Environmental Conservation 27 (4): 359–367 © 2000 Foundation for Environmental Conservation



Development of a coastal vulnerability index: a geomorphological
perspective
JOHN S. PETHICK1 & STEPHEN CROOKS1,2,3*
1
 Department of Marine Sciences and Coastal Management, University of Newcastle, Newcastle Upon Tyne NE1 7RU, UK, 2 Jackson
Environment Institute, University of East Anglia, Norwich NR4 7TJ, UK, and 3 Centre for Social and Economic Research on the Global
Environment (CSERGE), University of East Anglia, Norwich NR4 7TJ, UK
Date submitted: 1 March 2000 Date accepted: 6 July 2000


                                            Keywords: coastal management; shoreline management;
Summary
                                            geomorphology; vulnerability index; environmental change
Sustainable coastal resource management requires the
safeguarding and transmission to future generations of
                                            Introduction
a level and quality of natural resources that will
                                            Coasts are highly dynamic and geomorphologically complex
provide an ongoing yield of economic and environ-
                                            systems, which respond in a non-linear manner to extreme
mental services. All maritime nations are approaching
                                            events. Thus, in coastal management (CM) there is a critical
this goal with different issues in mind. The UK, which
                                            need to understand the geomorphological spatial and
has a long history of development and flood protection
                                            temporal aspects of coastal system response to perturbation.
in coastal areas, has chosen to adopt shoreline manage-
                                            While a vast literature exists detailing specific system
ment, rather than coastal management, so placing
                                            responses to perturbations, the tools to measure and commu-
coastal defence above all else as its primary and statu-
                                            nicate aspects of societal and environmental vulnerability has
tory objective. This paper aims to provide a
                                            been largely neglected. Only once we can measure vulner-
geomorphological perspective of long-term coastal
                                            ability can we then inform policy makers of the underlying
evolution and seeks to compare the UK approach with
                                            causes and the potential for ameliorating such vulnerability
wider interpretations of coastal management. Based
                                            (Adger 1999).
on a literature review, it is argued that coastal manage-
                                              To date the concept of, and research into, vulnerability
ment (CM) and shoreline management, as a subset of
                                            have been driven by global issues of climate change and its
CM, should share the same ultimate objectives, which
                                            impact on environmental and social systems. Prominent
are defined by many authorities as sustainable use. The
                                            amongst definitions of vulnerability is that of the
objectives, both strategic and pragmatic, which follow
                                            Intergovernmental Panel on Climate Change (IPCC),
from such an aim may appear to conflict with a reading
                                            according to whom vulnerability defines ‘the extent to which
of many of the texts for international and national CM
                                            climate change may damage or harm a system; it depends not
or designated area management which emphasizes
                                            only on system sensitivity but also the ability to adapt to new
stability rather than sustainability. The result is that
                                            climatic conditions’ (IPCC 1996). Sensitivity in this
coastal defence is seen not merely as a means to an end
                                            particular climatic context refers to the degree to which a
but as an end in itself. It is argued within this paper that
                                            system will respond to a change in climatic conditions. The
sustainable use of the coast, however, demands both
                                            concept of vulnerability, of course, extends across a whole
spatial and temporal flexibility of its component
                                            range of spatial scales including those at which coastal
systems, and management for change must therefore
                                            systems function. Here vulnerability may be defined as the
be the primary objective. Response of the natural
                                            exposure of social (and environmental) systems to stress as a
system to independent forcing factors must be encour-
                                            result of the impacts of environmental change (see Adger
aged under this objective, whether such forces are
                                            1999). This environmental change may be some combination
natural or anthropogenic. In achieving such an objec-
                                            of natural or anthropogenic forcing factors.
tive the concept of shoreline vulnerability may prove
                                              In focusing on the coast specifically, CM aims, through
useful. A simple and preliminary Vulnerability Index is
                                            holistic management for sustainable development, to main-
proposed, relating disturbance event frequency to
                                            tain a socially desirable mix of coastal zone products and
relaxation time (the time taken for the coastal feature
                                            services for current and future generations (Bower & Turner
to recover its form). This index provides a first order
                                            1998). At the same time, through adequate planning and
approximation of the temporal variability that may be
                                            control, CM must combine the maintenance of an optimal
expected in landform components of the shoreline
                                            level of environmental integrity, functioning and resilience,
system, so allowing management to provide more real-
                                            with reducing the level of vulnerability of coastal systems,
istic objectives for long-term sustainability in response
                                            and hence local populations, to catastrophic events and
to both natural and artificial forces.
