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Drowned Barriers as Archives of Coastal-Response to Sea-Level Rise

Chapter

Abstract

Advances in submarine technologies and increased exploration of continental shelves are revealing increasingly more submerged barriers that have drowned in response to early- to mid-Holocene sea-level rise. These coastal archives, when combined with information on sea-level trends, oceanographic conditions and palaeogeography, are valuable palaeo-evidence that can be used to understand the processes and drivers of coastal change. In this chapter, we synthesize documented examples of drowned barriers preserved on continental shelves across the world. Using these examples, we examine the relative significance of controls on barrier drowning (aka overstepping) whereby the barrier becomes drowned offshore of the advancing shoreline. Relative sea-level rise (RSLR), sediment supply and topography are the principal controls on shoreline retreat, but the interaction between these factors cannot readily be deconstructed as they are not in operation simultaneously, nor present along all coasts. However, it is possible to recognize local conditions that make barriers vulnerable to overstepping. It is shown that barrier retreat through overstepping is enhanced by one or more of the following; coarse grain size, cemented sediment, high sediment supply rates, topographic pinning and a rapid increase in accommodation. We emphasize that to gain a better understanding of the likely response of barrier coastal systems to future RSLR and to better constrain numerical models, we need to fully utilize the geological record left behind by former coastal systems that underwent accelerated RSLR in the past.

Keywords

Barrier Coast Overstepping Rollover Drowned Sea level Transgression Holocene Submerged landscape Coastal retreat Sediment supply Palaeoshoreline 

1 Introduction

In essence, barriers respond to relative sea-level rise (RSLR) by migrating landward when the creation of accommodation (space that sediment can occupy) by rising sea levels is outpaced by the availability and rate of sediment supply to the shoreline. With respect to observed accelerations in historical and modern sea-level data (Haigh et al. 2014; Jevrejeva et al. 2014) and projected rates of future sea-level rise (SLR) (Church et al. 2013), globally, barriers are expected to enter a phase of rapid landward retreat and begin encroaching (along with the shallow coastal bays behind them) on our heavily populated and strategically important coastal zones. Coastal degradation due to RSLR is already being observed at various locations around the world (e.g. Saito 2001; Thanh et al. 2004; Gibbons and Nicholls 2006; Blum and Roberts 2009) and the economic, environmental and social impacts of such ‘coastal squeeze’ are staggering. In order to plan strategically and to deploy resources effectively for the future resilience of coastal economies, it is essential to better understand the timescales and geomorphological response of barriers to rising sea levels.

Determining barrier response to RSLR on historical timescales (ca. the last 150 years) from cartographic, photographic and instrumental data (e.g. Fenster et al. 1993; McBride and Byrnes 1997; Lentz et al. 2013) provides only a snapshot of entire system response to longer term changes in relative sea-level (RSL). Whilst these data are of considerable importance in quantifying rates and scales of coastal geomorphic processes, there is a need for geological analogues in which barrier response to past RSLR can be examined in relation to other determinants. As the nature of transgression is commonly erosional, preservation of former shorelines is rare and typically biased towards scenarios where transgression was superseded by a regression of the shoreline (e.g. Goodman et al. 2008; Hein et al. 2014).

Renewed exploration of continental shelves due to the development of offshore renewable energy and mineral resource prospecting has led to the collection of high-resolution geophysical data that is uncovering a wealth of subaqueous geomorphic and sedimentary evidence of former barriers that were drowned below sea level during rapid post-glacial SLR. As the early Holocene is the most recent time period when rates of SLR were of similar magnitude to those predicted for the future under various emissions scenarios (Church et al. 2013), it is drowned barriers of this age that should be targeted as analogues to understand how modern barrier coasts will respond to projected global SLR.

This chapter provides a synthesis of known drowned barriers preserved on the continental shelf and uses them to identify a variety of scenarios/controls that determine the style of barrier shoreline retreat to RSLR.

2 Barrier Coastal-Response to Transgression

Shorelines have considerable capacity to respond to RSLR by migrating landward and upward (Cattaneo and Steel 2003). In barrier-dominated coastal settings, waves generally erode sediment from the shoreface and transport it landward to the back-barrier (e.g. Kraft 1971; Belknap and Kraft 1981; Roy et al. 1994), i.e. transgressive ravinement. If a barrier is in a state of equilibrium and there are no topographic constraints, the landward translation of sediment keeps pace with rising sea level and the barrier-lagoon system retreats landward in concert. In a state of equilibrium (or net sediment loss), the record of coastal retreat offshore is represented in the form of an erosion surface, or ravinement surface (Swift and Moslow 1982; Leatherman et al. 1983) (Fig. 1a). This style of coastal process-response to transgression is predominantly referred to as rollover (Swift 1968; Belknap and Kraft 1981; Swift et al. 1991) and coasts along the Gulf of Mexico and the US Atlantic are already displaying characteristics of this style of retreat (Pilkey et al. 1998; Feagin et al. 2005; Morton et al. 2005; FitzGerald et al. 2008; Odezulu et al. this volume; Rodriguez et al. this volume).
Fig. 1

Schematic illustration of different styles of barrier shoreline retreat during RSLR

Conversely, if a barrier coast is in disequilibrium with rising sea level, there is potential for all or part of the barrier to drown in-situ, becoming abandoned on the continental shelf seaward of the advancing shoreline (Fig. 1). This style of coastal-response is referred to as overstepping (a term we use synonymously with “drowning” throughout) (Curray 1964; Rampino and Sanders 1980), which can be recognized by the preservation of barrier-lagoon landforms and sediments offshore (Rampino and Sanders 1980, 1982; Leatherman et al. 1983; Forbes et al. 1991). Typically, only back-barrier sediments or landward-dipping barrier beach sediments are preserved during overstepping (low preservation; Rampino and Sanders 1980, 1982; Leatherman et al. 1983; Forbes et al. 1991) (Fig. 1b). However, there are scenarios in which the entire barrier-lagoon system is preserved (high preservation; Fig. 1c) with minimal reworking (e.g. Mellett et al. 2012a). While a barrier may re-establish landward of an overstepped barrier, this is not a requirement of the overstepping process; for example, re-establishment of a barrier landward where coastal slopes are steep (Fig. 1d) would be restricted. The extent (spatial and temporal) of preserved barrier-lagoon systems can provide information on the processes occurring during and after overstepping, and help to identify the controls driving this style of coastal change.

Here, we have defined the style of coastal retreat according to the presence (overstepping) or absence (rollover) of former barrier deposits or morphology offshore of the advancing shoreline. In this instance, any morphological or sedimentary remnant of the former barrier position is interpreted as barrier response through overstepping. However, it is important to recognize that barrier response to RSLR is dynamic on both spatial and temporal scales and it is expected during overall transgression a barrier has potential to switch between the two modes depending on local conditions. From a morphodynamic perspective, a barrier may be considered to be in a continuous state of rollover as sediment is translated from the nearshore to the backshore. However, here we assess the longer term response of barriers to relative sea-level rise over geological, rather than historical, timescales.

