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What is coastal subsidence?

Published online by Cambridge University Press:  15 January 2024

Torbjörn E. Törnqvist Open the ORCID record for Torbjörn E. Törnqvist [Opens in a new window]  and
Michael D. Blum
Torbjörn E. Törnqvist*
Affiliation:
Department of Earth and Environmental Sciences, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118-5698, USA
Michael D. Blum
Affiliation:
Earth, Energy and Environment Center, The University of Kansas, 1414 Naismith Drive, Lawrence, Kansas 66045, USA
*
Corresponding author: Torbjörn E. Törnqvist; Email: tor@tulane.edu
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Abstract

Major technological advances have made measurements of coastal subsidence more sophisticated, but these advances have not always been matched by a thorough examination of what is actually being measured. Here we draw attention to the widespread confusion about key concepts in the coastal subsidence literature, much of which revolves around the interplay between sediment accretion, vertical land motion and surface-elevation change. We attempt to reconcile this by drawing on well-established concepts from the tectonics community. A consensus on these issues by means of a common language can help bridge the gap between disparate disciplines (ranging from geophysics to ecology) that are critical in the quest for meaningful projections of future relative sea-level rise.

Topics structure

Topic(s)

Climate changeCoastal developmentGeomorphologyMonitoring

Subtopic(s)

Coupled natural and human dynamicsData collection, modelling and analysisSea level riseSustainability and resilience

Keywords

vertical land motionsubsidencecoastsea-level rise

Information

Type
Perspective
Information
Cambridge Prisms: Coastal Futures , Volume 2 , 2024 , e2
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2024. Published by Cambridge University Press

Impact statement

Coastal subsidence is increasingly recognized as a major threat for the rapidly growing populations in coastal lowlands worldwide. Monitoring and predicting coastal subsidence is challenging and requires a multitude of methods. As a result, researchers within this community have diverse backgrounds, which has resulted in differences in the conceptual framework for understanding this process, including differences in terminology and definitions. This paper proposes a unifying framework to understand coastal subsidence with the goal to enhance communication between these different research communities.

Land subsidence, defined herein as the downward motion of a specific reference horizon relative to a fixed datum, is a major compounding problem for low-elevation coastal zones that are feeling the effects of accelerating global sea-level rise, including some that host the world’s largest population centres. This “slow-motion disaster” is receiving rapidly increasing attention within the research community (Buffardi and Ruberti, Reference Buffardi and Ruberti2023) and is prevalent along depositional coastlines that are subject to long-term passive margin subsidence. Superimposed on this relatively steady (105 to 106 yr or more) and slow geologic process are non-steady components of vertical land motion (VLM), including glacial isostatic adjustment that affects every shoreline worldwide over shorter timescales (103 to 105 yr), plus compaction of recently deposited sediment as well as compaction due to fluid extraction that operate over timescales as short as 100 to 102 yr and exhibit large spatial variability but often dominate subsidence on local scales. Here we argue that the complexities of coastal subsidence are not always recognized and appreciated within the multidisciplinary subsidence research community, presenting an obstacle to subsidence projections and coastal policymaking. We therefore ask the simple question “What is coastal subsidence?”

Shirzaei et al. (Reference Shirzaei, Freymueller, Törnqvist, Galloway, Dura and Minderhoud2021) argued that within the context of VLM, distinction must be made between static and dynamic coastal landscapes. We use these terms in a morphodynamic sense: in the former, deposition and erosion have been essentially halted, whereas in the latter (e.g., coastal wetlands), these processes operate relatively uninhibited. Morphodynamically “static” landscapes include urban as well as agricultural settings with minimal relief, which are common in low-elevation coastal zones and characterized by geomorphic processes that are sufficiently slow that they can be neglected over human-relevant timescales. The implications of this distinction for subsidence measurements are profound: in static landscapes, VLM can be directly observed by measurements of land surface displacement with remote sensing and geodetic techniques. Therefore, subsidence studies in static settings are often relatively straightforward. The situation is entirely different in dynamic landscapes, where such methods measure surface-elevation change (SEC) rather than VLM (Figure 1). These are fundamentally different things.

Figure 1.

