the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jan 2025
Tracing suspended sediment fluxes using a glider: observations in a tidal shelf environment
Abstract. Underwater gliders equipped with current profilers and optical turbidity sensors offer a low-energy solution for high-resolution measurements of currents, suspended particle properties, and sediment transport in coastal waters. Because the spatial structure of hydrosedimentary processes often changes on short time scales (hours to weeks), especially in coastal areas, validating the distribution of glider observations is required to assess our capacity to represent hydrosedimentary processes. Here we propose to validate in a shelf tide-dominated environment, both i) glider-based currents, and ii) glider-based acoustic backscatters and optical turbidities in full resolution delayed mode, using in situ colocated and synchronous ancillary observations. The deployed glider system correctly measures the periodic pattern of the tidal current, with a satisfying RMSD of O(3 cm s -1). Glider optical turbidities highly correlate with the ancillary observations (R2 up to 0.83). They also correlate well with their glider acoustic counterpart for most of the campaign period (R2 = 0.76), allowing an estimation of suspended particulate matter concentrations from the acoustic. In this study, we showed the presence of bottom nepheloid layers of several mg·l-1 on the shelf probably due to advection of coastal turbid waters as evidenced by estimated glider sediment fluxes. These results highlight the potential of gliders for quantifying sediment fluxes and advancing our understanding of coastal hydrosedimentary processes.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(16290 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(16290 KB) - Metadata XML
- BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-4072', Anonymous Referee #1, 30 Apr 2025
- AC1: 'Reply on RC1', Sabrina Homrani, 14 Sep 2025
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CC1: 'Comment on egusphere-2024-4072', Jay Lee, 21 Jun 2025
#General Comments
This manuscript presents a detailed and comprehensive study of hydro-sedimentary dynamics on the French Armorican shelf using a glider equipped with acoustic and optical sensors. The authors validate the measurements of currents and suspended particulate matter concentrations (SPMC) obtained from the instruments mounted on the glider against those from moored ADCPs and CTD-Rosette casts, offering valuable insights into the potential of autonomous platforms for sediment flux estimation.
The study is well-motivated, methodologically sound, and based on a robust dataset. The authors' efforts to calibrate acoustic and optical sensors for SPMC estimation and decompose current signals into barotropic and baroclinic parts are commendable. However, the manuscript suffers from occasional linguistic awkwardness, convoluted sentence structures, and inconsistent terminology, which make the text hard to follow. The scientific content is solid, but the presentation needs refinement. I recommend minor to moderate revision before acceptance.
#Specific CommentsSection 4.2 Barotropic velocity computation:
- The description of the barotropic current as a depth-average from −98 m to −52 m is somewhat unclear. Why were these bounds chosen? Are they related to sensor coverage, the tidal boundary layer thickness, or data quality limits?Section 5.1.2 and Section 5.1.3:
- The manuscript states that the barotropic component accounts for nearly all of the total current (i.e., ratio ~1). In that case, is the subsequent discussion on the baroclinic component still necessary? It may help to clarify or simplify this section to avoid confusion and to emphasize the dominance of the barotropic signal
- The lower panel of Figure 8 shows that even in the mid-water column around 100 m depth, where glider data are expected to be most reliable (due to the highest overlap of measurements), the difference in baroclinic currents between glider and mooring still reaches up to ±6 cm/s. This implies that the relative error could be greater than (or equal to) the ground truth (mooring records).
- It seems the method used to derive the barotropic component is not clearly explained in the manuscript.Section 5.2.2:
- The author claims that fine sediment was advected from coastal waters but would benefit from further supporting evidence. A reference to satellite imagery, model hindcasts, or literature describing coastal sediment plumes in the region would strengthen the claim.Section 5.3.1:
- Line 456 attributes coarse surface signals to bubbles; this is unlikely below 20 m.
- LISST peaks in the highest bin may be artifacts. If claiming biological origin, cite evidence.Section 5.3.2:
- The use of a 25-hour low-pass filter to extract the tidal signal may be insufficient, as the right panel of Figure 12 still shows a significant periodic pattern. To more accurately separate tidal and subtidal components, I recommend applying a harmonic analysis (e.g., Foreman, 1977; see https://www.sciencedirect.com/science/article/pii/0309170889900171).
#Interpretation and physical assumptions:
- Line 188: The thickness of the bottom boundary layer, stated as 9–14 m in the manuscript, might be quite large according to Soulsby (1983).
- Eq. (3) presents only the u component of sediment flux. Please include the v component or clarify.
- The statement that 1 MHz acoustic sensors are “sensitive to particles of ~1 mm” may be incorrect. Please check this paper (https://www.nortekgroup.com/assets/documents/Monitoring-sediment-concentration-with-acoustic-backscattering-instruments.pdf).