3 Synthesis of Drowned Barriers

Here, we undertook a systematic review of scholarly articles to identify examples of drowned barriers using a combination of the following keywords; “Drowned Barrier”, “Overstepping”, “Drowned Shoreline”, “Early Holocene Barrier”, and “Transgressive Barrier”. Further articles were identified from citations within the returned results.

The systematic review returned examples of early Holocene transgressive barriers that are preserved onshore due to subsequent regression of the shoreline (e.g. Hein et al. 2014). These were not included in the review as, despite responding to a transgression initially, they have a different post-depositional history and are not directly comparable with transgressive barriers that have been overstepped. Literature searches also revealed a scenario in which sediments interpreted as being deposited in back-barrier or tidal inlet environments, thus indicating the presence of a former barrier, are preserved within incised valleys (e.g. Rodriguez et al. 2010). The distinction between back-barrier/tidal inlet and open estuarine sediments within an incised valley can be problematic; therefore, these examples were not included in the review.

A total of 25 examples of drowned barriers were discovered based on information published in 28 peer-reviewed articles. The articles were reviewed and the key information used in this synthesis is summarized in Tables 1 and 2.
Table 1

Drowned barriers documented in the published literature

ID

Location

Latitude

Longitude

Coastal setting

Reported Age

Elevation range of coastal deposits*

References

1

Gulf of Maine, USA

43°N

68°W

Lake/inland sea

9.5–8.2 cal. ka BP *

−28 m to −22 m

Kelley et al. (2010, 2013)

2

Adriatic Sea, Italy

44°N

14°E

Barrier-lagoon (Site A)

~14.3 cal. ka BP *

−82 m to −78 m

Storms et al. (2008); Maselli et al. (2011)

Adriatic Sea, Italy

44°N

15°E

Barrier-lagoon (Site B)

~10.5 cal. ka BP *

−39 m to −16 m

3

Bras d’Or Lakes, Canada

46°N

60°W

Lake/inland sea

Mid Holocene

−25 m to −15 m

Shaw et al. (2009)

4

Baltic Sea, Germany

54°N

11°E

Lake/inland sea

After 9.2 14C ka BP *

−17 m to −15 m

Novak (2002)

5

West-central Florida shelf, USA

28°N

83°W

Barrier-lagoon

8.3 to 5.9 14C ka BP *

~−12 m

Brooks et al. (2003); Hill et al. (2003)

6

KwaZulu-Natal shelf, South Africa

28°S

33°E

Barrier-lagoon

Late glacial to early Holocene

−100 m and

−60 m to −50 m

Salzmanm et al. (2013)

7

KwaZulu-Natal shelf, South Africa

29°S

31°E

Barrier-lagoon

Early Holocene

−65 m to −50 m

Green et al. (2012, 2013)

8

De Soto Canyon, Gulf of Mexico, USA

30°N

87°W

Barrier-lagoon

Early Holocene

−51 m to −26 m

Gardner et al. (2007)

9

De Soto Canyon, Gulf of Mexico, USA

29°N

85°W

Barrier-lagoon (shelf-edge delta)

Unresolved

−85 m to −55 m

Gardner et al. (2005)

10

Baltic Sea, Germany

55°N

12°E

Lake/inland sea

Early Holocene

−19 m to −12 m

Jensen and Stecher (1992)

11

New Jersey shelf, USA

40°N

70°W

Barrier-lagoon

Late glacial to early Holocene

~−70 m and −60 m to −50 m

Nordfjord et al. (2009)

12

Kattegat, Southern Scandinavia

56°N

11°E

Barrier-lagoon system

10.5 cal. ka BP to 9.5 cal. ka BP *

−35 m to −24 m

Bennike et al. (2000)

13

Southwest Florida margin, USA

25°N

83°W

Barrier-lagoon (capped with biogenic reef)

14.5 to 13.8 14C ka BP *

−72 m to −60 m

Jarrett et al. (2005)

14

Black Sea

43°N

31°E

Lake/Inland Sea

8.5 ka BP to 9.5 ka BP *

−100 m to −85 m

Lericolais et al. (2007)

15

New South Wales shelf, Australia

32°S

153°E

Barrier-lagoon

Early Holocene

Unknown

Browne (1994)

16

Rhine-Meuse, The Netherlands

52°N

5°E

Barrier-lagoon

8.3 cal. ka BP to 7.4 cal. ka BP *

−31 m to −14 m

Hijma et al. (2010)

17

English Channel, UK

51°N

0°E

Barrier-lagoon

8.4 ka to 5.3 ka *

−24 m to −14 m

Mellett et al. (2012a, b)

18

Chedabucto Bay, Canada

45°N

61°W

Barrier-lagoon

Early Holocene

~−38 m

Forbes et al. (1995)

19

Chezzetcook Inlet, Canada

45°N

63°W

Barrier-lagoon

Recent

−5 m to −2 m

Forbes et al. (1991)

20

Western Korea

35°N

126°E

Barrier-lagoon

Early Holocene

−27 m to – 8 m

Yang et al. (2006)

21

Sabine Bank, Gulf of Mexico, USA

29°N

94°W

Barrier-lagoon

~5.3 to 4.7 14C ka BP *

~−12 m

Rodriguez et al. (1999)

22

Heald Bank, Gulf of Mexico, USA

29°N

94°W

Barrier-lagoon

~8.4 to 7.5 14C ka BP *

~−15 m

Rodriguez et al. (1999)

23

Gulf of Valencia, Mediterranean

39°N

0°E

Barrier-lagoon

Pleistocene

~−60 m

Albarracín et al. (2013)

24

Old Rhine, The Netherlands

52°N

4°E

Barrier-lagoon

7.3 to 5.2 14C ka BP *

−15 to −31 m

Rieu et al. (2005)

ID refers to locations presented in Fig. 2. In reported age column * refers to examples that have been dated using chronometric methods (14C or OSL)

Table 2

Evidence used to identify drowned barriers

ID

Location

Evidence

References

1

Gulf of Maine, USA

Cores reveal glaciagenic deposits overlain by tidal flat deposits, washover fan deposits and marine sand. Rip up clasts of peat are present. Bathymetry shows ridges interpreted as spits and tombolos

Kelley et al. (2010, 2013)

2

Adriatic Sea, Italy

Channels observed in seismic showing oblique reflectors. Channel filled with interbedded silt and clay. Channels are erosionaly truncated by a ravinement surface. Channels interpreted as tidal inlets

Storms et al. (2008); Maselli et al. (2011)

Adriatic Sea, Italy

Bathymetry reveals a ridge interpreted as a barrier island. Cores from the ridge show coarsening upwards sediments. Barrier sediments are truncated by a ravinement surface

3

Bras d’Or lakes, Canada

Coastal landforms (tombolos, spits and barrier beaches) observed in bathymetry

Shaw et al. (2009)

