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Ž.
Catena 42 2001 291–305 www.elsevier.comrlocatercatena
Coastal sensitivity to environmental change: a view
from the beach
J.D. Hansom)
Department of Geography and Topographic Science, UniÕersity of Glasgow, Glasgow G12 8QQ Scotland, UK
Abstract
The coastal environment is home to a range of landforms and landscapes that include those at
either end of the sensitivity scale. The soft coastal landscapes of beaches, sand dunes, and
mudflats represent fast-responding and mobile geomorphic systems that are highly sensitive to
environmental change. On the other hand, coastal landscapes in areas of hard rock and inherited
Quaternary deposits represent relatively slow-responding systems that are, in the main, thought to
be relatively insensitive to change. This paper focuses upon both groups of systems and
demonstrates not only how soft coasts respond to environmental stimuli over a variety of time
scales but also how this inheritance shapes subsequent development. Using mainly Scottish soft
coast examples, the changes associated with sensitive coastal landscapes are assessed in terms of
the past timing and magnitude of changes in both sea level and sediment economy. Declining
abundance of coastal sediment has initiated a process of internal re-organisation that, because of
an inherent sensitivity to sea level change and low thresholds for the forcing of change,
fundamentally restricts future management options. The paper also attempts to show that hard rock
coasts are also responsive to particular types of event on short time scales and examples are
discussed that modify the accepted view of hard rock insensitivity to environmental change.
q2001 Elsevier Science B.V. All rights reserved.
Keywords: Soft coast inheritance; Hard coast HMLF events; Coastal sensitivity; Scotland
1. Introduction
Landscape or landform sensitivity can be defined as the ability of the landscape
or landform to undergo a recognizable change in response to changes in external
)Fax: q44-141-3304894.
Ž.
E-mail address: jhansom@geog.gla.ac.uk J.D. Hansom .
0341-8162r01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved.
Ž.
PII: S0341- 8 1 6 2 00 00142-9
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J.D. HansomrCatena 42 2001 291–305292
Ž.
controlling variables Brunsden and Thornes, 1979 . The nature of the response may
vary depending upon the amount and direction of change, the complexity of the system
Ž.Ž
and the time required for a new equilibrium to be achieved relaxation time Chorley
.
and Kennedy, 1971 . Many coastal landscapes contain elements that are viewed as being
Ž.
Fig. 1. a A well-adjusted soft coast, the gravels of Culbin Bar in the Moray Firth, Scotland extend westwards
Ž.
at a rate of 14 mryear. b The poorly adjusted hard coast. Plunging cliffs of Caithness flagstone, NE.
Scotland.
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J.D. HansomrCatena 42 2001 291–305 293
both sensitive and insensitive to environmental change. Soft sedimentary coasts are
representative of a responsive system that rapidly adjusts in a labile fashion to changing
environmental conditions, as evidenced by the degree of adjustment to prevailing
Ž.
conditions Fig. 1a . However, this may result in underestimation of the role of
inheritance in the subsequent evolution of soft coastal systems. Conversely, hard rock
coasts are generally thought of as slow-responding systems that persist unchanged for
long time periods and require major disturbances to the system controls to provide the
impetus for change. The argument that hard rock coasts are poorly adjusted to prevailing
Ž.
environmental conditions is also persuasive Fig. 1b , but risks underestimation of the
Ž.
role of extreme high-magnitude, low-frequency HMLF events in the forcing of change.
In some situations, dramatic responses to contemporary changes can be demonstrated via
an examination of cliff processes in relation to present day HMLF events.
A good example of the role of inheritance and its far reaching effect on subsequent
system performance is the Holocene history of sea level change and its effect on coastal
sediment budget change. Much of the development of the Scottish coast can be related
to fundamental changes over the Holocene and can be demonstrated at a variety of
scales. Events over three time scales are used here on the soft coast to demonstrate the
effect of adjustments to the forcing factors of change and the importance of coastal
inheritance. These are: mid-Holocene sediment switching, late-Holocene internal re-
organisation of coastal sediments, and present day coastal sediment deficits.
