| 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.
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