4

Baltic Sea, Germany

Buried ridges with mound and oblique reflectors observed in seismic. Cores reveal coarsening upward sequences of interbedded sand, silt and clay. Deposits interpreted as back-stepping barrier islands

Novak (2002)

5

West-central Florida shelf, USA

Cores reveal mud, organic muddy sand and muddy sand facies interpreted as back barrier deposits based on lithology and fauna assemblages. These are overlain by a coarse shell and sand facies with a sharp lower erosional boundary interpreted as a ravinement surface

Brooks et al. (2003); Hill et al. (2003)

6

KwaZulu-Natal shelf, South Africa

Buried ridges separated by depressions comprising draped reflectors are observed in seismic. These features are interpreted as barrier-lagoons and they are truncated by a strong reflector interpreted as a ravinement surface. Cores show the ridges comprise cemented shelly sand (beachrock/aeolianite). The ridge features are also visible in bathymetry

Salzmanm et al. (2013)

7

KwaZulu-Natal shelf, South Africa

Bathymetry reveals a series of arcuate ridges and associated depressions. The ridges and depressions are also observed in seismic data

Green et al. (2012, 2013)

8

De Soto Canyon, Gulf of Mexico, USA

Ridges interpreted as barrier islands identified from bathymetry

Gardner et al. (2007)

9

De Soto Canyon, Gulf of Mexico, USA

Ridges interpreted as barrier islands identified from bathymetry

Gardner et al. (2005)

10

Baltic Sea, Germany

Seismic facies show oblique landward prograding clinoforms that are truncated by an erosional surface. Cores comprise laminated clayey silt and coarsening upward sand. Both seismic and lithology are interpreted as a sequence of barrier beach ridges closely connected to back barrier lagoon/pond deposits

Jensen and Stecher (1992)

11

New Jersey shelf, USA

Topographic lows observed in seismic data interpreted as back barrier morphology filled with transparent seismic facies interpreted as tidal inlet facies

Nordfjord et al. (2009)

12

Kattegat, Southern Scandinavia

Seismic data reveal landward oblique dipping reflectors which comprise laminated clays and silts and contain macrofossils characteristic of lagoon sediments. These are interpreted as backstepping barrier-lagoon sediments and they are truncated by an erosional reflector representing a ravinement surface

Bennike et al. (2000)

13

Southwest Florida margin, USA

Ridges with oblique reflectors are observed in seismic. Recurved spit, tidal inlet channel and prograding beach ridges observed on bathymetry

Jarrett et al. (2005)

14

Black Sea

Linear ridges and depressions observed in bathymetry. Cores reveal laminated mud overlain by a shell hash and sand. Ridges interpreted as remnant beaches

Lericolais et al. (2007)

15

New South Wales shelf, Australia

Seismic data show seaward prograding oblique reflectors interpreted as shoreface deposits overlain by landward prograding oblique reflectors interpreted as washover fan, lagoonal and tidal deposits. An erosional reflector interpreted as a ravinement surface truncates deposits. Cores comprise interbedded sand and clay, peat beds, muddy sand and fine sand

Browne (1994)

16

Rhine-Meuse, The Netherlands

Channels observed in seismic data are infilled with interbedded sand and mud comprising fauna typical of back barrier environments. These deposits are overlain by lower shoreface sediment

Hijma et al. (2010)

17

English Channel, UK

Recurved ridge and associated landward depression observed in bathymetry. The ridge is represented in seismic by a mound deposit comprising convex reflectors. Cores from the ridge comprise gravel-pebbles and coarsening. These deposits rest unconformable on a seaward prograding sand unit. In the depression behind the ridge, parallel to draped reflectors on lap against the ridge facies. Cores in this unit comprise interbedded sand and mud. All facies are truncated by an erosional boundary interpreted as a ravinement surface

Mellett et al. (2012a, b)

18

Chedabucto Bay, Canada

A ridge/mound of sediment comprising seaward prograding oblique reflectors observed in seismic data. Feature interpreted as a gravel barrier/foreland

Forbes et al. (1995)

19

Chezzetcook Inlet, Canada

Morphological profiles reveal a ridge preserved offshore of the present day barrier. Samples of pebbles recovered from the ridge. Ridge interpreted as relict barrier

Forbes et al. (1991)

20

Western Korea

Seismic data show channels with oblique reflectors and cores show an overall coarsening upwards sequence and the presence of tidal rythmites. The channels are interpreted as back barrier tidal inlets. The channels are truncated by a sharp erosional boundary interpreted as ravinement surface

Yang et al. (2006)

21

Sabine Bank, Gulf of Mexico, USA

Banks observed in bathymetry. Seaward dipping seismic units comprising muddy sand rest unconformably on a seismic unit of landward dipping reflectors comprising interbedded mud and sand. Fauna assemblages in the lower unit are typical of bay/inlet environments

Rodriguez et al. (1999)

22

Heald Bank, Gulf of Mexico, USA

Shows the same characteristics as Sabine Bank

Rodriguez et al. (1999)

23

Gulf of Valencia, Mediterranean

Seismic data show buried mounds interpreted as barriers

Albarracín et al. (2013)

24

Old Rhine, The Netherlands

Channels recognized from seismic data with some oblique reflectors. Channels comprise fine to medium sand with cross-laminae and fauna characteristic of a back barrier setting. Channels interpreted as tidal inlets

Rieu et al. (2005)

The examples presented here are the preserved remnants of barrier-lagoon systems from open to embayed coastal settings or those representing the position of former shorelines in enclosed-lake or inland-sea basins. For each barrier example, the location was recorded—the distribution of drowned barriers presented in this review is given in Fig. 2. The maximum and minimum elevation of the barrier deposits/landforms was also documented (Table 1). Elevations have not been adjusted to a single datum and are assumed to be relative to mean sea level (MSL).
Fig. 2

Location of drowned barriers presented in this review. Bathymetry source: GEBCO_08 Grid, version 20091120, http://www.gebco.net. (1) Kelley et al. (2010, 2013). (2) Storms et al. (2008); Maselli et al. (2011). (3) Shaw et al. (2009). (4) Novak (2002). (5) Brooks et al. (2003); Hill et al. (2003). (6) Salzmanm et al. (2013). (7) Green et al. (2013a, b). (8) Gardner et al. (2007). (9) Gardner et al. (2005). (10) Jensen and Stecher (1992). (11) Nordfjord et al. (2009). (12) Bennike et al. (2000). (13) Jarrett et al. (2005). (14) Lericolais et al. (2007). (15) Browne (1994). (16) Hijma et al. (2010). (17) Mellett et al. (2012a). (18) Forbes et al. (1995). (19) Forbes et al. (1991). (20) Yang et al. (2006). (21) Rodriguez et al. (1999). (22) Rodriguez et al. (1999). (23) Albarracín et al. (2013). (24) Rieu et al. (2005)

The reported age of the drowned barriers is shown in Table 1. Where radiocarbon (14C) dates were available, the age of the drowned barriers was given in calibrated (cal. ka) or radiocarbon (14C ka) years before present (BP). For examples dated using optical stimulated luminescence (OSL), ages were reported in ka. Where possible, the ages were quoted as documented by the authors in the literature. However, in some cases (e.g. Kelley et al. 2010) chronological information had to be interrogated in relation to core and seismic data to establish the age of the drowned barriers. In the absence of chronological information, an age estimate (e.g. early Holocene) was extracted from the relevant manuscript according to elevation and local RSL history. It is important to note that the ages quoted in the literature are depositional ages and are not a representation of the timing of barrier drowning.