                                            change. Thus assessment and planning to minimize vulner-
                                            ability is a critical and central element within CM (Townend
* Correspondence: Dr Stephen Crooks Tel: +44 (0)1603 593738
                                            1990).
Fax: +44 (0)1603 593739 e-mail: s.crooks@uea.ac.uk
360   J.S. Pethick and S. Crooks

  By way of guidance on assessment of risk and vulnerability   this unregulated pre-20th century development, with, in
to sea-level rise the Coastal Zone Management Sub-Group      England alone, over 860 km of soft cliffs protected from
(CZMS) of the IPCC suggested a common methodology         erosion (23% of the coastline) and in excess of 1259 km of
                                 sea-defences protecting 2347 km2 of embanked lowlands
based on a seven step process (IPCC, CZMS 1992):
                                 from flooding (Barne et al. 1996). It is on these coastal flood-
(1) delineate the case study and specify the sea-level rise    plains that over 5% of the population live (more than 2
  boundary conditions;                     million people) and 50% of the highest grade agricultural
(2) inventory the study area characteristics;           land is found. The remaining coastal natural resources in the
(3) identify relevant development factors;            UK are suffering from a sustained net decline, largely related
(4) assess physical changes and natural system responses;     to coastal squeeze of intertidal habitats (Carpenter & Pye
(5) formulate response strategies and assess their cost and    1996). Given the long history of dyke construction, strength-
  effects;                           ening property rights on the landward side of this boundary
(6) assess vulnerability profile and interpret results; and    have shaped, limited and confined the subsequent developing
(7) identify relevant sections to determine long-term ICZM    UK coastal zone management strategy into one principally of
  planning.                           shoreline management, namely flood protection in low-lying
                                 areas and protection from erosion of soft cliffs.
  This process, particularly step four, is often difficult to     In England and Wales, shoreline management is techni-
complete in attempting to assess physical change and       cally the responsibility of the Ministry of Agriculture,
environmental response. As Capobianco et al. (1999)        Fisheries and Food (MAFF), but MAFF delegates to its
discussed in a modelling perspective for integrated assess-    operating authorities, namely local maritime district councils
ment, ‘most large-scale coastal problems concern complex     on high, potentially eroding, coasts and the Environment
geomorphic systems like estuaries, deltas and tidal inlets.    Agency for low, potentially flooding coasts. These auth-
The amount of sediment which circulates within these       orities, while they have a duty to consider the nature
systems is often large compared with the exchange of sedi-    conservation implications of their management activities,
ments with neighbouring systems. A variety of           nevertheless regard it as their primary duty to reduce risks to
processed-based models may be applied to such systems,      people and property (MAFF 1993). Thus, although shoreline
producing useful results from a diagnostic point of view.     management may be seen as a sub-set of coastal management
However, their predictive value remains limited, either      it possesses a very different set of aims and objectives. The
because they only describe a residual transport field and     aim of CM, as, for example, set out by the UK Department
ignore morphodynamic interactions, or because they only      of Environment (DoE) in its strategy for the management of
include some of the relevant physical processes.’ In this way,  the coast (DoE 1993), is to achieve sustainable use of the
process-level modelling fails to account sufficiently for both   coastal environment, while that of shoreline management is
the larger temporal and spatial scales that are associated with  to achieve shoreline stability. Although the distinction
natural system evolution.                     between these two aims may appear slight, nevertheless it is
  The objectives of this paper are to clarify the roles of    argued here that it is in fact fundamental.