2. Soft coast: inheritance effects
2.1. Mid-Holocene sediment switching
Central to the functioning of the coastal system are the interactions between relative
Ž.
sea level RSL , which affects the stability of coastal forms and sediment abundance or
deficit, which results in either progradation sufficient to reverse transgression or erosion
Ž.Ž.
even when RSL is falling Carter et al., 1987; Roy et al., 1994 Fig. 2 . In Scotland and
Ž. Ž
elsewhere, including much of Northern Europe Orford, 1987 , North America Boyd et
.Ž .
al., 1987 and Australia Roy et al., 1994 , sediment supply over the late-Glacial and
Holocene periods was controlled by availability on the continental shelf and by the sense
and magnitude of RSL change. Over the early part of the Holocene, the rapid rise in
RSL resulted in transgression which drowned moving shorelines rather than accomplish-
Ž.
ing significant shoreface modification Hansom, 1988 . Only when the rise in RSL
Ž.Ž.
began to slow down and stabilise at about 6.5 ka BP Firth et al., 1995 Fig. 3 did
wave activity establish widespread shoreface modification in association with the arrival
Ž.
of offshore gravels and then sand Carter, 1988 . However, in a static or slowly
Ž
changing RSL, the finite offshore sediment store declines through time Davies, 1974;
.
Bird, 1996 , leading to marked fluctuations in the coastal sediment economy. In Europe
and elsewhere, the resultant switch in sediment economy from surplus to decline at
Ž.
about 6.5 ka BP Carter, 1988, 1992 led to erosional shoreline tendencies irrespective of
RSL sense. However, the timing of the switch varied. In Eastern Scotland, RSL began to
fall in the mid-Holocene at a variety of start dates and rates depending on distance from
()
J.D. HansomrCatena 42 2001 291–305294
Ž.
Fig. 2. Coastal responses to the rate and sense of RSL and the availability of sediment after Carter, 1988 .
the centre of glaci-isostatic uplift and partly offset the negative effects of an overall
decline in the coastal sediment economy. In other areas, such as the Shetland Islands, the
Ž.
rise in RSL continued at a reduced rate Fig. 3 and led to a marked switch from
sediment surplus to deficit and increased shoreface erosion. This latter situation arose
under sediment deficit and rising RSL because shoreface sediments were transferred
offshore onto the shelf in order to maintain a nearshore gradient capable of dissipating
Ž.
wave energy Pethick, 1999 .
In the Scottish context, such erosion occurred first where high energy conditions and
rising RSL served to exacerbate the trend, conditions met in the north and west of
Scotland where foreshore freshwater peat, widespread erosion and tide-gauge evidence
Ž.
shows ongoing Holocene submergence Angus and Elliot, 1992 . In 1977, only 32 of the
Ž
466 Highland and Islands beaches were progradational the proportion of beaches with
.
purely erosional edges reaches a maximum in the Western and Northern Islands
Ž.
Mather and Ritchie, 1977 . Reversal of the positive sediment economy that once fed
dune development resulted in the removal of the frontal dunes and the recycling of dune
Ž.
sediment Hansom, 1999 . As a result, few dune and machair systems in Scotland are
Ž.
now fronted by anything other than eroding sub-vertical mature dune faces Fig. 4
Ž.
Hansom and Angus, in press .
Support for such an analysis of abundance followed by declining coastal sediment
supply over the Holocene comes from archaeological sites built originally within
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J.D. HansomrCatena 42 2001 291–305 295
Fig. 3. Modelled RSL curves from the Shetland Islands to the Firth of Forth, Scotland. Note the earlier onset
Ž.
of RSL rise in the north and the mid-Holocene RSL fall in the south after Lambeck, 1993 .
accreting coastal dunes now behind beaches that are erosional. The houses at Rosinish in
Ž.
Benbecula show an onset of sand deposition from 5.7 ka BP Ritchie, 1979 . Neolithic
houses within sand deposits at Skara Brae in Orkney were occupied around 4.7 ka BP
and then abandoned amidst environmental instability. Many other similarly aged sites
such as at Bornish and Baleshare in the Uists, and Traigh Varlish in Vatersay were also
likely abandoned due to wind blow and environmental instability. The onset of sand
arrival depended on local factors and at some sites may have been as early as 8.7 ka BP
Ž.
Ritchie, 1979; Gilbertson et al., 1996; Hansom and Angus, in press . The relatively
recent archaeological discoveries at many such sites is usually due to exhumation by the
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J.D. HansomrCatena 42 2001 291–305296
Fig. 4. Eroding dunes, such as these in the Isle of Iona, Argyll, are commonplace except where sediment is
locally plentiful, such as in estuaries.
wave-erosion of a sand dune edge. For example, Skara Brae was first revealed by
erosion of the dune front during storms in 1850.
It seems clear that the change from accretion to erosion was directly related to
mid-Holocene adjustments in sediment economy that have affected the sensitivity of the
coastal system and its subsequent performance. The role of inheritance appears all-per-
vasive.
