentioned in this paper in Figure 1, like the Nith River. Add scale, latitude and longitude.

2. use (a) (b) (c).... to replace (Top left) (Top right) (Middle).... in all figure captions?

3. Please add a Conclusion section.

4. Actually, there exist many estuaries worldwide characterized by strong tidal currents and frequent shifting of tidal channels. One typical example is the Qiantang Estuary, in which a significant relationship between high river flow and channel changes has been found. Discussion on this point would be helpful for a broader interest.

 

References:

River, tide and morphology interaction in a macro-tidal estuary with active morphological evolutions, Catena, 2022

Morphodynamics of the Qiantang Estuary, China: Controls of river flood events and tidal bores. Marine Geology, 2018

Comments on the Quality of English Language

I am not a English speaker. The authors are from UK and I think the writing is very good.

Author Response

Thank you for your feedback on the manuscript. We have made the following alterations in line with your suggestions.

1. Please add the main place names mentioned in this paper in Figure 1, like the Nith River. Add scale, latitude and longitude.

The River Nith is labelled more prominently in the figure and referenced in the caption. Scales, latitude and longitudes are added to the figure alongside other cosmetic improvements.

2. use (a) (b) (c).... to replace (Top left) (Top right) (Middle).... in all figure captions?

In general we have included (a) (b) etc when we have needed to refer to the separate plots in the text. That is not the case for figure 1 and it was difficult to add letters without notable changes to layout or overlaying the figures, so we have not incorporated this suggestion.

3. Please add a Conclusion section.

Conclusion section added.

“Synthetic Aperture Radar (SAR) imagery is a valuable tool for mapping dynamic natural areas, such as tidal zones. Images taken at low tide can reveal the structure and location of tidal channels, however the automatic detection of tidal channels is challenging due to variation in SAR imagery. Tidal channels presentation in SAR images is known to be susceptible to wind speed, but we found that the sand/mud in tidal flats is also influenced by wind speed among other tidal conditions. To mitigate against variation of SAR imagery, we developed a path routing algorithm that relies on the memory of previous sand and mud locations to inform the creation of a channel path for a given SAR image. This method has proven effective in mapping channel locations and has provided valuable support to the operation of the Nith Inshore Rescue service in the Solway Firth.”

4. Actually, there exist many estuaries worldwide characterized by strong tidal currents and frequent shifting of tidal channels. One typical example is the Qiantang Estuary, in which a significant relationship between high river flow and channel changes has been found. Discussion on this point would be helpful for a broader interest.

Thanks for this suggestion, you are correct to highlight that we could have better acknowledged existing work studying the relationships between river flow and channel changes. Including referencing the suggested papers:

“The role of river flow in shaping tidal channels and transporting sediment having been studied for similarly dynamic tidal environments [ 33 ,34 ].”