Nearshore Sandbar Classification of Sabaudia (Italy) with LiDAR Data: The FHyL Approach

Round 1

Reviewer 1 Report

This paper shows a good proposal of remote sensing methods integration and development for assist in interpretation of geomorphological forms of nearshore zone. Authors benefit from e.g. commonly used BPI method for description of concave/convex structures in the seabed. However, I suggest for authors to read the manuscript carefully again and explain or correct some shortcomings, like listed below, as well as some editorial and typo errors. Please, be careful with pompous statements, like 'first time ever' that several times appear in the text. I advise you to carefully check the literature again - you have to be absolutely sure if you use such strong statements. 

Line 59: can you explain, which traditional methods you are referring?

Line 69: did you mean context? 

Line 96: do you mean that hyperspectral data are useful only for detection of vegetation on the seafloor? Can you explain it? 

Line 127: please, provide model and manufacturer of the echosounder and some details of its parameters, e.g. operating frequency 

Line 227: you may consider reference of some more recent benthic habitat mapping studies involving bpi. (see e.g. Diesing & Thorsnes, 2018; Janowski, Tęgowski & Nowak, 2018; Fogarin et al., 2019; Misiuk et al, 2019; Toso et al., 2019)

Line 332 (figure 6): you should include a legend for bpi values and possibly provide some details about their distribution

Lines 391-408: I would argue, that these paragraphs (including figure) should be included in results section

Lines 415-417 are the same like lines 445-447.

Lines 467, 276 - I am afraid that acoustic eco-sounder doesn't exist 

Line 468-469: can you explain what 'logistical limitation due to wave action' means? 

Line 474: I suggest to add some recent references about seafloor characterization from acoustic remote sensing sources, like e.g. Brown et al., 2019; Masetti et al., 2018

I would like to see this paper published after considering abovementioned comments.

Author Response

We agree with reviewer about the fact that “STRONG STATEMENT” must carefully checked.

In particular, we want to reassure that we have written "for the first time ever" in relation to the 3D observation of the submerged sand bars. It would not be appropriate (after several years from the acquisition), to stress a novelty that is not scientifically or technically appropriate. Anyway, the present work has been a reference for the Italian Remote Sensing National Plan, because it provided a joint acquisition of LIDAR and MIVIS data (although this technical note does not elaborate and present them; see Valentini et al, this SI). More recently the presentation of the preliminary results influenced the local authorities to test a new maintenance approach of the beaches under erosion.

We have improve the manuscript following all suggestion provided by reviewer 1.

Response to details

Line 59: can you explain, which traditional methods you are referring?

RESPONSE -  this text was added at line 60: (i.e. total station/GPS and echo-sounder)

Line 69: did you mean context?

RESPONSE – Yes (see line 70)

Line 96: do you mean that hyperspectral data are useful only for detection of vegetation on the seafloor? Can you explain it?

RESPONSE: the sentence (lines 97-98) has been modified as follow:

 However, due to the absence of vegetation on the seafloor, hyperspectral data has not been included in the present research, but it has been used on other case studies and applications as part of the FHyL methodology.

Line 127: please, provide model and manufacturer of the echosounder and some details of its parameters, e.g. operating frequency

RESPONSE -  the sentence (lines 182-183) has been modified as follow:

An in situ campaign was performed before LiDAR acquisition survey, on April 2009, for acquiring bathymetric data by the use of a single beam acoustic sensor FURUNO’ FCV-600L (200 khz) along beach profile perpendicular to the shoreline (maximum length 1,5 km).

Line 227: you may consider reference of some more recent benthic habitat mapping studies involving bpi. (see e.g. Diesing & Thorsnes, 2018; Janowski, Tęgowski & Nowak, 2018; Fogarin et al., 2019; Misiuk et al, 2019; Toso et al., 2019)

RESPONSE -  the suggested references have been added (five references included; see line 240)

Line 332 (figure 6): you should include a legend for bpi values and possibly provide some details about their distribution

RESPONSE -  the suggested correction has been made on figure 6; now it is figure 7).

Lines 391-408: I would argue, that these paragraphs (including figure) should be included in results section

RESPONSE -  the suggested correction has been made and discussion now begin at line 415.

Lines 415-417 are the same like lines 445-447.

RESPONSE -  thank you. Lines 445-447 have been deleted

Lines 467, 276 - I am afraid that acoustic eco-sounder doesn't exist

RESPONSE -  thank you. We have corrected ”echo” and we checked the new version of the manuscript.

Line 468-469: can you explain what 'logistical limitation due to wave action' means?