Schematic cross section of two continental margins subject to subsidence (passive margin) and uplift (active margin), respectively. Note that InSAR measurements provide rates of surface-elevation change, which can be directly interpreted as vertical land motion only in static landscapes (e.g., urban areas). Also note the difference between the two GNSS stations: #1 is set in exposed bedrock and measures rock uplift, whereas #2 measures subsidence but misses the shallow compaction component because the instrument rests on a foundation several meters below the wetland surface. Insets (circles) provide details of the key processes operating in erosive uplifting settings, versus subsiding coastal wetlands where deposition can drive shallow sediment compaction. In the latter, there is subsidence despite net surface-elevation gain.

To progress towards a common understanding, we consider VLM and SEC in subsiding, coastal depositional landscapes to be the mirror image of what occurs in uplifting, erosional landscapes, as outlined in the seminal paper of England and Molnar (Reference England and Molnar1990) where rock uplift was differentiated from surface uplift. Rock uplift is the upward VLM of a specific reference horizon relative to a fixed datum, whereas surface uplift equals rock uplift minus erosion (exhumation). Surface uplift is therefore one variant of SEC as used herein, but if there is no erosion, or no deposition for that matter, rock uplift equals SEC. Subsidence is the opposite of rock uplift and represents the downward VLM (a negative number) of a specific reference horizon relative to a fixed datum, and SEC equals the difference between VLM of the reference horizon and vertical accretion (or erosion) of sediment. SEC will be a positive number if vertical accretion exceeds subsidence, SEC will equal zero if vertical accretion balances subsidence, and SEC will be negative if subsidence exceeds vertical accretion. As such, subsidence is defined in a way that is consistent with its long-standing use in the stratigraphic literature, where subsidence measurements can be referred to any horizon in the subsurface regardless of depth or timescale (e.g., Paola, Reference Paola2000; Frederick et al., Reference Frederick, Blum, Fillon and Roberts2019).

Measuring SEC and/or VLM can be accomplished by a wide range of methods, some of which strictly determine changes at the land surface, whereas others explicitly monitor subsurface processes. Combining both is essential to disentangling driving mechanisms. Space-based methods like Interferometric Synthetic Aperture Radar (InSAR) have become some of the most powerful tools to obtain spatially continuous data on SEC and/or VLM (e.g., Jones et al., Reference Jones, An, Blom, Kent, Ivins and Bekaert2016). Global Navigation Satellite System (GNSS) data can provide point observations on VLM in coastal settings (e.g., Hammond et al., Reference Hammond, Blewitt, Kreemer and Nerem2021) and are often used to ground truth InSAR measurements. While InSAR is potentially invaluable, it cannot differentiate the drivers, or causal mechanisms, if multiple processes contribute to SEC. Most importantly, InSAR fundamentally measures SEC, which will be equivalent to VLM in static landscapes only (Figure 1). Even in such settings, care must be taken that reflectors measure change at the land surface, given that large buildings often rest on foundations that may be tens of metres deep. As a result, significant differences may exist between velocities obtained from reflectors associated with buildings that rest on deep pilings compared to adjacent urban infrastructure, where the latter often exhibits higher rates (De Wit et al., Reference De Wit, Lexmond, Stouthamer, Neussner, Dörr, Schenk and Minderhoud2021).

Despite this caveat, ground truthing of InSAR data with GNSS measurements is comparably straightforward in static landscapes (e.g., Fabris et al., Reference Fabris, Battaglia, Chen, Menin, Monego and Floris2022). In contrast, as discussed by Keogh and Törnqvist (Reference Keogh and Törnqvist2019), this may be more challenging in wetland environments where subsidence in the shallowest portion of the subsurface is typically not captured by GNSS instruments (Figure 1). As a result, understanding subsidence in such settings requires independent measurements of vertical accretion and/or erosion to complement time series on SEC from InSAR. (Separate from these considerations, it should be noted that the collection of InSAR data in wetlands is challenging – a topic beyond the scope of the present paper.) It is therefore imperative that vertical accretion not be used synonymously with SEC as has been the case in recent, widely cited papers (e.g., Crosby et al., Reference Crosby, Sax, Palmer, Booth, Deegan, Bertness and Leslie2016; FitzGerald and Hughes, Reference FitzGerald and Hughes2019). Along the same lines, InSAR measurements that encompass both static and dynamic landscapes (e.g., Ohenhen et al., Reference Ohenhen, Shirzaei, Ojha and Kirwan2023) measure SEC, which can only be equated with VLM in the static case.

