Rock coast geomorphology: Recent advances and future research directions

Geomorphology 114 (2010) 3–11

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Review

Rock coast geomorphology: Recent advances and future research directions L.A. Naylor a,b,⁎, W.J. Stephenson c, A.S. Trenhaile d a

Department of Geography, University of Exeter, Cornwall Campus, Penryn, Cornwall, TR10 9EZ, UK Research Associate, Oxford University Centre for the Environment, University of Oxford, South Parks Road, Oxford, OX1 3QY, UK c Department of Geography, University of Melbourne, Victoria 3010, Australia d Department of Earth Sciences, University of Windsor, Canada b

a r t i c l e

i n f o

Article history: Accepted 5 February 2009 Available online 12 February 2009 Keywords: Rock coast Coastal geomorphology Shore platform Coastal vulnerability Theoretical geomorphology Coastal processes

a b s t r a c t There have been considerable advances in rock coast research in the past decade, as measured in terms of the number of active researchers and in the number of research papers being produced. This review, although not exhaustive, highlights many of the improvements that have been made in our ability to identify and measure the processes shaping rock coasts, at a range of spatial and temporal scales. We review how researchers are experimenting with new techniques; grappling with quantifying the effects of multiple processes on resultant landforms; and exploring how well rock coast systems relate to wider geomorphological and earth science debates. Recent research, including those in this special issue, aptly demonstrate the scientific benefits that can be accrued by studying rock coasts at a variety of spatial and temporal scales, by considering the effect of the wide range of processes that operate on them, and by the application of new measurement techniques and approaches. Despite these advances, there is ample scope for future research, which could profit from increasing collaboration with other coastal geomorphologists and allied earth science disciplines in order to identify and quantify linkages between rock coasts and other coastal systems. It is also important that new research considers how rock coasts will respond to extreme events and to risks associated with changing climate, and to how rock coast geomorphology might contribute, beyond coastal science, to wider debates in theoretical geomorphology. © 2009 Elsevier B.V. All rights reserved.

1. Introduction There has been growing interest in rock coast geomorphology in recent years, which has been reflected in both the number of published scientific papers and the number of active researchers. This is a welcome increase to a sub-discipline that had hitherto received comparatively limited attention by coastal scientists in general, and more specifically by geomorphologists. Although there have been recent reviews on aspects of rock coasts, such as on shore platforms with notable papers by Stephenson (2000) and Trenhaile (2002), and country specific reviews of rock coasts by Stephenson and Thornton (2005) and Kennedy and Dickson (2007), there has been no general review of rock coast geomorphology since the monographs of Trenhaile (1987) and Sunamura (1992). At a time when there is increased scientific interest and wider community concern about shoreline response to climate change, a review paper and special issue on rock coasts offer an opportunity to take stock of our present state of understanding, and where and how future research might be directed. This paper does not intend to provide an exhaustive review; instead we endeavour to provide insight into recent advances in rock coast geomorphology (extending beyond specific landforms such as ⁎ Corresponding author. Department of Geography, University of Exeter, Cornwall Campus, Penryn, Cornwall, TR10 9EZ, UK. Tel.: +44 1326 253617; fax: +44 1326371859. E-mail address: [email protected] (L.A. Naylor). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.02.004

shore platforms) and to contextualise this work within a more general coastal framework. In doing so, we seek to identify some of the research areas that remain under-represented, and present ways of improving the dialogue and interaction between rock coast geomorphologists and other coastal scientists.

2. Rock coast global distribution It is not really known how much of the world's shoreline is “rocky”? While Emery and Kuhn's (1982) 80% has been frequently repeated, there has been little or no research to substantiate this figure. In fact what appears to be a simple question is rather problematic. What constitutes a rocky shore (see discussion below)? For example, many cliffed coasts are fronted by beaches so that waves only reach the cliff foot under the most energetic conditions. How should such shorelines be categorised? Similarly, the percentage of coast having a shore platform is unknown. There is a need to work with GIS specialists and global datasets to determine both the amount and type of rock coast, but this requires the generation of both suitable data sets and classification schemes. Material could be derived from recent coastal mapping and vulnerability studies such as EUrosion (European Commission, 2004), Dinas-Coast (McFadden et al., 2007) and the geomorphic mapping of the Australian coast based on the method developed by Sharples (2006).