coastal and shoreline management and place them, along with
the understanding of coastal change, in the context of
                                 Sustainable use of the coast
sustainable resource utilization. Based on the wide recog-
nition that management must incorporate temporal and       Sustainability is now the dominant paradigm for the
spatial change, this paper concludes with suggestions for the   proposed management of the world’s coastal, and other,
creation of a simple and preliminary first order Vulnerability   environmental systems. The advancement of this paradigm
Index to aid identification of (1) whether a coastal system is   reflects growing and widespread political awareness of the
under threat of failure because of human perturbations, or (2)  need to manage the global environment (of which coastal
whether the change in coastal configuration of concern is part   zones are a key component) in a more holistic manner
of a natural or quasi-natural cyclical readjustment and will in  (WCED 1987; FAO 1992; OECD 1997). The scientific and
time return to a stable and resilient state.           policy literature abounds with definitions of sustainable
                                 development reflecting the broad range of world-views and
                                 the depth of debate over what sustainability means and
Shoreline management
                                 whether it can be achieved. The most familiar of the many
From the point of view of each maritime nation, the develop-   interpretations of sustainable development is the Brundtland
ment of coastal management strategies is largely a response to  definition: ‘Sustainable development that meets the needs of
existing problems within domestic coastal areas. In the UK,    the present without compromising the ability of future
many lowland shoreline boundaries were delineated clearly     generations to meet their own needs’ (WCED 1987).
into marine and terrestrial components, by the ad hoc        Two of the major issues to which such a definition has
construction of flood embankments to ‘improve’ coastal       given rise are, first, the confusion it engenders between the
floodplain areas. The present coastal configuration reflects     concepts of sustainability and permanence and, second, the
                                                                 361
                                       Development of a coastal vulnerability index

implication that the needs of the present generation may      coastal system may, as acknowledged in the excerpts given
indeed be met as a right (Turner 1993). Far from implying     above, be due to anthropogenic causes as well as natural
permanence, it can be argued that the primary objective of a    causes, but the primary response of the system is identical in
sustainable use policy must be management for change. It is    both cases. Cumulative anthropogenic forcing factors may,
only when the natural system is allowed time and space to     however, accelerate the process of coastal change and ‘trip’
develop naturally that true sustainability will be engendered.   positive or negative domain changes not previously encoun-
This concept has become known as ‘strong sustainability’,     tered. Whilst this is the case, it is also clear that attempting to
while the alternative view, that of merely preserving existing   stifle the response of a natural system to anthropogenically-
capital assets is referred to as ‘weak sustainability’ (Pearce &  induced changes may reduce system resilience further, so
Warford 1993; Turner 1993; O’Riordan 1995; Crooks &        emphasizing the negative impacts of human intervention
Turner 1999). Weak sustainability is that in which the       without allowing the system time to adjust to such modifi-
overall stock of natural capital, both natural and human-     cation.
made, be maintained, although allowing possibilities of
unlimited substitution between different forms of capital.
                                  Anthropogenic inputs
Theoretically, substitution may be between one ecosystem
form and another, or comparable functionality or via tech-     Instead of continuing this anthropophobic attitude,
nological advancement. However, complete substitution is      humankind should view itself as an integral part of the coastal
not always possible, and it is questionable whether human-     system: anthropogenically-introduced changes are perfectly
made advances can compensate fully for the loss of natural     acceptable as long the impacts changes are understood and
capital systems beyond critical threshold criteria. Strong     the temporal nature of infrastructure in the coastal zone is
sustainability is that in which the total stock of natural     recognized. The coastal system must be given sufficient time,
capital is non-declining, because of uncertainties over substi-  space and materials (usually in the form of sediments) to
tution possibilities and the adoption of a precautionary      adjust to a new equilibrium state. Given this change from one
approach, and the ecosystem is allowed to function in as      equilibrium change to another, attempts to re-engineer the
natural a manner as possible both for functional and ethical    former coastal system will result in reduced resilience as this
reasons.                              now lies within a non-equilibrium state. Thus, for example,
  The tendency, especially amongst shoreline managers, to     introduction of a hard coastal defence in order to protect an
adopt the former weak sustainability definition rather than     important asset may lead to the reshaping of adjacent shore-
strong sustainability may be, in some measure, attributable to   lines; this is a process which may take several decades to
a reading of the various documents providing guidance or      achieve but will eventually result in a new, stable, coastal
outlining legislation for coastal areas. In many cases,      morphology into which the artificial element has been incor-
temporal change at the coast is seen as the converse of      porated. Examples of such incorporation include many of the
sustainable use and, moreover, such change is often explicitly   18th century fishing harbours around the UK coast that now
defined as erosion or deposition (sometimes even referred to    appear to be moulded into their coastline but which, initially,
as degradation). For example Chapter 17 of Agenda 21 (UN      presented a major disruption to natural processes.