A related, widespread source of confusion is associated with the interplay between deposition and subsidence. For example, studies of river delta vulnerability have implied that a reduction of sediment supply increases subsidence (e.g., Becker et al., Reference Becker, Papa, Karpytchev, Delebecque, Krien, Khan, Ballu, Durand, Le Cozannet, Islam, Calmant and Shum2020; Glover et al., Reference Glover, Ogston, Fricke, Nittrouer, Aung, Naing and Lahr2023). However, the opposite is true: vertical accretion and subsidence from compaction in the upper portion of the sediment column are closely coupled (Saintilan et al., Reference Saintilan, Kovalenko, Guntenspergen, Rogers, Lynch, Cahoon, Lovelock, Friess, Ashe, Krauss, Cormier, Spencer, Adams, Raw, Ibanez, Scarton, Temmerman, Meire, Maris, Thorne, Brazner, Chmura, Bowron, Gamage, Cressman, Endris, Marconi, Marcum, St Laurent, Reay, Raposa, Garwood and Khan2022) due to the effective stress exerted by newly accumulated sediment (Zoccarato and Da Lio, Reference Zoccarato and Da Lio2021). In other words, the addition, not the reduction of, sediment will therefore increase subsidence. Nevertheless, even though deposition typically enhances subsidence, it still often results in net surface-elevation gain (Chamberlain et al., Reference Chamberlain, Shen, Kim, McKinley, Anderson and Törnqvist2021; Saintilan et al., Reference Saintilan, Kovalenko, Guntenspergen, Rogers, Lynch, Cahoon, Lovelock, Friess, Ashe, Krauss, Cormier, Spencer, Adams, Raw, Ibanez, Scarton, Temmerman, Meire, Maris, Thorne, Brazner, Chmura, Bowron, Gamage, Cressman, Endris, Marconi, Marcum, St Laurent, Reay, Raposa, Garwood and Khan2022). Put differently, wetlands that lose elevation compared to rising sea level may still gain elevation with respect to a fixed geodetic datum, as long as vertical accretion outpaces subsidence (Figure 1).

Why is this important? We argue that not recognizing these nuances could hinder progress, not just scientifically but also in terms of the policy implications of coastal subsidence. For example, it is not uncommon for river deltas to exhibit positive SEC due to ongoing sediment deposition. Data presented by Jankowski et al. (Reference Jankowski, Törnqvist and Fernandes2017) show that the rate of SEC in coastal Louisiana is 0.7 ± 6.9 mm/yr. This is due to vertical accretion that largely offsets subsidence (58% of their monitoring sites exhibit values of zero or higher). However, none of this means that subsidence is not occurring throughout the shallow and deep subsurface; Nienhuis et al. (Reference Nienhuis, Törnqvist, Jankowski, Fernandes and Keogh2017) reported a subsidence rate averaging 9 mm/yr for this area. Put differently, conflating subsidence with SEC in a case like this would make it very challenging to effectively communicate the subsidence problem in this region to practitioners. Given the increasing need for predictive, process-based subsidence models (Allison et al., Reference Allison, Yuill, Törnqvist, Amelung, Dixon, Erkens, Stuurman, Jones, Milne, Steckler, Syvitski and Teatini2016; Shirzaei et al., Reference Shirzaei, Freymueller, Törnqvist, Galloway, Dura and Minderhoud2021), it is critical to separate between elevation loss due to downward VLM (i.e., subsidence) and elevation loss due to a lack of sediment deposition or erosion, in the same way that tectonic geomorphologists separate between rock uplift and surface uplift (e.g., Gasparini and Whipple, Reference Gasparini and Whipple2014; Yang et al., Reference Yang, Willett and Goren2015). We advocate a similar theoretical framework for the coastal subsidence community.