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3. Erosional compared with depositional coastal geomorphology While we do not advocate a return to coastal classification as a preoccupation for those interested in rock coasts there is a need to establish consistent typologies that enable distinctions to be drawn between rock and soft sediment coasts. Here we refer to rock coasts as predominately erosional landforms while soft coasts are those which are primarily depositional in nature, such as unconsolidated or poorly consolidated sediments (e.g. beaches and dunes). We feel that these terms are more appropriate than referring to rock coast landforms as ‘hard’ or ‘resistant coasts’ in contrast to ’soft’, and by implication more vulnerable coasts, as is commonly done (e.g. Sherman and Gares, 2002). This is particularly important as typologies often form the basis of erosion vulnerability and sensitivity indices (e.g. Pethick and Crooks, 2000) that are increasingly used to assess risk under a changing climate (discussion below). Outputs from such indices can strongly influence perceptions on the relative importance of rock coast studies. For example, Pethick and Crooks (2000) argued that the vulnerability of coastal landforms can be characterised by relaxation times and return intervals for threshold events. Salt marshes and sand dunes are likely to respond almost instantaneously to direct wave attack caused by storm surges and would be defined as highly vulnerable environments, while the same storm may have no impact on a hard rock coast. On the other hand, while sand dunes and salt marshes may quickly recover from storms, changes to rock coasts are long lasting to permanent. An obvious problem lies in the distinction between coasts carved in rocks and those formed in less consolidated materials, such as cohesive clays, consolidated gravels and diamicts, that share some of the same morphological elements, including cliffs and shore platforms. In California, coastal workers study composite cliffs that are variable in hardness along their vertical profile, being harder at the base and softer at the top (Hampton et al., 2004). Additional problems occur with composite coasts such as beaches with shore platform foundations and cliffs that are fronted by beaches (which may or may not lie over a platform – are these rock coasts or soft coasts? Perhaps it would be better to think of coastal landforms as being part of a continuum ranging from depositional to erosional, rather than attempting to define where rock coast studies end and soft coast studies begin? There has been limited research on rocky coasts with beaches and other depositional elements, and rock coast geomorphologists have seldom studied coasts in softer rocks such as cohesive clays (i.e. tending towards soft, unconsolidated coasts). Notable exceptions include Davidson-Arnott (1986), Davidson-Arnott and Ollerhead (1995) and Davidson-Arnott and Langham (2000) who measured downwearing rates on submerged, nearshore glacial till in the Great Lakes; the erosion rate study Charman et al. (2007) and; Trenhaile's (2009) clay coast erosional model. Research is needed on these more complex morphologies to help to refine our typologies and to improve the sensitivity of vulnerability indexes. There also needs to be greater collaboration between those who predominately study depositional landforms (e.g. soft coasts such as beaches and dunes) and those who primarily focus on erosional landforms (i.e. rock coasts). There has been much less research conducted on rock coasts that on softer sedimentary coasts, in part because many sedimentary features, such as beaches, are perceived to have a high social and economic value (e.g. Finkl and Walker, 2002; Horn, 1997). Sherman and Bauer (1993, pg 242) in a review of coastal geomorphology (where only 4 of 139 references were on rock coasts) suggested that beaches will “retain topical dominance in coastal geomorphology over the next 20 years.” A literature search was carried out in 5-year intervals since this statement was made. It is obvious from Fig. 1 that there still remains a topical dominance on beach research, and that rock coast research, although a growing area, is still less wellexamined than other coastal landforms such as beaches and coastal

Fig. 1. Literature search illustrating the numbers of papers with beach, rock coast (shore platform or rock cliff) and coastal wetlands (including mangroves and salt marshes) in their title, in 5-year intervals since 1993 (i.e. after Sherman and Bauer, 1993). Searches conducted using Web of Science on 23rd December 2008, where subject areas were refined after the initial searches to restrict the output to geomorphology-related topics.

wetlands. Conclusions such as these perpetuate the research emphasis on those landforms. Rock coast research was conducted by a fairly small number of scientists in the 20th century and there was limited interest in rock coast processes and erosion by practitioners in allied disciplines such as coastal engineering. Research was typically carried out by small teams of geographically dispersed researchers, and limestone coasts and coral reefs were typically treated separately from other types of rock coast. The emphasis on particular types of landform has been skewed towards depositional coasts in most coastal textbooks. We analysed the tables of contents of 19 textbooks on coastal geomorphology to assess the amount of attention given to rock coasts, excluding corals due to their biogenic origin and their subtidal location (Table 1). Although this method of analysis is quite coarse and may underrepresent coverage of rock coasts by ignoring pages in other chapters that refer to rock coasts, or exaggerate their under-representation by including chapters on waves and tides, it provides a general illustration of their relative emphasis on depositional and erosional (i.e. rock) coasts. Depositional coast landforms are typically covered by several chapters on different types of landform, including beaches, dunes, and salt marshes, whereas rock coasts are typically treated in a single chapter. With the exceptions of Zenkovich (1969) and Trenhaile (1997), all general coastal texts devote one or fewer chapters to rock coasts and on average, 8.5% of the books (as a measure of the number of pages relative to book length) refer specifically to rock coasts; only six of the 19 books have more than 10% of their content devoted to rock coasts. Clearly, the attention placed on rock coasts in such texts is far below Emery and Kuhn's (1982) estimate of their global extent, relative to other types of coast. Brunsden (2002) has actively promoted the utility of geomorphology in engineering studies, and in doing so has increased the awareness and consideration of geomorphological science in slope stability investigations. His research in Lyme Regis (Brunsden and

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Table 1 Analysis of the coverage of rock coasts in textbooks on coastal geomorphology. Author

Title

No. of chapters

Rock coast

Book length (pages)

Rock coast (pages)

% of book on rock coasts

Bird (1976) Bird (2000) Carter and Woodroffe (1994) Carter (1988) Davis and Fitzgerald (2004) Davis (1977) Edwards (2008) Guilcher (1958) Hansom (1988) Haslett (2000) Johnson (1919) King (1972) Masselink and Hughes (2003) Pethick (1984) Steers (1969) Thom, B.G. (Ed.) (1984) Trenhaile (1997) Woodroffe (2002) Viles and Spencer (1995) Zenkovich (1969)

Coasts (2nd ed) Coastal geomorphology: an introduction. Coastal evolution Coastal environments Beaches and coasts Geographical variation in coastal development Shore processes and their palaeoenvironmental applications Coastal and submarine morphology Coasts Coastal systems Shore processes and shoreline development Beaches and coasts (2nd ed) Introduction to coastal processes and geomorphology An introduction to coastal geomorphology Coasts and beaches Coastal geomorphology in Australia Coastal dynamics and landforms Coasts Coastal problems: geomorphology, ecology and society at the coast. Processes of coastal development

