the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jul 2026
Geological controls on the formation and evolution of shoreface-connected ridges: new insights from the Stroombank sand ridge (Belgian coast)
Abstract. Shoreface-connected ridges are large bedforms of the lower shoreface that play a key role in sediment redistribution between the shelf and the coast, yet their origin and long-term dynamics remain debated and poorly constrained due to limited geological information. Here we integrate high-resolution acoustic data, sedimentological analyses, and new radiocarbon and optically stimulated luminescence dates to reconstruct the origin and evolution of the Stroombank, a shoreface-connected ridge on the Belgian continental shelf. The ridge consists of landward-dipping clinoforms overlying an irregular basal disconformity that locally truncates Pleistocene and Holocene deposits. Chronological constraints indicate that ridge development began ca. 1.3 ka ago, followed by sustained shore-parallel elongation and landward migration. We infer that initial ridge formation was fuelled by sediment derived from a waning late-Holocene tidal inlet, after which elongation and migration were maintained through sediment distribution within the active shoreface. We propose a formation model in which wave-driven cross-shore transport, combined with flood-dominant residual sediment transport, explains the coupled landward migration and alongshore elongation reconstructed in the internal architecture. The Stroombank is therefore interpreted as an actively evolving sedimentary body rather than a relict feature. This interpretation is supported by the coexistence of active and inactive (preserved) sectors within the ridge. The preserved inactive sector allowed us to date the inception of the ridge, while the occurrence of both active and inactive sectors indicates that ridge development is spatially heterogeneous and reflects a continuum from antecedent to active phases. These results demonstrate that shoreface-connected ridges can form composite geomorphic features controlled by the interplay of sediment supply, hydrodynamic forcing and shoreface retreat, and provide geological constraints to evaluate existing conceptual and process-based models of ridge formation and evolution.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-2791', Robert W Dalrymple, 30 Jul 2026
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AC1: 'Reply on RC1', Víctor Cartelle, 28 Oct 2026
We thank Robert Dalrymple for his detailed and constructive review, and for recognising the publication value of the dataset and its potential contribution to understanding nearshore sediment dynamics. The comments highlight several places where the manuscript needs a clearer distinction between observations, interpretations, and hypotheses. We respond below to the principal concerns and outline the revisions we propose.
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Context and models of ridge formation
We acknowledge that the introduction places disproportionate emphasis on the tidal-inlet and ebb-tidal-delta model. Although alternative mechanisms are mentioned, they are not developed sufficiently or used consistently in the discussion. In particular, the implications of the Sable Island study by Dalrymple and Hoogendoorn (1997) deserve greater attention.
We agree that tidal inlets are not a necessary condition for shoreface-connected ridge formation. We will revise the introduction and discussion to present alternative mechanisms more fully, including the flow–topography feedback mechanism discussed for Sable Island. We will also distinguish the processes organising and maintaining a ridge from the possible sources of its sediment. Any contribution from former inlet deposits at Stroombank should be considered a site-specific sediment-supply hypothesis, rather than a general requirement for ridge genesis.
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Regional geomorphic and hydrodynamic setting
We agree that the regional setting needs fuller description. Some relevant information is included in the manuscript, but it is too brief and insufficiently connected to the interpretation. We will expand the description of the Belgian shoreface-connected ridges, the adjacent tidal banks, and the relationship between Stroombank and Balandbank.
Numerous observational and modelling studies are available for the Belgian coastal zone. We will make greater use of these to describe tidal-current velocities and directions, wave conditions, and sediment-transport pathways. We will also integrate the historical bathymetric reconstructions more explicitly into the discussion of ridge behaviour. Throughout, we will distinguish regional transport patterns from local processes around Stroombank and its neighbouring banks and troughs.
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Oldest preserved deposits and ridge initiation
We agree with the distinction between the age of preserved ridge deposits and the age of ridge initiation. Our wording assigns the oldest dated deposits too directly to the time and location of initial formation, without adequately accounting for the incomplete preservation of the ridge’s earlier history.
We will therefore describe approximately 1.3 ka as the age of the oldest preserved deposits that can be clearly associated with the ridge in our dataset. These deposits establish that the ridge was present by that time, but do not determine when or where it first formed. This qualification will be carried through the abstract, discussion, conclusions, and conceptual model.
The proposed relationship with the waning Nieuwpoort inlet will also be reconsidered in this light. The spatial and chronological association may support a hypothesis concerning sediment availability, but it does not establish a causal connection between inlet abandonment and initial ridge formation.