2.2. Late-Holocene internal re-organisation of coastal sediments
On a shorter time scale, an important repercussion of the mid-Holocene decline of
sediment supply has been the late-Holocene re-organisation of coastal sediment into
progressively smaller coastal cells and sub-cells. Under conditions of sediment abun-
dance, coastal systems may adjust via the alongshore exchange of sediments between
Ž.
sections of the coast Roy et al., 1994 and by headland by-passing. However, in
Scotland and elsewhere the decline in the sediment economy and the resultant landwards
migration of the shoreline post 6.5 ka BP, favoured the emergence of barriers such as
headlands to alongshore exchanges. As a result, smaller and more discrete coastal
Ž.
sediment cells and sub-cells subsequently began to emerge Fig. 5 that are dominated
by the internal re-organisation processes of erosion and deposition.
A useful case study of the partitioning of a large coastal sediment cell into smaller
ones is the evolution of Culbin Sands in the Moray Firth, a beach and dune system
intimately linked to the longshore supply of sand and gravel from the east, including
both the Findhorn and Spey Rivers. About 6.5 ka BP, sediment from Spey Bay was
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J.D. HansomrCatena 42 2001 291–305 297
Ž.
Fig. 5. Coastal cells and sub-cells in Scotland as identified by Brampton et al. 1999 .
by-passing the headland of Burghead in its westwards movement into the Findhorn and
Ž.
Culbin area Comber et al., 1994 , leading to the construction of spectacular raised
gravel ridges that now underlie large areas of this coast. Based on estimates of the
amounts of sediment delivered to these beaches and their downdrift connections within
the same cell, a palaeosediment budget has been established that demonstrates former
Ž.
connectivity Comber, 1993 . However, at some time between 6.5 and 4.6 ka BP
Ž.
Comber et al., 1994 , widespread decline in the sediment economy contributed to the
emergence of headland barriers and the cessation of sediment supply from the Spey to
Culbin. Lack of sediment from the west and ongoing westwards tracking waves led to
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J.D. HansomrCatena 42 2001 291–305298
up-drift erosion of existing beaches, sediment transfer westwards, and down-drift
Ž
deposition in spits which deflected the exit of the River Findhorn west spits 1, 2 and 3,
.
Fig. 6 . Radiocarbon dating of peat shows the abandonment of spit 3 to have occurred
between 4.6 and 3.3 ka BP as the River Findhorn breached the eroding proximal end
Ž.
Comber et al., 1994 . This internal re-organisation of sediment within the cell continued
y1Ž
through a series of spits, the present one extending west at a rate of 16 m a Comber
.
et al, 1994 . Similar re-organisation occurred throughout Scottish coasts and led to the
establishment of the series of small and well-organised sediment cells observed today
Ž.Ž .
Fig. 5 Brampton et al., 1999 .
Further north in the Dornoch Firth, sediment re-organisation led to shifts in the wave
climate of the firth that fundamentally affected the subsequent performance of the
system. In the inner Dornoch Firth, a series of raised spits developed as a result of
Ž
changing relative sediment influx amounts from offshore and alongshore sources Fig.
.Ž .
7a Firth et al., 1995 . At the mid-Holocene high sea level before about 6.5 ka BP, the
waters of the inner Dornoch Firth were open to storm waves from the north east and
prominent westwards-trending raised gravel spits up to c10m OD developed at Meikle
Ž.
Ferry and Ness of Portnaculter Fig. 7b . At about the same time large sand spits began
to develop in the outer firth at Dornoch Point and, at 6.8 ka BP, at the Morrich More
Ž.Ž .
Fig. 7a Hansom and Leafe, 1990 . The development of the outer spits resulted in the
progressive isolation of the inner spits from wave energies sufficient to move their
constituent gravels and in a change to the relative importance of wave directions within
the firth. On the Scottish east coast, westerly waves are usually insignificant relative to
easterly waves, but the growth of Morrich More and Dornoch Point in the mid-Holocene
limited the ingress of easterly waves and led to a switch to favour westerly waves in the
central firth. As a result eastwards-trending, spits composed of small gravels were
constructed in the lower energy wave climate at c4m OD at Ard na Cailc and
particularly at Ardjachie Point. The inherited product that resulted from this energy shift
and now affects modern beaches, is erosion of the east side of the Morrich More due to
an enhanced easterly transfer of sediment.
Both of the above examples are the direct result of the creation of the smaller and
more isolated coastal sediment cells inherited from post 6.5 ka BP decline in the
sediment economy. Landwards translation of the shoreline, together with internal
re-organisation of its sediment store, has created a highly sensitive, and sensitised,
coastal system.