10 12 13 14 21 12 11 8 8 6 10 21 11 12 8 14 12 10 8 14

1 1 1 0.3 1 1 1 0.2 1 0.15

282 322 515 617 419 204 519 274 96 218

1 1 1 1 0 1 1 1 1.5

570 354 260 136 349 366 623 350 738

36 47 25 15 16 21 33 13 9 8 estimate: 33 27 19 20 0 24 45 45 54 Mean

12.8 14.6 4.9 2.4 3.8 10.3 6.4 4.7 9.4 3.7 25.0 5.8 7.6 7.3 14.7 0.0 6.6 7.2 12.9 7.3 8.5

Moore, 1999) identified linkages between anthropogenic shore platform lowering, cliff recession rates and increased slope stability risk. Recognition of the contribution that rock coast geomorphologists can make to engineering practice has generally been poor. For example, in the new edited book by Fookes et al. (2005) on geomorphology for engineers, the chapters on coastal geomorphology (Orford, 2005) and karst terrains (Waltham, 2005) make little or no reference to rock coasts. Meanwhile, Trenhaile (2009) argues that an ignorance of geomorphological research on rock coasts is to the detriment of the coastal engineering studies. It is important for all coastal geomorphologists and geomorphology more generally, that we seek to redress this imbalance in our communication with coastal managers and engineers. Therefore, we encourage rock coast geomorphologists to consider working more closely with coastal engineers and engineering geologists as well as publishing their research efforts beyond traditional geomorphology journals. 4. Recent progress in rock coast research Rock coast research has tended to explore landforms either in process-based investigations or in evolutionary-scale studies. The former are predominately designed to evaluate the relative (or more recently combined) contribution of different forces and processes to shaping modern coastal landforms whilst the latter have been preoccupied increasingly with elucidating, through modelling and to a lesser extent dating, whether shore platforms are contemporaneous or partly inherited features with a polygenetic origin. Thus, research on rock coasts also illustrates Summerfield's (2005) argument about the divergent scales at which recent geomorphology research is taking place. One of the most significant contributions to understanding rock coasts, at least based on the number of researchers and participating countries and institutions that were involved, was the European Shore Platform Erosional Dynamics (ESPED) project. A number of papers from this project were published in a special volume (Robinson and Lageat, 2006), which provided erosion rates on shore platforms from micro-erosion meter and small scale (1600 cm2) laser scanner measurements. The wide variety of erosion rates from different lithologies and morphogenetic environments was clearly demonstrated. Cliff recession rates and erosion processes, particularly on chalk cliffs, were also well represented. This research project was primarily focussed on examining current processes and recent landform evolution; rather than on unravelling larger scale questions

such as whether shore platforms are contemporaneous or polygenetic in origin. A discussion of recent rock coast research on inheritance (i.e. at the evolutionary scale) is presented in Section 4.2 below. Other individuals and groups have made significant contributions to process geomorphology research on rock coasts in recent years. Researchers have sought to improve quantification and understanding of individual processes. For example, Stephenson and Kirk (2001), Stephenson et al. (2004), Trenhaile (2006), Trenhaile et al. (2006), Gómez-Pujol et al. (2007), Porter and Trenhaile (2007) and Hemmingsen et al. (2007) have conducted field and laboratory experiments to examine the role of wetting and drying and on the nature and variability of surface expansion and contraction on shore platforms. Research has ranged from field experimental colonisation trials of rock blocks coupled with novel microscopy techniques (Naylor and Viles, 2002) to laboratory wetting and drying experiments by Kanyaya and Trenhaile (2005). Moses et al. (2006) tested all the rock types studied as part of the ESPED project to a range of geomechanical (i.e. rock material) tests. Increasingly, researchers are expanding the range of processes examined as part of research projects and/or coupling field and laboratory or laboratory and modelling studies. Trenhaile and colleagues are also investigating, in the laboratory and in the field, the effect of salt weathering, tidally induced freezing and thawing, and abrasion. Despite this flurry of process research activity in recent years, some processes have received comparatively little attention. Assessment of the potential for and impacts of wave quarrying and abrasion (or corrosion) on rock coast development has received very limited attention by geomorphologists. Stephenson and Kirk (2000) measured waves on a subhorizontal platform in New Zealand. They found that only between about 5 and 7% of the wave energy reaching the seaward edge of the platform reaches the cliff foot, and they concluded that wave erosion was not effective in this area. Trenhaile and Kanyaya (2007) measured waves in eastern Canada and calculated that they are able to quarry large joint blocks and other rock fragments from sloping platforms in this region, although not from subhorizontal surfaces, where the waves break at the seaward edge. Swantesson et al. (2006) examined the impacts of wave quarrying by process measurements, rather than by measuring the impacts of waves. They conducted repeat block and scar inventories, to calculate the volume and number of rock fragments over a set length of time. Among the few attempts to measure the effect of abrasion on rock coasts are those by Robinson (1977) and Blanco-Chao et al. (2007), and the recent laboratory experiments on abrasion potential by Moses et al. (2006).

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This special issue is set in the context of wider questions related to rock coast geomorphology. Some questions remain long standing but further questions arise as new techniques are applied (i.e. terrestrial laser scanners). Not unsurprisingly, research continues to be directed at understanding the processes responsible for rock coast development, although there is a clear bias towards shore platforms in these endeavours and the traditional argument over waves versus weathering continues. However researchers are perhaps not as far apart on this issue as in the past, with work increasingly being directed towards understanding the interplay of processes and their relative contribution, rather than in trying to identify a single process responsible for platform development (Trenhaile et al., 2006).

4.1. Erosion and time There are increasingly better data on rates of change on rock coasts, with micro-erosion meter records now reaching decadal scales. Stephenson et al. (this volume) report rates calculated over 30 years that were the same as data from 2 and 20 years (previous intervals of measurement), but they note that the loss of monitoring sites (to erosion) means that longer records are not necessarily representative of the range of rates operating across platforms. The application of laser scanners to measure cliff erosion has provided new opportunities to determine the frequency and magnitude of erosional events (Lim et al., this volume). These papers represent a widening of the temporal scales over which erosion is measured by both shortening and lengthening measurement intervals to months and inter-decadal scales, respectively. Despite such techniques and records of erosion, understanding how rock coasts develop over longer timescales of hundreds to thousands of years remains problematic.