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Northeastward development and landward migration
We appreciate the reviewer’s point that ages must be interpreted in their stratigraphic context. Comparisons between cores occupying different positions within migrating clinoform packages can confound alongshore and cross-shore trends.
We would nevertheless like to clarify that our interpretation of northeastward development was based on the combined evidence from absolute dating and the wider spatial coverage provided by luminescence profiling, considered alongside the historical bathymetric reconstructions reported by Dujardin and colleagues. It was therefore not based solely on the comparison between two dated cores. We recognise, however, that the manuscript does not explain the contribution and limitations of these different lines of evidence sufficiently clearly.
We will carefully reassess their relationship to the internal architecture and revise the presentation to show which observations support landward accretion and which support northeastward development. In doing so, we will account explicitly for the limitations of bulk luminescence intensity as a relative-age proxy and for differences in the stratigraphic positions of the sampled deposits. We will also distinguish alongshore migration of the ridge as a whole from elongation, and reassess whether the available evidence justifies the proposed long-term elongation rate.
The revised discussion will allow for migration oblique to the ridge crest and consider how the landward component recorded by the internal architecture relates to the alongshore behaviour documented in the historical reconstructions.
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Buried and active sectors and the relationship with Balandbank
Our intention was to distinguish contrasting expressions of ridge development, rather than to imply two discrete episodes separated by inactivity. We recognise that the terminology of “Phase I” and “Phase II” conveys a stronger temporal subdivision than the evidence supports.
We will replace this terminology with descriptions of the buried southwestern sector and the active sector associated with the present ridge morphology. We will revise the accompanying discussion and conceptual model accordingly, avoiding any implication of a hiatus or two separate episodes of ridge formation.
We will also examine the possible relationship between the buried ridge geometry and Balandbank more explicitly. The reviewer’s suggestion concerning spatially variable transport around the neighbouring tidal banks provides a useful mechanism to consider alongside the available hydrodynamic studies. We will distinguish possible transport pathways from those supported directly by observations or modelling.
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Sedimentary processes and the basal erosional surface
We agree that tidal currents receive insufficient attention in the current interpretation. The revised discussion will consider both tidal-current erosion and wave reworking as possible contributors to the basal surface. Variations in its relief will be discussed in relation to local processes and substrate characteristics, without assigning them to discrete developmental phases.
We will also clarify the evidence used to identify former tidal-inlet and delta deposits, distinguishing preserved channel fills from deposits interpreted as remnants of an ebb-tidal-delta platform. Their possible contribution to sediment supply will be discussed separately from the mechanisms responsible for ridge formation and migration.
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Radiocarbon calibration and detailed comments
Marine shell and foraminiferal ages were calibrated using Marine20, which accounts for the general marine reservoir effect. No additional local reservoir correction (ΔR) was applied. We will state this explicitly in the revised methods.
We will also address the remaining annotations, including clearer explanation of the interpretation and limitations of luminescence profiles, possible coring disturbance, and the requested figure corrections and clarifications.
These revisions will strengthen the presentation of the study’s principal contribution: the integration of internal architecture, sedimentology, and chronology to constrain the preserved development of Stroombank. We thank the reviewer for identifying opportunities to improve both the interpretation and its connection to the wider literature on shoreface-connected ridges.
Citation: https://doi.org/10.5194/egusphere-2026-2791-AC1 -
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AC1: 'Reply on RC1', Víctor Cartelle, 28 Oct 2026
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RC2: 'Comment on egusphere-2026-2791', Ad van der Spek, 01 Oct 2026
This is a well written, nicely illustrated and highly interesting paper that sheds new light on a long-standing debate. I highly recommend it for publication. For a few minor comments and some suggestions, see the attached document.
Best, Ad van der Spek
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AC2: 'Reply on RC2', Víctor Cartelle, 28 Oct 2026
We thank Ad van der Spek for his positive assessment of the manuscript and recommendation for publication. We appreciate the detailed comments and suggestions, which will help us clarify both the geological interpretation and its presentation.
An important point concerns the nature and preservation of the deposits attributed to former tidal inlets and ebb-tidal deltas. We will clarify the criteria underlying these interpretations and distinguish preserved channel fills from any deposits interpreted as remnants of a delta platform. We will also explain more explicitly how shoreface erosion may have modified these deposits and qualify the proposed contribution of inlet-derived sediment to Stroombank development.