- The sentence “especially in winter” (Line 300) should be elaborated in details.
- Figure 9 shows two high-SPMC events. If the authors interpret these as resulting from advection of suspended particles from coastal sources, it would strengthen the argument to include flow direction or progressive vector plots to illustrate the possible transport pathways.
- Please ensure that Durand et al. (2018) supports the claims described in the manuscript, particularly the statement regarding riverine export of particles (does particle show similar grain size class?)#Specifications and formatting inconsistency:
- Please clarify the operating range of the LISST-100X Type C, which is stated as 2.5-500 μm by the manufacturer, while the manuscript claims 2-380 μm.
- Include schematics or detailed configuration for the mooring setup if it is accessible.
- Ensure the consistency of linear regression results throughout the manuscript. Some plots show R² values while others do not. Also, the p-value should be included as the linear regression is used.
- The term "single yo" should be defined for readers unfamiliar with glider-specific terminology
- Why are Tables 4–7 placed after the references instead of being integrated into the main text? It would improve readability and context if these tables were inserted closer to where they are discussed.
- Table and figure formatting is inconsistent: some figure captions refer to left/right incorrectly (e.g., Figure 11), and colorbar units/locations vary. Table 4’s caption appears below the table.
- Please standardize equation references (e.g., use consistently “Eq. (9)” rather than a mix of formats in the manuscript).
- Verify the time boundaries (magenta line) in Figure 5 for BU and VM periods. I thought the BU was between the 14th and 18th of February, and VM is after the 18th of February, according to Figure 1b.
-Several redundant statements are present in the manuscript (e.g., Line 475: ‘thanks to Equation 3’). The authors are encouraged to revise for conciseness and eliminate unnecessary descriptions.Citation: https://doi.org/10.5194/egusphere-2024-4072-CC1 - AC2: 'Reply on CC1', Sabrina Homrani, 14 Sep 2025
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EC1: 'Comment on egusphere-2024-4072', Matt Rayson, 04 Jul 2025
I note that the Community Comment posted by Jay Lee was from a selected referee so is fine to be counted as valid Referee Comment.
Citation: https://doi.org/10.5194/egusphere-2024-4072-EC1 -
AC3: 'Reply on EC1', Sabrina Homrani, 14 Sep 2025
Dear Dr Matt Rayson,
Please find herewith our revised manuscript, ‘Tracing Suspended Sediment Fluxes from a glider: Observations in a tidal shelf environment’ by S. Homrani et al., following the two reviewers’ comments.
Please note that, in the revised manuscript, added text appears in magenta colour, and removed text appears in light grey (the same for the Figures and Tables)
Yours sincerly,
Dr Sabrina Homrani et al
Citation: https://doi.org/10.5194/egusphere-2024-4072-AC3
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AC3: 'Reply on EC1', Sabrina Homrani, 14 Sep 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-4072', Anonymous Referee #1, 30 Apr 2025
In general the manuscript presented is interesting and of scientific significance. There are a few major comments that require attention. There are many minor comments. Some of these are more writing style / language suggestions, others are more impoartant and require a response.
- AC1: 'Reply on RC1', Sabrina Homrani, 14 Sep 2025
-
CC1: 'Comment on egusphere-2024-4072', Jay Lee, 21 Jun 2025
#General Comments
This manuscript presents a detailed and comprehensive study of hydro-sedimentary dynamics on the French Armorican shelf using a glider equipped with acoustic and optical sensors. The authors validate the measurements of currents and suspended particulate matter concentrations (SPMC) obtained from the instruments mounted on the glider against those from moored ADCPs and CTD-Rosette casts, offering valuable insights into the potential of autonomous platforms for sediment flux estimation.
The study is well-motivated, methodologically sound, and based on a robust dataset. The authors' efforts to calibrate acoustic and optical sensors for SPMC estimation and decompose current signals into barotropic and baroclinic parts are commendable. However, the manuscript suffers from occasional linguistic awkwardness, convoluted sentence structures, and inconsistent terminology, which make the text hard to follow. The scientific content is solid, but the presentation needs refinement. I recommend minor to moderate revision before acceptance.
#Specific CommentsSection 4.2 Barotropic velocity computation:
- The description of the barotropic current as a depth-average from −98 m to −52 m is somewhat unclear. Why were these bounds chosen? Are they related to sensor coverage, the tidal boundary layer thickness, or data quality limits?Section 5.1.2 and Section 5.1.3:
- The manuscript states that the barotropic component accounts for nearly all of the total current (i.e., ratio ~1). In that case, is the subsequent discussion on the baroclinic component still necessary? It may help to clarify or simplify this section to avoid confusion and to emphasize the dominance of the barotropic signal
- The lower panel of Figure 8 shows that even in the mid-water column around 100 m depth, where glider data are expected to be most reliable (due to the highest overlap of measurements), the difference in baroclinic currents between glider and mooring still reaches up to ±6 cm/s. This implies that the relative error could be greater than (or equal to) the ground truth (mooring records).