2.3. Present day coastal sediment deficits
The decline in the relative importance of offshore sediment sources has direct
implications for the short-term sensitivity of the present coast in the face of changing
Ž.
conditions, not only in Scotland but worldwide Bird, 1993 . As noted above, ongoing
erosion of up-drift sites serves to fuel down-drift accretion in those places affected by
the internal re-organisation of coastal sediment cells. To the east of Culbin at Spey Bay
Ž.
Fig. 6 , the declining importance of offshore sand and gravel sources and the switch to
a negative sediment economy has led to enhancement of the importance of gravels
sourced from the Spey River which now contributes about 6400 m3ay1of gravel to the
()
J.D. HansomrCatena 42 2001 291–305 299
Ž. Ž.
Fig. 6. Culbin Sands Fig. 1 has developed via internal re-organisation of sediments from updrift to fuel downdrift distal extension after Comber, 1995.
()
J.D. HansomrCatena 42 2001 291–305300
()
J.D. HansomrCatena 42 2001 291–305 301
Ž.
Fig. 8. Modern sediment supply to Spey Bay comes largely from the River Spey Profile 1 . Sediments eroded
Ž.
from here are redistributed alongshore e.g. to Profile 2 etc. and provide temporary volumetric additions to the
beach.
Ž.
coast in addition to sediment eroded from the shoreface Gemmell et al., 1997 .
Sediment is no longer added areally from offshore sources to the entire length of the
Spey Bay beaches. Instead, reduced volumes of sediment are now added as point
Ž.
sources at the Spey-mouth delta and at erosional sites Fig. 8 . Sediments from both
sources are periodically redistributed alongshore as discrete plugs of gravel by storm
events and can be identified as longshore-moving areas of sediment accreted to beach
Ž.
profiles Fig. 8 . Those profiles affected by the transit of a sediment plug undergo
temporary changes reflected in a seawards shift in the shoreline resulting from foreshore
accretion, but inter-plug areas are characterised by a landwards shift in the shoreline and
overtopping of the main beach ridge. Protection to particular profiles is afforded only for
the residence time of the sediment plug. Similarly, during the intervening periods of
local sediment deficit, the beach volume reduces to levels that threaten the integrity of
the main beach ridge and by implication, the land behind. This condition of locally
reduced sediment volumes now renders sections of the main beach ridge at Spey Bay
Ž.
Fig. 7. a In the early Holocene, the wide entrance of the Dornoch Firth allowed unrestricted access of high
Ž.
energy waves into the inner firth. b Early Holocene development of high energy gravel spits in the central
Dornoch Firth was curtailed by the growth of Dornoch Point and Morrich More and resulted in the
development of an eastwards drift of sediment.
()
J.D. HansomrCatena 42 2001 291–305302
progressively more susceptible to storm events of lesser magnitude than would previ-
Ž.
ously have been the case under a regime of plentiful sediment Gemmell et al., 1997 .
Such heightened sensitivity of the modern soft coastal system appears to be driven
largely by sediment deficits inherited from earlier in the Holocene. The human response
to such sensitivity is usually to stabilise the shoreline position using structures, a
response which may exacerbate the original condition by reducing the local supply of
Ž.
eroded sediment Bird, 1993, 1996; Viles and Spencer, 1995 . As a result, many
AstabilisedBsections of soft coast are now rendered increasingly insensitive to all but the
most dramatic of environmental change. In this light, to acknowledge the role of
inheritance and supply the beach with sediments to restore sensitivity seems a logical
management strategy.
3. Hard coast: sensitivity to HMLF events?
3.1. Present day HMLF eÕents and cliff processes
In marked contrast to the soft systems described above, the hard systems of rocky and
cliff coasts are seemingly insensitive to all but the most dramatic of changes and are
Ž.
generally understood to adjust at much slower rates Trenhaile, 1997 . For example,
plunging cliffs have altered little over long time periods and some are interglacial in age.
In such situations only large-scale disturbances in system controls, such as significant
Ž
sea level changes, or infrequent HMLF events may stimulate change Bryant et al.,
.
1992; Bryant and Young, 1996 . However, in some locations there is evidence to
suggest that the amount of change produced by HMLF events may be greater than
supposed and produces landforms that show a substantial amount of sensitivity to
environmental extremes.