Careful attention was paid to the seismic and lithological evidence underpinning drowned barrier interpretations and articles that did not present a convincing argument or sufficient raw data to test interpretations were excluded from the review. The sedimentological and/or geomorphological evidence used to support interpretations of features as drowned barriers is summarized in Table 2. However, it is advised that original articles are consulted for more comprehensive descriptions.

4 Characteristics of Drowned Barriers

Drowned barriers have a morphological and stratigraphic expression that can be determined from geophysical data (multibeam bathymetry and sub-bottom seismic) and/or borehole data and sediment cores (Fig. 3). In addition, fauna and flora assemblages can be used to characterize depositional environment (e.g. Hill et al. 2003).
Fig. 3

Example of key morphological, stratigraphic, seismic and lithological features of a representative drowned barrier. Images modified from Mellett et al. (2012a). (a) Annotated seismic cross section running onshore–offshore perpendicular to the shoreline. Key seismic reflector patterns are highlighted and three phases of overstepping are documented. (b) Bathymetry of the most seaward ridge (Overstepping Event 1). (c) Representative lithostratigraphically extracted from core records (note: not all facies are present in each core). Core photographs show key facies and erosion surfaces

Multibeam bathymetry is used to distinguish the morphology of a barrier-lagoon system, although it would be possible to identify morphological components buried in the sub-surface using high-resolution 2D or 3D seismic data. The barrier beach element of the depositional system is represented morphologically by one or more elongate to recurved ridges (e.g. Jarrett et al. 2005; Mellett et al. 2012a; Salzmanm et al. 2013). The ridges are commonly parallel to the palaeoshoreline and may be segmented alongshore and/or cross-shore, showing evidence of breaching or shoreline retreat (Storms et al. 2008). Topographic depressions corresponding to the back-barrier environment may be present on the landward side of the ridges (Fig. 3). It is important to recognize that relict barrier ridges can exhibit a similar morphological expression to shelf sediment ridges forming underwater in response to hydrodynamic processes operating post-transgression (e.g. Lericolais et al. 2007) and therefore interpretation of landforms using morphology alone is ambiguous. Furthermore, landforms are not always recognizable on the seabed due to poor data resolution or full/partial burial.

Sub-bottom seismic data are used to recognize barrier-lagoon features that are buried, or to unravel the internal structures and stratigraphy of barrier-lagoon systems. Using high-resolution seismic data, cross-shore seismic profiles of the barrier beach exhibit oblique reflectors that dip both landward and seaward (Fig. 3). If data are of lower resolution, the barrier beach is represented by a mound in cross-shore profiles (e.g. Kelley et al. 2010; Salzmanm et al. 2013). In scenarios where the barrier has been partially reworked and the shoreface eroded, oblique landward-dipping reflectors represent the transgressing barrier (e.g. Browne 1994). These can rest unconformably on pre-transgression deposits (Storms et al. 2008; Kelley et al. 2010) or shoreface deposits characterized by seaward-dipping reflectors (Browne 1994; Rodriguez et al. 1999) that are the product of deposition in relatively deeper water when the shoreline was farther landward, prior to emergence of the barrier beach and creation of the back-barrier environment.

Washover (or overwash) fans are another component of the barrier-lagoon system that can be preserved, i.e. the barrier beach backshore (Mellett et al. 2012a; Kelley et al. 2013). Chaotic seismic reflectors that dip predominantly landward are diagnostic of these fans and borehole data show poorly sorted coarse sediments become thinner and finer landward. These may be recognizable using multibeam bathymetry as topographic lows or breaches in the barrier. However, blow outs in dune systems have a similar morphological expression (Lericolais et al. 2007).

Preservation of entire barrier-lagoon systems is rare; typically, the existence of a former barrier is inferred from the presence of lagoon or tidal inlet sediments below a ravinement surface that can be diagnosed from sub-bottom seismic data and/or boreholes (e.g. Fig. 3). In seismic data, these deposits exhibit low amplitude, parallel to low-angle oblique landward-dipping reflectors. Where the back-barrier is dissected by tidal inlets, multiple lateral and stacked channels can be recognized (e.g. Rieu et al. 2005; Hijma et al. 2010). In boreholes, fine grained sediments with structures indicative of tidal influence or organic deposits are characteristic of back-barrier lagoon and tidal inlet environments. Stratigraphically, vertical deepening of facies (lagoon-shoreface-marine) is diagnostic of barrier-lagoon systems that have been overstepped (Cattaneo and Steel 2003).

During transgression, barrier-lagoon systems are at least partly reworked through ravinement as the shoreline advances. This erosion surface is represented in seismic data by a strong reflector that truncates underlying strata (e.g. Storms et al. 2008). In cores it can be identified by a sharp erosional contact above which lies a shell hash or coarse shelly sand which can exhibit fining upwards representing water deepening (e.g. Brooks et al. 2003).

Identification of drowned barriers should ideally be carried out through the integration of morphological, sub-bottom seismic, lithological and palaeoecological data and a lesser degree of confidence is placed on interpretations underpinned by a single line of evidence. An example of an integrated approach has been given in Fig. 3. While the characteristics of seismic, bathymetry and lithofacies at this site are not representative of all drowned barriers, they clearly demonstrate the key morphologicical, seismic and lithological signatures of drowned barriers. It is important to bear in mind that coastal barrier systems are highly dynamic and their style of retreat can switch on many timescales during an overall transgression leading to high degrees of spatial variability and preservation potential. This was demonstrated at Hastings Bank, UK (Fig. 3) where at least three phases of overstepping were recognized and preservation of barrier-lagoon deposits became progressively lower as the shoreline retreated (Mellett et al. 2012a).

5 Patterns of Drowned Barrier Distribution and Behaviour

5.1 Distribution of Drowned Barriers in Space and Time

The majority of the drowned barrier examples are located in the Northern Hemisphere (Fig. 2). This hemispheric bias may be related to the availability of suitable geophysical and borehole data which is used to underpin interpretations of drowned barrier. It is perhaps also due to the greater areas of shallow shelf seas that comprise significant accumulations of sand and gravel to support barrier development.