RESPONSE -  we modified the sentence at line 481-486 as follow:

Beside this general trend to the overestimation of bathymetric data-set by using LiDAR technology, we also notice (Figure 6c) that the accuracy of bathymetric data-set acquired in the surf zone (between 1 and 2,5 m of depth) by the use of the echo-sounder may be affect by the wave action in the surf zone [74-77].  (where the waves are higher even during calm condition;). It is commonly observed phenomena during bathymetric survey where above crest, reduction of water depth increases the incident waves height, even during calm condition.

Line 474: I suggest to add some recent references about seafloor characterization from acoustic remote sensing sources, like e.g. Brown et al., 2019; Masetti et al., 2018

RESPONSE -  the following references have been added….

Line 227: five references included.

Line 474: two references included.

Lines 467-468: three references included.

I would like to see this paper published after considering abovementioned comments.

RESPONSE -  of course. Thanks for all suggestion provided. They have been very useful!

Reviewer 2 Report

General Comments:

The technical note is well structured, however in some paragraphs, proper formatting is missing. In general, english language should be checked. The abstract must be more clear.

Specific comments:

Figures 5 and 8 should be in a higher resolution.

Line 20: "Analyses" should change to "Analysis"

Lines 21-23: needs rephrasing

Line 49: References are not hyperlinked.

Line 60: Nearshore ... (something is missing here)

Lines 61-64: Regarding bathymtery from remote sensing data, please cite these, which are among the most recent works that can be found in the literature:

Satellite (SDB):

• Traganos, D., Poursanidis, D., Aggarwal, B., Chrysoulakis, N., & Reinartz, P. (2018). Estimating satellite-derived bathymetry (SDB) with the google earth engine and sentinel-2. Remote Sensing, 10(6), 859.
• Kasvi, E., Salmela, J., Lotsari, E., Kumpula, T., & Lane, S. N. (2019). Comparison of remote sensing based approaches for mapping bathymetry of shallow, clear water rivers. Geomorphology, 333, 180-197.
• Misra, A., Vojinovic, Z., Ramakrishnan, B., Luijendijk, A., & Ranasinghe, R. (2018). Shallow water bathymetry mapping using Support Vector Machine (SVM) technique and multispectral imagery. International journal of remote sensing, 39(13), 4431-4450.

UAV (SfM-MVS/Photogrammetry):

• Agrafiotis, P.; Karantzalos, K.; Georgopoulos, A.; Skarlatos, D. Correcting Image Refraction: Towards Accurate Aerial Image-Based Bathymetry Mapping in Shallow Waters. Remote Sens. 2020, 12, 322.
• Dietrich, J. T. (2017). Bathymetric structure‐from‐motion: extracting shallow stream bathymetry from multi‐view stereo photogrammetry. Earth Surface Processes and Landforms, 42(2), 355-364.

Lines 72-73: In general, LiDAR systems are problematic in this shallow area because of the multi-path reflections due to the water surface. Can you justify this statement ? Check also this: Schwarz, R., Mandlburger, G., Pfennigbauer, M., & Pfeifer, N. (2019). Design and evaluation of a full-wave surface and bottom-detection algorithm for LiDAR bathymetry of very shallow waters. ISPRS journal of photogrammetry and remote sensing, 150, 1-10.

Line 80: Please add some references here about the Geomatics techniques.

Line 106: Do you mean coastal ?

Lines 152-153: "... submerged environment above mean sea level..." Do you mean below sea level ?

Line 294: It's extension. Please add " 's " to "it"

Lines 467-468: This a well known problem in the literature. Please add some references here.

Regrading the overestimation of LiDAR depths of about -0.20m, are you sure that this is not caused by a difference in the reference systems or a different water level when measuring with the echo sounder? Can you elaborate more on that?

Author Response

Thanks for comments and suggestions. We accepted all of them with great pleasure because we believe they improved the overall quality of the work. Changes are reported within the manuscript. Abstract has been improved and English has been checked. Figures 5 and 8 have been published at higher resolution.

Line 65: five references included

Line 73: two references included

Line 80: three references included

Line 481-488: three referenceshave been added)

About LIDAR-echosounder overestimation we checked the dataset and we are sure it does not depend by variation of water level during acquisition. We are studying an area with a microtidal environment and about ± 30/40 cm between LWL and HWL occur in about 6 hours. During the field work a single bathymetric profile was collected in less than one half-hour.