For example, in exposed sites in the north and west of Scotland, cliffs that plunge
Ž
vertically into y20 to y30 m water depths are not unusual May and Hansom, in
.
press . As a result, these cliff coasts are subject to relatively unattenuated deepwater
waves, the dimensions of which are impressive. For example, analysis of wave records
to the west of Shetland show that waves exceeding 25 m in amplitude can be expected
Ž.
in major storms Hall et al., in preparation . Where plunging cliffs occur, such waves
may undergo only limited attenuation and access of breaking waves seems likely at
Ž.
heights of about 15–25 m above sea level asl . Consequently, in such situations
wave-wash stripping of vegetation is known to result in significant modification of the
Ž
cliff edge to considerable altitudes and distances inland Hall et al., in preparation; May
.
and Hansom, in press . Similar effects have been noted from Australia as a result of
Ž.
catastrophic wave or tsunami impact Bryant et al., 1992; Young et al., 1996 . Where
the coastal slope is more gentle, then wave attenuation is greater and the run-up altitude
of waves may be reduced. However, although they occur at several sites in Scotland, the
development of wave-placed accumulations of large boulders at heights of 15–25 m asl
Ž.
on top of such cliffs is less well known Steers, 1973; May and Hansom, in press .
At several sites in the Northern and Western Islands, cliff-top deposits composed of
boulders of c2.1m a-axis, 1m b-axis and 0.3m c-axis are arranged in prominent ridges.
()
J.D. HansomrCatena 42 2001 291–305 303
The boulders are arranged in ordered imbricate clusters that show almost perfect
Ž.
relationships with the intricacies of cliff-edge morphology Fig. 9 . For example, at
Eshaness, in the Shetland Islands, large slabs of ignimbrite have been quarried from the
adjacent cliff-top edge, and transported 50 m landwards to be deposited at 15 m asl in
Ž.
one of three cliff-top boulder ridges, the largest of which reaches 3.5 m Fig. 9 . The
boulders that comprise the ridges are fresh, with sharp and serated edges that can be
Ž
matched perfectly to fresh scars on the cliff edge quite unlike the adjacent rounded,
.
lichen-encrusted surfaces . It is likely that these boulders have been recently quarried
and moved to their present locations by HMLF events. Even higher, but less distinct,
deposits exist in Shetland up to 35 m asl. In Australia, similar wave-tossed boulder
masses and wave erosion of rock surfaces have been related to tsunami wave impact up
Ž.
to 32 m asl Bryant et al., 1992; Bryant and Young, 1996; Nott, 1997 .
Key questions remain as to whether the waves are storm- or tsunami-related, whether
the wave heights and forces generated at altitude are sufficient to quarry and move large
boulders, the date of emplacement of individual boulders and thus, the frequency of the
events that moved them. Mathematical and hardware modelling is under way to address
the first two issues and to establish the sensitivity of cliff edges to wave impact. A
variety of dating techniques are also being employed to secure dates for the emplace-
ment of the boulders. Nevertheless, field evidence in Shetland indicates that a series of
relatively recent events have deposited, or at least re-organised the fresh cliff-top
boulder ridges. In the absence of recent tsunamis in the North Atlantic, it seems
probable that the quarrying and movement of the fresh boulders may be the result of
Ž.
storms in the 1980s and 1990s Hall et al., in preparation .
Fig. 9. A wave-placed boulder ridge atop a 50-m-wide rock platform quarried into a 15 m asl cliff in northern
Shetland. The boulders of the 3.5-m-high ridge have been freshly quarried from the adjacent cliff edge.
()
J.D. HansomrCatena 42 2001 291–305304
Such processes of modification of cliffs and cliff-top deposits are by no means
commonplace, but their occurrence and degree of development strongly indicates that
the processes and periodicity of HMLF events may either be greatly underestimated or
the concept of low frequency needs to be process-qualified and quantified. The
environmental significance of extreme waves has relevance not only for coastal sensitiv-
ity in a variety of settings from Australia to the North Atlantic, but also for the safety of
offshore installations in such environments.
4. Conclusion
In many respects, the endless adjustments of the coastline should represent a perfect
example of the concept of sensitivity and adjustment to the present morphogenetic
environment, yet within the labile soft coast the role of inheritance is a central control on
subsequent system performance. On the other hand, hard rocky coasts should represent
lack of adjustment to present processes and classic slow responses, yet in some places a
surprising degree of sensitivity to HMLF events can be demonstrated. It follows that in
order for the sensitivity concept to fulfill its promise in contributing to the improved
understanding of coastlines and their management, the effects and timing of both
inheritance and process extremes on both soft and hard coastal landforms and develop-
ment need to be more fully understood.
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