The elevation range of drowned barrier landforms and sediments was plotted against latitude (Fig. 4). A clustering of drowned barriers (Group 1; Fig. 3) is observed at elevations between ca. −35 m and −15 m and latitudes of 43–56°N, i.e. shallow shelf seas that might be regarded as experiencing both RSL fall and rise over glacial-interglacial timescales (cf. Zone II from Clark et al. 1978), suggesting a potential temporo-spatial control on drowning and/or preservation of barriers. No barriers were identified in water depths <15 m as these correspond to a period of time in the late Holocene when the rate of RSLR was very low, allowing sufficient time for reworking through ravinement of any former barrier systems. Elsewhere there are no strong relationships between elevation and latitude.
Fig. 4

Elevation of drowned barriers in relation to latitude. ID numbers and locations are presented in Fig. 2 and Table 1

As noted in Sect. 3, where no chronological information is available, the age of drowned barriers is often estimated by comparing the elevation of the feature to local RSL history (e.g. Green et al. 2013). If high-resolution local RSL data are available and there has been minimal post-depositional reworking then a degree of confidence can be placed in these age estimates. However, preservation is often partial and, given reworking, it is unlikely the elevation of the feature preserved today reflects the original barrier morphology and elevation prior to submergence. Furthermore, the relationship between RSL and the morphology and elevation of any given barrier is difficult to establish unequivocally without accompanying palaeoecological or sedimentary evidence (Rodriguez and Meyer 2006; Tamura 2012; Hede et al. 2013; Billy et al. 2015). The drowned barriers discussed here are from a period of time (Early Holocene) where local RSL data are often sparse as these sites occupy elevations that are now submerged. This must be considered when interpreting sea level-related controls on barrier response where chronological data are absent.

Of the documented drowned barriers included in the systematic review, 13 have been dated using chronological methods (see Table 1). The reported ages and elevations are presented in Fig. 5. With the exception of The Black Sea (ID 14.), there is a relationship between elevation and age with those located at greater depths being the oldest. This broad relationship is likely a function of post-glacial SLR . The drowned barrier preserved in the Black Sea is an outlier in this respect due to changes in water(sea) level being controlled by intermittent connection to the Mediterranean Sea and water balance in the surrounding drainage basin (Lericolais et al. 2007). Ten of the drowned barriers span the time period from early Holocene to mid− to late Holocene, whilst two are of Late Glacial age.
Fig. 5

Age of chronometrically constrained (14C and OSL) drowned barriers and their elevation. (1) Kelley et al. (2010, 2013). (2) Storms et al. (2008); Maselli et al. (2011). (4) Novak (2002). (5) Brooks et al. (2003); Hill et al. (2003). (12) Bennike et al. (2000). (13) Jarrett et al. (2005). (14) Lericolais et al. (2007). (16) Hijma et al. (2010). (17) Mellett et al. (2012b). (21) Rodriguez et al. (1999). (22) Rodriguez et al. (1999). (24) Rieu et al. (2005). MWP1A: −96 m to −76 m from 14.3 to 14.0 ka BP (Liu and Milliman 2004). MWP1B: −58 m to −45 m from 11.5 to 11.2 ka BP (Liu and Milliman 2004). 8.2 ka sea-level jump: 8.5–8.3 ka (Tornqvist and Hijma 2012). Early Holocene defined as 11,650–7000 years BP after Smith et al. (2011)

Evolution of a barrier coast during RSLR can be broadly split into three phases: (1) barrier formation; (2) barrier retreat; and (3) preservation post-submergence. The role of RSL, sediment supply and topographic/antecedent controls in governing each of these evolution phases, as described in the cited articles following interpretation of the presented evidence, are given in Table 3.
Table 3

Controls on barrier formation, drowning and preservation determined from drowned barrier examples

RSL components given in blue, topographic components in green and sediment supply controls represented in orange

5.2 Barrier Formation

Twelve of the 24 drowned barrier examples acknowledged that RSL stillstand or slowdown was required to enable the barrier to form (Table 3). However, barrier systems can develop during RSLR where the rate of sediment supply is greater than the rate at which accommodation is created by the rising sea (e.g. Mellett et al. 2012a). Sediment supply is highlighted as an important control, particularly in high latitudes where the supply of coarse clastic sediment from previously glaciated terrains supports barrier formation (Jensen and Stecher 1992; Jarrett et al. 2005; Storms et al. 2008; Kelley et al. 2010, 2013).

5.3 Barrier Retreat Through Overstepping

The role of RSLR in driving barrier retreat through overstepping is complex. High rates of RSLR (or shoreline transgression driven by RSLR) is the most commonly cited driver of barrier retreat through overstepping (Table 3). At two locations, barrier overstepping has been attributed to high rates of RSLR associated with post-glacial meltwater pulses (Storms et al. 2008; Green et al. 2012, 2013; Salzmanm et al. 2013).

Transgression is the landward movement of the shoreline and whilst it can be driven by RSLR, it can be moderated by sediment budget (Curray 1964). Under any given rate of RSLR where there is no significant change in sediment supply, transgression will be more rapid on a shallow slope when compared to a steep slope. This important topographic influence on the rate of transgression has been acknowledged as a driver of barrier overstepping (Nordfjord et al. 2009; Mellett et al. 2012a). Rapid transgression can also occur if a topographic barrier is breached/overtopped. For example, barrier overstepping in Bras d’Or Lakes, Canada is interpreted to have occurred when a topographic sill was exceeded or breached, allowing the basin in which the barrier was located to flood rapidly (Shaw et al. 2009).

Based on the systematic review, the role of sediment supply is considered subordinate to topographic and RSL controls in driving barrier retreat. Sediment supply is recognized as a control at only four locations (Table 3). Two modes of sediment supply are identified as a driver of overstepping. Traditionally, barriers are interpreted to drown when RSLR outpaces sediment supply (Curray 1964; Swift 1968; Rampino and Sanders 1980). This mode of overstepping is referred to here as ‘sediment deficit’ overstepping and has been identified at two locations (Forbes et al. 1991; Mellett et al. 2012a). A ‘sediment surplus’ mode was also recognized by Mellett et al. (2012a) where sediment supply to the shoreface during transgression is sufficient to prevent substantial reworking of the barrier, and retreat is achieved through overstepping (i.e., in this case a new barrier rapidly becomes established at a more landward position, see additional discussion in Sect. 6.2). High sediment supply in relation to accommodation driven by a small tidal amplitude or prism was recognized as a driver of barrier overstepping at two locations (Rieu et al. 2005; Yang et al. 2006). In these examples, local hydrodynamics are considered alongside the more common drivers of barrier response.

Whilst topography (coastal slope) can influence the rate of transgression, the morphology of the back-barrier, which partly governs accommodation and rate of transgression, is acknowledged for its role in influencing barrier retreat. When back-barrier accommodation is large, sediment reworked from the shoreface, and transported across-shore, fills the space, preventing the barrier from retreating through rollover, effectively pinning the barrier in place (Mellett et al. 2012a). Tidal amplitude moderates back-barrier accommodation and, as a result, is recognized as a control on barrier retreat (Storms et al. 2008; Hijma et al. 2010).