We will clarify the proposed sediment sources and transport pathways, including the possible supply from the southwest and the reworking of antecedent shoreface deposits. This will include a clearer explanation of the younger deposits overlying the buried ridge sector and the proposed former sediment source. Where the available evidence does not distinguish among sources or establish their relative contributions, we will make that uncertainty explicit.
Thank you also for suggesting the Texel Inlet example described by Elias and Van der Spek (2017). We will assess its relevance as a comparison for sediment redistribution associated with ebb-tidal-delta erosion, while considering the differences between that setting and Stroombank.
The suggestion that waves approaching from the northwest could contribute to landward migration is also useful. We will consider this in the discussion of combined wave and tidal forcing, drawing on the available observational and modelling studies and distinguishing a plausible mechanism from one demonstrated by the available evidence.
Seismic profiles were collected across the detached northeastern sector of Stroombank, as shown in Fig. 1B. However, these profiles do not reveal distinguishable internal structure, so they do not allow us to extend the architectural interpretation established for the main study area into this sector. We will clarify this limitation in the revised manuscript.
We will also check the number of cores with luminescence profiles, correct the transect reference, and address the remaining textual and figure annotations.
We thank the reviewer again for the encouraging assessment and helpful suggestions.
Citation: https://doi.org/10.5194/egusphere-2026-2791-AC2
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AC2: 'Reply on RC2', Víctor Cartelle, 28 Oct 2026
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After a flurry of research on shoreface-attached ridges in the 1980s and 1990s, interest in them appeared to decrease, so I was eager to review this manuscript because I anticipated that I would learn much about how the field has advanced over the last 25 years. In general terms, the subject of this manuscript is of considerable importance for coastal dynamics, as demonstrated by the unpublished background research referenced in this manuscript. I must express some surprise, therefore, to learn that the still prevailing idea for the genesis of shoreface-attached ridges was their supposed connection with tidal inlets and ebb-tidal deltas. This was an idea that I thought had been significantly weakened, if not disproven, by my student’s work on the shoreface-attached ridges at Sable Island, eastern Canada, a location where ~80 ridges exist where there are NO tidal inlets or ebb-tidal deltas (Dalrymple and Hoogendoorn, 1997, Geoscience Canada, v. 24, p. 25-36). Unless it is believed that there is more than one type of such ridge, then the occurrence of even one area where there are no ebb-tidal deltas surely negates this model for their origin. The attention given to this model in the early part of the manuscript is, therefore, very surprising, especially as it is concluded by the end of the manuscript that ebb-tidal deltas play only a secondary role in ridge genesis. The more recent ideas about ridge genesis, including the ideas presented in our 1997 paper, are not considered although they are mentioned briefly in the introductory material. This for me gives the manuscript the appearance of being based on old ideas with no significant effort to make use of newer work. I suggest, therefore, that less emphasis be placed on the supposed genetic connection with ebb-tidal deltas, and that more attention be given to other suggested origins for shoreface-connected ridges.
Although I have a general understanding of the coastal-zone dynamics and sedimentation in Belgium, the Netherlands and Germany, that knowledge is relatively superficial, so I felt disoriented when reading the manuscript because, for me, there was insufficient description of the larger setting of the single studied ridge. For instance, there are three shoreface-attached ridges along the Belgian coast and yet there is no mention of this, nor is information that is known about the other ridges (e.g., their eastward migration) used to help understand the dynamics of the studied ridge. Similarly, the studied ridge is flanked on its seaward side by a series of tidal ridges that are never mentioned. The dynamics of the setting are also not described in any detail. For instance, the speeds of the flood and ebb-tidal currents are never given, nor are wave sizes and approach directions specified. Even the fact that there is an eastward-directed tidal residual in some unspecified part of the study area is only mentioned briefly, almost as an afterthought near the end of the manuscript to explain the supposed (questionable) deduction of an eastward elongation of the studied ridge. Overall, the geomorphic and process settings are in serious need of significant expansion.
Unfortunately, I also believe that there is a relatively serious error with regard to the location where the studied ridge originated. The oldest date obtained from the southwestern (oldest) preserved part of the ridge is repeatedly given as the location and time of initiation of the ridge, the implication being that it did not exist prior to that date. However, this is clearly incorrect as these oldest deposits contain inclined bedding that has been truncated on its seaward side, an architecture that indicates without a doubt that the ridge originated further offshore and is older than the oldest preserved deposits. This age does NOT indicate the initiation of the ridge; instead, it indicates where it arrived at that location. Its previous history is lost, so its age and location of origin cannot be determined with the available data. Thus, the proposed link with the formation of the ebb-tidal delta at the mouth of the Niewpoort channel is questionable at best, given that the studied ridge originated farther offshore. [As an aside, I note that muddy deposits are not a typical facies associated with classic ebb-tidal deltas (cf. Fitzgerald et al., 2010, in Principles of Tidal Sedimentology). Thus, the use of the term “ebb-tidal delta” might need to be reconsidered.]