1992) states that: ‘Current approaches to the management of      This laissez faire approach to the development of strong
marine and coastal resources have not always proved capable    sustainability is appealing, but it may involve the loss of
of achieving sustainable development and coastal resources     existing infrastructure, natural, industrial or urban, found on
and the coastal environment are rapidly being degraded and     adjacent coastlines, which may be threatened by erosion,
eroded in many parts of the world’ (UN 1992, para 17.3). ‘As    deposition, flooding or other processes set in motion by the
concerns physical destruction of coastal and marine        introduced change. Building ‘permanent’ infrastructure on
areas…priority action should include control and prevention    eroding unstable cliffs, which supply sediment to down-
of coastal erosion and siltation due to anthropogenic factors’   stream coastal lowlands, is a clear indication of the impacts of
(UN 1992, para 17.29).                       inappropriate development. Such losses may be considered
  The Ramsar Convention includes the following enjoinder     unacceptable, but, in order to assess whether or not to
to guard against change: ‘Each contracting party shall arrange   proceed with coastal defence construction there is a need to
to be informed at the earliest possible time if the ecological   first be able to predict the medium to long-term effects of any
character of any wetland in its territory has changed, is     modifications to the natural system. Unfortunately we do
changing or is likely to change as the result of technological   not, at present, possess such predictive power for the
developments, pollution or other human interference’        medium to long-term development of coastal morphology. A
(Ramsar 1971, Article 3:2).                    strong sustainable coastal system is, therefore, one which
  These statements raise two major propositions. First,      cannot be designed since prediction of an optimum
erosion and siltation should be seen, not as degradation, but   morphology is not possible and, since managers are forced to
as natural responses to external changes leading to a steady    continue making adjustments to the coastal system a protec-
state which should be encouraged by managers who wish to      tionist philosophy of weak sustainability is constantly
achieve strong sustainability. Second, these changes to the    adopted.
362   J.S. Pethick and S. Crooks

Monitoring for sustainable use
There is, however, a middle road that may be adopted,
allowing progression towards strong sustainable utilization of
coastal systems that function with a mix of anthropogenic and
natural inputs. Careful monitoring of the coastal system can
be used to fine-tune anthropogenic inputs so that progressive
deterioration of natural and human assets is avoided while at
the same time allowing the system to change in response to
variations in inputs. This implies that, while prediction or
design of sustainable coastal systems is not possible, it is
possible to recognize an unsustainable system and take appro-
priate remedial action. This middle road considers the
attainment of sustainable coastal systems as a process not as a
plan, a process that manages change in the coastal system.
  The recognition of an unsustainable coastal system is,
however, more complex than may at first be supposed.
Deterioration cannot be equated simply with erosion as
suggested in Agenda 21 (UN 1992); the erosion of a sand
dune or salt marsh may be part of the internal functioning of
the wider coastal system, allowing it to adjust to changes in
energy or sediment caused by natural or anthropogenic
factors. Alternatively, the progressive loss of coastal land-
forms such as dunes, marshes or mudflats may be seen as
deterioration, in that the resulting system is less capable of
responding to imposed changes. The problem facing coastal
managers is to be able to distinguish between progressive
deterioration and system adjustments to changing inputs.


Temporal change in coastal systems
                                  Figure 1 (a) A diagrammatic representation of response to a
Natural coastal systems have to respond to constantly       step-like change in control variable, where the solid line of
changing environmental conditions that are imposed by such     the response curve indicated the mean condition about which
factors as tidal cycles, wave action or biological seasonality   fluctuations occur (dashed line). (b) Diagrammatic represen-
(Carter 1988). If perturbed, a coastal system in equilibrium    tation of potential coastal responses to large storm events
with environmental conditions will accommodate a distur-      (after Knighton 1998).