4.2. Unravelling inheritance One of the greatest challenges confronting researchers is to determine the age of rock coasts and their rate of development in different types of rock and morphogenic environments. The occurrence of wide shore platforms in resistant, slowly eroding rocks has led many observers to question whether they are contemporary features that were formed since the sea reached its present level. Dating is generally difficult because of the lack of suitable sediments and tidal inundation which reduces the potential for the application of cosmogenic dating. Nevertheless, it has been demonstrated that some platforms were inherited from one or more interglacial stages when sea level was similar to today (Brooke et al., 1994; Stone et al., 1996; Trenhaile et al., 1999; Blanco Chao et al., 2003). Conversely, elevated erosional marine terraces in California, New Zealand, and on other coasts along convergent plate boundaries show that wide shore platforms can develop within the few thousand years of a single interglacial stage (Muhs and Szabo, 1982; Pillans, 1983). Modelling confirms that initial platform development is quite rapid, and that fairly wide surfaces can therefore develop within a few thousand years, and it also provides support for the contention that many platforms were at least partly inherited (Trenhaile, 2001). Modelling also suggests that in Australasia and other areas, particularly in the Southern Hemisphere, platforms may have partly developed during the early to middle Holocene, before the sea was raised, temporarily, above its present level (Trenhaile, this volume). Often we are unsure whether a coast is polygenetic or contemporaneous with current sea level. Kennedy (this volume) related the shore platform characteristics around Sydney Harbour to other studies which have dated similar raised platforms to the last interglacial. Clearly, an area of future work is to date raised platforms (or suitable deposits) to unravel more precisely whether shore platforms are of contemporaneous or polygenetic origin.

4.3. Considering theoretical geomorphology Little geomorphological theory is based on our understanding of rock coast systems; nor have rock coast geomorphologists contributed much to wider theoretical developments in geomorphology, until recently. For example, although there has been reasonable consideration of coastal environments as part of rock control theory (e.g. Yatsu, 1966; Sunamura, 1994), the theory has not been widely considered or adopted by the wider geomorphological community. For example, Yatsu's (1966) work on rock control theory has only been cited 27 times (ISI Web of Science, accessed 19 December 2008). Whalley (2007) suggests that the lack of citations may partly be attributed to the fact that the book was published in Japan and as such, had limited circulation. Similarly, until recently, rock coast geomorphologists have made few attempts to contribute to theoretical debates in the wider geomorphological community. Rock coast research can make significant contributions to a number of theoretical concepts, including disturbance and thresholds, complexity and scale issues, rock control theory, and contingency and emergence. Rock coast systems are wellplaced to do this, in part because of the fairly small spatial scales over which they occur (i.e. compared to landform evolution of mountainous regions and hillslope erosion process studies). Rock coasts are investigated at a variety of scales, however, including emerging research at the meso-scale (see Naylor and Stephenson, this volume), and may therefore help to bridge the “two scales of geomorphology” outlined by Summerfield (2005). In this special issue, authors are making linkages to, and in most cases stretching the boundaries of, particular theoretical debates in geomorphology. For example, several papers (e.g. Cruslock et al.; Kennedy; Naylor and Stephenson) explicitly examine aspects of rock control theory (Yatsu, 1966; Sunamura, 1994), with one also considering equifinality (Cruslock et al., this volume). Recently, Phillips (2004) suggested that geomorphic systems are contingent on a suite of parameters, such as past development, human activity and/or geological parameters. Rock coast geomorphologists are examining such contingencies. For example, researchers are debating whether platforms are contemporaneous or polygenetic in origin (i.e. temporal contingency) and on which parameters are current platform geomorphologies contingent (e.g. geological contingency). 4.4. Tackling disturbance thresholds Another problem, which is related to the long-term evolution of rock coasts, is the role of high magnitude events, such as extreme storms or tsunamis. Storms are known to play a large role in influencing the susceptibility of coastal systems to erosion (Betts et al., 2004). Stone and Orford (2004) edited a special issue of Marine Geology on “Storms and their significance in coastal morphosedimentary dynamics”. This special issue focused entirely on depositional coastal environments (Stone and Orford, 2004, p. 2), with scant mention of any the rocky components within the coastal margins of the North Atlantic Ocean (e.g. reference to “friable sandstone bedrock” in Forbes et al., 2004, p. 173); yet these features supply sediments to the depositional coasts focused on the Special Issue. No similar volume exists on the response of rock coast systems to storms. Hansom (2001) demonstrated that rock coast responses to extreme (high magnitude, low frequency) events are more variable than hitherto supposed. He noted that some rock coast systems are responsive to current forces, namely tsunamis and large storm events. For example, Hall et al. (2008) recently demonstrated that cliff erosion and subsequent deposition of large boulder debris has been driven by large storm, rather than tsunami events. Importantly, Hansom (2001) argued that further research is required to better understand the sensitivity of rock coast systems to environmental extremes generally, and more specifically, to evaluate whether particular deposits were