- It seems the method used to derive the barotropic component is not clearly explained in the manuscript.Section 5.2.2:
- The author claims that fine sediment was advected from coastal waters but would benefit from further supporting evidence. A reference to satellite imagery, model hindcasts, or literature describing coastal sediment plumes in the region would strengthen the claim.Section 5.3.1:
- Line 456 attributes coarse surface signals to bubbles; this is unlikely below 20 m.
- LISST peaks in the highest bin may be artifacts. If claiming biological origin, cite evidence.Section 5.3.2:
- The use of a 25-hour low-pass filter to extract the tidal signal may be insufficient, as the right panel of Figure 12 still shows a significant periodic pattern. To more accurately separate tidal and subtidal components, I recommend applying a harmonic analysis (e.g., Foreman, 1977; see https://www.sciencedirect.com/science/article/pii/0309170889900171).
#Interpretation and physical assumptions:
- Line 188: The thickness of the bottom boundary layer, stated as 9–14 m in the manuscript, might be quite large according to Soulsby (1983).
- Eq. (3) presents only the u component of sediment flux. Please include the v component or clarify.
- The statement that 1 MHz acoustic sensors are “sensitive to particles of ~1 mm” may be incorrect. Please check this paper (https://www.nortekgroup.com/assets/documents/Monitoring-sediment-concentration-with-acoustic-backscattering-instruments.pdf).
- The sentence “especially in winter” (Line 300) should be elaborated in details.
- Figure 9 shows two high-SPMC events. If the authors interpret these as resulting from advection of suspended particles from coastal sources, it would strengthen the argument to include flow direction or progressive vector plots to illustrate the possible transport pathways.
- Please ensure that Durand et al. (2018) supports the claims described in the manuscript, particularly the statement regarding riverine export of particles (does particle show similar grain size class?)#Specifications and formatting inconsistency:
- Please clarify the operating range of the LISST-100X Type C, which is stated as 2.5-500 μm by the manufacturer, while the manuscript claims 2-380 μm.
- Include schematics or detailed configuration for the mooring setup if it is accessible.
- Ensure the consistency of linear regression results throughout the manuscript. Some plots show R² values while others do not. Also, the p-value should be included as the linear regression is used.
- The term "single yo" should be defined for readers unfamiliar with glider-specific terminology
- Why are Tables 4–7 placed after the references instead of being integrated into the main text? It would improve readability and context if these tables were inserted closer to where they are discussed.
- Table and figure formatting is inconsistent: some figure captions refer to left/right incorrectly (e.g., Figure 11), and colorbar units/locations vary. Table 4’s caption appears below the table.
- Please standardize equation references (e.g., use consistently “Eq. (9)” rather than a mix of formats in the manuscript).
- Verify the time boundaries (magenta line) in Figure 5 for BU and VM periods. I thought the BU was between the 14th and 18th of February, and VM is after the 18th of February, according to Figure 1b.
-Several redundant statements are present in the manuscript (e.g., Line 475: ‘thanks to Equation 3’). The authors are encouraged to revise for conciseness and eliminate unnecessary descriptions.Citation: https://doi.org/10.5194/egusphere-2024-4072-CC1 - AC2: 'Reply on CC1', Sabrina Homrani, 14 Sep 2025
-
EC1: 'Comment on egusphere-2024-4072', Matt Rayson, 04 Jul 2025
I note that the Community Comment posted by Jay Lee was from a selected referee so is fine to be counted as valid Referee Comment.
Citation: https://doi.org/10.5194/egusphere-2024-4072-EC1 -
AC3: 'Reply on EC1', Sabrina Homrani, 14 Sep 2025
Dear Dr Matt Rayson,
Please find herewith our revised manuscript, ‘Tracing Suspended Sediment Fluxes from a glider: Observations in a tidal shelf environment’ by S. Homrani et al., following the two reviewers’ comments.
Please note that, in the revised manuscript, added text appears in magenta colour, and removed text appears in light grey (the same for the Figures and Tables)
Yours sincerly,
Dr Sabrina Homrani et al
Citation: https://doi.org/10.5194/egusphere-2024-4072-AC3
-
AC3: 'Reply on EC1', Sabrina Homrani, 14 Sep 2025
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Sabrina Homrani
Orens Pasqueron de Fommervault
Mathieu Gentil
Frédéric Jourdin
Xavier Durieu de Madron
François Bourrin
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(16290 KB) - Metadata XML
In general the manuscript presented is interesting and of scientific significance. There are a few major comments that require attention. There are many minor comments. Some of these are more writing style / language suggestions, others are more impoartant and require a response.