5.4 Barrier Preservation

The style of barrier retreat in part governs the preservation of the barrier during RSLR (Sect. 2). However, a number of conditions have been identified that increase the preservation potential of the barrier during drowning or after submergence. These conditions create a bias that is independent of the processes that drive barrier overstepping.

A scenario in which overall RSLR is punctuated by a short-lived phase of RSL fall has been interpreted to increase preservation of barrier systems (Jensen and Stecher 1992; Browne 1994). In this case, as the shoreline moves seaward (regression) the barrier system becomes stranded on land and becomes at least partly disconnected from the sea (or lake). If the subsequent transgression is rapid and the barrier does not have time to equilibrate morphodynamically (i.e. retreat by rollover), the barrier is drowned and becomes stranded offshore of the advancing shoreline. After submergence, near- and offshore hydrodynamics rework the barrier system, removing or degrading evidence of its existence. Local topography can enhance preservation potential where it shelters a barrier from, or modifies, the hydrodynamic regime (tides and waves) (Jensen and Stecher 1992; Forbes et al. 1995; Kelley et al. 2010, 2013).

The characteristics of sediment can support preservation of the barrier either during or after submergence. Coarse clastic (gravel-dominated) barrier systems have greater morphological resilience to rising sea levels which must be overcome for the barrier to retreat through rollover (Forbes et al. 1995; Orford and Anthony 2011). As a result gravel barriers are more likely to retreat by overstepping when compared to sand-dominated ones. Barriers preserved in subtropical latitudes have been cemented by biological and chemical processes (Albarracín et al. 2013; Green et al. 2012, 2013; Salzmanm et al. 2013) or capped by reef communities (Jarrett et al. 2005). This geochemical or biogenic cementation has been interpreted to increase barrier preservation, making it more difficult to rework the barrier during or after transgression.

6 Relative Significance of Controls on Barrier Overstepping

Reflecting on the examples outlined in Sect. 5, and the evidence on which barrier response has been interpreted, here we consider the relative importance of RSLR, sediment supply, and topography/antecedence in determining the style of barrier retreat.

6.1 Relative Sea-Level Rise

A general assumption is that barriers drown when the rate of RSLR is high (Swift and Moslow 1982; Leatherman et al. 1983). Eleven of the 13 dated examples described in Sect. 5 have depositional ages ranging from 10.5 to 4.5 ka spanning both the early Holocene when rates of RSLR were high, and the mid Holocene when rates began to slow (Fig. 4). As is it not always possible to date drowning events due to their erosive nature, these ages record the existence of a barrier prior to overstepping, hence the minimum age of drowning is taken as the maximum age of deposition of barrier sediments. It is recognized that this assumption does not account for spatial variations in barrier behaviour (e.g. alongshore progradation or erosion ). The age of drowning may also be overestimated due to removal of younger sediment during and after submergence. Furthermore, the lag time between forcing and barrier response is unknown; some barriers may respond immediately to enhanced forcing from RSLR, others might exhibit substantial delay before breaking down, thus making it hard to constrain the timing or duration of overstepping. In light of this, it can be inferred from the examples presented here that barrier drowning occurred during the early to mid-Holocene transition, incorporating episodes of both rapidly rising and decelerating global sea-level rise. The compilation of barrier age and elevation in relation to relative sea-level history is of interest as: a) it implies that barriers are able to develop even under rapid RSLR, as occurred in the early Holocene, although their persistence and thickness is expected to be low; and b) barrier overstepping can occur when rates of RSLR are slowing. These observations can be tested through further research into the threshold RSLR rates for retreat through overstepping.

Aside from sediment supply and topography, regional RSLR can explain the variability observed in the age of drowned barriers shown in Fig. 4, particularly when the effects of glacio-isostasy are considered. For example, in the Gulf of Maine, despite rapidly rising global sea level during the early Holocene, isostatic rebound generated a local relative sea-level stillstand that lasted ca. 3.5 ka (Kelley et al. 2010). During this stillstand the barrier system developed and was later overstepped as rates of RSLR began to rise. At this location, local RSLR can be isolated as a driver of barrier overstepping (Kelley et al. 2010, 2013). However, this control can only be identified where well-constrained (vertical and temporal) RSLR data are available.

The timing of barrier drowning identified above overlaps with the timing of initiation of worldwide marine deltas from ca. 8.5 to 6.5 ka (Stanley and Warne 1994), where radiocarbon-dated deltaic sequences document the landward migration or ‘pinning’ of coastal depositional environments. The dated barrier sequences reviewed here corroborate this global landward advance of shorelines during the early- to mid-Holocene transition as rates of SLR decelerate and ‘modern’-day barrier systems become established.

Chronological information implies that some of the barriers drowned in the mid-Holocene when rates of RSLR were waning. Despite this deceleration, it is likely that rates of RSLR remained high enough during this time to exceed a modelled threshold for overstepping of c.3 mm/year (cf. Storms et al. 2002). Whilst the rate in itself is important in controlling the mode of barrier response, a change in rate driven by a sudden pulse or ‘jump’ in sea level, such as those associated with meltwater events (Liu and Milliman 2004), may provide additional impetus for barrier overstepping. To test ‘sea-level jumps’ as drivers of barrier process-response, a high-resolution barrier chronology (e.g. Hijma and Cohen 2010) is essential. In the absence of such chronological constraint, it is not possible to exclude other factors in moderating barrier drowning.

6.2 Sediment Supply

During transgression, sediment supply rates can substantially alter barrier response to RSLR (cf. Curray 1964; Murray and Moore this volume). For example, during sea-level rise, continued barrier rollover is in part driven by a net sediment loss where the barrier has to migrate landward to extract sediment from the shoreface to maintain its geometry despite the rising sea level (Moore et al. 2010; Murray and Moore this volume). Using the drowned barrier examples in Sect. 5, sediment supply can be separated into a number of components that condition overstepping, namely sediment availability, sediment transport (wave and tide regime), sediment volume (relative to accommodation) and sediment properties (grain size and cementation). However, the relative significance—or, indeed, combination—of these sediment-related factors in governing the style of barrier retreat cannot be determined from the sedimentological or stratigraphic evidence alone.

The elevations at which the drowned barrier examples are found implies that they formed during post-glacial RSLR and are not relicts from sea-level stillstand(s) during the last glacial (Fig. 4). Sediment supply to these barriers must therefore have been sufficient to outpace rapidly rising post-glacial RSLR , allowing the barrier systems to aggrade or even prograde (e.g. Mellett et al. 2012a). During transgression when the overall trajectory of shoreline migration is from offshore to onshore, the availability of sediment on the continental shelf can be of greater importance than that being delivered from land due to coastal erosion and/or riverine input (e.g. Long et al. 1996; Cowell and Kinsela this volume). These seabed sediment ‘reserves’ are the product of processes and environments that prevailed before and during RSLR. For example, sediment availability is high in formerly glaciated or paraglacial areas (Forbes et al. 1995; Kelley et al. 2010, 2013) and in basins connected to large deltas where significant thicknesses of sediment associated with falling stage systems tracts have accumulated on the continental shelf (e.g. Anderson et al. 2014), though deltaic sediments may contain limited sand. These environments may be predisposed to barrier retreat through overstepping in that full recycling of the barrier sediment volume, i.e. rollover, is not a requirement as there are large volumes of sediment available to facilitate coastal-response without significant reworking.