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derived from storms or tsunamis. Noormets et al. (2004) modelled mega-clast transport in Hawaii and determined that large clasts can be transported by either tsunamis or storm-waves. Switzer and Burston (this volume) extends Hansom's (2001) debate by illustrating the difficulty of identifying the type of high energy events (tsunami or storm waves) impacting on the New South Wales coast, a site of contentious debate over the role of tsunami in shaping rock coasts. Etienne and Paris (this volume) examine boulder deposits on rock coasts in Iceland and consider storm waves more likely than tsunami to be the transport mechanism. They note that severe storms have received comparatively little attention by researchers compared with tsunamis. The application of laser scanners to measure cliff erosion has created new opportunities to consider the frequency and magnitude of erosive events (Lim et al., this volume). A fruitful area for further study is investigating the frequency of erosion events, the rates of boulder abrasion, and examining how both weathering and erosion processes contribute to threshold changes in rock coast systems. 4.5. Applying rock control theory The role of geology in coastal geomorphology continues to be an area requiring further work. The relationships between geomorphology and geology and the complexity that variations in geology generate, are well represented in this issue (papers from Cruslock et al.; Kennedy; Naylor and Stephenson, all in this volume). Research by Cruslock et al. (this volume) ably demonstrates that comparisons made on the basis of varying lithological composition as the primary rock control parameter are not necessarily the best indicators of dominant processes. Instead, geological structure appears to play a greater role (than composition) in controlling the nature and scales of erosion of shore platforms in Sweden and Wales. Meanwhile, Kennedy (this volume) examines how platforms vary as one extends up an estuary and he concluded, based on qualitative observations, that rock structure plays a significant role in the morphology and spatial extent of shore platforms in Sydney Harbour. He aptly demonstrates that platform width is often governed by geological controls as well as waves. Future work may benefit from examining the interplay between rock control, waves and platform geomorphology. Naylor and Stephenson (this volume) take this theme one step further, by providing statistically robust, quantitative evidence of the powerful role discontinuities can play in shaping shore platforms, and the nature and scales at which erosion processes are likely to occur. 4.6. Complexity and scale This brief synopsis of recent papers in rock coast geomorphology, including those in this special issue, illustrate some of the complexity and scale issues that confront rock coast geomorphologists. Temporally, there is a need to better understand whether platforms are contemporaneous or polygenetic. Dating and modelling are needed to answer this question. The varying roles of different resisting forces and processes on platform morphologies, and on the possibility of different processes producing similar landforms are also well represented by the papers in this special issue. For example, the polygenetic origin of boulder deposits and their environmental significance will continue to be the subject of debate for some time. Similarly, Trenhaile et al. (2006) have illustrated the complex relationships that exist between processes and importantly, how these can change over time. As much as this recent work advances our understanding of rock coasts, it also raises a series of new questions within geomorphology and beyond. For example, rocky shore ecologists are increasingly interested in the ecosystem-value of rocky shore communities (e.g. Rius and Zabala, 2008) and habitat

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complexity (e.g. Archambault and Bourget, 1996) – that is the geomorphological and geological complexity of the intertidal zone. 5. Lessons from the inaugural working group session A few key messages emerged from the session on rock coasts and subsequent lively discussions held by members of the AIG/IAG rock coast working group at the IAG Regional Conference in Kota Kinabalu, Malaysia, June, 2007. Most notably, shore platforms are now a much less neglected landform than previously (e.g. Trenhaile, 1980), owing to the growth in research in recent years (e.g. Stephenson and Brander, 2003, 2004; Stephenson, 2006). Although there is still considerable work to be done on shore platforms, other coastal landforms such as stacks, beaches on shore platforms and consolidated clay coasts have received limited attention from coastal geomorphologists. Another important recognition is that greater integration is needed between rock cliff and shore platform science, as well as between studies of depositional and erosional landforms, and between those concerned with contemporary landform/process studies and palaeoenvironmental reconstruction. There is also a need to advance our ability to date and model rock coasts and to interact with those in allied disciplines. 5.1. Less studied landforms require greater consideration Despite considerable interest in shore platforms in the last decade there has been a general lack of current process research on other elements of primarily rocky coasts, or on the morphodynamics of coasts that are both erosional and depositional. There has been considerable Holocene coastal change research which focuses on the relationships between erosion and deposition (see for example, Long et al., 2006). However, although we now know much more about Quaternary sea levels and climate, and have much greater ability to date the associated sediments, little work has been conducted since the middle portion of the last century on the origin of certain landforms such as high cliffs with multiple slope elements (e.g. hogs backs, multistory, slope-over-wall cliffs). 5.1.1. Other types of rock coast The occurrence of arches, stacks, caves, keyhole inlets (Clark and Johnson, 1995) and related erosional features are described in most textbooks and they hold particular fascination for seaside vacationers, yet there has been limited research on their relationship to joints and other elements of rock structure (with the notable exception of Trenhaile et al., 1998), or to patterns of wave refraction, or attempts to determine their age and rate of development (see Roberts and Plater, 1999). Re-focussing our research efforts on these landforms may also serve to increase the awareness of rock coast geomorphology in the wider geomorphological context (e.g. Pfeffer, 2007), as well as helping to ensure that geomorphological sites of national and international importance are promoted for geoconservation and World Heritage Site designation (Schneider, 2004). 5.1.2. Beaches on shore platforms Although there has been a lot of research on the response of beaches to storm and post-storm conditions in the last 30 years, many beaches are on rock or clay foundations that limit the degree to which the cross-shore profile can adjust to ambient wave conditions. There has been little research on beaches with rigid foundations, their potential susceptibility to rising sea level and on whether they can be viewed conceptually using the morphodynamic model of Wright and Short (1984) and its variants. Trenhaile (2004) modelled the occurrence and morphodynamics of beaches on shore platforms based on the gradient of the platform, which is constant in the shortterm, and the equilibrium gradient of the beach, which varies in the short-term according to changes in wave conditions and possibly