In Figure 6, one profile is reported as example. We believe that the most interesting part of this comparison (LIDAR-echo sounder bathymetry) is that the scattering of data in the surf zone but the changing response at higher depth (more than 7,5-8m) that surprisingly correspond to the closure depth (i.e. grain size of superficial sediment). We are carrying out further elaboration on this research topic but it is far beyond the purpose of the present article as we need a more robust dataset to find an algorithm or a correlation coefficient.

Reviewer 3 Report

General comment:

This paper employs LiDAR data to characterise the coastal morphology (both sub-merged and overland) of sandy beach in Sabaudia, Central Italy. After reading the manuscript, I’m not fully convinced that this should be classified as a scientific article, but rather a technical note or a report. This is because the authors mainly employ well-established methods and approaches in a new geographical context, accentuating  the science-policy interface several times. Some results on the geomorphology of the study area are shown, implications of these findings (both from the geomorphological point of view as well as from the point of view of the operational services) are, however, not discussed and, thus, not clear. The authors mentioned challenges and opportunities of this work (L487), but fail to present them in an consistent way – I recommend to introduce a separate section on that. On the other hand the authors provide some information which are not so relevant for the scientific article (e.g. on costs, L491-516).

Specific comments:

L17: delete 'along the'

L26-29: this is coming back to my general comment – is this a expected purpose of a scientific article?

L78: focused

L104: some field photos would be helpful to introduce the study area

L152: what is the total area scanned wth LiDAR? How many points?

L172: please provide a proper reference to Bloom’s work

L285: delete '()'

L332: please explain colour scale used in Fig. 6 (and other figures where the colour scale is missing)

L354: Outer?

L373: delete 'of'

L381: please consider separate sub-section starting from L381

L404: is this drift evidenced by data of this study?

L409: please provide a reference

L427: please provide examples of these studies / authors

L453: echo-sounder (the same on L468)

L487: this should be elaborated more rigorously and profoundly (see my general comment)

L491-516: I don’t find this usefull in the scientific article

L529-530: I don’t think this is really addressed in the manuscript

L532: dynamics

Author Response

We agree with reviewer about the comments and the present paper was initially thought as a technical note. However, we forgot to write it in the header of the first page and this may have confused the reviewer and the editor.

We are very sorry for the inconvenience and the fact that rev 3 is incline to TN is not surprising us.

However, it has to be considered that the work is longer compare to standard T.N. and all reviews suggested to lengthen the manuscript. In addition, we have recently known that, based on the elaborations of the present paper, and thanks to the fact that we were able to “classify” and map (3D) the submerged sand bar system, stakeholders and competent Authorities have decided to use the results to perform coastal erosion mitigation actions. So, scrapers will dredge the crest of the Inner/Outer Bar (IB and OB) and will spill the removed sand over the trough (between the shoreline and the crest of the Inner Bar; IB).

In this way it is expected that emerged beach will be larger during summer season (2020) and this will support tourism. It is quite rare that some scientific activity and obtained results of a RS methodology drive stakeholders and Public Administration to a new approach to contrast a word wide problem such as coastal erosion.

So, we are more inclined to deliver the paper in its present configuration as an article considering that :

(1) there are already a lot of speculation in the present  discussions and we have added two subsections;

(2) we now have (as requested by the reviewers) 80 references;

(3) we are going to submit a second work in the same study area related to the emerged dunes;

(4) we have added two figures to the article.

We will of course summarize it (and shift it to a technical note) in case the reviewer (n. 3) will not consider the paper still eligible as an article, but we hope you will consider the additional effort to enrich the manuscript and its novelty related to management implications.

We know it is a very unusual way to proceed, but we feel that can be an extra value for RS journal and community to publish a work whose results have strongly influenced the coastal management and the local administration/stakeholders.

Response to your comments are reported as follow:

in several parts of the manuscript we have tried, to highlight the implication of our findings for geomorphology of the study area as well as from the point of view of the operational services (following also suggestions and comments of other reviewers). In particular, we have added a fig. showing photos of the study area. N-S drift of sediment is well known and has been discussed by other Authors [40, 45].

Response to details and specific comments

L104: some field photos would be helpful to introduce the study area

RESPONSE – Field Photohave been added in Fig. 2

L152: what is the total area scanned wth LiDAR? How many points?

RESPONSE –Line 162  “Dataset (~24 km²; 6x10⁶ pts ) was used to interpret features… “

L172: please provide a proper reference to Bloom’s work

RESPONSE –Reference has been added (ISPRA, 2009) and one web page has been  included

L332: please explain colour scale used in Fig. 6 (and other figures where the colour scale is missing)

RESPONSE – Figure has been corrected

L381: please consider separate sub-section starting from L381

RESPONSE – Discussion has been changed and start at line 417. A subsection has been considered starting from L381 (part of the results and discussion have been reallocated as suggested by another reviewer).