An important motivation for this contribution is the increasing recognition of coastal subsidence as an existential threat that adds to the risks posed by global sea-level rise for millions of people worldwide. In fact, along many deltaic coastlines the magnitude of coastal subsidence can equal or exceed current and projected rates of geocentric sea-level rise (Jelgersma, Reference Jelgersma, Milliman and Haq1996; and many subsequent studies). In this sense, coastal subsidence research has enjoyed rapid progress, not least due to a wide range of technological advances and an increasingly high spatial and temporal resolution in the detection of VLM at or near the Earth’s surface (e.g., Da Lio et al., Reference Da Lio, Teatini, Strozzi and Tosi2018; Steckler et al., Reference Steckler, Oryan, Wilson, Grall, Nooner, Mondal, Akhter, DeWolf and Goodbred2022; Zoccarato et al., Reference Zoccarato, Minderhoud, Zorzan, Tosi, Bergamasco, Girardi, Simonini, Cavallina, Cosma, Da Lio, Donnici and Teatini2022; Zumberge et al., Reference Zumberge, Xie, Wyatt, Steckler, Li, Hatfield, Elliott, Dixon, Bridgeman, Chamberlain, Allison and Törnqvist2022). However, as shown by the examples discussed above, new and/or increasingly sophisticated measurements have not always gone hand in hand with progress on our understanding of the relevant surface and subsurface processes: the vital question of “what exactly is being measured?” must never become an afterthought.

In closing, we note that increased concern about subsidence will be addressed by the recently established International Panel on Land Subsidence (IPLS; Minderhoud and Shirzaei, Reference Minderhoud and Shirzaei2022; https://sites.google.com/view/iplsubisdence/home). One of the key objectives of the IPLS will be to produce subsidence projections that can be combined with IPCC-style sea-level projections (Oppenheimer et al., Reference Oppenheimer, Glavovic, Hinkel, Van de Wal, Magnan, Abd-Elgawad, Cai, Cifuentes-Jara, DeConto, Ghosh, Hay, Isla, Marzeion, Meyssignac, Sebesvari, Pörtner, Roberts, Masson-Delmotte, Zhai, Tignor, Poloczanska, Mintenbeck, Alegría, Nicolai, Okem, Petzold, Rama and Weyer2019; Fox-Kemper et al., Reference Fox-Kemper, Hewitt, Xiao, Ađalgeirsdóttir, Drijfhout, Edwards, Golledge, Hemer, Kopp, Krinner, Mix, Notz, Nowicki, Nurhati, Ruiz, Sallée, Slangen, Yu, Masson-Delmotte, Zhai, Pirani, Connors, Péan, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekçi, Yu and Zhou2021) to generate more powerful forecasts of relative sea-level change. A recent community paper on sea-level terminology (Gregory et al., Reference Gregory, Griffies, Hughes, Lowe, Church, Fukimori, Gomez, Kopp, Landerer, Le Cozannet, Ponte, Stammer, Tamisiea and Van de Wal2019) has reduced confusion on critically important concepts from this neighbouring field. In that spirit, we hope that this brief paper can be an initial contribution towards a more clearly defined conceptual framework for understanding and projecting subsidence in the coastal zone.

Open peer review

To view the open peer review materials for this article, please visit http://doi.org/10.1017/cft.2024.1.

Acknowledgements

Jaap Nienhuis and an anonymous reviewer provided numerous thoughtful comments that enabled us to clarify key elements of the subject matter, but we stress that the opinions expressed herein are our own. We also thank Philip Minderhoud and Nicole Gasparini for valuable feedback.

Competing interest

The authors declare none.

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Figure 0

Figure 1. Schematic cross section of two continental margins subject to subsidence (passive margin) and uplift (active margin), respectively. Note that InSAR measurements provide rates of surface-elevation change, which can be directly interpreted as vertical land motion only in static landscapes (e.g., urban areas). Also note the difference between the two GNSS stations: #1 is set in exposed bedrock and measures rock uplift, whereas #2 measures subsidence but misses the shallow compaction component because the instrument rests on a foundation several meters below the wetland surface. Insets (circles) provide details of the key processes operating in erosive uplifting settings, versus subsiding coastal wetlands where deposition can drive shallow sediment compaction. In the latter, there is subsidence despite net surface-elevation gain.

Author comment: What is coastal subsidence? — R0/PR1

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr1 [Opens in a new window]
Torbjorn Tornqvist [Opens in a new window] Department of Earth and Environmental Sciences, Tulane University, United States
Revision round: 0
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Comments

Dear Tom,

We here submit a short manuscript intended as a Perspective for “Cambridge Prisms: Coastal Futures” that focuses on coastal subsidence, a globally relevant issue that is rapidly gaining interest.