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changes in grain size; however, this model has not yet been verified or tested in the field. 5.1.3. Integrating rock cliff and shore platform research While it may appear that studies of cliffs and platforms should be integrated, there has been a tendency for the two features to be treated independently, and they have often have been studied by different types of scientists. There is considerable scope for integrated cliff and platform studies which may perhaps be best achieved in collaboration with coastal engineers. For example, understanding wave dynamics on platforms and the delivery of energy to the cliff toe leads to improved modelling of cliff failure. Similarly engineering models of cliff failure could inform geomorphic models of platform development. Substantial benefits can be obtained by linking modelling with field studies, which can provide data for model calibration and validation (e.g. Hall et al., 2002). Dickson et al. (2007) provide an example of a recent collaboration between geomorphologists and engineers. The application of rock mechanics to cliff and platform erosion has particular promise with respect to understanding how rock coasts erode and how that erosion is controlled by geological factors. Cohesive clay coasts are well-suited for such collaboration. The majority of work on the erosion of consolidated clay coasts has been conducted by engineers, whilst work on rock coasts has generally been conducted by geomorphologists (see exceptions outlined in Section 3). We believe that the traditional distinction that has been made between these types of coast and harder rock coasts is artificial and untenable. Although clay coasts generally erode more rapidly than rock coasts, they share the fundamental characteristic that the removal of material by erosion, and subsequent coastal retreat, are irreversible. Rock coasts vary enormously in morphology and in the dominant processes that operate on them, and consolidated clay coasts simply represent a subset of this broad spectrum of forms. Wave erosional efficacy and the nature of the dominant processes on rocky coasts vary in space and time, according to surface gradient and roughness, and particularly as to whether there are seaward facing scarps or upstanding beds of dipping rock to promote water hammer and air compression in joints, bedding planes, and other discontinuities. Weathering efficacy also varies from one rock type to another, depending on its physical and chemical characteristics, and from place to place according to the prevailing climate and the elevation of the site with respect to the tides. As on many rock coasts, wave quarrying on clay coasts is inhibited by smooth, gently sloping intertidal surfaces, and the dominant wave erosional processes on clay platforms are therefore abrasion, where there is suitable loose material, and downwearing by granular breakdown owing to weathering and to shear stresses generated by turbulence and hydrostatic variations in pressure beneath shoaling waves and surf (Trenhaile 2009). 5.2. Modelling Modelling provides useful insights into the long-term evolution of slowly changing rock coasts. Trenhaile's (2000) wave erosional model has been used to study the development of shore platforms, erosional continental and volcanic island shelves, and elevated marine terraces, and in a modified form to consider the additional effect of surface downwearing by weathering (for example, Trenhaile in this volume, and references listed therein). Nevertheless, despite the ability of mathematical models to simulate the morphology and evolution of erosional rock surfaces, model coefficients should be calibrated in the field. Future improvements in our ability to model platform development therefore require the acquisition of field data on factors such as: (1) rates of surf attenuation and their relationship to water depth, bottom gradient and roughness; (2) the effect of wind shear and barometric pressure on the relationship between wave height and tidal

elevation and; (3) the relationship between wave forces and rates of rock backwearing. Modelling would also benefit from collaboration with engineering geologists, who are making considerable progress in our ability to predict how rock masses are likely to erode. 5.3. Linking rock coasts to wider coastal systems The contribution of cliff and platform erosion to coastal sedimentary budgets has rarely been investigated. Sediment pathways between erosional and depositional coasts have not been clearly identified or examined in detail. For example, what volume of sediments is derived from platform erosion as distinct from cliffs? Dornbusch et al. (2006) estimated that 10% of the 7700 m3 of flint delivered to East Sussex beaches is derived from shore platform erosion with the remainder coming from the cliff. Their findings are based on a geometric calculation relating platform width, the rate of cliff recession and downwearing of the platform to generate a ratio of cliff-derived to platform-derived materials. However, difficulties in determining cliff position and platform width from historical charts introduced significant errors (due to lack of metadata), and the authors emphasised the need to improve our capacity to quantify the volumes, relative contribution and timescales over which platformderived sediments are added to the sediment budget of a region. The following questions illustrate further issues that require investigation (and highlight the complex spatial-temporal issues in this area). How long do sediments yielded by rock erosion remain on rock coasts as, for example, in beaches on shore platform foundations, before being transported to adjacent areas? Similarly, is there sediment exchange between beaches and rock coasts or is the sediment pathway one way, from rock coasts to beaches? Sediment budgeting is an integrated model that has a direct management application (Bray et al., 1995; Komar, 1996), and understanding how, when, and in what quantities cliffs (and arguably platforms) yield sediments is a vital component of most sediment budgets. Failure to understand the link between cliff erosion and beaches has resulted in management responses that have starved beaches of sediment. For example, the eastern beaches of Port Philip Bay, Australia, were lost following seawall construction intended to prevent cliff erosion, and the subsequent need for beach renourishment has been at a considerable cost to the community (Bird, 1990, 1993). Better understanding of the sedimentary relationships between erosional and depositional coasts will also become more important as we seek to predict coastal response to changes in climate and rising sea level. Faster erosion of cliffs and shore platforms may threaten some infrastructure but the increased supply of sediment may partially off set net beach erosion in cases where inputs of locally derived sediments are important. The potential contribution of rock coast sediments to the provision of sediments of an appropriate competency to maintain sedimentary landforms (e.g. sand and gravel to beaches), is spatially variable and dependent on local geology as well as on wave forcing. However, the availability of such rock coastderived sediments is dependent on the coastal zone management decisions that are made; particularly, on whether cliffs are reinforced to prevent the loss of valuable assets sitting above them, rather than being left unprotected with the potential to provide sediments to nearby beaches, dunes, salt marshes and mud flats. Related to this, are ecosystem-based evaluations of rock coasts. Rock coast systems are important habitats for biota, thereby having value as a provisioning ecosystem service. Thompson et al. (2002) refer to this potential in their review and prospect on rocky shore ecology by stating that an understudied area in need of future research are complex habitats such as crevices, rock pools and sand-scoured areas – those habitats which require a good understanding of geomorphology, geology and rock control theory. Likewise, biota can also mediate rock coast processes (Fornós et al., 2006) and as such, climate change impacts such as rising sea surface temperatures (e.g.

