Coastal Evolution - Late Quaternary Shoreline Morphodynamics

431 Pages·1997·28.26 MB·431\431

by  Carter, R.W.G., Woodroffe, C.D. (Eds.)

COASTAL EVOLUTION
Late Quaternary shoreline morphodynamics
Edited by
R. W. G. CARTER
University of Ulster at Coleraine
C. D. WOODROFFE
University of Wollongong,N ew South Wales
a contribution to IGCP Project 274: Coastal Evolution in the Quaternary
CAMBRIDGE
UNIVERSITY PRESS
PUBLISHED BY THE PRESS SYNDICATE OF THE UNIVERSITY OF CAMBRIDGE
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CAMBRIDGE UNIVERSITY PRESS
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0 Cambridge University Press 1994
This book is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without
the written permission of Cambridge University Press.
First published 1994
First paperback edition 1997
Printed in the United Kingdom at the University Press, Cambridge
Typeset in Times 10/13pt
A catalogue record for this book is available from the British Library
Library of Congress Cataloguing in Publication data
Coastal evolution: Late Quaternary shoreline morphodynamics/
edition by R. W. G. Carter, C. D. Woodroffe.
p. cm.
“A contribution to IGCP Project 274: Coastal Evolution in the Quaternary.”
Includes bibliographical references.
ISBN 0 521 41976 X
1. Paleogeography-Quaternary. 2. Coast changes. I. Carter,
Bill. (R. W. G.) 11. Woodroffe, C. D. 111. IGCP Project 274.
QE501.4.P3C63 1994
551.4’57-dc20 94-7763 CIP
ISBN 0 521 59890 7 paperback
ISBN 0 521 41976 X hardback
Contents
List of contributors page ix
Dedication J.D. Oford xi
Foreword 0. van de Plassche xvii
1 Coastal evolution: an introduction R. W.G. Carter and
C.D . Woodroffe 1
2 Morphodynamics of coastal evolution P.J. Cowell and
B.G. Thorn 33
3 Deltaic coasts J.R. Suter 87
4 Wave-dominated coasts P.S. Roy, P. J. Cowell,
M.A. Ferland and B.G. Thorn 121
5 Macrotidal estuaries J. Chappell and C.D. Woodroffe 187
6 Lagoons and microtidal coasts J.A.G. Cooper 219
7 Coral atolls R.F . McLean and C.D . Woodroffe 267
8 Continental shelf reef systems D. Hopley 303
9 Arctic coastal plain shorelines P.R. Hill, P. W. Barnes,
A. Hkquette and M-H. Ruz 34 1
10 Paraglacial coasts D.L. Forbes and J.P.M. Syvitski 373
11 Coastal cliffs and platforms G.B. Griggs and A.S. Trenhaile 425
12 Tectonic shorelines P.A. Pirazzoli 45 1
13 Developed coasts K. F. Nordstrom 477
Index 51 1
vii
Coastal evolution: an introduction
R.W.G. CARTER AND C.D. WOODROFFE
‘ifthe environment is the theatre, then evolution is the play’
G. Evelyn Hutchinson
Studies of coastal evolution examine and explore the reasons why the position
and nature of the shoreline alter from time to time. Although this type of
approach has been practised for generations - by geomorphologists, geologists
and engineers -events over the last two decades have brought a new immediacy
to the subject. Generally there has been realisation that many of the world’s
coastlines are under ‘threat’ (see, for example the recent US Geological Survey
Publication Coasts in crisis (Williams, Dodd & Gohn, 1990) or the
Intergovernmental Panel on Climate Change (IPCC) report Global climate
change and the rising challenge of the sea (1992)) and that environmental
change is the consequence of human occupation of shorelines, to which
adjustment (of some kind) is inevitable. Specifically, the spectre of rising sea
levels has induced a strong political response as well as raising inevitable
questions among scientists as to the state of our knowledge and understanding
(processes that are not always convergent). It would not be hard to conclude
that our knowledge is woefully thin. Despite many excellent studies from a
wide range of environments, we are still well short of understanding how a
coastline will respond or react to secular variations in forcing functions such as
sea-level rise, storm intensity and magnitude variations or shifts in the sea state
pattern. The commonest conclusion is to predict flooding or coastal erosion,
yet such processes are clearly only part of much broader responses, which need
to be viewed over a range of scales, in both space and time. It is important to
remember that erosion simply releases sediment for deposition elsewhere, and
that sediment budgets need to be considered in order to explain coastal evolution.