Another important point that I question is the inference that the ridge becomes younger toward the northeast. This is based on the comparison of dates in cores VC06-1 and VC26. I agree that the first of these lies to the southwest of VC26, BUT… from the internal structure of the ridge shown in Figs. 2 and 3, it is obvious to me that core VC06-1 comes from the northern edge of the ridge whereas VC26 comes from the youngest deposits, closest to the actively accreting southern lee face of the ridge. This, for me, these dates reflect the southward migration of the ridge, NOT an eastward extension! To demonstrate an eastward younging, more than two dates would be needed, spanning the preserved range of southward-dipping internal strata at the two ends of the ridge. It has been known for several decades that shoreface-connected ridges as a whole (i.e., their entire body) migrates alongshore, a behaviour documented especially clearly in the Sable Island ridges cited above. A similar along-shore migration (to the east) has been documented for the three ridges on the Belgian coast (Dujardin et al., 2023, 2024), but this universal (?) behaviour is never mentioned. I do admit that the pronounced southward (onshore) migration direction of the studied ridge is difficult to reconcile with this behaviour, but this presumably tells us something about the nature of the hydrodynamic setting in which this ridge exists. This thus offers a unique opportunity to add to our fundamental understanding of shoreface-attached ridges.
I also question the assertion that the studied ridge has a two-stage evolutionary history. The presence of two ridges is based solely on the isopach map of ridge deposits (Fig. 5A). In the seismic sections, there is no evidence of a two-stage evolution of the ridge: the clinoform architecture is continuous with no hint of a period of inactivity between the two phases. I note that the NE extension of the “Phase I” ridge is in exactly the same place as the present-day Balandbank. Why is it not related to that tidal ridge, which is apparently active today as indicated by the clearly defined sand waves at its NE end? This possible connection with Balandbank is something that needs to be explored further. It suggests a very complex flow field in the area immediately north of the studied ridge, a complex flow field that might contribute to the southward migration of the studied ridge. For example, one possibility is that there is net westward (ebb) flow in the depression immediately to the north of the studied ridge, based on the widespread observation that tidal ridges have opposite directions of net sediment transport on either side of their crest. Perhaps this westward flow “spills over” the ridge crest, depositing sediment on the southern flank of Stroombank. [Another aside… It appears that the authors implicitly assume that the sedimentary bodies migrate in a direction orthogonal to their long axis. This is incorrect, as there are many situations where sediment bodies such as ridges and sand waves (dunes) migrate in an oblique direction. This is well known for tidal ridges and is built into the existing models for their genesis (cf. Huthnance (1982) and subsequent workers).]
In conclusion, the data presented in this manuscript are most definitely worthy of publication and I wish I could have given a more positive review. However, the manuscript requires considerable additional work to reach its full potential. More information is needed about the morphologic and hydrodynamic setting, and greater use should be made of previous work, with more analysis of what the documented internal structure and ages suggest with regard to the hydrodynamics responsible for the maintenance and behaviour of the ridge. The errors and questionable deductions need to be rethought and corrected or argued more cogently. If all of this can be done, then I believe that this study has the potential to make a valuable addition to our understanding of nearshore sediment dynamics. Please also consider the smaller points inserted into the annotated copy of the manuscript that I have provided.
My answers to the questions posed to reviewers are provided below using capital letters.
Does the paper address relevant scientific questions within the scope of ESurf? YES
Does the paper present novel concepts, ideas, tools, or data? QUESTIONABLE
Are substantial conclusions reached? QUESTIONABLE
Are the scientific methods and assumptions valid and clearly outlined? GENERALLY YES
Are the results sufficient to support the interpretations and conclusions? NO
Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? YES
Do the authors give proper credit to related work and clearly indicate their own new/original contribution? YES
Does the title clearly reflect the contents of the paper? YES
Does the abstract provide a concise and complete summary? IN PART
Is the overall presentation well structured and clear? GENERALLY YES
Is the language fluent and precise? YES
Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? NOT NECESSARY
Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? YES
Are the number and quality of references appropriate? GENERALLY YES, BUT THEY ARE NOT USED SUFFICIENTLY
Is the amount and quality of supplementary material appropriate? YES