Prevailing nearshore and coastal hydrodynamics are also an important consideration in relation to barrier sediment supply. For example, significant vertical and shore-normal changes in water level associated with a large tidal amplitude (or prism) can encourage barrier overstepping (Rieu et al. 2005; Storms et al. 2008). Reconstructing past hydrodynamics to understand their interaction with different styles of barrier retreat is difficult, because there is little evidence of past hydrodynamic conditions available in the geologic record.

With respect to sediment grain size, it is expected that gravel-dominated barriers will be more morphologically resistant to RSLR (Orford 2011), because their larger grain size makes rollover less likely. Thus, these barrier systems exhibit greater potential for overstepping. In cases, where a barrier system has been overstepped during post-glacial RSLR and the barrier beach component of the system is preserved, it is possible to determine the predominant sediment composition (gravel vs. sand). However, when a barrier retreats and only back-barrier sediments are preserved, i.e. the barrier beach component has been removed, it is not possible to establish the sediment composition of the former barrier. The best preserved drowned barriers are gravel dominated (e.g. Forbes et al. 1995; Mellett et al. 2012a) or have been cemented into beachrock (e.g. Green et al. 2012, 2013; Salzmanm et al. 2013).

When sufficient sediment is available and nearshore/coastal hydrodynamics have the ability to transport it, the evolution of a barrier system—and, indeed, its preservation on the sea bed—depends on the interaction between sediment supply and RSLR. Where there is a deficit in sediment supply (or translation potential) relative to RSLR, the barrier degrades and drowns as it cannot meet the pace of retreat through rollover, which relies on sediment translation from the shoreface to the back-barrier. Alternatively, barrier overstepping can be supported by a ‘surplus’ of sediment where high sediment supply maintains the shoreface and prevents the barrier from recycling itself through erosion of the underlying substrate. Under these conditions, the barrier shoreface maintains its seaward position and elevation despite rising sea levels. Meanwhile, overwash continues to transport sediment landward increasing barrier width until a morphodynamic threshold is reached and the barrier ‘jumps’ landward. In this case, a new barrier begins to form landward and the former features become stranded below the influence of waves and tidal currents (e.g. Mellett et al. 2012a). This ‘sediment surplus’ mode of overstepping appears to support exceptional preservation of drowned barriers. The thickness or volume of sediment within the barrier-system deposits relative to the depth of reworking (or depth of ravinement) during transgression also plays a role in determining preservation of the barrier. It is apparent that there is no single style of barrier response with respect to interactions between RSLR and sediment supply, especially as the latter can be both a limiting and enabling factor.

6.3 Topography/Antecedence

Topography that is largely an artefact of past geological and glacial-interglacial processes is considered antecedent and is a fixed control on barrier evolution. Antecedent topography governs substrate slope which, for a given rate of RSLR, determines the rate of transgression (cf. Curray 1964), i.e. the pace and distance over which a barrier migrates. Many of the barriers discussed in this review rest unconformably on different types of deposits from the last glacial stage (e.g., bedrock planation surfaces, Mellett et al. 2012a; undulating glacigenic landscapes, Kelley et al. 2010, 2013; incised valleys, Rodriguez et al. 1999; and deltaic systems, Gardner et al. 2005) and thus barrier form and thickness vary considerably. This variation in substrate slope and antecedent lithology, which is a local phenomenon, plays a significant role in determining the style of coastal retreat .

In addition to being static, topography can also be dynamic, changing as the barrier morphodynamically adjusts to RSLR (Rieu et al. 2005). Topography essentially provides the ‘space’ for sediment to occupy as a barrier responds to RSLR, i.e. accommodation. In this respect, barrier retreat is fundamentally governed by the balance between the evolving back-barrier accommodation and sediment supply. Rapid barrier drowning by overstepping can be assisted by large back-barrier accommodation due to disequilibrium between sedimentation at the shoreface and in the back-barrier (e.g. Storms et al. 2008; Hijma et al. 2010). In this case, the barrier becomes anchored as any overwash sedimentation is lost to the accommodation space, whilst RSLR continues, relocating the shoreline further landward.

Accommodation created by tidal inlets may be antecedent if the barrier occupies former lowstand fluvial valleys (Rodriguez et al. 1999; Anderson et al. 2014), or may become modified as the barrier evolves morphodynamically (Rieu et al. 2005). In either case, the presence of a large inlet can restrict sediment in both cross-shore and alongshore directions and act as a sink in a similar way to a back-barrier lagoon (FitzGerald et al. 2008, this volume; Mellett et al. 2012a). Whilst the inlet remains unfilled, barrier migration is interrupted. If RSLR continues whilst cross-barrier sediment flux is diverted to inlet infilling, accommodation in the back-barrier is maintained or increases. Therefore, depending on the duration of this infilling phase, barrier overstepping may be encouraged by creation of ‘excess’ accommodation that would otherwise be met by sediment supply if it were not being diverted to infilling.

Topography can therefore play an important role in barrier response by ‘trapping’ the barrier in place and preventing retreat through rollover. Furthermore, should a barrier experience progradation, for example due to an increase in sediment supply, the topography offshore can also pin a barrier in place if shoreface accommodation is too great (Mellett et al. 2012a).

7 Prerequisites for Barrier Retreat Through Overstepping

A number of conditions have been identified that lead to barrier retreat through overstepping. Isolating a single, predominant driver or control is problematic because the sedimentological, stratigraphic and chronological data obtained from the offshore geological record provide evidence of net landform response. In short, the interaction of RSLR, sediment supply and topography cannot readily be deconstructed, especially as wave climate and storm magnitude/frequency have an important moderating effect on barrier response. Despite this limitation, it is still possible to recognize local conditions that make barriers vulnerable to overstepping.

In a shore-normal sense, barrier response to SLR can be framed simply as an interaction between barrier forcing and barrier inertia (cf. Carter 1988). Barrier forcing mechanisms include coastal hydrodynamics (waves and tides), superimposed on an underlying trend in RSLR and punctuated by aperiodic storms (and in some locations, tsunami). As such, not all forcing mechanisms produce the same barrier response because they operate over different scales of time and space (Cowell and Thom 1994) and induce different morphodynamic responses (Wright and Thom 1977; Prime et al. 2016). For example, a change in storm regime and/or wave climate is likely to cause significant changes in the translation of sediment from the shoreface to the back-barrier. Where cross-shore sediment transport is enhanced, or depth of ravinement is higher, retreat through rollover would be expected. However, while one component of barrier forcing can encourage rollover, another component, RSLR, has the potential to drown a barrier through overstepping in the absence of any substantial landward translation of sediment (Roy et al. 1994; Plater et al. 2009).