bance unless a critical threshold level is exceeded. A coastal
                                  long-term sustainability of the coast, since, if insufficient
system in an unstable state will, by contrast, not return to its
                                  time elapses for recovery between threshold events, the coast
pre-disturbance state. Many of these low-level, quasi-
                                  will suffer progressive change (Fig. 1b). Thus, after pertur-
continuous changes in the environment do not result in
                                  bation a coastal system may fluctuate about a stable state
morphological changes in the coast, since their energy levels
                                  equilibrium, be out of equilibrium or be approaching a new
lie below the threshold strength of the coastal form (Pethick
                                  equilibrium, depending on its initial status and the size of the
1996). In most cases, this threshold coastal strength has
                                  disturbance. The ratio between relaxation time and the
developed as a direct response to such environmental inputs,
                                  return interval for threshold events, referred to here as
for example, as in the depositional mudflat environment, so
                                  the Vulnerability Index, provides an important measure of
that a short-term balance is achieved between environmental
                                  the manner in which coastal landforms respond to imposed
inputs and system response. Infrequent but high-energy
                                  changes and can allow assessment of the potential for long
events such as extreme storms may, however, exceed this
                                  term progressive change in the system:
threshold strength and cause changes in the coastal
morphology (Pethick 1996). If such large disturbance events      Vulnerability Index = relaxation time/return interval (1)
were to persist, such change would represent a natural devel-
opment towards a sustainable form, but since they are
                                  A coastal vulnerability index
relatively rare, they are separated by periods of low energy
                                  Despite the crucial importance of the assessment of change in
during which the coast can recover from the effects of the
                                  coastal systems (such as the seven step ‘common method-
extreme event. This period of recovery, often referred to as
                                  ology’ for assessment of coastal vulnerability; IPCC, CZMS
the relaxation time of the system (Fig. 1a), is crucial to the
                                                                  363
                                       Development of a coastal vulnerability index

1992), no extensive research has been undertaken into the      sensitivity may explain why sand dune systems are
response of these landforms to imposed change. Table 1       commonly observed with eroding seaward margins and yet
shows some documented estimates of the return intervals and     with extensive dune ridges landwards (e.g. Carter 1988).
corresponding relaxation times of a range of coastal forms.
These data are collected from locations around the world and    Sand dune vulnerability
it must be borne in mind that event frequency will be highly    A vulnerability index and its component data could be used
variable by geographical area and by the local exposition of    as the basis for interpretation of databases assembled as part
the sites. Given that so much effort is related to short term,   of monitoring programmes based on remote, terrestrial or
site-specific studies obtaining appropriate data for the       marine sources. Simple observation of the temporal vari-
construction of a generic vulnerability index at present is     ability of coastal landforms is insufficient to allow assessment
difficult. Nevertheless, by way of example, a vulnerability     of long-term deterioration; instead, temporal changes in each
index constructed from such data (Table 1) provides a first     landform must be assessed in conjunction with the expected
order indication of the sensitivity of the landform to slight    behaviour as indicated by the vulnerability index. Based on
changes in its environment. Construction of a vulnerability     the discussion above, observation of erosion of the foredune
index for specific coastal regions will need locally specific     ridge in a sand dune system need not be taken as an indica-
data, the monitoring requirements of which are discussed      tion of long-term deterioration of the system. The sensitivity
below.                               of sand dunes to slight random changes in environmental
                                  conditions means that such change is to be expected and may
                                  continue for several years before a period of accretion ensues.
Small scale coastal landforms
                                  By way of example, Ritchie and Penland (1990) documented
Using the examples within this preliminary index, salt       storms to induce erosion on the Louisiana barrier coast dunes
marshes in southeast England were shown by Pethick (1992)      with a return interval of eight years, yet the dunes recovered
to suffer surface erosion under vegetation cover only during    within four years. Similarly, Orford et al. (1999) attributed
rare events when a high tide is combined with storm-wave      long-term dune erosion at Inch Spit, SW Ireland, to extreme
conditions. The return interval for such an event was calcu-    storm events (30–50 year frequency) with intervening
lated as being >30 years, but the marsh recovered from the     periods of recovery. The Inch study highlighted the fact that
erosion by rapid deposition in less than 5 years (Pethick      small, annual and sub-decadal scale erosion events had little
1992). The vulnerability index of this saltmarsh of 6.0 (Table   influence on dune field stability in the system concerned.