Current practices and paradigms
Most coastal scientists concerned with the processes, products and patterns of
shoreline evolution are aware of the need to investigate the fundamental issues
1
2 R.W.G. Carter & C.D. Woodrofe
which control sediment transport. This can be done directly, by the association
of fluid flows and fluid flow structures to the movement of individual particles
or mass aggregates, or indirectly through the interpretation of sediment
textures, structures and bedforms. A few studies combine both approaches, but
regardless of which approach is used there are many drawbacks, particularly in
developing robust, generic models of coastal behaviour that might be useful in
predicting future evolution - certainly of the type requested by planners and
shoreline managers. Part of the problem relates to the current database; with
very few exceptions data are sparse, particularly on long-term change.
Commonly, one is forced to use information of dubious or unreliable quality,
in which error is often acknowledged but seldom analysed. All too often a
coastal-change study will use a few historical maps, perhaps one or two aerial
photographs and some ground surveys, developing a discursive discussion
from this inherently unstable mix of information. If coastal change is obvious
and explainable then this approach can be relatively powerful and indicate
some underlying symptomatic process. However, in nature such examples are
rare, although, paradoxically, they may be relatively abundant in the coastal
literature, simply because both authors and editors would view them more
favourably. No-one wishes to publish a paper in which the information
(however carefully collected) fails to support a clear conclusion.
A second problem perhaps relates to a common scientific inability to break
with convention - this type of reluctance to shift to new ‘paradigms’ has been
discussed by Kuhn (1962). For many years, coastal evolution studies have
tended to be stereotyped, assuming that there is some kind of observable and
straightforward explanation for most changes (the example of the Bruun Rule
is discussed below). The universal presence of simple linkages is manifestly
not the case (just consider how many studies fail to find such links). This
problem is not confined to coastal studies; recently, Schumm (1991) has
examined a wide range of geomorphic examples and extracted many common
failings of this type, which lead inevitably to misinterpretation. As a matter of
some urgency, researchers concerned with coastal evolution should consider
the alternative models, even if there are few supporting data. The ideas of non-
linear response, stochastic development, deterministic chaos, catastrophism
and criticality all deserve investigation.
One, almost Pavlovian, response to the dilemmas listed above, is to collect
more and more data. There is some rationale in this as the bigger database may
well provide better insights, although more data do not equal better data.
Indeed, consistency of information, especially between different coastal workers
and their projects, is rarely questioned, yet it is evident that a modicum of
effort in standardisation might pay dividends in terms of better interpretation.
Coastal evolution: an introduction 3
Another recent trend (epitomised by the International Geological
Correlation Programme Project 274 which provided the impetus for this book)
has been to both widen the context of coastal studies and to move towards
integration between the two dominant spheres of research activity, namely
process studies of wave-morphology interaction and Holocene-scale
investigations of environmental change (especially charting the course of
former sea levels). This consideration of the co-adjustments of process and
form is termed morphodynamics (see Chapter 2).
The widening of context arises from the view of the shoreline as part of a
much larger system, within which the parts cannot be easily divorced.
Obviously, there are critical links between the oceans and the shore, but the
exact nature of these was practically unexplored until studies, initiated by
Swift, Hayes, Pilkey, Wright and others along the Gulf and eastern US
seaboards in the 1970s and 1980s, began to reveal the very intimate
connections between the beach, estuary, nearshore, shoreface and shelf. These
have a profound effect on sediment exchange (see Wright, 1987, for review).
Often, this type of sediment budget has developed on a long-term (Holocene or
Pleistocene - -lo3 to lo6 years) time scale, although there is no guarantee that
recent accelerations in the rates of global change will not reactivate and re-
establish such mass movements. A further important broadening of scope
comes from the view that terrestrial environments play a profound role in
coastal evolution. In some ways, this view has been implicit in studies since
the mid-nineteenth century, but nonetheless the increasing amount of
empirical evidence, especially where off-land sediment yields have been
decimated by such processes as shoreline ‘protection’ and river regulation, has
brought a new recognition to the subject. Again, time scale is important, as
there is evidence that such processes as sediment amount, transport and
delivery to the coast may vary by several orders of magnitude both through
natural, climatically controlled factors as well as through those mediated by
human activity. For example, Mediterranean coastal evolution shows distinct
episodes, which may be related not only to regional climatic change, but also
to anthropogenic influences such as agricultural expansion and dam building
(Vita-Finzi, 1964; Sestini, Jeftic & Milliman, 1989).
Over the last 15 000 years, first-order mediation of coastal evolution has
been related to global sea-level history, which in turn is coupled to global
climatic change and, to a lesser extent at this time scale, to tectontic activity
(Carter, 1992). There are strong latitudinal differences in climate response as
well as hemispheric ones, with the result that predictability of events is low.