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Helmuth et al., 2006) and ocean acidification (e.g. Bibby et al., 2007) may in turn affect the nature and rates of biogeomorphic processes operating on rock coasts. 5.4. Rock coasts and climate change Traditionally rock coasts have been viewed as having slow rates of erosion and consequently as being relatively resilient to climate change. However, recent work has shown that they can erode quite quickly, if not episodically, as illustrated by the laser scanning work of Lim et al. (this volume). Nevertheless, the response of rock coasts to rising sea level has received little attention in recent reviews of the possible effect of global warming on coastal erosion. For example, Zhang et al. (2004) made no reference to rocks, platforms or cliffs in their paper, nor did they refer to any rock coast research on the susceptibility of either cliffs or platforms to erosion. Although, Stive (2004) alluded to the potential importance of rock coasts as a source of sand, he did not comment on potential climate change impacts on these landforms. Most recently, Nicholls et al. (2007, p. 325–326) noted that “hard rock cliffs have a relatively high resistance to erosion, while cliffs formed in softer lithologies are likely to retreat more rapidly in the future” in response to rising sea level. Critically, the authors did not define the difference between soft and hard cliffs and on the basis of this ‘distinction’, all further discussion on coastal cliff vulnerability was focused on “soft” cliffs. As we have noted, the distinction is artificial and does not take account of the range of sensitivities within a broad grouping such as “hard cliffs”. Yet, this view is persuasive in recent government-funded research projects, such as EUrosion in Europe where the key results publication (aimed at end users) stated, “the implications of coastal resilience vary depending on the coastal type. For hard rock coasts resilience may not be critical because the rocks themselves are resistant to erosion. Conversely active erosion of ‘softrock’ cliffs (bluffs) is often a natural phenomenon contributing material to the coastal sediment volume,” (European Commission, 2004, p. 28). The view that “hard” cliffs are resistant to erosion is derived from a comparison with “soft” sedimentary coasts and using anthropomorphic frames of reference. For example, Sherman and Gares (2002, p. 2) argued that, “because these systems tend to change slowly, they are more difficult to study on human time scales. However, the widespread occurrence of these coastal features demands our attention.” Cliffs and platforms by definition are erosional features and any climate change that increases the efficiency of process acting on them will lead to increased rates of erosion. Furthermore “sensitive” coastal systems such as coral reefs, mangroves and salt marshes can respond dynamically to sea level rise and adjust their elevation upwards through sedimentation and biogenic growth; in contrast rock coasts can only respond through erosion. It follows then that the response of shore platform lowering rates to sea-level rise needs to be coupled with understanding the likelihood of increased cliff erosion, owing to reduced rates of wave attenuation caused by platform lowering. Similarly, and perhaps more importantly, hard rock cliffs may actually be far from ‘resilient’ since they have limited capacity for dynamic response to climate change pressures. Once a platform is lowered or a cliff retreats, it is very difficult to rebuild such features – especially when compared to management measures such as beach recharge. Consequently, there is an urgent need to better understand and quantify historic rates of cliff and platform change to help elucidate whether rock coasts are actually as ‘resilient’ as commonly thought (e.g. Hammar-Klose and Thieler, 2001). At the same time, it is also important that we better understand how (and for how long) features in rock coast environments, such as cliff fall debris, protect cliffs from the assailing force of waves and how platform widening buffers these impacts. Rising sea-level is just one of the factors that controls cliff retreat and platform lowering. Increased water depths will reduce wave

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attenuation rates, leading to greater expenditure of wave energy at the cliff foot, but changes in climatic conditions will also be important. Potential changes in storm frequency, duration or intensity will therefore have an impact on rock coast processes and as such, studies of the impacts of past, current and future storms on rock coast dynamics are required. Consideration must be given to changes in weathering regimes, such as increased wetting and drying if warming leads to more suitable conditions, or increased frost shattering in more extreme cold conditions (Hutchinson, 1998). It is conceivable that changes in climatic conditions could also reduce the efficiency of subaerial processes. Changes in precipitation and ground water will need to be considered (Nicholls et al., 2007). Future work on the impact of climate change on rock coasts should seek to model the changes in erosion rates following the predicted changes in the full suite of processes acting on cliffs and platforms. In order to attenuate climate change impacts from general rock coast dynamics, “there is a need for further reliable, quantitative data on processes and rates of erosion,” (Edwards, 2008, pp. 399) so that the impacts of climate change can be contextualized by empirical data on Holocene to current rates of erosion. It will also be important to link these geophysical studies to ecosystem impacts, to examine the implications of these predictions for rock coast ecosystems. 6. Future goals for rock coast geomorphology We see several interrelated goals for rock coast geomorphologists, primarily related to how we might better conduct our research, thereby leading to a step-change in the types of questions we can answer. We suggest the following: • Encourage rock coast scientists to contribute to wider theoretical debates and developments in geomorphology, as suggested by Stephenson (2006). • Foster interdisciplinary partnerships with geologists, engineers and Quaternary geomorphologists to advance scientific understanding of shore platforms (see Stephenson and Brander, 2003). This could build on increasingly widespread integration between ecologists and geomorphologists, as represented, for example, by three special issues in Geomorphology on these topics since 2002 (Viles and Naylor, 2002; Urban and Daniels, 2006; Renschler et al., 2007). • Design future studies to forge better conceptual and working links between erosional and depositional geomorphologists. This would usefully include research on the morphodynamics of beaches with rigid foundations and on the contribution of sediments from rock coasts to local sediment budgets. • Expand the repertoire of field and laboratory techniques and the scales at which they are deployed to better quantify the relative (and combined) contributions of different processes and resisting forces. These data can then be used to calibrate and evaluate models of coastal evolution, thereby working to link the evolutionary and process scale approaches. 7. Conclusions There are four key points that emerge from our review. First, research interest in rock coasts, and especially on shore platforms, is greater today than it has been for about 60 years. Second, considerable scientific advances have resulted from this research effort. In terms of advances there have been significant improvements in our ability to map these coasts, to measure erosion rates, and to identify and measure the processes that are operating on rocky substrates. Third, scientific benefits can be derived from increasing the spatial and temporal scales, the range of processes that are examined and through the application of new measurement techniques. The papers in this special issue illustrate that the wider consideration of such parameters can lead to improved understanding of the complex interactions that operate on rock coasts as