1) suggests that an increase in the return interval of such     These examples also serve to highlight the site-specific
storm events or a decrease in the deposition rate would not     nature of the vulnerability index for which locally referenced
result in progressive deterioration of the marsh system. The    data must be collected.
marsh system may thus be described as robust.
  On the other hand, sand dunes located in high-energy      Saltmarsh vulnerability
environments and with relatively slow recovery time, have a     Observations of the continued erosion of a salt marsh should
vulnerability index close to 1.0 (Table 1). This implies that    be cause for greater concern than those of a sand dune
even slight random variation in the return interval of erosive   system, since the high vulnerability index of this landform
wave events could mean that re-erosion of the foredune ridge    implies that only relatively massive changes in the return
may occur before the system has recovered fully from        interval of erosive events, or the ability of the marsh to
previous erosive events. However, the opposite is also true, in   recover from such events, can result in progressive loss of the
that a slight increase in the return interval of threshold events  marsh surface. Determining whether observations of erosion
can allow the sand dune system to prograde seaward. This      are progressive or merely part of a periodic adjustment to

Table 1 Example Vulnerability Indices for a range of coastal features. * Recovery refers to the form of the cliff itself not the
location of the form.
Shoreline      Event Frequency     Relaxation time     Vulnerability index   Example
           (yr)           (yr)
           100–103         >100–103
Cliffs                                 1           Brunsden & Chandler (1996)*; Moon
                                                and Healy 1994
Beaches          1             0.7        1.5          Bascom (1954); Gunton (1997)
Sand dunes         8             4         2           Ritchie and Penland (1990); Orford et
                                                al. (1999)
Mudflats          2             1         2           Pethick (1996)
Spits          500             50        10           De Boer (1988)
Salt marshes       33             5         6           Pethick (1992)
Estuaries      100 000           10 000        10           Metcalfe et al. (2000)
Shingle ridges    10–100            1–10         10           Forbes et al. (1995); Orford et al. (1995)
364   J.S. Pethick and S. Crooks

storm damage is, however, extremely difficult in this case due   steady state in the 10 000 years of the Holocene period.
to the time intervals involved. The probability of a measure-   Assuming that such major changes in sea level have taken
ment of the aerial extent of a salt marsh falling in a period of  place only in inter-glacial periods, the return interval for the
recovery from storm damage is 0.16 (five years in 30 years;     threshold event here may be as long as 100 000 years. This
Pethick 1992) Comparison of a single measurement taken       would mean that the vulnerability index for such estuaries
during this period of five years when a reduced marsh area is    could be 0.1, well within the range for landforms that are
present, with one taken during a previous stable period when    much smaller in scale and this implies that slight changes in
marsh area would be at a maximum, would immediately, but      sea level or deposition rates would have no long-term
mistakenly, be construed as marsh deterioration. Repeated     progressive impact on the estuarine system.
annual observations over at least 10 years would be needed to
establish a progressive change in marsh extent.          Open coasts
  It is clear that monitoring coastal landforms such as salt   At the opposite extreme, rocky cliffed coastlines also respond
marshes requires that the periodicity of measurement be      to major changes in sea level, again perhaps with an inter-
carefully adjusted to fit the vulnerability index and the relax-  glacial return interval, but here the strength of the cliffs
ation time period of the landform. Monitoring programmes,     prevents any rapid response to such changes, so that the
which do not incorporate such periodicity, may result in      relaxation time is extremely long. In some cases, cliff erosion
erroneous conclusions and stimulate coastal managers to take    appears to have continued throughout the last inter-glacial
wholly inappropriate defensive action resulting in a reduction   period, implying that the relaxation time of these large-scale
in the ability of the marsh to respond to future events.      hardrock systems is greater than the threshold return
                                  interval, and that the vulnerability index might be <1.0. If
                                  this is the case, then continued erosion of cliff coasts may be
Beach vulnerability
Similar conclusions could be made in the case of beaches, the   expected during the present inter-glacial period, despite the
high vulnerability index of which implies robustness of      negative feedback in the system whereby widening abrasion
response to environmental changes. Recovery of beaches       platforms reduce incident wave energy at the foot of the cliff.