The motivation for this piece is the widespread confusion in the recent literature on this subject, which we believe can be traced to misunderstandings about key concepts. These misunderstandings are exemplified by recent papers in high-profile journals (PNAS, Nature Communications) as well as earlier and frequently cited synthesis papers. This presents an obstacle to our understanding of a critical environmental problem that affects hundreds of millions of people worldwide.

Some of these issues can be linked to the highly multidisciplinary nature of subsidence studies and the lack of communication between research communities in different fields. We therefore believe that a multidisciplinary journal would be the best fit for our purpose. Our contribution is deliberately meant to be somewhat provocative, and we hope to solicit input from the wider subsidence community on the preprint that we plan to post at EarthArXiv. We have been in contact with a colleague (Philip Minderhoud) who recently launched the International Panel on Land Subsidence, and he is committed to reach out to this community for this purpose. Our hope is that feedback from the wider coastal subsidence community can help us shape this paper into a document that will put us on a path toward consensus.

We hope you will find this work of interest.

Torbjorn Tornqvist

Review: What is coastal subsidence? — R0/PR2

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr2 [Opens in a new window]
Jaap Nienhuis [Opens in a new window] Utrecht University Faculty of Geosciences, Netherlands
Date of review:  08 November 2023
Revision round: 0
Role: reviewer
Recommendation/decision: minor-revision

Conflict of interest statement

Tornqvist is an active collaborator. I signed my review to be fully transparent about this.

Comments

Tornqvist and Blum present on a widespread terminology issue surrounding VLM, subsidence, and surface elevation change. I find it well-written and easy to read. I fully agree that a better conceptual framework would benefit the diverse subsidence community, most importantly any new student entering this field.

I have one comment, and that is, in my view, that there is not always a “miscommunication” (L12) about subsidence, but that part of the issue is simply that it is approached differently in different scientific disciplines. As far as I understand, the distinction is that for the geodesy community, sedimentation/erosion is part of SLR, whereas in geomorphology it is not. Although I am very much part of the geomorphology community and I follow the same definitions as Tornqvist and Blum, I would not say that either science discipline is wrong.

This comes back in several instances in their paper. For example, in L71, perhaps Becker et al and Glover et al follow the geodesy definitions. Then this is not necessary a misconception and a reduction in sediment supply (resulting in erosion) would lead to “subsidence”? In another instance, when reading the Gregory et al paper that Tornqvist and Blum cite (L94), it seems to me that these authors (Gregory) also follow the geodesy definition when they write: “The level of the sea floor varies due to solid-Earth tides, accumulation of sediment (with eventual compaction) and vertical land movement on a range of timescales.” They differentiate consolidated and unconsolidated sediments but they do not specify a degree of consolidation.

Note that in a recent paper of mine (Nienhuis et al., Annual Reviews 2023, co-authored by Tornqvist), we also briefly discussed this point. Here we wrote that “some studies consider sedimentation and erosion to be contributors to VLM and, therefore, RSL (e.g., Dalca et al. 2013). Here we follow the geomorphologic literature and consider sedimentation and erosion to be separate from VLM such that surface-elevation change is the sum of VLM, sedimentation, and erosion.”

To make a long story short: I fully agree with the content of the paper and I think it has great merit in the scientific literature. One aspect I would personally like to see added (or changed) is the fact that it may not be a “misconception” but rather a different use of terminology stemming from different scientific disciplines. This, of course, does not imply that the terminology is used correctly in every paper and there may yet be many studies where subsidence/VLM/SEC was actually used incorrectly.

Two minor points:

L59; perhaps add why the foundation depth is relevant in this case (i am aware this is because of vertical changes in the subsidence rate, but this may not be obvious to all readers).

L68: be equated with VLM in [the static case]

Best wishes,

Jaap Nienhuis

Review: What is coastal subsidence? — R0/PR3

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr3 [Opens in a new window]
Date of review:  13 November 2023
Revision round: 0
Role: reviewer
Recommendation/decision: minor-revision

Conflict of interest statement

Reviewer declares none.

Comments

Dear Tor and Michael,

I read your ms with interest. I agree that a clarification on the subject and the related terminology is really needed. I would like to add my point of view, i.e, that of an engineer and/or hydrogeologist, to this contribution with the hope of enlarging the “validity” of the definitions. In fact, in the present form people/researchers with background like mine (which started working on the topic since the first decades of the last century) do not feel comfortable.