Nonetheless, it is possible to speculate that, for example, fluctuations in the
glacial mass balance of Antarctica would have a worldwide impact. Anderson
4 R. W.G. Carter & C.D. Woodrofle
& Thomas (1991) suggest such ice-balance changes may well lead to rapid,
episodic sea-level variations around the world, which can be detected in
coastal sediments and morphology from a range of environments, assuming
one is looking for such evidence.
Coastal evolution also has an importance within the biological sciences. At
one level coasts need to be viewed as functional ecosystems playing an
important role in the recycling of nutrients and minerals (Brown &
McLachlan, 1990). It is not too far fetched to describe some coastal
environments, such as estuaries, as ‘reactors’, involved in energy production,
which is then exported to sustain adjacent, energy-deficient areas. For this
function alone it is necessary to maintain coastal dynamics. There is, however,
a broader function, related to long-term biological diversity and genetic
evolution. Woodroffe & Grindrod (1 99 1) indicate that sea-level change and
the consequent extent of intertidal habitat has been a major control on the
distribution of mangrove forests, and perhaps other intertidal communities,
which in turn gives rise to biodiversity. As such we have a vested interest in
coastal evolution as one means of providing global stability! Deceleration of
sea-level rise around 6000 years ago may have triggered Predynastic
agriculture in Egypt (Stanley & Warne, 1993a), setting the scene for the dawn
of western civilisation.
There is an opposite tendency to proselytise ideas which at one stage proved
interesting, but which through time become largely unsustainable. Such
concepts often have a large number of adherents, who are reluctant to abandon
what, in truth, is a simple, easily applicable procedure. There are plenty of
examples of such occurrences in the Earth Sciences - readers are referred to
Ginsburg’s (1973) book Evolving Concepts in Sedimentology for a series of
case studies and Dury (1978) for a more cynical view focusing on
geomorphology. Two examples in coastal evolution studies are the formula
devised by Bruun (1962), which claims to predict coastline retreat under sea-
level rise, and the ‘equilibrium profile concept’, which has a far longer history,
but which was calculated by Dean (1977, 1991; Dean, Healy & Dommerholt,
1993) to be modelled by the expression y = PX*‘~. (It is worth noting that both
Bruun and. Dean are engineers, with an interest in quantifying clients’
problems - an absolute value will always beat statistical caution.)
The original 1962 paper of Bruun’s is almost certainly a citation ‘classic’ as
it is widely and persistently quoted. Unfortunately, in the intervening 30 years
the basic idea has never been fully vindicated (certainly not in the field), for the
rather obvious reason that coastlines do not behave in a strict two-dimensional
sense. Although Bruun himself (Bruun, 1988) has tried to widen the rule’s
applicability, it remains in most situations an inadequate oversimplification.
Coastal evolution: an introduction 5
The equilibrium profile debate is a far more complex one. There is an
intuitive feeling that if a wave-formed slope (like a hillslope) is given enough
‘time’ it will come into some kind of equilibrium with the forces applied to it.
Most authors cite Cornaglia (1891) as being the first to introduce the concept.
However, as with the Bruun problem, a wave-formed slope is subject to
multidirectional forces of considerable spatial and temporal variability, so that
the simple ‘time to equilibrium’ models of, for example, Schumm & Lichty
(1965) and Carson & Kirkby (1972) cannot apply (see Fig. 1.la).
Alternatively, one anticipates a noisy model, in which there is constant
movement across and along the coast.
I I
time ---f time
I
i d H olocene
time time
Figure 1.1. (a) the concept of time as expressed by Schumm & Lichty (1965). Cyclic
time is represented by the evolution of landform attributes over geological time; graded
time is a shorter timescale, over which landforms may be considered to be in a steady
state. (b)A n example of dynamic equilibrium of reef islands on a Pacific atoll (after
Bayliss-Smith, 1988). Coarse-grained islands, termed motu, react to the passage of a
storm (hurricane) through an increase in size as rubble ramparts are formed; these break
down and are redistributed during the periods between storms. Cays, sandy islands, are
eroded by storms, but gradually recover after the passage of the storm. In mid-Holocene
times sand cays may have been in a steady state, and motu may have been undergoing
gradual net enlargment. During the late Holocene, as a result of less-frequent storms,
lower wave energy and reef productivity, less island vegetation and rising sea level,
islands may have less capacity to recover and may gradually become smaller.
6 R. W.G. Carter & C.D. Woodroffe
The recent work of Wright and others mentioned above (Wright, 1987,
1993; Young & Pilkey, 1992; Wright et al., 1991) is very important in this
context, as it begins to provide extensive data and new insights into the
problem of long-term nearshore response of sandy beach and barrier systems.