after storm events can take hours and thus gives an apparent
permanence to this highly volatile environment. Cyclical
                                  Nested responses
changes on beach morphology may take place with seasonal
change (Gunton 1997). Beach erosion that is not matched by     In both these cases the large-scale landform, whether estuary
such recovery must therefore be taken as an indication of a    or open coast, has nested within it a number of smaller-scale
major environmental shift that is leading to progressive      forms such as beaches, shingle ridges, sand dunes, mudflats
deterioration. Such changes are often ascribed to anthro-     or marshes. Each of these components must respond to the
pogenic interference in the system, particularly to sediment    changes in environment that result from the adjustment of
inhibition by coastal defence works that can significantly     the larger unit and adjustment of its own. Thus, beaches
increase the relaxation time of adjacent beach systems       located on a cliff coast must respond to the gradual changes
(Cooper et al. 2000).                       in sediment supply and wave energy that are imposed upon
                                  them as the cliffs recede and abrasion platforms widen. It
                                  may be speculated that the gradual, world-wide erosion of
Large-scale coastal landforms
                                  beaches that has been noted over the past 100 years (Bird
Large-scale coastal landforms, such as estuaries, respond to    1985) could be a response to this large-scale change in
environmental changes in the same way as do the smaller-      environment. If this is so, then the causes of the present
scale forms already discussed. The difference in spatial scale,  observed deterioration in many beaches (Bird 1985) may be
however, is reflected in the temporal scale of such adjust-     internal to the larger system and not exclusively to any recent
ments and, since the smaller-scale forms are nested within     changes in environmental inputs.
the larger systems, this means that a complex series of        In the case of estuarine systems, the robustness implied by
internal responses is set up.                   the high vulnerability index means that if the system reaches
                                  an age at which it has recovered from its postglacial sea level
                                  shock, further minor changes in energy or sediments will
Estuaries
Monitoring and assessment of the long-term sustainability of    become less significant. To the small-scale components of the
larger coastal landforms such as estuaries or cliffed coasts    estuary, however, these environmental changes may repre-
presents much greater problems than the smaller-scale       sent major shifts in the environmental conditions and the
marshes, beaches or sand dunes, although speculative relax-    components will respond accordingly. Thus, an increase in
ation times and event return intervals for such large-scale    sea level may be expected to increase the joint probability of
systems (Table 1) may indicate some of the issues involved.    wave and tide events that threaten salt marshes and intertidal
Estuaries for example are a response to the change in post-    mudflats. The result may be a progressive decrease in inter-
glacial sea level and in many cases, especially those estuaries  tidal area, such as is now being observed (Viles & Spencer
with high sediment loads, appear to have achieved a form of    1995) in many coastal areas. Although the change in sea level
                                                               365
                                      Development of a coastal vulnerability index

itself may not affect the wider estuary system directly, such    to pollution (e.g. Ly 1980; Carter 1988; Stanley & Warne
reduction in the intertidal zone may have an indirect effect by   1993).
widening the estuary at a given point.              • A decrease in the area available for coastal landform devel-
                                  opment. This is perhaps the most prevalent cause of
                                  system deterioration and can involve such structures as
Monitoring and management
                                  reclamation banks for agriculture, coastal defences, estu-
It is apparent from these examples that the recognition of     arine training walls, or port and harbour constructions
progressive change in coastal systems depends to a large      (e.g. Bird 1985; Carpenter & Pye 1996).
extent on the ability to determine the periodicity of regular
adjustments to threshold events for coastal landforms and to     In each case, such interference results in a lowering of the
interpret any longer periods of change as deterioration.     vulnerability index until the critical threshold is attained
Provision of a suitable database, the measurement periodicity   when progressive deterioration is initiated. In the case of
of which is designed to coincide with coastal adjustments and   extremely responsive landforms, for instance sand dunes,
which will therefore allow such interpretation, must be seen as  such a process may be initiated by relatively small changes in
a major challenge for the immediate future. The rigorous     the environment. In other cases such as beaches, major
requirements of such a database call into question the utility  changes must be introduced before deterioration is affected.
of infrequent or randomly-spaced observations. For this      These considerations may allow a more positive approach to
reason, many types of remote sensing which depend upon      the introduction of artificial elements into coastal systems. It
cloud cover or tidal states can only be used if they are     is not necessary to attempt to return to some ideal natural
combined with more frequent and regular terrestrial obser-    system in order to provide a sustainable coastal morphology.