Two are the main issues:

1) the definition of “land subsidence” cannot differ from that of “land subsidence in coastal areas” (or coastal subsidence). In some why, the problem of “what is actually been measured” cannot rule the definition land subsidence. In other words, we cannot define what land subsidence is based on the monitoring techniques presently available. The techniques must adapt to the concept;

2) in the present form of the ms, the distinction between “displacements” and the “processes” responsible for these displacements is somehow lacking. There is a sort of mixture between the terminology used to represent “movements” and that used to represent “processes”. From the perspective of a reader aimed at disentangling / modelling (not only monitoring) the various physical processes causing the movements, this issue is worth to be clarified.

I think the ms should start with the definition of what “land subsidence” is. In the first paragraph (lines 30-44, p1) the term “land subsidence” is mentioned several times but without explaining what the word means (or what is the definition for the word). For example: “it is a compounding problem for low-elevated coastal zones (only?)”, or “long-term passive margin subsidence”, or “dominate subsidence on local scale”. But what is land subsidence? After the first paragraph the reader is really confused.

From an engineering point of view, “land subsidence” is the loss of elevation (with respect to a geocentric reference system, coherently with the sea-level rise) of the actual land surface, independently of all the processes that can superpose to generate the movement (and if the actual land surface gains elevation, we observe a “land uplift”) and irrespectively where this movement occurs (in “static” or “dynamic” landscapes). It is not the movement of a specific grain in the soil stratigraphy or the thickness loss/gain of a certain (shallow or deep) layer.

The variety of processes that superpose to cause land subsidence/uplift is large, ranging from deep and slow (tectonics), to intermediate (GIA, SIA), to (relatively) shallow and fast such as compaction due to subsurface fluid removal, shallow auto-compaction, erosion, oxidation, sedimentation, surface loading. And they can act independently on the site, in inner basins and coastal zones.

In view of understanding the fate of coastal area to future sea level rise, definitions such as “long-term passive margin subsidence” (p.1), “downward VLM of a specific reference horizon relative to a fixed datum” (p2), and “shallow subsidence” (figure 1) seem inconsistent and make confusion. Three different terms, the first and the latter referring to processes, the second one to the movement of a specific level.

Conversely, the “engineering” definition is consistent with what is reported in the last paragraph of your ms: “increasing recognition of coastal subsidence as an existential threat that adds in the risks posed by global sea level rise”. It is the movement of the actual land surface of a certain coastal area that must be considered when we aim at evaluating coastland resilience to sea level rise. It is not a matter of which displacement affect the geologic horizon dated 1000 yr BP (i.e., the VLM) or the thinness loss of the sediments deposited over the last 1000 yr (i.e., vertical accretion minus shallow subsidence, see Figure 1).

Indeed, a wetland that loose elevation with respect to a rising mean seal level may still uplifting or subsiding depending if processes contributing to gain elevation (e.g., deposition of new sediments, deep tectonics, GIA, or expansion of aquifers after well shutdown) prevails or not to processes causing elevation loss (such as sediment auto-compaction, compaction due to hydrocarbon exploitation, biodegradation of organic matter, deep tectonics, etc).

I agree that the outcomes of different monitoring systems must be properly interpreted depending on what they really measure. And this can differ in static and dynamic coastal landscapes. And a huge warning must be advanced, especially today when thousands of young scientists apply remote sensing without any or with a small knowledge of what they are measuring.

Please consider these notes in your ms. This way, the definition of land subsidence (uplift) is unique and has a general validity, independently on the study site (coastlands, inner plains, mountains). Eventually, at least make a clear distinction between the terminology related to processes and that related to (vertical) movement of surfaces. E.g., the term “shallow subsidence” seems wrong to me, while “shallow compaction” is unequivocal irrespective of the scientific background.