Pilkey et al. (1993) provide a recent review and analysis of the equilibrium
slope debate, and interestingly show the almost complete inapplicability of the
Dean formula to field data.
In many cases there is likely to be a dynamic equilibrium. One such
example is shown in Fig. 1. lb. On Pacific atolls there are two types of islands;
motu are islands composed of coral shingle and boulders on the higher-energy
reef flats, and cays are sandy islands in less-exposed situations. These respond
differently to the impact of tropical storms, with a net input of coarse material as
rubble ramparts on motu, but storm cut on cays. Motu subsequently alter with
redistribution of material between storms, while cays rebuild between storms.
Bayliss-Smith (1 988) has suggested that in mid-Holocene times these reef
islands may have existed in a steady state, with storm effects averaging out over
time, but in the late Holocene, under different conditions of storm occurrence
and with reduced vegetation cover, the islands may be exhibiting a dynamic
equilibrium, but gradually reducing in size (Fig. 1. lb; see also Chapter 7).
History of coastal evolution
For as long as shorelines have been occupied it is likely humans have taken
note of coastal changes. The oral traditions (dreamtimes) of Australian
aboriginals, perhaps extending back tens of millennia, identify clearly the early
Holocene transgression and the consequences of the rising sea in destroying
food sources and disrupting communications (Flood, 1993). Biblical evidence
also emphasises the impact of coastal change, particularly associated with
extreme events (Bentor, 1989). Geomorphological reconstructions indicate
how the site at Troy, and other cities referred to in the Iliad, have altered (Kraft,
Kayan & Erol, 1980). In this, and subsequent Old World periods, shoreline
changes were especially noteworthy where they interfered with commercial or
defensive activities around river mouths, harbours or ports (e.g. Inman, 1978;
Masters & Flemming, 1983).
The development of geographical studies of coastlines in the mid-
nineteenth century quickly acknowledged the need to understand how coastal
changes occurred. The apotheosis of this early movement came with the works
of Gilbert on Lake Bonneville, and Davis on Cape Cod, and later the studies of
Johnson (1919), of which his book The New England - Acadian Shoreline,
published in 1925, represents perhaps the finest example of the genre. In this
Coastal evolution: an introduction 7
Johnson attempts to argue from morphological evidence how a complex
shoreline has evolved. In the years immediately after Johnson’s monograph
was published, J. Alfred Steers began his career in Cambridge. Steers’ impact
on British - and to some extent Australian - coastal research was significant.
Steers (along with Vaughan Lewis) began a detailed investigation of coastal
changes around the British Isles, culminating in the publication of The
Coastline of England and Wales in 1946 (later extensively revised in 1964).
This work frequently refers to coastal evolution, as for instance in Steers’
interpretation of the evolution of Dungeness in southern England. Steers was
also responsible, along with biologists such as Frank Oliver, for the initiation
of long-term studies on Scolt Head Island off the Norfolk coast, and the
maintenance of records which have proved invaluable to present-day studies.
It is somewhat invidious to name only a few individuals, as many people
have been influential in the development of coastal evolution studies.
However, honourable mention should be made of Axel Schou who published
his monograph Det Marine Forland immediately after the Second World War.
This remarkable contribution - sadly in Danish - provides a concise and
detailed exposition of the evolution of the Danish coast. Although Schou
lacked much of the chronological support that modern coastal evolutionists
take for granted, he still managed to develop coherent arguments, which stand
the test of time. Slightly later comes the contribution of Andrt Guilcher in
France. Again, working under wartime restrictions, Guilcher was to bring a
broader perspective to coastal evolution studies through his oceanographic
(and to a lesser extent geological) interests. Guilcher’s text Coastal and
Submarine Morphology (published in French in 1954 and in English in 1958)
marked an important diversification, which has continued to find echoes in
many contemporary studies. Finally, it would be remiss not to mention the
Russian work of V. P. Zenkovich. Although occasional Russian studies had
appeared in the English literature, it was the 1967 translation and publication
of Zenkovich’s book which provided the first real insight into the parallel (and
in places, divergent) developments in coastal evolution studies between the
western and communist scientists. Many of Zenkovich’s examples clearly
found their original impetus in the pre-Revolution studies of Gilbert, Davis and
Johnson, but nonetheless a very distinctive praxis had developed. For example,
work on sediment tracing to ascertain sediment transport rates was progressing
in the 1930s, well before similar studies were attempted in the USA or UK.
Zenkovich’s own ideas on shoreline evolution - often based on spectacular
examples from inland sea coasts - are still worthy of consideration, although
many were completed by the 1950s.
During the 1950s and early 1960s, coastal research was still influenced by a