vations. Such a monitoring programme, composed of a variety    Anthropogenic changes can be introduced as long as the
of different types of observation, could be devised to allow   impacts allow the vulnerability index to remain above the
minimum cost commensurate with maximum temporal cover.      critical threshold. This means more careful management is
  Identification of deterioration, as opposed to adjustment,   required in treatment of sensitive coastal areas but less so in
is the primary goal of monitoring and the complexities of     robust areas. In cases where progressive deterioration is
such an assessment have been outlined here in an introduc-    noted, then it will be necessary to adjust anthropogenic
tory way. If progressive changes in a coastal system are     impacts to a safe level, rather than remove the impact
suspected, then identification of possible causal factors is    altogether.
necessary before any remedial action may be taken.
Heightened vulnerability of a coastal landform, reflected by a
                                 Conclusions
decrease in index value towards 1.0, may be caused by one or
both of two groups of factors, namely a decrease in return    This paper has attempted to provide a deterministic approach
intervals of energy events or an increase in the length of the  to the problems involved in the attainment of a geomorpho-
response time of the landform (Eq. 1). This is of course a    logically-sustainable coastal system. This approach is based
geomorphological perspective and it must borne in mind that    upon recovery-time vulnerability for coastal landform at
social vulnerability may stem from habitation in locations    given geographical locations. Human interference upon the
where disturbance occurs on a low frequency and high       resilience of the natural system must be viewed much in the
magnitude basis. Here shortening of relaxation time in      same way as any other driving force of environmental change.
coastal landform recovery may well induce a chain of       This is not to say that all human-induced activity in coastal
additional damages and disturbances that increase social     systems is acceptable, but that given the means to determine
vulnerability. Progressive change in a natural system cannot   deterioration in system geomorphological vulnerability the
be defined as deterioration, since eventually some new       possible need to adapt management strategies to reduce social
steady-state form will be attained. Progressive change that is  vulnerability becomes apparent.
due to anthropogenic causes may, however, be defined as        The prediction of a sustainable coastal morphology over
deterioration and here interference in natural coastal systems  long time periods must remain an elusive goal for the present;
can have three effects:                      instead, it is recommended that a programme of monitoring
                                 is initiated which allows the identification of coastal deterio-
• An increase in the frequency or magnitude of energy       ration in time for remedial action to be taken. This implies
 inputs (waves or currents). This may be caused by such     that the attainment of sustainable coastal systems is more of
 modifications as an increase in water depth due to        an ongoing process than the implementation of a well-defined
 dredging, or channel straightening or reclamation in an     plan. Such a programme does not entail protectionism or
 estuary (e.g. Inglis & Kestner 1958; Price & Kendrick      preservation but is intended to manage change in the coastal
 1963; O’Connor 1987).                      system so that it is allowed to adjust to the various environ-
• A decrease in the sediment supply to a coastal area caused   mental changes that occur, both natural and human. In order
 by coastal defence of eroding cliffs, aggregate dredging,    to determine the levels at which human interference might be
 fluvial dams, or loss of sediment-trapping vegetation due    sustainable, a vulnerability index, to be based upon site-
366   J.S. Pethick and S. Crooks

specific data, is proposed that would allow the sensitivity of      S.C. (1995) Morphodynamic evolution, self-organisation, and
                                    instability of coarse clastic barriers on paraglacial coasts. Marine
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                                   Gunton, A. (1997) Upper foreshore and sea wall stability, Jersey,
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                                   Inglis, C.C. & Kestner, F.J.T. (1958) The long-term
to suffer long-term deterioration. The task that faces society
                                    effects of training walls, reclamation and dredging on estuaries.
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research base that is capable of defining the precise limits of    IPCC (1996) Second Assessment Report: the Science of Climate
such tolerances, only then will we be able to utilize our coastal    Change. Cambridge, UK: Cambridge University Press: 564 pp.
resources in a sustainable manner over the long term.         IPCC, CZMS (1992) Global Climate Change and the Rising Challenge
                                    of the Sea. Geneva, Switzerland: Meteorological Organization and
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