Barrier inertia is largely controlled by local topography and sediment grain size (Cowell et al. 1991; Roy et al. 1994). These are factors that moderate the translation of sediment from the foreshore to the backshore either geometrically, e.g. barrier elevation and cross-section, or dynamically, by slowing sediment-transport rates. Barrier rollover occurs when the landward translocation of sediment by waves and storms (a component of barrier forcing) exceeds barrier inertia, enabling coastal hydrodynamics to relocate and reshape a transgressing barrier without any restrictions from topographic constraints or grain size limitations. In comparison, overstepping is facilitated when conditions support high barrier inertia, making it difficult for a barrier system to be translated dynamically under any given combination of forcing factors, and especially when rates of RSLR are high. First order controls considered to enhance barrier inertia include; (1) coarse grain size, (2) cemented barrier sediments, (3) high sediment supply (positive net sediment budget), (4) topographic pinning (e.g. morphological obstruction, barrier thickness), and (5) rapid increase in accommodation driven by back-barrier topography and coastal slope. In this respect, the morphological resistance of barriers to RSLR is not directly comparable to barrier ‘resilience’ because any dynamic response is limited by high barrier inertia. Resilient barriers are those that can respond dynamically to perturbations such as change in the rate of RSLR and return to their pre-existing state, hence they have a greater chance to retreat by rollover.

Here, we demonstrate that the relationship between barrier forcing and barrier inertia is complex, making it difficult to predict coastal-response under given conditions. However, we recognize that forcings can both encourage (e.g. via high waves and frequent storms) and limit (e.g. via high RSLR rates) the dynamic, landward translation of barrier sediment, and we predict that barriers with high inertia are more likely to be overstepped under the same barrier forcing as those with low inertia.

Because the examples we present are early- to mid-Holocene drowned barriers from the continental shelf, the influence of human activity on the coast has not been recognized in this review. However, it is possible to evaluate scenarios in which human activity can potentially increase barrier inertia by modifying one or more of the first order controls. For example, coastal defence strategies such as beach replenishment provide an additional supply of sediment of potentially more resistant grain size that may enhance barrier inertia, increasing the potential for overstepping and making coastal resources more vulnerable to catastrophic exceedance/breakdown. In addition, emplacement of engineering structures, such as rock armouring or seawalls, can ‘pin’ a barrier, preventing it from responding to RSLR dynamically; this activity increases barrier resistance whilst reducing its resilience (see also McNamara and Lazarus this volume; Murray and Moore this volume).

To ensure sustainable management of the coast, it is important to identify environmental conditions that would cause barrier retreat through overstepping. It is possible to parameterize antecedent topography (onshore and offshore) and apply RSLR projections to calculate accommodation . The much greater challenge lies in understanding sediment regimes and how they interact with accommodation under different RSLR scenarios, especially where the rate of RSLR is potentially punctuated and sediment supply is modified by human intervention. The dynamic component of sediment supply in terms of the ability of the barrier to rework and resupply itself through ravinement (governed by hydrodynamics and barrier inertia), and the availability of sediment determined by barrier size and thickness, needs to be considered (see Murray and Moore this volume). Furthermore, the relative importance of dynamic vs. antecedent topography in moderating barrier retreat needs to be evaluated. Predicting barrier retreat relies heavily on numerical and simulation models (Moore et al. 2010; Williams et al. 2012; Lorenzo-Trueba and Ashton 2014; McCall et al. 2014; Brenner et al. 2015) but these should be calibrated with geomorphological and sedimentological evidence from drowned barrier archives preserved on the continental shelf.

The role of barrier forcing processes other than RSLR, such as wave climate and storm magnitude/frequency, is underrepresented in this review. Whilst the importance of waves in determining barrier behaviour is widely recognized (e.g. Orford et al. 1991, 2002; Roy et al. 1994; Masselink et al. 2010), it is not possible to resolve storm history using the offshore evidence base and palaeotidal and palaeowave models are replied upon (e.g. Storms et al. 2008). Here, we consider RSLR, sediment supply and topography/accommodation as the main drivers of barrier response. However, this is because our analysis does not capture event-based (storm-driven) coastal-response. Indeed, this is a general shortcoming of current chronological constraints on submerged barriers and is a key limitation of applying lessons over geological and glacial/interglacial timescales to contemporary, resource management timescales. Despite this limitation, we can still provide detail on coastal settings where barrier retreat through overstepping is more likely, although it is not possible to predict whether this overstepping will be temporary (storm impacts, mendable breaches) or permanent (longer-term change that is beyond the reach of engineering solutions).

Overstepping as a scenario of shoreline retreat is currently not considered in shoreline management strategies and the impact of such process-response to RSLR on the coastal zone is unknown. Depending on the coastal setting, overstepping may have a positive or negative influence. For example, a barrier stranded offshore of the shoreline may act as a shoal, intercepting and dissipating incoming wave energy and hence offering natural morphological protection. Alternatively, a barrier (or barrier sediments) that become stranded would serve to remove large volumes of sand from the coastal sediment budget (e.g. Anderson et al. 2014) which can perturb the system with feedback implications.

8 Conclusions

There are increasingly more examples of drowned barriers being discovered on continental shelves suggesting these features are not as rare as once thought. This style of coastal process-response to RSLR, i.e. in situ drowning by overstepping, which was previously relatively poorly understood due to a lack of suitable examples, should be considered in shoreline management plans. Evidence from drowned barriers preserved on the continental shelf shows there is no predominant driver of barrier overstepping. Whilst it is possible to identify conditions that would facilitate barrier retreat through overstepping, these conditions are not in operation simultaneously nor are they evident across all coastal settings. Site-specific local conditions, such as antecedent topography and sediment supply, may therefore outweigh any widespread forcing of change, e.g. global SLR. Barrier drowning can be facilitated by the rapid rates of sea-level rise that may be achieved under future climate change projections. However, it is the interaction between RSLR, topography and sediment supply on a local scale that conditions the style of retreat. As such, it is difficult to establish a widely applicable ‘recipe’ for barrier overstepping. Aside from the value of submerged, offshore examples in framing our understanding of barrier overstepping and testing numerical models, resolving system interactions should be a priority for coastal research to improve the reliability of predictions of barrier response to RSLR and thus ensure sustainable management of the coast.

Notes

Acknowledgements

The authors would like to thank the editors of this volume for inviting this review and their comments on the manuscript. We would also like to thank John Anderson and an anonymous reviewer for their input which improved the manuscript. Chris Thomas is thanked for his review. This research is published with the permission of the Executive Director of the British Geological Survey (BGS) and was supported in part by BGS’s Marine Geoscience research programme.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.British Geological Survey, The Lyell CentreResearch Avenue South, EdinburghUK
  2. 2.School of Environmental Sciences, University of LiverpoolLiverpoolUK

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