A few minor suggestions:

- l24 : “sediment compaction and fluid extraction” cannot be listed together. “Sediment autocompaction and compaction due to fluid extraction” are more consistent processes to be consecutively listed;

- l36: remote sensing techniques do not measure a “deformation” but “displacements”. Extensometers and fiber optics measure a deformation;

- l55 to l58: these lines seem a bit unclear and confuse the reader. Does GNSS differentiate drivers contributing to SEC? I don’t think so, GNSS has the same capability of InSAR, i.e. they both measure displacements. Only with GNSS antennas or radar reflectors founded at different depths give the possibility to differentiate the drivers. Moreover, it is reported that “InSAR measures SEC, which is equal to VLM in static landscapes”; but what about “land subsidence”? InSAR does not measure land subsidence? With these definitions the goal of defining what is land subsidence in coastal areas seems resolved by removing the word “land subsidence”. Similarly, in l67 to l69;

- l76 to l78: it is unclear to me what do you mean with “Wetlands that lose elevation with respect to a relative sea-level rise”. It seems a condition that refers to a “double references”: the sea that rises with respect to a certain land surface and the wetland that loses elevation with respect to this relative sea level movement. Very complex. Is it better to write “wetlands that lose elevation with respect to a rising sea-level”?

- figure 1: “shallow subsidence” is a wrong concept from my point of view, and it should be substituted with “shallow compaction”. Otherwise, there is the risk of increasing the misunderstanding about land subsidence instead of clarifying the topic.

Recommendation: What is coastal subsidence? — R0/PR4

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr4 [Opens in a new window]
Aimee Slangen [Opens in a new window] Royal Netherlands Institute for Sea Research Division Yerseke, Netherlands
Date of review:  13 November 2023
Revision round: 0
Role: Handling Editor
Recommendation/decision: minor-revision

Comments

Dear Tor and Michael,

Two reviewers have now seen your manuscript. As you see, both reviewers are positive about the manuscript and applaud its goals, but they also provide some useful pointers on how to further refine&clarify the definitions, in particular keeping in mind how there may be differences between various fields/backgrounds. I therefore return the manuscript to you for minor revisions.

Kind regards,

Aimée

Decision: What is coastal subsidence? — R0/PR5

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr5 [Opens in a new window]
Tom Spencer [Opens in a new window] Geography, Cambridge University, United Kingdom of Great Britain and Northern Ireland
Revision round: 0
Role: Editor in Chief
Recommendation/decision: minor-revision

Comments

No accompanying comment.

Author comment: What is coastal subsidence? — R1/PR6

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr6 [Opens in a new window]
Torbjorn Tornqvist [Opens in a new window] Department of Earth and Environmental Sciences, Tulane University, United States
Revision round: 1
Role: author

Comments

Tom, Aimee:

We have revised our manuscript and submit it here. The reviews were extremely helpful and gave us plenty to think about. We came to the conclusion that we do not agree with the definitions favored by reviewer #2, however, their comments have enabled us to better articulate why. To this end, we have added a paragraph that explains why the approach proposed by the reviewer is bound to be problematic.

We also include a Word-document with tracked changes which shows that the additional revisions that we have made in response to the comments. We have also added a few more references.

We feel that the manuscript is significantly improved and we look forward to your decision.

Best Regards,

Tor

Review: What is coastal subsidence? — R1/PR7

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr7 [Opens in a new window]
Jaap Nienhuis [Opens in a new window] Utrecht University Faculty of Geosciences, Netherlands
Date of review:  27 December 2023
Revision round: 1
Role: reviewer
Recommendation/decision: accept

Conflict of interest statement

The first author (Prof. Tornqvist) was my postdoctoral advisor and we still publish together.

Comments

I thank the author for their clear responses to my comments and I have no further remarks.

Review: What is coastal subsidence? — R1/PR8

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr8 [Opens in a new window]
Date of review:  30 December 2023
Revision round: 1
Role: reviewer
Recommendation/decision: accept

Conflict of interest statement

Reviewer declares none.

Comments

The revised ms is ok for me. Best, Pietro Teatini

Recommendation: What is coastal subsidence? — R1/PR9

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr9 [Opens in a new window]
Aimee Slangen [Opens in a new window] Royal Netherlands Institute for Sea Research Division Yerseke, Netherlands
Date of review:  05 January 2024
Revision round: 1
Role: Handling Editor
Recommendation/decision: accept

Comments

No accompanying comment.

Decision: What is coastal subsidence? — R1/PR10

Published online by Cambridge University Press:  15 January 2024
DOI: https://doi.org/10.1017/cft.2024.1.pr10 [Opens in a new window]
Tom Spencer [Opens in a new window] Geography, Cambridge University, United Kingdom of Great Britain and Northern Ireland
Revision round: 1
Role: Editor in Chief
Recommendation/decision: accept

Comments

No accompanying comment.