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well as providing contributions to geomorphological theory. Further work is required. We need to improve our ability to determine the age and long-term development of rock coasts in different environments and, to better understand the relative effects of multiple processes (often at different scales) on erosion risk. Fourth, we need greater “collaborative work between researchers from a number of morphogenetic environments” (Stephenson 2000, p. 311) and with scientists in allied disciplines. The papers in the special issue illustrate the point that researchers are experimenting with new techniques and improving our capacity to quantify process – landform responses at a broad range of spatial and temporal scales. Perhaps most importantly, we appear to be moving from explaining processes according to theories and typologies of the 20th century to conceptualizing and contextualizing our work in the wider geomorphic and earth science literature. Acknowledgements The Guest Editors of this Special Issue (WJS and LAN) were responsible for the review process, and the preparation of this Special Issue would not have been possible without the help of a number of selected reviewers. The work and valuable contributions by the reviewers are greatly acknowledged. Also the support of journal editors Andrew Plater and Adrian Harvey as well as Elsevier's production staff is greatly acknowledged. This review paper benefited from lively discussions between the three authors at the IAG Regional Conference in Kota Kinabalu, 2007. The manuscript was improved by the constructive comments of Andrew Plater and Stephan Harrison. Funding from the UK Resource Centre for Women in SET is much appreciated, as it facilitated Larissa Naylor's participation at the conference and henceforth, this paper. References Archambault, P., Bourget, E., 1996. Scales of coastal heterogeneity and benthic intertidal species richness, diversity and abundance. Marine Ecology Progress Series 136, 111–121. Betts, N.L., Orford, J.D., White, D., Graham, C.J., 2004. Storminess and surges in the South-Western approaches of the eastern North Atlantic: the synoptic climatology of recent extreme coastal storms. Marine Geology 210, 227–246. Bibby, R., Cleall-Harding, P., Rundle, S., Widdicombe, S., Spicer, J., 2007. Ocean acidification disrupts induced defences in the intertidal gastropod Littorina littorea. Biology Letters 3, 699–701. Bird, E.C.F., 1976. Coasts. Australian National University Press, Canberra. 282 pp. Bird, E.C.F., 1990. Artificial beach nourishment on the shores of Port Phillip Bay, Australia. Journal of Coastal Research SI 6, 55–68. Bird, E.C.F., 1993. The Coast of Victoria. Melbourne University Press, Melbourne. 324 pp. Bird, E.C.F., 2000. Coastal Geomorphology An Introduction. John Wiley & Sons, Chichester. 322 pp. Blanco Chao, R., Costa Casais, M., Martínez Cortizas, A., Pérez Alberti, A., Trenhaile, A.S., 2003. Evolution and inheritance of a rock coast: western Galicia, northwestern Spain. Earth Surface Processes and Landforms 28, 757–775. Blanco-Chao, R., Perez Alberti, A., Trenhaile, A.S., Costa Casais, A., Valcarcel-Diaz, M., 2007. Shore platform abrasion in a para-periglacial environment, Galicia, northwestern Spain. Geomorphology 83, 136–151. Bray, M.J., Carter, D.J., Hooke, J.M., 1995. Littoral cell definition and budgets for Central Southern England. Journal of Coastal Research 11, 381–400. Brooke, B.P., Young, R.W., Bryant, E.A., Murray-Wallace, C.B., Price, D.M., 1994. A Pleistocene origin for shore platforms along the northern Illawarra coast. New South Wales Australian Geographer 25, 178–185. Brunsden, D., 2002. Geomorphological roulette for engineers and planners: some insights into an old game. Quarterly Journal of Engineering Geology and Hydrogeology 35, 101–142. Brunsden, D., Moore, P., 1999. Engineering geomorphology on the coast: lessons from west Dorset. Geomorphology 31, 391–409. Carter, R.W.G., 1988. Coastal Environments: An Introduction to the Physical, Ecological and Cultural Systems of Coastlines. Academic Press, London. 617 pp. Carter, R.W.G., Woodroffe, C.D. (Eds.), 1994. Coastal Evolution. Cambridge University Press, Cambridge. 515 pp. Charman, R., Cane, T., Moses, C.A., Williams, R.B.G., 2007. A device for measuring downwearing on cohesive shore platforms. Earth Surface Processes and Landforms 32, 2212–2221. Clark, H.C., Johnson, M.E., 1995. Coastal geomorphology of andesite from the Cretaceous Alisitos Formation in Baja California (Mexico). Journal of Coastal Research 11, 401–414.

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