L404: is this drift evidenced by data of this study?

RESPONSE – No it is not. It is a well know process and we refer to other papers that already described this general trend of long shore transport. At line 420-421 we synthetically resume results of our investigation and wht is represented in Figures 7, 8, 10.

L409: please provide a reference L427: please provide examples of these studies / authors

RESPONSE –reference have been added [40,42,45] at line 426 and [40]at line 445.

L487: this should be elaborated more rigorously and profoundly (see my general comment) and L491-516: I don’t find this usefull in the scientific article L529-530: I don’t think this is really addressed in the manuscript

RESPONSE – we have added more details to discussion and add two subsections (management implications and cost analysis). This is way instead of reduce the manuscript length (appropriate for a TN, we kindly ask to consider the present version eligible as an Article.

We do consider the cost estimation a key part highlighting that most policy decisions are still taken by governments at the national and local level interacting with researchers working in the field. The opportunity to engage with policy makers and science communicators and to reflect on the way science can support good policy decisions including cost is an important issue for science paper (https://spp2019.github.io/conference/).

The cost of this research are reported as they represent a benchmark for local authorities, but in this final version we specified that and why they decrease through time. We believe that part of the discussion dedicated to cost of acquisition and processing of Remote Sensing data, can be a useful information due to its direct application to coastal survey. Time series of RS dataset can represent the base for coastal erosion projects and long term monitoring scheme.

In addition, we have recently known that, based on the elaborations of the present paper, and thanks to the fact that we were able to “classify” and map (3D) the submerged sand bar system, stakeholders and competent Authorities have decided to use the results of this mapping to perform coastal erosion mitigation actions. Basically, for the first time on the Tyrrhenian Sea, scrapers will dredge the crest of the Outer Bar (OB) and will spill the removed sand on the trough (between the shoreline and the crest of the Inner Bar (IB). In this way it is expected that emerged beach will be larger during summer season and this will support tourism. It is quite rare that some scientific activity and obtained results of a RS methodology drive stakeholders and Public Administration to a new approach to contrast a word wide problem such as coastal erosion. So, considering that : (1) there are already a lot of speculation in the present discussions; (2) we now have (as requested by the reviewers) 80 references; (3) we are going to submit a second work in the same study area related to the emerged dunes … We are more inclined to deliver the paper in its present configuration as an article. We will of course summarize it (and shift it to a technical note) in case the editor and reviewer (1/3) will not consider the paper still eligible as an article.

Round 2

Reviewer 3 Report

Thank you for your edits and clarification. I recommend acceptance of this manuscript.

Author Response

We have :

1. checked everything again and changes (few) are reported below.
2. improved the readability and layout of the work.
3. created a cover for the article
4. listed the mayors changes below …

Line 22: parameters

Line 23: provides

Line 32: FHyL, sand bar, bedform classification, beach, morphology, LiDAR, DSM

Line 40-42: Therefore ….. scenarios. (Some changes to improve the english).

Line 54: requires

Line 58-59: ranging from single to seasonal events

Line 62-66; 73-74; 118-121: minor changes

Line 76: construction production

Line 144: showing, suggests

Line 161-162: LiDAR survey for collecting elevation data both in emerged and submerged environment …

Line 170-177: minor changes

Line 184-185: in order to collect  for acquiring + some typo

Line 189: About 1,000m of survey…

Line 237-238: LiDAR point cloud

Line 247: using by

Line 250: Focal-Statistic

Line 273: [see  66, 67]

Line 328: right

Line 422-431:

Line 458: neglectable negligible

Line 486: conditions. More

Line 491: auto classification

Line 501: health of 

Line 516: reference [82] has been added

Line 537-551: text has been added

Line 562-576: text has been removed

Line 565-567: The study of the coastal morphology through innovative techniques implemented remotely and represent a new systematic…. Change of style

Line 572: (... first attempt to of identify their complex variability...

Line 586: this the present

Line 593: The corresponding author S.C. and the co-author E.V. are the contacts to access the dataset.

Line 596-598: This research has been funded by the Lazio Region within the CAMP Project (2009) and the 5publication of this manuscript is supported and exploited by the ISPRA agreement with the Italian Space Agency for the 'Habitat Mapping' (Agreement number A0HMASI2).

Line 601-605: text has been updated.

Line 800: empty line deleted

Line 841-848: references have been re-numbered and formatted according to RS rules.